Speed Density vs. Mass Air and performance with intake mods.

**snickle** · 08-31-2004, 10:46 AM

A good (if brief) article on Speed Density and Mass Air EFIs

http://www.carcraft.com/techarticles/24549/

Explains some of the comments that we won;t get improvements with a cold-air kit. And why we do.

Basically, the computer looks up the fuel value in a large TABLE based on engine speed air, temp and manifold pressure. Changes in the intake cause these values to be wrong. An 02 sensor in the exhaust measures remaining oxygen in the exhaust and the computer tries to compensate.

So we will get improvements, but I bet we could easily provide more air than the system could compensate for.

So mild intake changes are probably best until someone reprograms the system for us.

**MagnumFreak** · 08-31-2004, 11:45 AM

Good article. FROM what I have read FROM the service manual it appears to me that the computer in our cars actually calculates the values. Here is the first paragraph of the section about the PCM:

"The PCM operates the fuel system. The PCM is a pre-programmed, triple microprocessor digital computer. It regulates ignition timing, air-fuel ratio, emission control devices, charging system, certain transmission features, speed control, air conditioning compressor clutch engagement and idle speed. The PCM can adapt its programming to meet changing operating conditions."

That last statement is one that makes me believe that the values are calculated or that there a great deal of room for adjustment automatically by the computer. There are also many "modes" of operation that the PCM runs under. Depending on what mode it is in determines what sensors it looks at and which ones it ignores. The service manual is pretty interesting. It gives some good insight INTO how our PCM operates.

**snickle** · 08-31-2004, 12:10 PM

It calculates adjustment to the tables. SInce this is a speed density system, it can't totally calculate the values, as it has no DATA on how much air it is really getting.

**MagnumFreak** · 08-31-2004, 12:12 PM

Ahh I see said the blind man. I wonder how much room they left for adjustments?? Only one way to find out..... Modify modify modify and test to see the results.

**C4me** · 08-31-2004, 06:55 PM

OK, think Denver in the summer, then think uhm someplace low and cold in the winter. There HAS to be a wide range of values available.

**C-Man** · 08-31-2004, 11:19 PM

Here's something I pulled out of the service manual

MODES OF OPERATION
As input signals to the Powertrain Control Module (PCM) change, the PCM adjusts its response to the output
devices. For example, the PCM must calculate different injector pulse width and ignition timing for idle than it does
for wide open throttle (WOT).
The PCM will operate in two different modes: Open Loop and Closed Loop.
During Open Loop modes, the PCM receives input signals and responds only according to preset PCM programming.
Input FROM the oxygen (O2S) sensors is not monitored during Open Loop modes.
During Closed Loop modes, the PCM will monitor the oxygen (O2S) sensors input. This input indicates to the PCM
whether or not the calculated injector pulse width results in the ideal air-fuel ratio. This ratio is 14.7 parts air-to-1
part fuel. By monitoring the exhaust oxygen content through the O2S sensor, the PCM can fine tune the injector
pulse width. This is done to achieve optimum fuel economy combined with low emission engine performance.

**one_raven** · 08-31-2004, 11:20 PM

Originally Posted by snickle

this is a speed density system

Are you sure about that?
I'm not saying otherwise, just checking if you have the specs FROM anywhere.

**snickle** · 08-31-2004, 11:35 PM

Every review I have read on the engine has called it such and no MAF sensor that I can find. So it has to be a Speed Density setup, unless someone has invented something new.

**one_raven** · 08-31-2004, 11:57 PM

Originally Posted by snickle

Every review I have read on the engine has called it such and no MAF sensor that I can find. So it has to be a Speed Density setup, unless someone has invented something new.

That's enough for me, thanks.
I just wasn't sure if you were basing it on the article saying that most Mopar engines have Speed Density.

**Rev.Hammer** · 09-02-2004, 02:02 AM

well, in the end, someone is going to have to come up with higher flow injectors. I would bet my sister in laws underwear that the stock injectors wont be good for more than about 20% over stock Horsepower ratings.

**one_raven** · 09-02-2004, 02:06 AM

Originally Posted by HammerFoot

I would bet my sister in laws underwear that the stock injectors wont be good for more than about 20% over stock Horsepower ratings.

I think you'll have to post a picture of your sister-in-law before anyone will take that bet.

Is that her in your avatar?

**7thS0N** · 09-02-2004, 01:03 PM

**C4me** · 09-02-2004, 02:35 PM

Originally Posted by HammerFoot

well, in the end, someone is going to have to come up with higher flow injectors. I would bet my sister in laws underwear that the stock injectors wont be good for more than about 20% over stock Horsepower ratings.

Uhm, and a 20% increase won't make you smile? Just what do you want/need?

**Rev.Hammer** · 09-02-2004, 10:44 PM

I am prone to my excesses, I want as much as I am able to squeeze out and still maintain some reliability and driveability. I am thinkinnnggg, oooh, about 500hp at the wheels would be a good, happy medium.
As for my sister in law,

http://www.chinaeastwind.com/trailer...gly****er2.jpg

Take your chances!
HAHA!!