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dacaman12
12-01-2009, 10:16 AM
Throttle Body Selection With a Pocket Calculator by: dacaman12



An LX member sent me a message asking, “What size TB do I need?”, and I realized that there really wasn’t a clear answer to his question. Oversize TBs are readily available, but they don’t come with any sort of recommendations or technical info to help with the decision. The vendors that offer these things aren’t even providing actual flow numbers for their products. The lack of info has cause TB selection to be quite the debatable topic. Therefore, I’m going to attempt to make some sense of the matter with the aid of an ordinary scientific calculator, a few common formulas, and even a little geometry.

First, I’ll explain in detail how flow area of each TB will be established. We’ll essentially determine effective flow area for each TB we want to use in our calculations. We are going to look at 80mm, 85mm, and 90mm. First we figure the total area of the round “hole”. We, then subtract the area of the 10mm throttle shaft. To improve accuracy, we’ll multiply this area by a value for discharge coefficient (cd). A reliable source tells me a 90mm TB flows 1006cfm at 20”/H2O. We’ll use this info to get our “cd”, and assume that all TB sizes exhibit this value. The math………


(90mm / 25.4)^2 x pi / 4 = 9.86in^2 bore area
(90mm / 25.4) x (10mm / 25.4) = 1.395in^2 throttle blade area
9.86in^2 – 1.40in^2 = 8.46in^2 total area


calculating theoretical max cfm from area…..


(20in/H2O)^.5 x 66.2 = 296.1fps
9.86in^2 x 299.1fps / 2.4 = 1054cfm (theoretical max)
1006cfm / 1054cfm = .955 cd
8.46in^2 x .955cd = 8.08in^2 effective flow area


….Calculating for the other two sizes gives us the following values for our three TB sizes….


80mm = 6.26in^2
85mm = 7.14in^2
90mm = 8.08in^2


In order to calculate the TB size we need, we must figure how much air we are using. ? For this, we can use the infamous “carb sizing” formula.


Eq.1 cfm = (CID x RPM x VE%) / 3456


If we look at the term “cfm”, or ft^3/m, we see that it is the product of an area(ft^2) and a velocity(ft/m), resulting in a simple flow analysis equation that can help determine the size we nee, based off of flow area.


Flow (cfm) = Area(ft^2) x Velocity(ft/m)


...adding a constant to convert units…..


Eq.2 cfm = in^2 x fps / 2.4


So we now have a simple way to solve for TB size, based on area, given that we have values for velocity and cfm. We’ve already tackled cfm. So, how do we our velocity values? 4bbl carbs are typically flowed at 1.5 inches of Hg. However, unrestricted carbureted engines typically see 0.8-1.2 inches of Hg at peak power. We’ll use these values, multiplying by 13.61 to convert from Hg to H2O. To get our velocity, we’ll use a formula common to any head porters who uses a pitot probe. Here are the formulas in action…..


Eq.3 (in/Hg x 13.61)^.5 x 66.2 = Velocity (fps)
(0.8 x 13.61)^.5 x 66.2 = 218.4fps
(1.2 x 13.61)^.5 x 66.2 = 267.5fps


Let’s take a look at what the factory did. We’ll use 5400rpm for the 5.7L engine and 6200rpm for the 6.1L mill. Let’s also use 100% VE for stock performance of 345hp and 440hp, respectively. Using Eq.1…..


(345 x 5400 x 100%) / 3456 = 539cfm
(370 x 6200 x 100%) / 3456 = 664cfm


Now we calculate for area. We’re going to use 218.4fps for the 5.7L and 267.5fps for the 6.1L. I’m wondering if the same TB that was designed from scratch for the 5.7L engine was used on the 6.1L simply to satisfy the accounting department. Let’s use Eq.2 and see…….


539cfm x 2.4 / 218.4fps = 5.923in^2
664cfm x 2.4 / 267.5fps = 5.957in^2


…We can also work the math backwards, plugging in 6.26in^2 for the stock TB….


539cfm x 2.4 / 6.26in^2 = 206.6fps
664cfm x 2.4 / 6.26in^2 = 254.6fps


It appears that the decision to use the same TB on both engines was, indeed, made by the accountants. It also appears that they left a bit of room for a few bolt-on performance mods. Most importantly, the factory engineers agree with our .8-1.2 “/Hg range. Feeling confident with the math, let’s run the numbers for our fellow LX member and make a recommendation. He has a 423 stroker with ported heads and a custom grind cam suited for a 3600 stall TC, 5600 hp peak and a 6400 shift point with excellent VE%. Calculating for 0.8 and 1.2, or 218.4fps and 267.5fps, respectively….


(423 x 5600 x 116%)/3456 = 795cfm
754cfm x 2.4 / 218.4 = 8.73in^s
754cfm x 2.4 / 267.5 = 7.13in^2


That particular engine would need a full 85mm at minimum and could make use of a TB even larger than the 90mm piece. I wouldn’t even recommend one of the ported stock jobs for this particular application. The stock throttle blade diameter is still the restriction. Of course, with the 85mm and the 90mm being the same price, he might as well go for the 90mm and be done with it.


At this point, someone is thinking, ”yeah, yeah, but how power is it worth? And for how much?” Well, the dyno numbers and e.t. slips speak for themselves. However, power is determined by the air MASS we move through the engine and effectively burn. So, I done a little research and found a formula that calculates air density, based on barometric pressure and temperature. If temperature and VE% stay the same, we should be able to calculate, with fair accuracy, the change in air mass, thus power output. If VE% changes, then the difference will be even greater, meaning that our calculation is on the conservative side. We’ll use 29.921 “/Hg for standard barometric pressure. Doing all the math for the stock 80mm TB and the 90mm piece, we get 1.56”/Hg and .945”/Hg, respectively. 80°F seems like a reasonable number for intake temp. The calculation….


Eq.4 1.325 x (inches of Hg / (temp°F + 460)) = air density
1.325 x (29.921-1.56) / (80+460) = .0696lbs/ft^3
1.325 x (29.921-.945) / (80+460) = .0711lbs/ft^3
.0711 / .0696 = 1.02


So, we would have a difference in mass of 2%, and that same difference in output. But, remember, this is assuming that VE% remains constant. In reality, the smaller TB would, indeed, cause a restriction in flow and have an adverse effect on VE%, making the difference even greater. Also, we would incur additional pumping losses from running under greater vacuum.

