I new but I've been reading your questions and replies and I thank you for the knowledge and confidence you've given me. I have come across a point of confusion though and I'd like to get to the bottom of this.

I've read on the forum that if you adjust your fuel pressure you will need to re-program the ECM to realize the increased fuel pressure and performance. Is this true? This isn't a simple plug in mod that you can do on your own? You have to own "HAL" to modify and program your chip?

Also, can you simply pull the vacuum line off the fuel pressure regulator then plug it and get 4lbs of increase? Why is the vacuum line attached in the first place?

I know I asked a lot of questions but it's all of your fault. You have created curiousity.

I have a 95 coupe. Thanks

 

The ECM sets fuel injector pulse width based on engine parameters and stored tables. In closed loop mode (normal part throttle operation) it also uses feed back from the O2 sensor(s) to trim the data in the stored tables; in open loop mode (during WOT operation) the O2 sensors are ignored.

During WOT operation injector pulse width only depends on engine parameters like RPM and load...the ECM doesn't know about fuel pressure. Also when you go to WOT there is no vacuum (or nearly none) to the FPR to fuel pressure is at the maximum. So, we remove the vacuum line to simulate the WOT condition when checking/adjusting fuel pressure. The vacuum line allows more latitude for the ECM in adjusting fuel flow at part throttle to meet driving needs as it provides a way to increase fuel pressure (and so fuel flow)as engine load increases.

Now if you increase the fuel pressure with an AFPR the ECM will sense more fuel flow in closed loop and make corrections to achieve the correct A/F ratios. However at WOT the ECM uses the old injector pulse width tables so fuel flow can be increased. Remember that fuel flow capability increases as the SQRT of fuel pressure increases. So if you increase fuel pressure by say 10% flow capability at WOT is increased by about 5%. This is an easy method to add more fuel to balance out air flow increase mods like cut-lid & K&N or exhaust mods. So the AFPR mod doesn't add power of itself, but allows us to tune the engine to take advantage of air flow increasing mods.

When we do some more extensive mods like head porting or cam sometimes the increased fuel needs vary with rpm so we may need to burn a chip to adjust the pulse width at each RPM step. But, IMHO, this should usually be saved for the last step in fine tuning your engine after all air flow mods have been done, and then only if an AFPR is incapable of properly tuning fuel flow at all RPMs or fuel flow must be increased to the point where the ECM can no longer make proper adjustments at part throttle when in closed loop mode.

 

This is the conventional wisdom, very well explained, but it misses one point, 65Z01. On my 1985 the WOT values are based on a % increase of the normal closed loop values. So changes in fuel pressure can only make temporary gains in WOT enrichment. Soon the normal closed loop tables update to the new fuel pressure, and start leaning out WOT. If I reset the computer, WOT is once again enriched, until the closed loop normal run values get corrected, and then they are used to calculate new, leaner, WOT mixtures.
Fuel pressure changes only help if you change the pressure curve - increase pressure in a disproportionate fashion, and only at WOT. This can be done, but not with a simple regulator change.
OBII systems might be different - I never had reason to research that system. But I suspect they are also percent based enrichment, not mere pulse width table. This is because GM wants the system to be tolerant of normal car to car fuel pressure variations.
I logged gains from fuel pressure changes, but they do not last. I recently ran 60psig as a test, 8 degrees base advance. After 2 hours of driving, WOT leaned back out to the point of detonation. Disconnect the battery for 15 minutes, start it back up, and WOT was rich again.
IMO fuel pressure changes are not for the faint of heart. The computer will correct for it.

See "mixture tables and WOT" in "Scan and Tune"

George

 

The ECM uses tables stored in EPROM which are effectively "hard coded" to determine injector pulse width.

It then uses O2 sensor data to adjust values in the BLM cells as well as in the Integrator for closed loop control.

When I go to WOT my scan tool shows that the ECM is using BLM cell #15, which always has the quiescent value of 128 and at WOT the Integrator is always 128. So the ECM is not applying any corrections to the hard coded table values for injector pulse width at WOT.

