Does adjusting the fuel psi help performance?What is the best way to find the correct psi your car needs?Thanks for any info

 

i had an adjustable regulator but ended up taking it off in favor of the stock unit. I'm still a little confused on why it wouldn't work :confused: the stock one is doing the trick for now :cheers:

 

The best way to adjust fuel pressure is on a dyno, but you can use drag strip time slips, assuming you can get consistent data from run to run.

First put a fuel pressure gauge onto the Schrader valve, disconnect and plug the vacuum line to the FPR, start the engine and record the fuel pressure at idle.

Install the AFPR and set it to stock fuel pressure.

Go the the strip and make a few passes at stock fuel pressure, bump pressure about 4psi and make a few more passes. Repeat till trap speed drops off, then back off about 2psi.

For reference, my L98 is now making about 290-300hp at the crank with a fuel pressure of 48psi and an A/F ratio of 12.6:1 at WOT.

 

From what I understand, the adjustment on the afpr is temporary. What I mean by this is the computer senses the higher pressures and adjusts the pulse width of the injectors accordingly. Someone with better info will chime in soon!

 

If the car is for the mostly stock I would leave it alone unless you have done some modifications that would affect the air/fuel ratio to begin with like introducing more air into the mix. Even different air conditions can affect your a/f.

 

That is only true for Closed Loop conditions ( highway driving). At WOT the computer goes to Open Loop and there is not feedback from the sensors (O2s). So changing the FP can effect the overall peformance.

I would also like to add that even in closed loop, you can make the performance sluggish. yeah at a constant pedal position and airflow, the computer will trim the injectors to achieve 14.7 AFR, but as soon as you change your gas pedal it has to react and trim again. This can cause the throttle response to regress to less than perfect. That is why when tuning the ideal situation is to have as many cells at 128 BLM, anything above that number the car is lean, so the ECM is adding fuel. Anything below and your rich, so the computer is pulling fuel. What happens at 128 is the computer is not doing anything with fuel and as soon as you get into and out of the gas, no matter where you are at its at its perfect fuel mixture for a crisp snappy response. Hope this makes sense. :cheers:

 

So, if for instance, I am adjusted at 46 psi, the computer trims the pulse width of the injectors accordingly while highway driving, and as soon as I mash the gas and go to open loop, it's at a full 46 psi with full injector firing?

 

The computer does not measure fuel pressure. It ASSUMES stock pressure unless tables are changed. In Wide Open Throtle (WOT), it goes to default programming and oxygen to fuel ratios are not considered, the ratio is generally rich versus lean. Typically, with most set ups, leaning is required for max HP. Adjustable fuel pressure regulators and injector changes are appropriate for major changes in camshaft and heads. Simply put, if you have not increased air flow (heads and cam), no more fuel pressure/injectors pulse is required. Like buying racing tennis shoes for someone with one lung. If you wish to increase performance at WOT, you MAY have to decrease fuel pressure at WOT. A dyno will tell for sure.

Keep the faith. Good cars.

dlmeyers 90 coupe zf6 3 speed shocks




[Modified by dlmeyers, 9:41 PM 11/30/2002]

 

My .02
the factory spec for the L98 is 41 to 49 psi
out of the factory My car was @ 41
I adjusted the pressure to 49 and found a seat of the pants incr. in power
in all throttle pos. and rpm's.
the boys at tpis recomend 51 psi for the l98
this was my first mod on the black beast when she was new :thumbs:
the best bang for the buck except for the k&n


[Modified by L98 DRIVER, 2:08 AM 12/1/2002]

 

L98 DRIVER the 49 psi is that with the vacum line to the regulator connected?

 

I did all my adjustments with the eng off
turn on ignition sw (fuel pump runs for about 2 sec)
to recycle turn off the ignition sw for 10 sec, then back on
dont forget to fully bleed the gauge before testing or the reading will be false
ref 91 corvette factory serv manual page 6e3-a-23
:yesnod: :yesnod: :yesnod:

 

Adjusting fuel pressure can help performance. BUT it depends on your set up and how you have changed other variables, like air flow, injectors, compression, cubic inch displacement, chip tables, etc.

