Ok, here's the situation. I just tested my fuel pressure and came up with 42psi with the key on and 48psi started (Both with the vaccume line disconnected and plugged). Haynes Manual calls for 34 - 39psi, so it looks like I'm already above that for some reason. Here are the mods that I've done that may affect your decision. I have straight dual exhaust w/Borlas (No Cats), K&N, open lid, open shroud w/custum ram air, modified MAF, air foil, smoothed plennum walls, new fuel filter, and stock injectors. Car runs great, but I have always wondered if an AFPR would improve performance.
For those of you who have one, what is your psi set at? :flag

 

The fuel pressure specs are different for different years. If my memory is correct, you are okay on your FP.

In my opinion, from your list of mods, you probably would not gain much if anything by adding a AFPR. If you increased the pressure you will get a better spray (atomization) pattern but the computer will adjust the fuel injector constant so that no additional fuel will enter the engine. The computer is trying to maintain a 14.7:1 oxygen/fuel ratio and additional fuel disturbs that balance, hence the correction.

A legitimate use for an AFPR is to correct for fuel injectors that are rated different than stock. For example, Ford injectors are rated at a different fuel pressure than GM. This results in a different flow rate when installed in a Corvette. A 19 lb Ford injector would probably require about 48-50 lbs pressure to be equal to a GM 22 lb. Even using an AFPR to correct for the difference only puts you in the "ball park". For really close adjustments, one must reprogram the chip.

A negative effect of the computer adjusting the fuel injector constant with stock injectors is that the "instant MPG" readout will be in error. You might see readings like 45-60 MPG at a steady 65 miles per hour. This is because the computer uses the fuel injector constant to calculate the fuel flow. This along with the speed you are traveling is used to calculate the MPG. All of this can be corrected with a chip reprogram.


[Modified by Wildride, 6:46 AM 6/13/2002]

 

Hey fella!

Without some other mods I think it would just be a neat toy. This neat toy could also hurt you some too. As stated above, if you increase/decrease the FP you car will just correct for the increase or decrease with the BLM. With that being said, you would not see a difference in NON-PE situations. However, during WOT the ECM goes into Open Loop and does not make corrections to fuel delivery. As a result you could either lean the engine out or richen it up. As to whether you will gain or loose performance is anyones guess.

The one key to tuning is to first get the BLMs as close to 128 as possible. Then when the system goes to WOT its easier to tell what the PE parameters are doing.

Lastly, before you spend 50-100 bucks on a AFPR I would go to ebay and pick up a cheap laptop for 150-200 bucks. This will at least let you see what changes you are making and how they are effecting the engine. You will be amazed at how neat it is. Really makes tuning easier. Like trying to play pin the tail on the donkey with and without a blindfold.

Let me know if I can help you out in any way!

Later!

 

I agree with ski_dwn_it 100%. Get a laptop, add a scan program and interface to connect to the ALDL. This will allow you to really see what the engine is doing. Craig Moates has a nice scan program that is free. Look at the ECM/PCM board or thirdgen.org for more info.

Another comment on bumping the fuel pressure. A lot of times people have done just that and have seen immediate gains on the dyno; BUT on later runs the gains all dissapeared because the computer adjusted for the change. If you change your injectors to a different rating, a AFPR is almost a necessity but to do the job right you are going to have to scan and then make the appropriate adjustments to the chip.


[Modified by Wildride, 12:15 PM 6/13/2002]

 

i believe the 34-39 psi referenced in the haynes manual is with the vac line connected. i checked the stock fuel pressure on my brothers 91' with the vac line disconnected and came up with 48 psi. your fuel pressure is good

 

Thanks guys for the advise, sounds like I don't need an AFPR with my setup.
Jesse, its good to hear from you again. First the easy question. What does BLM stand for? As far as the Laptop, that's not the problem, I have that. The problem is knowing how to use it to tune my engine. I will look into getting the software, and then seek help from you and the forum.
Thanks again... :flag

 

The way to be sure about fuel pressure (i.e. fuel delivery vs air flow) would be to get a dyno pull with a wide band O2 sensor. I got three dyno pulls last fall and my 47psi was just right for best power at WOT.

Our OBD-1 systems use Block Learn Mode (BLM) cells and an Integrator to achieve correct air/fuel ratio in closed loop mode. There are 15 cells, one for each of several engine/load conditions with the 15th for WOT operation only. Pick up a copy of "Chevrolet Fuel Injection" by Ben Watson for about $17 at Amazon for a good explanation of how this all works as well as diagnostics for each trouble code.

 

If you have the laptop then you are halfway to connecting to the car. I use this cool program: http://www.blacky.co.nz/efilive/

I only have the demo version though. You will need to fabricate the proper serial cable to convert the TTL data into standard RS232. Here is a site with some info on that: http://www.techedge.com.au/vehicle/aldl8192/8192hw.htm

You can just aligator clip in the wires or order the cable from summit like I did. The darn thing costs like $30! You still need to make the data converter circuit though. Keep the posts comming if you have anymore questions.

 

I checked out the links, and read the forums tuning /FAQ's and I realized that this stuff is way out of my league. I appriciate the help, but this is way over my head.
For now I'll just stay with my "Supertuner" aka SOTP!!! :lol: :flag