Alas, we shall remain conservative and take a look at this 2% from a cost perspective. It just so happens that the guy with the 6.1L Super Stock football upgraded his mill with heads and a cam, only to capture the 6.1L modified football. $3200 worth of parts netted him 59hp, or an increase of 13%. Add another $1000 for installation, and we get $323 per 1% increase. Cast TBs can be had for $500, resulting in the upgrade cost of $250 per1% increase. This means that a 90mm TB would have a higher bang for buck value than even a head/cam swap on a 423 stroker. If VE% differs between the stock and 90mm, even the shiny billet TBs begin to have a favorable bang for buck value on this size engine.

It is kind of unfair to do all the work for one guy, and leave everyone else out. Therefore, I made a nifty excel sheet to calculate for the most common Gen III Hemi displacements. I used VE% of 110 for the chart, suggesting a fairly stout, efficient package. Here, for the first time ever, is a chart to help with TB selection, based on rpm.

Using the chart is simple. If peak power is higher than the rpm in the 1.2 “/Hg column, or peak torque is higher than the rpm in the 0.8”/Hg column, then you should really consider an upgrade. If peak power is higher than the rpm in the 0.8”/Hg column, or your shift point is is higher than the rpm in the 1.2”.Hg, an upgrade can be of benefit in an “max-effort” application, but isn’t absolutely necessary. I even included accompanying power outputs, so you can also make a selection based off of expected N/A power.


Throttle Body Selection Chart
-----------80mm ----------85mm ---------90mm
---------0.8-1.2"/Hg ----0.8-1.2"/Hg ----0.8-1.2"/Hg
345_____5190-6360_____5920-7250_____6700-8200
370_____4840-5930_____5520-6760_____6250-7650
392_____4570-5590_____5210-6380_____5900-7220
426_____4200-5150_____4790-5870_____5420-6640
440_____4070-4980_____4640-5680_____5250-6430
N/A hp____360-500_______440-580_______510-660



Now, let’s take a look for the forced induction guys. A lot of people seem to think that the blower cars could benefit from a larger TB even more than the N/A guys. Let’s take a look. I will be using some additional formulas, mainly intended to help with compressor selection. The main differences are in air density and temperature, although I’ll be looking at flow before the compressor, too, for those who are entertaining the idea of relocating the TB to this place. The standard “carb sizing” formula is used here as well, suggesting that the engine determines the actual volume of air that is moved. The compressor only changes density, by way of temperature and boost. We must find a way to determine the temp of the air going in the engine. To do this we first figure temp of the air exiting the compressor. Then, we use our intercooler efficiency to get the actual temp. Here are those formulas….


Eq.5 (boost psi + 14.7) / 14.7 = pressure ratio
Eq.6 ((Inlet temp°F + 460) x (pressure ratio)^.283) - 460 – Inlet temp°F = Ideal temp°F rise
Eq.7 Ideal temp rise / adiabatic efficiency + Inlet temp°F = actual compressor outlet temp
Eq.8 -1 x ( intercooler(IC) efficiency x ( IC temp°F in – temp°F ambient) – IC temp°F in) = IC temp°F out
Eq.9 ( Inlet temp°F + 460) /( IC temp°F out + 460 ) x pressure ratio = Density ratio
Eq.10 Outlet cfm x Density ratio = Inlet cfm


Although these formulas have a variety of uses, we’re only interested in using them for TB selection. Looking at Eq.10, we see that the changes in boost pressure and temp, therefore air density, only changes the inlet cfm requirements. Outlet cfm remains the same and is cal’d, using Eq.1. This is good news for those who have their TB after the compressor, like the Procharger systems. Because of the increased “outlet” temperature, it gets even better. The higher temps raise the speed of sound. The higher “mach speed” makes the engine think everything is larger in size, and will literally move your power band up in rpms. In essence the same size TB, after the compressor, will support even more rpms when used with boost. Sticking with the “calculation” theme, here is a formula to estimate the effects….


Eq.11 old rpm peak x ((outlet temp°F + 460) / (inlet temp°F + 460))^.5 = new rpm peak


We’ll use our friend’s 423 again for the example, adding 10 psi boost at 80°F ambient. We’ll also use 75% for both compressor adiabatic efficiency and IC efficiency.


(10 + 14.7) / 14.7 = 1.68 pressure ratio
(80+460) x (1.68)^.283 – 460 – 80 = 85°F ideal temp rise
85 / .75 + 80 = 193°F actual compressor outlet temp
-1 x ( .75 x (193-80)-193) = 108°F final temp out
(80 + 460)/(108 + 460) x 1.68 = 1.597 density ratio
5600 x ((108 + 460) / (80 + 460))^.5 = 5740 new rpm peak
5740 / 5600 x 1.597 = 1.636


…Or a 64% increase in output at approx. 150rpms higher with the exact same TB. What happens if we use a roots-type blower, or move the TB to the inlet side of the compressor? We can use our density ratio to figure this out, keeping in mind that the inlet side is still at ambient temp. Remember that demand was calc’d to be 795cfm…


795 outlet cfm x 1.64 = 1300 inlet cfm


Woah!!!! Vacuum at WOT just jumped to 2.5”/Hg with a 90mm TB. Relocating the TB to the front of the compressor would cause a huge restriction….Unless you were to use two of them.

It is my hope that all of these equations and examples will help make sense of TB selection for a variety of applications, and helps to explain “why” the larger TBs work. I also hope that the readers of this write-up will become a little more knowledgeable about their hotrods and the parts they are purchasing for them. Most importantly, when someone asks about a 90mm TB, we’ll have some real info to go with all of our “marketing” claims.

dacaman12
12-08-2009, 09:56 AM
Wow, no replies? I put a lot of work into this. Do I need to change something?

lafrad
12-08-2009, 10:12 AM
The numbers and formula's look great, facts can't be argued...

awesome! Time for me to upgrade, it seems.

(There is a lot here, might take a week or two more for people to REALLY chime in :-)

MAGFX
12-08-2009, 12:13 PM
I read over this a couple of times and am trying to ingest some of your information. I think that I may be going in the right direction on my build.