Merely increasing fuel pressure with no other modifications is marginal at best. Several tuners will tell you at wide open throttle, many engines will gain horspower by DECREASING fuel pressure. This is because factory mapping tables (as stated above, at Wide Open Throtle-WOT, you are no longer in closed loop) are somewhat rich and actually have more fuel than necessary. This protects engines from overly lean conditions. The current chip assumes a factory setting. Increasing pressure with no software changes will simply add more fuel, adding to an already rich Air to Fuel ratio.

Some will state you get a finer spray with higher fuel pressures. True statement. Your get a finer spray of too much fuel. The air to fuel ratio is likely too high. (WOT) General comment. Dynos use a very sensitive Air to Fuel meter and figure out exactly where you are. This can cost. In stock form, engineers spent lots of time getting those mapping tables balanced for emissions, driveability, fuel economy, and performance. If you have stock air flow, stock fuel flow will likely be just fine.

Further, injectors have to react against fuel pressure in order to open. If you increase fuel pressure beyond a certain point, this will slow injector response time. This problem will vary depending on age, design, voltage, fuel pressure, etc. Additionally, the fuel pump will be working harder against a higher spring pressure provided by the adjustable pressure regulator. Remember, fuel pumps provide a volume against the pressure set by the regulator. Most pumps can handle some increase. If you have a serious engine, you will need serious changes. As a system, not little bits here and there.

Barnes and Noble will usually offer several books on Chevy Tuned Port Injection. Good reading for very little money. The amount of information available is simply quite amazing.

Great cars.

Keep the greasy side down.

dlmeyers 90 coupe zf6 3 speed shocks

 

I put an AFPR on my stock engine and cranked it up to 50psi. say what you will about useless mods..... it feels really good to me :D

 

Here's a link to a page on thirdgen.org with "dyno results" for playing with fuel pressure. But I can't vouch for how accurate they are...

http://www.thirdgen.org/newdesign/fa...n.shtml#engine

42 PSI (stock) 46 PSI 50 PSI
RPM Torq HP Torq/diff HP/diff Torq/diff HP/diff
------------------------------------------------------------------------
2250 237.7 101.8 254.9/17.2 109.2/ 7.4 275.0/37.3 117.8/16.0
2500 241.7 115.1 262.1/20.4 124.8/ 9.7 283.3/41.6 134.9/19.8
2750 241.5 126.5 275.2/33.7 144.1/17.6 297.0/55.5 155.5/29.0
3000 237.1 135.4 278.2/41.1 158.9/23.5 305.5/68.4 174.5/39.1
3250 233.8 144.7 278.9/45.1 172.6/27.9 311.3/77.5 192.6/47.9
3500 241.0 160.6 278.2/37.2 185.4/24.8 309.5/68.5 206.3/45.7
3750 246.6 176.1 283.4/36.8 202.4/26.8 303.3/56.7 216.6/40.5
4000 250.4 190.7 278.3/27.9 212.0/21.3 298.0/47.6 227.0/36.3
4250 251.6 203.6 272.5/20.9 220.5/16.9 289.1/37.5 233.9/30.3
4500 253.5 217.2 257.3/ 4.2 220.5/ 3.3 277.0/23.5 237.3/20.1
4750 245.0 221.6 242.1/-2.9 219.0/-2.6 266.4/21.4 240.9/19.3


5000 227.6 216.7 225.0/-1.4 214.2/-2.5 237.8/10.2 226.4/ 9.7 :chevy

 

There may well be an increase in performance in certain applications and conditions. Speculation here. Another computer mode is the acceleration or enrichment mode. The computer will add additional fuel programming to preclude "flat spots" or hesitation. For carb folks, we would call this the accelorator pump. Would seem quite possible for the computer, using mapping tables, to spray fuel, coming off 14.7, dropping down to a slightly rich mixture more favorable to performance, then correct back to 14.7.

Again, haven't paid for dyno time, research and development, etc to prove or disprove. And unlikely to do so. Some areas for increased performance are quite possible.

This is my problem with ECM/PCM guesswork. Without wideband sensors, better measurement tools, etc, tough to say. I just like knowing what, how and why things are happening.

Enjoy the cars.

dlmeyers 90 coupe zf6 3 speed shocks