It may be possible for the OBD-II and later systems that use flash memory instead of EPROMs to be more "intelligent", but out L98's OBD-I cannot correct for fuel pressure changes at WOT.

I've been running 48psi for over a year and the ECM has not corrected for it at WOT, as seen by the O2 sensor readings down the 1/4 mile at WOT. However the BLM cells 1-14 and Integrator at part throttle show that the ECM is correcting while in closed loop mode.

George, check your scan tool to be sure your ECM is indeed going into closed loop at WOT and check the BLM and Integrator values. I'd be very interested to know if the early ECMs operate differently than the later ones.

Hmm, I was just comparing my '85 Corvette Shop Manual with my '88 Corvette Shop Manual and indeed there is a difference in the Drivability and Emissions sections. My '88 manual has a section under Diagnosis called Fuel Control System where it discusses the use of the BLM cells but the '85 manual does not have that discussion under Diagnosis. So, the '85 ECM may indeed differ from later L98 ECM in WOT fuel control.


[Modified by 65Z01, 1:41 AM 11/4/2003]

 

This has been a good explaination of fuel pressure except for one point. The PWs are NOT hardwired in the ECM. They can be adjust many, many different ways. Too many ways for me. I can't figure out all the tables in TunerCat 2. You can adjust Pulse Width, or when the pulse starts. You can change PW in relation to Power enrichment, temperature, battery volts and heaven knows what else. It's insane.

 

I didn't mean to imply that the computer goes closed loop at full throttle - it does not. But the WOT pulse width does look at the fuel trims in it's calculations. So when I first power up with high fuel pressure, the pulse width is (simplified) VALUE * TRIM * POWER ENRICHMENT. But after some time goes by (many minutes, not many days) the trim value goes negative (due to high fuel pressure), and WOT pulse width gets "corrected".

I cannot be certain that any one else's CPU works like that - but mine does fight me when it comes to WOT mixture.

George

 

there are a bunch of hacks that people do to fool the integrator from trying to adjust the BLMs. But at WOT the integrator doesn't change anything, anyway. It is locked. I usually just set the A/F ratio to something less than 14. And adjust the lower RPM stuff accordingly. Right now I'm going to try and get it working top notch at 14.7. I don't know if it is possible, but time or a brain transplant will tell.

 

At Wide Open Throttle (WOT), are we seeking the best air to fuel ratio for horsepower, (around 12.5 to 1)- or the "correct" (stoic) air to fuel ratio of 14.7 to one.

A dyno owner once told me every tuning effort for max HP thus far has required adjustment of fuel downward at WOT. Stock, modified, you name it. Settings were too rich. A decrease in fuel pressure was required (or burn a chip). On carb cars, they typically lean out the jets. I have also read GM has the mixture a little rich at WOT for stock applications, thus providing a measure of protection. If that is the case, why the need for increased fuel pressure at WOT?

Increased air will require more fuel, but even stock fuel systems are relatively robust. Respectfully submit there are far more cars running too rich at WOT vs lean. Then again, I'm not selling adjustable fuel pressure regulators for relatively stock cars. Oh well.

dlmeyers 90 coupe zf6 3 speed shocks

 

This info is correct, but like Paul Harvey " here's the rest of the story":

First, I assume this dyno tuner does many different kinds of cars, and many different years. Even the C-4 vette ECM and sensors change alot. MAF to MAP back to MAF. Some ECMs will adjust the engine based on nominal input. Others, like your (and mine) ECM use alot of variables. One advantage of MAP ECMs is that altitude has no effect on the air/fuel program.

Comparing a fuel-injected computer car with a carb car is not very useful. Way different efficiencies. A carb must atomize fuel before reaching intake runners and the head intake, while injectors sit at the head intake and can be adjusted much finer.

Efficency of the injectors will determine how much fuel pressure will effect WOT. If an injector is running 90% to 100% of "duty cycle" (full open-full closed) then fuel pressure will have a larger effect on fuel used than an injector that is running at 60% to 70%.

While the GM ECM may be rich, it is only SLIGHTLY rich. If you start pushing alot more air through your V8 pump it will be lean at higher RPM.