Thanks you.

Cam
12-08-2009, 12:15 PM
Wow, no replies? I put a lot of work into this. Do I need to change something?

You might start with your shorts...........but that's another thread. :doh:





Great job bud!. :not_worth

dacaman12
12-08-2009, 12:19 PM
You might start with your shorts...........but that's another thread. :doh:





Great job bud!. :not_worth

OHHHHhhh...That's why it smells funny in here, lol.:doh:

Thanks, guys.

oakcharger
12-08-2009, 02:49 PM
very good info, spot on in my opinion !!!! :thumbs_u:

CT-MSRT
12-08-2009, 03:19 PM
holy crap that's a lot of solid info. Thanks for the writeup.

Mymopar
12-08-2009, 04:31 PM
Very interesting stuff, but your chart makes it seem that the stock throttle body even for the built engines will do. You mention if peak power falls within the RPM range then this will do good for you, but peak power can be adjusted with a cam degreeing.
So in this example below:

392_____4570-5590_____5210-6380_____5900-7220

If you used a 85mm because peak power was at 5250 but moved the power curve lower to 4900 then the stock throttle body would do. In that sense you wouldn't even need a ported TB with a 392 at all.

I guess I'm just confused in understanding. If I used the largest TB even though my power range wasn't dictated by it, would it still be beneficial or not?

dacaman12
12-08-2009, 05:03 PM
Very interesting stuff, but your chart makes it seem that the stock throttle body even for the built engines will do. You mention if peak power falls within the RPM range then this will do good for you, but peak power can be adjusted with a cam degreeing.
So in this example below:

392_____4570-5590_____5210-6380_____5900-7220

If you used a 85mm because peak power was at 5250 but moved the power curve lower to 4900 then the stock throttle body would do. In that sense you wouldn't even need a ported TB with a 392 at all.

I guess I'm just confused in understanding. If I used the largest TB even though my power range wasn't dictated by it, would it still be beneficial or not?

You're thinking right. If a 392 nosed over at 4900, you would only need the stock TB.......Of course, you'd only be making about the same power as a stock 6.1L.

However, I'd expect a fairly stout 392 to have no problems going a bit above 6000 before nosing over. That would make for a good place to use an 85mm TB. We're still above the ".8" column for the 90mm with a hp peak that high, making the big boy a nice pick for a max-effort deal, even with the 392. Alas, with the extra tuning involved, modifying the manifold, and utilizing a shift point below 7000, I'd go with the 85mm just to make things easier and cheaper. Of course, some are more :rock: than others and could justify the extra hassle with the 90mm. Larger than a 392, then I'd go straight to the 90....unless my checkbook disagreed, lol.

MAN WGN
12-08-2009, 05:10 PM
:shock::shock::shock::shock:

WOW!!!!
Thanks for going over my head. LOL
I need to read this many more times to fully grasp all that you just spit out. Great info and thank you for taking the time to do this.

AZTorRed
12-08-2009, 05:15 PM
Wow, I was waiting for pictures. Great work.

dacaman12
12-08-2009, 05:24 PM
Wow, I was waiting for pictures. Great work.

Sorry, no pics for this one. Just Imagine a skinny red-headed dude sitting in front of a TI-83 calculator.:mrgreen:

blkbelttech
12-08-2009, 07:55 PM
but i got a 200 mm t/b on my v6 and i run 10's. :mrgreen:




er, i mean, great info. Too much science for me but you sounded like you knew what ya meant. :not_worth

Now can you recalculate everything taking into account the loss of VE due to the iat sensor? :mrgreen:
thanks for taking so much time to help others

Super T
12-08-2009, 08:14 PM
Very cool write-up! I'm still a little jealous of the Mustang GT that get two butterflies - lets you go big (error on the side of too much) without having crappy flow control at low load conditions.

ujokin2
12-08-2009, 08:37 PM
Looking at data you provided .... and trying to comprehend it... it does seem I could make use of the 100MMTB.. but then that means I would then have to port my intake manifold as well. Which results in sort of a delima for me as I have already a diminished wallet...

Eric Bryant
12-09-2009, 12:48 AM
Excellent write-up - nicely done!

Something to add, however - your calculations work fine for calculating the average airflow through the TB, but engines don't inhale smoothly - they take big gulps of air every time an intake valve opens. This effect is more pronounced as the engine displacement increases, or the cam ramp angles increase, or with manifolds with small-ish plenum volumes. It's really exaggerated with individual-runner throttle bodies, which is why a 150 HP 1000cc V-twin has a 54-58 mm throttle body for each cylinder.

The net effect of this is that "simple" calculations may underestimate the optimum throttle body size. This is why dyno results will sometimes contradict theoretical calcs.

dacaman12
12-09-2009, 09:20 AM
Now can you recalculate everything taking into account the loss of VE due to the iat sensor? :mrgreen:

Relocating the IAT sensor won't change the engines VE%........It could very well make the computer "think" the VE% is different, however. A lil tweak to the tune will put you right back where you started.

I bet playing around with the air density formula would give an idea of what relocating the IAT actually does to the computer. After all, the inputs are pressure and temperature.

Eq.4 1.325 x (inches of Hg / (temp°F + 460)) = air density

dacaman12
12-09-2009, 09:32 AM
Excellent write-up - nicely done!

Something to add, however - your calculations work fine for calculating the average airflow through the TB, but engines don't inhale smoothly - they take big gulps of air every time an intake valve opens. This effect is more pronounced as the engine displacement increases, or the cam ramp angles increase, or with manifolds with small-ish plenum volumes. It's really exaggerated with individual-runner throttle bodies, which is why a 150 HP 1000cc V-twin has a 54-58 mm throttle body for each cylinder.

The net effect of this is that "simple" calculations may underestimate the optimum throttle body size. This is why dyno results will sometimes contradict theoretical calcs.

Good point.

FWIW, the constant "3456" in the carb sizing formula is 12^3 to convert from in^3 to ft^3.........times 2, suggesting that each cylinder only inhales on every OTHER down-stroke.

To calc TB size for an individual runner deal, you would have to take your cam duration into consideration for starters. You would need the arrive upon a different constant to take the pulsing effect into consideration.

