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J to da Weezy..Baby!!
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4,249 Posts
Discussion Starter · #1 ·
Sup guys...

It really BOTHERS the HELL out of me...all the neons, rsx, and GTO, mustangs, and a couple other CHUMP CARs...have higher compression ratio engines then we...DONT WE HAVE A HEMI...one of the best ever made...

IT really bothers me, for the fact when the turboz come out, and other things, our compression ratio will affect how high we can go...

An example is the Twin Turbo..we are fighting on gettin OVER 4 psi...SAFETLY...WTF...

my friends EVO has 30-45 psi..

and my friends srt-4 runs at 25 all the time...no issues, and DOESNT RUN HOTTER THAN OUR ENGINES...without a TURBO...

SO my question is CAN WE INCREASE THIS...sorry if this is a stupid question? but it really pisses me off, that this mustang or this GTO you add a turbo or supercharger, and YOU HAVE CRAZY HORSEPOWER...with no issues...

Jweezy
 

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Speed on... Hell ain't half full
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24,673 Posts
i'm not sure what the supercharger from gsm adds in terms of pounds of boost, but it's way over 4. They stopped at 4 with the turbo setup because they were ironing out the tuning. Problems in fuel or spark curves can be accentuated, and therefore more harmful, at higher compressions. Both of these setups can easily yield over 100 hp increase. I don't doubt our block could handle 20 psi boost, but it'd probably require high-yield head gaskets and it'd be smart to replace your crank and connecting rods before you got into that realm of power. Besides, not to burst the communal bubble, but the legendary hemi doesn't really have all that much more to offer than other 'high-performance' engines these days, other than a legendary namesake.
 

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Bane of Zombiehood
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1,861 Posts
i thought the weak block was why they stopped boring it out for a 6.1? or was the boring what was causing them to go weak?

i'm with jweezy on this. i mean i KNOW i have a 4400lb stationwagon...but i'd also really like to have 700hp and 850+ ft/lbs at the wheels pushing that monkey. if i have to replace all the internals to run 10lbs of boost...maybe i can deal with that. if there's no way to make the engine work with 10lbs of boost...i can't deal with that.

it's my understanding morpheus will make all this a non-issue. maybe MF can chime in?
 

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and there's no going back
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846 Posts
I believe the GSM boost is in the 8-10psi range. I don't think the main source of extra displacement for the 6.1 was additional bore, but extra stroke. That's length, not girth, gentlemen. ;) Perhaps someone can correct me or tell the whole story.
 

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15 Years of AWD fun
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14,454 Posts
iamkungfool said:
i thought the weak block was why they stopped boring it out for a 6.1? or was the boring what was causing them to go weak?

i'm with jweezy on this. i mean i KNOW i have a 4400lb stationwagon...but i'd also really like to have 700hp and 850+ ft/lbs at the wheels pushing that monkey. if i have to replace all the internals to run 10lbs of boost...maybe i can deal with that. if there's no way to make the engine work with 10lbs of boost...i can't deal with that.

it's my understanding morpheus will make all this a non-issue. maybe MF can chime in?
Well there is the "weak block" fairy tale again. I thought we killed that last year.
 

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See ya in anotha life brotha
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14,677 Posts
Justin - A few things you need to understand about compression and Boost.

Compression is the mechnical compression put on the air within the cylinder. So at Bottom Dead Center ( BDC ) you have lets say 1 cubic foot of air, and at Top Dead Center ( TDC ) that air is compressed to 1 cubic inch that is effectly 12:1 compression.

Boost is the ammount of pressure that is put on the air into the engine. This packs more air into the cylinder. This will make power because it gets more air/fuel into the cylinder and by doing that your raising the pressure in the cylinder to higher levels making more power ( basically )

Now, I would be willing to bet that those Evo's and Subies are running clost to 8:1 vs our 9.5 10:1 With a lower mechnical compression, and the boost your packing more air/fuel and making more power.

With a higher mechnical compression ratio you can boost it to the sme levels BUT you will have more of a problem with detonation with out race gas, so the cure is running lower boost levels.

All of this is also effectived by the cam and how much valve over lap is there. You can effect cylinder pressure but lower overlaps or longer to tune how things run.

With our engines I wouldnt want to run lots of boost because of detonation problems with 93 octane fuels.

-Robert
 

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High Power Junkie
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16,628 Posts
RobAGD said:
Justin - A few things you need to understand about compression and Boost.

Compression is the mechanical compression put on the air within the cylinder. So at Bottom Dead Center ( BDC ) you have lets say 12 cubic inches of air, and at Top Dead Center ( TDC ) that air is compressed to 1 cubic inch that is effectively 12:1 compression.