Here's an example:
My 90 with Big mouth set-up/headers, etc. needed to have heavy PE (power enrichment) increases and basic fuel air/ratio set at 13.125. It needed 48-49 lbs of fuel pressure. It was dead on from 1800 rpm to 5600 RPM, and faster than a Z-01. It took awhile to find this "perfect" fuel pressure.

When I put in my new heads and intake with 30 lb injectors, it was so lean at WOT throttle and at higher RPM that if I hadn't noticed right away, I would have fried the engine. The fuel pressure was at 46 lbs. Compared to the 22lbs @ 48 lbs, this was much higher pressure, but it didn't do spit.

I don't think vette fuel pumps are "robust"? Your fuel pump (if stock) is only rated for 300 hp by GM. I could never have achieved my numbers on the big mouth with the stock pump. I replaced it with a turbo Grand National pump rated at 400 hp. I now run 2 pumps - total rating 560 hp. (the numbers are not added together).

Enough of running my mouth - I need to run the vette instead. I still won't take it above 5800 RPM because it is still too LEAN.

 

My general comments were for stock or near stock applications. Heads and cam are a different story. The air pump known as an engine may well require different injectors with SIGNIFICANT changes in air flow. The term I used was "relatively stock" with regards to adjustable fuel pressure regulators.

We certainly agree the stock fuel pump is not the most durable piece of equipment. That is why partially clogged fuel filters eat them up. My implication for "robust" is that even stock injectors could likely handle an additional 40 horsepower before getting well north of the 80% injector duty cycle. I am too lazy to do the math. As a note, even the stock 48mm throttle body is good for some healthy horsepower additions, north of 300 total HP. Sure, somewhat subjective, but if the stock fuel system can provide fuel for an additional 40 HP, I called that "relatively robust".

My basic point is this, for stock or near stock applications, with no change in chips, dyno the car and the tuner will likely have to lean out the mixture for max horsepower at WOT. Throwing another 4-8 psi at the cylinders will likely decrease performance IF already running rich.

Keep the greasy side down.

dlmeyers 90 coupe zf6 3 speed shocks

 

The GM ECM looks at the last closed loop cell before PE to determine how it will run in PE.
If it's running rich it will run rich in PE. If it's running lean then it will run lean in PE. Thats why you were running rich with the 22 lb/hr and are running lean with the 28lb/hrs. There is no other explanation. This is also why afpr only works for a short time. I also experience pulse width shortening in PE mode when I adjust the F.P.. When I drag race and play with the f.p. I must always dump the memory for it to be effective. If I leave the memory in, it will slow down as the computer relearns and trys to compensate by lowering the pulse width. All of my GM performance books state that the fuel pressure mod is short lived and that the memory must be dumped for it to be effective.
There is a Buick GN site out there that describes this in full detail. I think it's Thrasher Engineering or something like that. Sorry it's been a while since I've been there.

John

 

I have given up on static fuel pressure adjustments for enhancing performance, due to the transient nature of any gains that I get. It is maddening to make an adjustment, run a good strong 8.4 second 85 MPH 1/8 mile - and have the car ping and stumble on a later run!

Presently I have the fuel pressure regulator adjusted for about 40psig. That provides decent fuel atomization without excessive strain on the injectors (shortening their life). When full throttle is sensed, the line from the fuel pressure regulator that normally goes to the intake manifold is switched over to the output of a small 12 volt air compressor. The air pressure is set to 15psig, which raises the fuel pressure to ~ 50psig. Since the fuel pressure is raised only at WOT, and drops to normal as soon as the throttle is released, there is no effect on normal closed loop running. So the fuel trims are unaffected.

This method of dynamic fuel pressure adjustment is helping me dial in the mixture that I want at WOT, with reliable and predictable results. The static adjustment of fuel pressure was causing the car to have unreliable run times. I think I now understand why. Thanks to all who contributed to the discussion!

George

 

I agree with George West...

I have noticed on my car, whenever adding more fuel to the chip at WOT, Ill get black smoke out the exhaust the first couple times I hit it.

Then, a few days later the exhaust is fine. Something is correcting itself here.

Dave