86TpiTransAm

Ok, I know the stock fuel pressure for an L98 is 43psi, that's with a 22# injector constant and 22# injectors.... or maybe it was 21#, please correct me. I also know that most people raise their fuel pressure to about 46-47psi for optimal performance.

Now, currently I'm running 24# injectors on my TPI setup and have the injector constant set to 24# in the ECM. My question is, what would be the optimal fuel pressure to run with this setup. I know the 305s with smaller injectors ran different fuel pressure than the L98s with the larger injectors so I'm guessing that the 46-47psi won't be optimal with the 24# injectors. Should I go higher or lower with the FP?

Jim 86 IROC

Fuel flow depends on injector pulse width and fuel pressure as you know. You realy need to trim the fuel pressure to match the air flow, which depends on your setup.

I have tables on my site that show flow & HP supported vs fuel pressure. If you have time slips and know the weight of your car these can give you some idea on a pressure setting.

The best way is to vary pressure for best HP, as shown by dyno pulls or trap speed.

drive it

iTrader: (1)

You really need a scanner or scan software to run on a laptop to dial it in. The whole idea is to get your blms at 128 across the board; by setting your fp where you want it, then adjust the inj. constant to achieve 128's. Then WOT tuning for best power....you really need a wideband.

86TpiTransAm

I "need" alot of things. Unfortunately most of those things require spare money that I don't have. I have an AutoXray scan tool but I don't program my own chips so the way it's going to be is 24# injector constant and 24# injectors and basically all I'm going to be able to change is the FP and I just wanted to know a good starting point for tuning. Like I said earlier, I know with a 350 with stock injectors 45-46psi is a good place to run. I guess I'll just start at about 45psi and go from there.

As it is right now I'm running 24# injectors with a 22# injector constant and I have my fuel pressure set to 38 1/2 psi (with vaccuum unplugged) so I don't run too rich. With 24# injectors and a 19# injector constant in my stock 305 chip I had to run 36psi and I was still running too rich.

Grim Reaper

86, the only way to accomplish what you want is by burning your own eproms - plain and simple. But with the equipment (since you have a scan tool, you just need the burner and either WinBin or TunerCat) the following is the correct way to do it.

On my car I run 48 psi with stock 22# injectors. My BLMs/INTs was quite low (1008-110). To get my BLMs "overall" around 128/128, I needed to "lean/shorten" the pulse width. So I kept increasing my "Injector constant" until most of my BLM/INTs were around 128 (overall) and then "tweaked" my VE Tables to get ALL my values exactly at 128.

I did a lot of playing with the effects of fuel pressure and how I needed to compensate within the eprom. I found that raising fuel pressure has the LEAST EFFECT at "light part-throttle/low load" and GREATEST EFFECT at "WOT/High Load". This corresponds to the fuel pressure readings I observed when I drove around with the "vacuum line" reconnected.

With the Vacuum line reconnected, you will always show less fuel pressure at a high vacuum/low load situation because this was the way the fuel pressure regulator was designed. And, I found it was pretty close to the original fuel pressure I was running before I ever installed an AFPR.

Only at WOT/High Load do I see the higher fuel pressure. But, the bottom line, I do believe in the "theory" that higher fuel pressure promotes better atomization (but not so high as to pooch the fuel pump). And 46-48 psi is probably as high as you should run (if you don't want to be replacing fuel pumps all the time).

The only way you can run fuel pressure that high and NOT be running too rich is to adjust the EPROM - and you often have to change the Injector Constant. But you should adjust the Injector Constant to something that gets you close to 128/128 - not what the "computed" fuel flow would be based on the Injector rating and fuel pressure.

That's a common mistake made by guys when they start "tuning" their own eproms - which is what it sounds like the person that adjusted your eprom to 24# did in the first place, since no eprom comes "stock" with 24#. This is one of those things where you need "hands on access" to the car - and why "eprom's by specs" are seldom "optimal" and often "far from perfect".

Jim 86 IROC

Since you have a scanner, start at say 43psi and collect some data at part throttle and at WOT.

If you can keep the BLMs and Int not too far from 128 and the O2 values near 920 at WOT you will have a decent point to go the the strip and fine tune with time slips.

86TpiTransAm

Thanks Glenn for all the info. The whole DIY thing continues to make more and more sense to me but I'm just having the problems getting the extra cash for the burning equipment. Someday soon I hope.

When I first popped in my Ed Wright chip with 22# injector constant and using 24# injectors, I had very very close and sometimes right at 128/128 at WOT and O2 readings aroung 900 at WOT. Only problem was that with this, I was running about 110-112 at idle and part throttle. I think I'm just going to give in and spend he extra money on some burning equipment... when/if I ever get any extra money.

Grim Reaper

See my comment on the "Ed Wright Custom Chip". I say how you can get into it "cheaply". $149.00 to be exact (plus another $10 for a Flash Prom to avoid using a UV Eraser and MUCH FASTER re-programming).

86TpiTransAm

How does the Flash Prom work exactly?

Grim Reaper

Do a search on "Flash Prom" and "29C256" on the DIY Prom Board. A lot of great info on it and once you do a little reading, you'll say "I have to have one"!

Z_Ghost

The computer locks the BLM and INT to 128 when you go WOT (PE), so that is why you need to be running close to 128 when not in WOT (PE). Also the O2 sensor is only accurate around 14.7 to 1 when you go above or below it just tells the computer that you are either lean or rich.