On a stock or near stock motor, what is the relationship between lbs of boost and AF? If you slapped a Procharger, for instance, on a motor with nothing to regulate fuel (leave out timing), what would the AF ratio be? What if you started with 8:1 ratio? What would the supercharger bump the AF ratio to with no adjustment? What boost levels change the AF ratio how much? If clarification is needed just say so.

 

I would say clarification is needed.

A Stoich burn is 14.7:1 , 14.7 parts are to One part fuel. You can figure out how much more air your flowing to figure the offset of the fuel and how lean you would be. Idealy the ecu would account for the extra o2 in the combustion chamber with more fuel.

 

Well, yeah, I know that. What I am asking is very elementary I think so make sure to keep your thought process simple and generic. Simply, with no ECU tune, what would, theoretically, AF ratio go to if a Procharger was installed. Just the mechanicals forcing air as it does. No added fuel. Starting with, say, a 12:1 ratio, what would AF go to at any given RPM with the added boost? How many AF ratio points is each lb of boost worth? I could probably think about it, but I don't feel like it. Just feed me.

 

To answer you..Not under Wide open throttle..The Stock ECU will go into CLosed loop mode and run one of its pre-tuned maps..It may adjust to a certain point but no enough for a procharger..KABOOM!!

 

exacctly..the ecu cannot see boost at all as it has no MAP sensor. now, ditch the MAF and convert to MAP, and you might have a kick *** little system going, though fuel flow is stilla concern (ie pump volume)

 

Daking and Z1, I know that. Picture the setup not even in a car. Just put it on paper. It isn't something I want to try. I just want to understand the relationship. You guys are making this too difficult.

 

So when you reflash the ECU you are actually just giving it a different Fuel/Timing map to follow once the MAF reads max airflow and throws the ecu into closed loop..Correct???

 

i think i know what your asking.......but i could be wrong.......i guess i'll give it a shot

assuming the RPM's increased very, very slowly.........slower than the ECM's adaptation rate.....then it should have you somewhere in the 13's to 14's up until 5K, the approximate MAF max point

 

not exactly.......there are 3 maps in the ECM.......open loop, closed loop, and failsafe mode.......for OL and CL, the baseline AF map is changed.....the correction rate is changed for CL.....there are some other functions the ECM performs on the AF for various situations, that were applicable in NA mode, and now need to be corrected for FI.........some programmers are having problems dealing with these other functions even in NA applications

 

Ok, well here's a little walk through of some very rough calculations.

1st we need to know our starting point. So, what kind of vacuum does the car create @ WOT? I have no idea, but I'll use -7.35 psi for simplicity (since -7.35 psi gauge pressure is about 7.35 psi absolute pressure)

So, using the ideal gas law PV=nRT we see that pressure (P) is directly proportional to the amount of air (n). So if we make some huge assumptions about the rest of the variables (volume is the same - true, since the engine isn't changing size, temperature is the same - probably not true since the air gets heated as it is compressed, but let's just say we have an intercooler that is very good, and that we have no restriction in the intake or heads so that our cyliners are actually seeing the same pressure as the "boost" ) then we can eaily figure out how much more air the cylinders are seeing.

So, lets say we boost to 0 psig (the g=gauge pressure so 0 psig= 14.7 psi absolute pressure = normal atmosphere). Now we've gone from 7.35 psi to 14.7 psi and effectively doubled our pressure. Therefore we will see twice as much air. Now lets say we boost to 7.35 psig (22.05 psi) and we've tripled our pressure. So, using all the huge assumptions we've made above, we should have tripled the amount of air in our cylinders.

So, if the ECU is putting in the same amount of gas as it did before, and we are seeing 3X the amount of air, then we should have an A/F ratio 3X as high.

Really, if you can know the difference between your cylinder pressure in N/A mode compared to the cylinder pressure in F/I mode (at similar piston positions) you should be able to quickly get an idea of the amount of air and therefore the new A/F ratio. Throw in some temp measurements and you can finish off the equation for real.

-D'oh!

 

So are changing volume and temperature the only two variables to alter the equation? What are others and how do they all affect AF? Also, I can see temp being variable, but how often is volume variable? How much extra fuel is needed to balance out 3X more airflow? 3x more fuel? Also, when you have tripled the volume in your cylinders, how much boost is that on the VQ? Does it take more boost or less on a larger displacement motor to triple air volume?

 

The only time anything in the engine would change in volume is the cyls durring the intake stroke. But you can assume that it's always going to burn the same volume of air so the only variable in that would be absolute pressure. Like D'oh said.

Nice D'oh, I couldn't have explained it that well myself.

 

D'oh....
You just fried my brain man!!

 

Hehe, I think I overloaded a couple circuits as well, trying to think all the way back to high school chemistry.

Just to clarify one thing about volume: don't confuse volume (physical space) with amount (mass). Since air is compressible, volume and mass are not equal (ie: 3.5L of air @ atmosphere is not the same amount as 3.5L of air @ 7 psi of boost, even though both are 3.5L in volume). We are often used to dealing with incompressible fluids, like water, in which case volume and amount are don't change with pressure, but with compressible fluids they vary, so you need to be careful about what you mean when you start talking about volume.

Anyhow, in the PV=nRT equation you have:
P=pressure
V=volume
n=amount (moles)
T=temperature
R=constant

Here's a website that will either take you back to your high school days (for those who are finished with that phase of your life) or give you something to look forward to (for those about to begin).

http://hyperphysics.phy-astr.gsu.edu...ic/idegas.html

-D'oh!

PS - Hi, my name is D'oh! and I'm a nerd...

D!

PPS - If you go to that link and click on the thermodynamics section, you can learn all about the I/C engine (go to the "heat engine" portion).

D!

 

Does that mean ECU count only the signal from MAF for fuel feeding ?

How about those signals from O2 sensors in CATs
ECU read & check A/F there, right ?

 

Fly by Z, pm me and I will give you my friends email addy. I think he may be able to answer your question. Considering he built a stoked and blown 5.0, and now he has a twin turbo lingenfelter (sp) Z06.

He is the Z06 Power Adder Authority on zo6vette.com. He sprayed his zo6 b4 the turbos went on. He also tuned my WS6 TA with his laptop.

 

fx, the o2 sensors will tell the ecu how rich or lean the burn was so that it can compensate. The maf tells the ecu how much air is coming in so that it can make an educated guess on the amount of fuel required for combustion based on an algorithm programmed into the ecu by nissan.

 

Is that the dude that was on American Muscle?He had the 1200hp Vette and he tuned that custom Mustang Fastback for SEMA??

 

This is correct but once you go into Closed Loop mode all bets are off..The ECU gois into a pre-programmed a/f and timing map..

 

No but it sure sounds like him. lol! Here is his website.

http://hometown.aol.com/wa2fst/home.htm