In that situation, the TB becomes part of your intake port. You'd also have to look at things like runner length, taper, and cross section from a head porting standpoint.

PM or post your intake duration and rpm of peak power, and I'll see if I can arrive upon 54-58mm with the TI-83. It's worth a shot, anyway.

blkbelttech
12-09-2009, 08:50 PM
Relocating the IAT sensor won't change the engines VE%........It could very well make the computer "think" the VE% is different, however. A lil tweak to the tune will put you right back where you started.

I bet playing around with the air density formula would give an idea of what relocating the IAT actually does to the computer. After all, the inputs are pressure and temperature.

Eq.4 1.325 x (inches of Hg / (temp°F + 460)) = air density

oh god man, i was just kidding. Relocating iat's is a joke :mrgreen: i meant take into account the interference of the iat in your calculations, and meant that only as a joke. :wink:

Cam
12-09-2009, 08:55 PM
oh god man, i was just kidding. Relocating iat's is a joke :mrgreen: i meant take into account the interference of the iat in your calculations, and meant that only as a joke. :wink:

Confuses say: Men with closet full of brown suites neber joke.

QuickRT2SRT
12-15-2009, 09:28 PM
Great writeup!

For the poster who called it too much "science", it's really not so much science as it is Math... Of course even science is math at its core... everything is.

:not_worth:not_worth:not_worth

Bryali
12-15-2009, 10:31 PM
Great write-up. I've got a headache trying to understand it all, but it's very informative. Beans to you!

BlueBee 6.1
12-27-2009, 10:33 PM
Wow what a write up, puts some theoretical structure to the empirical evidence of the dyno.
Reps to you for taking the time to do the work and post it for all to see and benefit.

I would be willing to wager that it will take a lot of us many reads and re-reads to fully comprehend the theory and calculations.

Good work!

Hemi Family
12-27-2009, 11:03 PM
Beans to you my skinny redheaded friend!

Just to throw something new into the mix...the intake manifold is port matched to the 80mm, wouldn't additional work at this location would be required to acheive the flow calculations described? Perhaps this is why most dyno results do not support the increased performance?

Alterocks
02-22-2010, 02:00 AM
wow, cool thread. no idea what it means but i think i should be good with a stock TB for quite sometime.

white lightning
03-06-2010, 05:19 PM
dacaman12, great technical info! thank you for taking the time to post this.:thumbs_u:

CplDaugherty
03-06-2010, 05:27 PM
Wow... I need to try reading that again when I'm not all hopped up on pain meds...
Great work.

Posted via LXFMobile http://media.lxforums.com/images/system_images/mobile/mobile.png

dudeiwin86
04-27-2010, 05:21 PM
where are you getting the constant you use of 2.4?
danke

lxmodguy
04-27-2010, 08:22 PM
very good info. I didn't read every post but I do have a question. The inital post used 100% VE as your base for calculations. What about engines that exceed 100% VE?

climberccm
04-28-2010, 06:59 AM
So what's all that mean: Here is an article done by PWR_Andy on throttle bodies:

85mm vs 90mm vs 100mm!
I see lots of talk about what size TB people should be using. I also see tons of vendors recomend smaller tb's stating that bigger is not always better. Well, I decided it was time to prove them wrong.

About 2 weeks ago, we tested a stock tb that had been ported to 85mm vs a 90mm and a 100mm tb. All three units used the same CAI and we revised the tune each time in order to dial in each tb as best we could.

First up are the pics.

90mm vs 100mm
http://www.challengertalk.com/forums/images/statusicon/wol_error.gifThis image has been resized. Click this bar to view the full image.http://i861.photobucket.com/albums/ab175/PW-Racing/Throttle%20Body/012.jpg
http://www.challengertalk.com/forums/images/statusicon/wol_error.gifThis image has been resized. Click this bar to view the full image.http://i861.photobucket.com/albums/ab175/PW-Racing/Throttle%20Body/013.jpg
http://www.challengertalk.com/forums/images/statusicon/wol_error.gifThis image has been resized. Click this bar to view the full image.http://i861.photobucket.com/albums/ab175/PW-Racing/Throttle%20Body/014.jpg



Stock TB vs 100mm
http://www.challengertalk.com/forums/images/statusicon/wol_error.gifThis image has been resized. Click this bar to view the full image.http://i861.photobucket.com/albums/ab175/PW-Racing/1000rwhp/DSCN0543.jpg


Dyno Sheet
Red: 85mm
Blue: 90mm
Green: 100mm
http://www.challengertalk.com/forums/images/statusicon/wol_error.gifThis image has been resized. Click this bar to view the full image.http://i861.photobucket.com/albums/ab175/PW-Racing/Throttle%20Body/TB-1.jpg

Data Chart
http://www.challengertalk.com/forums/images/statusicon/wol_error.gifThis image has been resized. Click this bar to view the full image.http://i861.photobucket.com/albums/ab175/PW-Racing/Throttle%20Body/TB.jpg


As you can see, the 100mm tb was good for 25rwhp/33rwtq over the ported 85mm TB that so many vendors try to push. The 100mm was also worth 12rwhp/12rwtq over the 90mm TB.

Draw your own conclusions, but 100mm is all I will be selling.

Car was an SRT8 Charger with heads/cam/intake/exhaust.



__________________

slideway
04-28-2010, 09:47 AM
^^ Wow! Pretty incredible that the 100mm made more gain over the 90mm then the 90mm made over over the ported 85mm(80mm butterfly) on top! I thought I read somewhere the 100mm actually has a 93mm butterfly.
Now maybe it's just the dyno but this engine is well down relative to it's PWR-PPP peers' with any of those TBs'. Did the cam had to be installed a bit retarded?

lxmodguy
04-28-2010, 09:52 AM
How is the average Joe going to put a 100MM throttle body on their car?

Also, why was a heads/cam/intake/exhaust SRT8 only making 400-425 wheel horsepower on a dynojet? Seems like low numbers considering we have cam only cars making 400 through a converter and gears on my mustang dyno.

manny72
04-28-2010, 10:22 AM
I'm just trying to figure out what size would be the best bang for your buck on a Daily driven 3.5 with a CAI and exhaust..

lxmodguy
04-28-2010, 10:30 AM
I'm just trying to figure out what size would be the best bang for your buck on a Daily driven 3.5 with a CAI and exhaust..