Boost is the amount of pressure that is put on the air into the engine. This packs more air into the cylinder. This will make power because it gets more air/fuel into the cylinder and by doing that your raising the pressure in the cylinder to higher levels making more power ( basically )

Now, I would be willing to bet that those Evo's and Subies are running close to 8:1 vs our 9.5 10:1 With a lower mechanical compression, and the boost your packing more air/fuel and making more power.

With a higher mechanical compression ratio you can boost it to the same levels BUT you will have more of a problem with detonation with out race gas, so the cure is running lower boost levels.

All of this is also effected by the cam and how much valve over lap is there. You can effect cylinder pressure but lower overlaps or longer to tune how things run.

With our engines I wouldn't want to run lots of boost because of detonation problems with 93 octane fuels.

-Robert
That belongs in the Knowledge Base. :rock:
 

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Builders of the 500rwhp 426's
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5,341 Posts
Exactly...........

I remember years ago I was "keeping up with the Jones".....

Someone else would go fast, so I would try and go faster..........it never ends.

You'll find you need to have expectations and build a car to THOSE. If the sky is the limit then I think you just answered your own question.

If you want a ton of boost, then you need to LOWER your mech compression..........if you want mech compression you need to lower your boost.

The SRT motors run (i think) 10.5:1 while we 5.7-guys are running 9.5:1.

My combo, if we get this tuning thing figured out, will be to run around 10.25 - 10.5:1 in a 5.7 with a little N20.

As I spray, it will bump up the compression to maybe around 12:1 due to the expansion and density of the N20.........

We control this with "cooler" plugs (as do turbo and blower guys), drop timing, and up octane.

Its not a "one thing fixes all" issue............

More timing = more HP
Too much timing = detonation
More compression = more HP
too much compression = detonation
Hot plugs = good burn = HP
Too hot = detonation
Smaller LSA = up the power in the RPM
More LSA = lowers the power in the RPM
More lift = volume flow increase (cfm)
Too much lift = volume flow decrease (psi - speed)

****(these are layman examples and a generalities ONLY)

You'll find, as you get into "cars" is that its all about the COMBO and not really about "bigger is better".

You may find that a 5.7 with 10:1 (or so) with 4 PSI of boost and a small 25 shot of N20 runs all over a 2.0 - 2.5 litre running 25 lbs of boost @ 8:1.
 

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Betrayer of Mjolnir
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3,111 Posts
RobAGD said:
Compression is the mechnical compression put on the air within the cylinder. So at Bottom Dead Center ( BDC ) you have lets say 1 cubic foot of air, and at Top Dead Center ( TDC ) that air is compressed to 1 cubic inch that is effectly 12:1 compression.
If I may inject something...there's a math error here. 1 cubic foot = 1728 cubic inches (12" x 12" x 12"). So compressing 1 cubic foot down to 1 cubic inch would be a compression ration of 1728:1. I guess you meant to say 12 cubic inches instead of 1 cubic foot?
 

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Bane of Zombiehood
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1,861 Posts
DawsonMagnum said:
If I may inject something...there's a math error here. 1 cubic foot = 1728 cubic inches (12" x 12" x 12"). So compressing 1 cubic foot down to 1 cubic inch would be a compression ration of 1728:1. I guess you meant to say 12 cubic inches instead of 1 cubic foot?
lol i was gonna say that.

but otherwise you're spot on
 

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05 Magnum RT and 10 Challenger
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7,415 Posts
Details details.

Anyways, I've always heard you either want a Normally aspirated High Compression engine OR a lower compression supercharged or turboed engine. The higher compression negates what the supercharger and turbos do for you somewhat.
 

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Builders of the 500rwhp 426's
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5,341 Posts
BrilliantBlackHemi said:
Details details.

Anyways, I've always heard you either want a Normally aspirated High Compression engine OR a lower compression supercharged or turboed engine. The higher compression negates what the supercharger and turbos do for you somewhat.
No, not negate, rather will add a bit too much to handle in many cases.

There are true race cars that are running 10:1 motors with 20 PSI turbos......but these are RACE CARS that could handle 14:1 - 15:1 mechanical.

Remember, you can compress as much as you want..........the turbo or blower just puts more there to compress.
 

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Betrayer of Mjolnir
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3,111 Posts
RobAGD said:
yea little brain fart going on there. I am still a little punchie from the drive lastnight.
Understandable... But you still managed to clearly and concisely convey a somewhat esoteric concept of the internal combustion engine quite well. Better than I could have, for sure.
 
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