Experts all seem to agree that increasing the FP for LT1s is ineffective for stock or near stock fuel systems. I have installed an adjustable FPR and tried pressures above 41 psi. I have never been able to measure any significant improvements with a VERICOM. I have the stock injectors, cam, etc. What do LT1 folks recommend? Turn it back to 41 psi?

 

People say LT1's run rich from the factory, so I never understood why you would raise the fuel pressure on a mostly stock motor, seems that it would make it even richer. But I haven't done it myself so I don't know, hopefully someone who has experimented can chime in.

 

You can tweak a WOT air/fuel mixture with the AFPR but the computer regulates the injector pulse width and frequency to account for any pressure differences. The AFPR can gain some HP for you at WOT, which would benefit you at the track, but you'd probably want to tweak in on a dyno with a wideband O2 sensor.

 

My stock LT1 ran best at 39 psi.

 

This is what I understand the LT1 does with fuel pressure. This is an autocross car so I tune it for every hundredth, but without a reliable dyno tune, I think I want to dial it back - maybe 39 psi as someone mentioned. Would like to hear if others have other opinons.

 

Don't turn up the fuel pressure on a fairly stock LT1. You'll run rich and end up replacing the cats. Been there, done that. If you're going to mess with fuel pressure do it on a dyno with A/F readings otherwise you're just guessing.

 

I have one of these on my basically stock 95. I also understand that the computer will adjust pulse width to compensate for greater or lesser pressure. However, it still seems like there would be a benefit to increasing the pressure a bit since that should improve fuel atomization. Seems like that combo of higher pressure/shorter pulse could be better than lower pressure/longer pulse due to the atomization issue?

 

Here's what happens....During idle and part throttle cruise, the PCM adjusts the injector pulse width based on o2 sensor feedback. Some claim better idle quality with the higher fuel pressure and that may have been true on the old pintle style injectors vs the Multecs used on later C4s. None the less, cruise and part throttle performance is unaffected unless the pressure is increased in such an extreme manner that the required adjustments are out of the PCMs range.

During WOT, the PCM ignores the o2 sensors. Assuming a slightly rich mixture (BLMs less than 128), the BLMs will snap to 128. You now have a situation where the base fuel calculation is using BLMs of 128 but with a higher than stock fuel pressure (that the PCM doesn't "know about" since the PCM hasn't been reflashed). By definition, this makes the WOT A/F ratio richer. Since the good General errors on the side of conservatism (read: rich), you now have an overly rich condition that will likely decrease hp.

Of course with enough mods, the engine may be lean and the increased fuel pressure will help. Without a wb o2 sensor to measure the A/F ratio, making adjustments is like playing the lottery. To give you an idea of the mods it takes to get an engine to go lean, look at my sig line. My first set of datalogs after adding the HOT Cam and prior to reflashing my PCM showed I was ~7% lean. Translated to OBD I, this would be equivalent to having BLMs of about 137. Basic mods like an exhaust, open air filter lid, & 52mm throttle body won't get you there.



[Modified by 96GS#007, 9:14 PM 4/2/2003]

 

According to the Helms manual for a '96, the stock fuel pressure (measured key on, engine off) should be between 41-47psi.

My stock FPR read 44 with this test. With LT4 injectors (28lb), LT4 heads, Hotcam, and mostly LT4 PCM programming, my fuel trims would bounce between slightly negative (PCM leaning fuel) to slightly positive (PCM richening fuel).

I got an Aeromotive AFPR and bumped up the pressure to about 49-50psi. Now my fuel trims reads slightly negative (-1 to -5%) all the time. This shows that the PCM does compensate for higher fuel pressures (it has to). But it will make for more consistent Power Enrichment mode fueling (easier to fine tune this way) and I get the "supposed" benefit of finer fuel atomization due to the higher pressure.

Basically, unless you've got the data logging/reprogramming software needed OR you can spend time at a dyno with a wideband O2, then the utilization of an AFPR will just leave you guessing.