I would recommend a ported stock V6. It is really the only option I am aware of for the V6

S. Artee
04-28-2010, 11:38 AM
very good info. I didn't read every post but I do have a question. The inital post used 100% VE as your base for calculations. What about engines that exceed 100% VE?


How is the average Joe going to put a 100MM throttle body on their car?

Also, why was a heads/cam/intake/exhaust SRT8 only making 400-425 wheel horsepower on a dynojet? Seems like low numbers considering we have cam only cars making 400 through a converter and gears on my mustang dyno.


Two very good points. I'd like to know as well. :thumbs_u:

Cam
04-28-2010, 11:50 AM
So what's all that mean: Here is an article done by PWR_Andy on throttle bodies:

85mm vs 90mm vs 100mm!
I see lots of talk about what size TB people should be using. I also see tons of vendors recomend smaller tb's stating that bigger is not always better. Well, I decided it was time to prove them wrong.

About 2 weeks ago, we tested a stock tb that had been ported to 85mm vs a 90mm and a 100mm tb. All three units used the same CAI and we revised the tune each time in order to dial in each tb as best we could.

First up are the pics.

90mm vs 100mm
http://www.challengertalk.com/forums/images/statusicon/wol_error.gifThis image has been resized. Click this bar to view the full image.
http://www.challengertalk.com/forums/images/statusicon/wol_error.gifThis image has been resized. Click this bar to view the full image.
http://www.challengertalk.com/forums/images/statusicon/wol_error.gifThis image has been resized. Click this bar to view the full image.



Stock TB vs 100mm
http://www.challengertalk.com/forums/images/statusicon/wol_error.gifThis image has been resized. Click this bar to view the full image.


Dyno Sheet
Red: 85mm
Blue: 90mm
Green: 100mm
http://www.challengertalk.com/forums/images/statusicon/wol_error.gifThis image has been resized. Click this bar to view the full image.

Data Chart
http://www.challengertalk.com/forums/images/statusicon/wol_error.gifThis image has been resized. Click this bar to view the full image.


As you can see, the 100mm tb was good for 25rwhp/33rwtq over the ported 85mm TB that so many vendors try to push. The 100mm was also worth 12rwhp/12rwtq over the 90mm TB.

Draw your own conclusions, but 100mm is all I will be selling.

Car was an SRT8 Charger with heads/cam/intake/exhaust.



__________________

Hey boss, that's some great info there. But I think it would benefit everyone if we knew exactly what build this test was completed on. There are far to many variables to lay out a blanket statement that the 100mm is the only way to go.

And yes Bill, the 100mm is only a 92mm butterfly.

The gains viewed on that dyno graph are a very good linear indication of the cross sectional area of the TB. That must be one hell of a nice breathing motor you got there. :thumbs_u:

slideway
04-28-2010, 04:56 PM
http://i861.photobucket.com/albums/ab175/PW-Racing/1000rwhp/DSCN0543.jpg
This is the pic from Dad 6.1L's thread, Is this his motor getting tuning?
http://www.lxforums.com/board/showthread.php?t=198484&page=3

dacaman12
04-28-2010, 06:03 PM
where are you getting the constant you use of 2.4?
danke

Dude, the 2.4 constant converts the area figures from sq.ft to sq.inches and the time element from minutes to seconds.

The 100mm is, indeed, 92 or 93 at the blade. I think the gains result from a SHORTER choke point. I'd say the larger opening makes the elbow in the intake tube affect the throttle blade area less. The result would be a more even velocity profile at the blade, which would allow a higher average velocity.

dacaman12
04-28-2010, 06:05 PM
very good info. I didn't read every post but I do have a question. The inital post used 100% VE as your base for calculations. What about engines that exceed 100% VE?

There is an input for VE% in the requred cfm calc, labeled "Eq.1".:wink:

whitehemi
10-15-2010, 04:25 AM
Without VE being over 100% Would you need that 90mm or a 100mm tb?

So get that VE up.... Then buy your Ginormous TB:racing:



Volumetric efficiency in internal combustion engine (http://www.lxforums.com/wiki/Internal_combustion_engine) design (http://www.lxforums.com/wiki/Design) refers to the efficiency with which the engine can move the charge into and out of the cylinders (http://www.lxforums.com/wiki/Cylinder_(engine)). More specifically, volumetric efficiency is a ratio (or percentage) of what quantity of fuel and air actually enters the cylinder during induction to the actual capacity of the cylinder under static conditions. Therefore, those engines that can create higher induction manifold pressures - above ambient - will have efficiencies greater than 100%. Volumetric efficiencies can be improved in a number of ways, but most notably the size of the valve openings compared to the volume of the cylinder and streamlining the ports. Engines with higher volumetric efficiency will generally be able to run at higher speeds (commonly measured in RPM (http://www.lxforums.com/wiki/RPM)) and produce more overall power due to less parasitic power loss moving air in and out of the engine.
There are several standard ways to improve volumetric efficiency. A common approach for manufacturers is to use larger valves (http://www.lxforums.com/wiki/Poppet_valve) or multiple valves (http://www.lxforums.com/wiki/Multi-valve). Larger valves increase flow but weigh more. Multi-valve engines combine two or more smaller valves with areas greater than a single, large valve while having less weight. Carefully streamlining the ports increases flow capability. This is referred to as Porting (http://www.lxforums.com/wiki/Cylinder_head_porting) and is done with the aid of an air flow bench (http://www.lxforums.com/wiki/Air_flow_bench) for testing.
Many high performance cars use carefully arranged air intakes and tuned exhaust systems to push air into and out of the cylinders, making use of the resonance (http://www.lxforums.com/wiki/Resonance) of the system. Two-stroke engines (http://www.lxforums.com/wiki/Two-stroke_engine) take this concept even further with expansion chambers (http://www.lxforums.com/wiki/Expansion_chamber) that return the escaping air-fuel mixture back to the cylinder. A more modern technique, variable valve timing (http://www.lxforums.com/wiki/Variable_valve_timing), attempts to address changes in volumetric efficiency with changes in speed of the engine: at higher speeds the engine needs the valves open for a greater percentage of the cycle time to move the charge in and out of the engine.
Volumetric efficiencies above 100% can be reached by using forced induction (http://www.lxforums.com/wiki/Forced_induction) such as supercharging (http://www.lxforums.com/wiki/Supercharger) or turbocharging (http://www.lxforums.com/wiki/Turbocharger). With proper tuning, volumetric efficiencies above 100% can also be reached by naturally-aspirated engines (http://www.lxforums.com/wiki/Naturally-aspirated_engine). The limit for naturally-aspirated engines is about 137%[1] (http://www.lxforums.com/board/#cite_note-0); these engines are typically of a DOHC (http://www.lxforums.com/wiki/Overhead_camshaft#Double_overhead_camshaft) layout with four valves per cylinder (http://www.lxforums.com/wiki/Multi-valve).
More "radical" solutions include the sleeve valve (http://www.lxforums.com/wiki/Sleeve_valve) design, in which the valves are replaced outright with a rotating sleeve around the piston, or alternately a rotating sleeve under the cylinder head. In this system the ports can be as large as necessary, up to that of the entire cylinder wall. However there is a practical upper limit due to the strength of the sleeve, at larger sizes the pressure inside the cylinder can "pop" the sleeve if the port is too large.
Volumetric Efficiency is frequently abbreviated as "VE" when discussing engine efficiency.

inferno6.1
10-15-2010, 06:13 PM
Jeeez I can't stop drooling over that 100mm TB and the numbers it puts out!! I'm thinking the Arrington 90 would be my best bet for now though. Both are great TB's!

inferno6.1
10-15-2010, 06:17 PM
Actually this seems like a good place to ask a TB question...I have my manifold snout ported to 88mm...What is the opening on the Arrington 90 TB in mm?? Would that be a good match?

8yourM5
10-15-2010, 06:30 PM
Ok that was way wAy way to much info!! Amazing post man, two huge thumbs up.

Now if I wanted a 392 with 18 psi what t. Would you suggest since I can't understand a word of your math/ charts lol

inferno6.1
10-15-2010, 07:01 PM
I'll try again...lol

Actually this seems like a good place to ask a TB question...I have my manifold snout ported to 88mm...What is the opening on the Arrington 90 TB in mm?? Would that be a good match?

slideway
10-15-2010, 07:32 PM
I'll try again...lol

Actually this seems like a good place to ask a TB question...I have my manifold snout ported to 88mm...What is the opening on the Arrington 90 TB in mm?? Would that be a good match?
The opening on the 90mm TB is 90mm :) The lip for the seal on the manifold has to be machined off for the 90mm T.B. Then you use a gasket to mate the TB and manifold. But 88mm is big enough for the 85mm cast TB.

inferno6.1
10-15-2010, 07:37 PM
The opening on the 90mm TB is 90mm :) The lip for the seal on the manifold has to be machined off for the 90mm T.B. Then you use a gasket to mate the TB and manifold. But 88mm is big enough for the 85mm cast TB.

Ah...Thank you. I'm actually running a ported 88mm stock TB on her now. Runs strong...always looking to pick up a few extra whp though.:beerchug:

dudeiwin86
10-16-2010, 10:59 PM
I didn't know the stock unit could even take 88mm. Hmm

Sent from my DROIDX using Tapatalk

inferno6.1
10-16-2010, 11:08 PM
I didn't know the stock unit could even take 88mm. Hmm

Sent from my DROIDX using Tapatalk

Yep...Well the 87mm MM TB's are using stock cores...Mine's ported a little past that(by hand)...If you saw how thin the outside edge was you'd understand...haha. Scott saw it.:thumbs_u:

lashlarue
07-08-2011, 12:06 AM
GM had a ton of problems with both my 98 Buick GS, and my 98 C5 auto, both had screens in front of the throttle bodies to reduce turbulance of the intake, one being the Eaton supercharged Buick GS and the Ls1 powered Camaro and Corvette.I tried running without the screens and saw no difference in performance, although I felt 13.80s were somewhat anemic for a 345hp, 3300lb car.With a cai and zo6 catback.Dodge in its wisdom does not have screens or anything else to control excessive turbulance, if it indeed exists.

Hemifried
11-15-2011, 12:27 PM
all I can say is WOW!! this has got to be one of the most informative post's I have ever read! You deserve a HUGE pat on the back for taking what I'm guessing was a fare bit of time out of your day(day'sssssss) figuring out the mathematics for this. Adding the link to this page to my desktop... I need to come back a few more times and absorb more of this! wouldn't by grade 10 math teacher be proud of me trying to use formula's in a real world situation 25 years later! haha

Cheers dacamn12!!!:beerchug: and thanks again



Throttle Body Selection With a Pocket Calculator by: dacaman12



An LX member sent me a message asking, “What size TB do I need?”, and I realized that there really wasn’t a clear answer to his question. Oversize TBs are readily available, but they don’t come with any sort of recommendations or technical info to help with the decision. The vendors that offer these things aren’t even providing actual flow numbers for their products. The lack of info has cause TB selection to be quite the debatable topic. Therefore, I’m going to attempt to make some sense of the matter with the aid of an ordinary scientific calculator, a few common formulas, and even a little geometry.

First, I’ll explain in detail how flow area of each TB will be established. We’ll essentially determine effective flow area for each TB we want to use in our calculations. We are going to look at 80mm, 85mm, and 90mm. First we figure the total area of the round “hole”. We, then subtract the area of the 10mm throttle shaft. To improve accuracy, we’ll multiply this area by a value for discharge coefficient (cd). A reliable source tells me a 90mm TB flows 1006cfm at 20”/H2O. We’ll use this info to get our “cd”, and assume that all TB sizes exhibit this value. The math………


(90mm / 25.4)^2 x pi / 4 = 9.86in^2 bore area
(90mm / 25.4) x (10mm / 25.4) = 1.395in^2 throttle blade area
9.86in^2 – 1.40in^2 = 8.46in^2 total area


calculating theoretical max cfm from area…..


(20in/H2O)^.5 x 66.2 = 296.1fps
9.86in^2 x 299.1fps / 2.4 = 1054cfm (theoretical max)
1006cfm / 1054cfm = .955 cd
8.46in^2 x .955cd = 8.08in^2 effective flow area


….Calculating for the other two sizes gives us the following values for our three TB sizes….


80mm = 6.26in^2
85mm = 7.14in^2
90mm = 8.08in^2


In order to calculate the TB size we need, we must figure how much air we are using. ? For this, we can use the infamous “carb sizing” formula.


Eq.1 cfm = (CID x RPM x VE%) / 3456


If we look at the term “cfm”, or ft^3/m, we see that it is the product of an area(ft^2) and a velocity(ft/m), resulting in a simple flow analysis equation that can help determine the size we nee, based off of flow area.


Flow (cfm) = Area(ft^2) x Velocity(ft/m)


...adding a constant to convert units…..


Eq.2 cfm = in^2 x fps / 2.4


So we now have a simple way to solve for TB size, based on area, given that we have values for velocity and cfm. We’ve already tackled cfm. So, how do we our velocity values? 4bbl carbs are typically flowed at 1.5 inches of Hg. However, unrestricted carbureted engines typically see 0.8-1.2 inches of Hg at peak power. We’ll use these values, multiplying by 13.61 to convert from Hg to H2O. To get our velocity, we’ll use a formula common to any head porters who uses a pitot probe. Here are the formulas in action…..


Eq.3 (in/Hg x 13.61)^.5 x 66.2 = Velocity (fps)
(0.8 x 13.61)^.5 x 66.2 = 218.4fps
(1.2 x 13.61)^.5 x 66.2 = 267.5fps


Let’s take a look at what the factory did. We’ll use 5400rpm for the 5.7L engine and 6200rpm for the 6.1L mill. Let’s also use 100% VE for stock performance of 345hp and 440hp, respectively. Using Eq.1…..


(345 x 5400 x 100%) / 3456 = 539cfm
(370 x 6200 x 100%) / 3456 = 664cfm


Now we calculate for area. We’re going to use 218.4fps for the 5.7L and 267.5fps for the 6.1L. I’m wondering if the same TB that was designed from scratch for the 5.7L engine was used on the 6.1L simply to satisfy the accounting department. Let’s use Eq.2 and see…….


539cfm x 2.4 / 218.4fps = 5.923in^2
664cfm x 2.4 / 267.5fps = 5.957in^2


…We can also work the math backwards, plugging in 6.26in^2 for the stock TB….


539cfm x 2.4 / 6.26in^2 = 206.6fps
664cfm x 2.4 / 6.26in^2 = 254.6fps


It appears that the decision to use the same TB on both engines was, indeed, made by the accountants. It also appears that they left a bit of room for a few bolt-on performance mods. Most importantly, the factory engineers agree with our .8-1.2 “/Hg range. Feeling confident with the math, let’s run the numbers for our fellow LX member and make a recommendation. He has a 423 stroker with ported heads and a custom grind cam suited for a 3600 stall TC, 5600 hp peak and a 6400 shift point with excellent VE%. Calculating for 0.8 and 1.2, or 218.4fps and 267.5fps, respectively….


(423 x 5600 x 116%)/3456 = 795cfm
754cfm x 2.4 / 218.4 = 8.73in^s
754cfm x 2.4 / 267.5 = 7.13in^2


That particular engine would need a full 85mm at minimum and could make use of a TB even larger than the 90mm piece. I wouldn’t even recommend one of the ported stock jobs for this particular application. The stock throttle blade diameter is still the restriction. Of course, with the 85mm and the 90mm being the same price, he might as well go for the 90mm and be done with it.


At this point, someone is thinking, ”yeah, yeah, but how power is it worth? And for how much?” Well, the dyno numbers and e.t. slips speak for themselves. However, power is determined by the air MASS we move through the engine and effectively burn. So, I done a little research and found a formula that calculates air density, based on barometric pressure and temperature. If temperature and VE% stay the same, we should be able to calculate, with fair accuracy, the change in air mass, thus power output. If VE% changes, then the difference will be even greater, meaning that our calculation is on the conservative side. We’ll use 29.921 “/Hg for standard barometric pressure. Doing all the math for the stock 80mm TB and the 90mm piece, we get 1.56”/Hg and .945”/Hg, respectively. 80°F seems like a reasonable number for intake temp. The calculation….


Eq.4 1.325 x (inches of Hg / (temp°F + 460)) = air density
1.325 x (29.921-1.56) / (80+460) = .0696lbs/ft^3
1.325 x (29.921-.945) / (80+460) = .0711lbs/ft^3
.0711 / .0696 = 1.02


So, we would have a difference in mass of 2%, and that same difference in output. But, remember, this is assuming that VE% remains constant. In reality, the smaller TB would, indeed, cause a restriction in flow and have an adverse effect on VE%, making the difference even greater. Also, we would incur additional pumping losses from running under greater vacuum.

Alas, we shall remain conservative and take a look at this 2% from a cost perspective. It just so happens that the guy with the 6.1L Super Stock football upgraded his mill with heads and a cam, only to capture the 6.1L modified football. $3200 worth of parts netted him 59hp, or an increase of 13%. Add another $1000 for installation, and we get $323 per 1% increase. Cast TBs can be had for $500, resulting in the upgrade cost of $250 per1% increase. This means that a 90mm TB would have a higher bang for buck value than even a head/cam swap on a 423 stroker. If VE% differs between the stock and 90mm, even the shiny billet TBs begin to have a favorable bang for buck value on this size engine.

It is kind of unfair to do all the work for one guy, and leave everyone else out. Therefore, I made a nifty excel sheet to calculate for the most common Gen III Hemi displacements. I used VE% of 110 for the chart, suggesting a fairly stout, efficient package. Here, for the first time ever, is a chart to help with TB selection, based on rpm.

Using the chart is simple. If peak power is higher than the rpm in the 1.2 “/Hg column, or peak torque is higher than the rpm in the 0.8”/Hg column, then you should really consider an upgrade. If peak power is higher than the rpm in the 0.8”/Hg column, or your shift point is is higher than the rpm in the 1.2”.Hg, an upgrade can be of benefit in an “max-effort” application, but isn’t absolutely necessary. I even included accompanying power outputs, so you can also make a selection based off of expected N/A power.


Throttle Body Selection Chart
-----------80mm ----------85mm ---------90mm
---------0.8-1.2"/Hg ----0.8-1.2"/Hg ----0.8-1.2"/Hg
345_____5190-6360_____5920-7250_____6700-8200
370_____4840-5930_____5520-6760_____6250-7650
392_____4570-5590_____5210-6380_____5900-7220
426_____4200-5150_____4790-5870_____5420-6640
440_____4070-4980_____4640-5680_____5250-6430
N/A hp____360-500_______440-580_______510-660



Now, let’s take a look for the forced induction guys. A lot of people seem to think that the blower cars could benefit from a larger TB even more than the N/A guys. Let’s take a look. I will be using some additional formulas, mainly intended to help with compressor selection. The main differences are in air density and temperature, although I’ll be looking at flow before the compressor, too, for those who are entertaining the idea of relocating the TB to this place. The standard “carb sizing” formula is used here as well, suggesting that the engine determines the actual volume of air that is moved. The compressor only changes density, by way of temperature and boost. We must find a way to determine the temp of the air going in the engine. To do this we first figure temp of the air exiting the compressor. Then, we use our intercooler efficiency to get the actual temp. Here are those formulas….


Eq.5 (boost psi + 14.7) / 14.7 = pressure ratio
Eq.6 ((Inlet temp°F + 460) x (pressure ratio)^.283) - 460 – Inlet temp°F = Ideal temp°F rise
Eq.7 Ideal temp rise / adiabatic efficiency + Inlet temp°F = actual compressor outlet temp
Eq.8 -1 x ( intercooler(IC) efficiency x ( IC temp°F in – temp°F ambient) – IC temp°F in) = IC temp°F out
Eq.9 ( Inlet temp°F + 460) /( IC temp°F out + 460 ) x pressure ratio = Density ratio
Eq.10 Outlet cfm x Density ratio = Inlet cfm


Although these formulas have a variety of uses, we’re only interested in using them for TB selection. Looking at Eq.10, we see that the changes in boost pressure and temp, therefore air density, only changes the inlet cfm requirements. Outlet cfm remains the same and is cal’d, using Eq.1. This is good news for those who have their TB after the compressor, like the Procharger systems. Because of the increased “outlet” temperature, it gets even better. The higher temps raise the speed of sound. The higher “mach speed” makes the engine think everything is larger in size, and will literally move your power band up in rpms. In essence the same size TB, after the compressor, will support even more rpms when used with boost. Sticking with the “calculation” theme, here is a formula to estimate the effects….


Eq.11 old rpm peak x ((outlet temp°F + 460) / (inlet temp°F + 460))^.5 = new rpm peak


We’ll use our friend’s 423 again for the example, adding 10 psi boost at 80°F ambient. We’ll also use 75% for both compressor adiabatic efficiency and IC efficiency.


(10 + 14.7) / 14.7 = 1.68 pressure ratio
(80+460) x (1.68)^.283 – 460 – 80 = 85°F ideal temp rise
85 / .75 + 80 = 193°F actual compressor outlet temp
-1 x ( .75 x (193-80)-193) = 108°F final temp out
(80 + 460)/(108 + 460) x 1.68 = 1.597 density ratio
5600 x ((108 + 460) / (80 + 460))^.5 = 5740 new rpm peak
5740 / 5600 x 1.597 = 1.636


…Or a 64% increase in output at approx. 150rpms higher with the exact same TB. What happens if we use a roots-type blower, or move the TB to the inlet side of the compressor? We can use our density ratio to figure this out, keeping in mind that the inlet side is still at ambient temp. Remember that demand was calc’d to be 795cfm…


795 outlet cfm x 1.64 = 1300 inlet cfm


Woah!!!! Vacuum at WOT just jumped to 2.5”/Hg with a 90mm TB. Relocating the TB to the front of the compressor would cause a huge restriction….Unless you were to use two of them.

It is my hope that all of these equations and examples will help make sense of TB selection for a variety of applications, and helps to explain “why” the larger TBs work. I also hope that the readers of this write-up will become a little more knowledgeable about their hotrods and the parts they are purchasing for them. Most importantly, when someone asks about a 90mm TB, we’ll have some real info to go with all of our “marketing” claims.

10ChallSRT8
11-19-2011, 02:01 PM
This is alot of data to ingest.... what does this mean for the avg joe with an internaly stock 5.7 or 6.1? My thought process based on reading various posts on multiple forum threads is that:
- Stock Motor = stay with stock TB or maybe go 85mm bit no larger since a 90mm on a stock motor is dificult to tune. I've also read that an 85mm on a stock motor is a direct bolton and the pcm will auto adapt to the slight change granting a slight (if any) increase in performance
- Cam/heads/headers/intake or Supercharger = 90mm with a fair amount of tuning that would be needed anyways due to the build.

Am I off on this?

EDIT 2Dec2011: Based on my own Dyno numbers from Spring 2011 and the TB Chart...


Using the chart is simple. If peak power is higher than the rpm in the 1.2 “/Hg column, or peak torque is higher than the rpm in the 0.8”/Hg column, then you should really consider an upgrade. If peak power is higher than the rpm in the 0.8”/Hg column, or your shift point is is higher than the rpm in the 1.2”.Hg, an upgrade can be of benefit in an “max-effort” application, but isn’t absolutely necessary. I even included accompanying power outputs, so you can also make a selection based off of expected N/A power.


Throttle Body Selection Chart
-----------80mm ----------85mm ---------90mm
---------0.8-1.2"/Hg ----0.8-1.2"/Hg ----0.8-1.2"/Hg
345_____5190-6360_____5920-7250_____6700-8200
370____4840-5930____5520-6760___6250-7650
392_____4570-5590_____5210-6380_____5900-7220
426_____4200-5150_____4790-5870_____5420-6640
440_____4070-4980_____4640-5680_____5250-6430
N/A hp____360-500_______440-580_______510-660



My peak TQ happens at about 4800rpm (0.8 column) and peak HP at about 5700rpm (1.2 column). Given that data with my current mods, an 85mm TB would not reap any HP increases and might boost TQ by 2 or 3. I do not know what my shift point is. Based on Dyno and graph data, an 85+mmTB not worth the $450+ for a mod. Challenge me on the data if you like. I've sparred with some vendors on this.

srt8-in-largo
12-10-2012, 01:18 AM
Bump for an interesting read.