Still putting the finishing touches on my 383. Just installed a Adj FPR. I have 36# injectors fueled by a 255 walbro, what should I set my fuel pressure for? Thanks.

 

You have to set the fuel pressure to the value that you told your tuner it was going to be set to. If you have 36# injectors, and aren't making too much more than 500 flywheel HP, you would use the stock setting of 43.5psi, WITHOUT the vacuum compensation line connected.

If you are making substantially more than 500 flywheel HP, you either need larger injectors, that will supply the required fuel at 43.5psi, or you need to bump up the fuel pressure. But you can't change the fuel pressure unless you put the correct flow rate for the injectors AT THAT PRESSURE in the PCM.

 

Thanks Fred. Wasn't sure if I should stay with stock pressure or not. I haven't gotten a dyno tune yet. With my setup, I don't think I am making more than 500fwhp. I will be sure to tell the tuner what pressure I am running. Thanks again.

 

The flow rate of an injector varies with the pressure differential across the injector orifice. The rated FP is just a convenience number. The point when setting fuel pressure is that when you change it, you change injector flow. So, injector flow is a variable. When tuning, to make the motor run richer or leaner the two main options for making gross adjustments are to either change the injector flow rate in the ECU program or to change the FP. Do not do what a guy who brought his car into the shop for us to fix his fuxxored tune did. He kept chagning FP, calculated the new injector flow rate at the new pressure, and then re-programmed his ECU with the new flow rate. Of course, the AF ratio did not change as the changes offset one another.

FWIW, the formula is (sq.rt (new pressure/old pressure)) * old flow rate = new flow rate

Example: you have injectors which flow 50#/h @ 43.5psi and set the FP up to 60psi. They will now flow 58.7#/h, provided the fuel pump is up to the task.

Rich

 

Put the stock regulator back on and call it good.
Why did you think you needed an adjustable?

 

Id start with the stock psi. Then once you get it on a dyno, Id up it a pound, at WOT. See if that helps. I have also seen dyno sheets/track times, to where a pound either way, has improved.

 

Honestly, I figured an adj FPR would be needed since I increased injectors and CIDs. Since the stock was probably original and/or with unknown miles, I played it safe and just replaced it. I don't have money to throw on parts just for the heck of it but I'd rather throw money at an unknown part than at a tow truck........

Rich - Good read, appreciate the education. The members here never cease to help out!!

 

If changing fuel pressure makes better power the tune was wrong.

Thing is MOST cars even heavily modified ones are very well served by the stock regulator and several of the aftermarket ones are VERY trouble prone.

Regulastors should be treated as a fuel system component not a tuning tool as most guys want to use them.

 

Id PM FASTFATBOY, and ask him if adjusting fuel pressure, matters or not. He's got proof..

 

If adjusting the fuel pressure makes it run faster that means the tune was off. There is nothing to debate about that.

 

STOCK tune is a little rich for max power. If you lean it out a little by lowering the fuel pressure at the track it will run a little quicker until the PCM "learns" and adds the fuel back. This can be avoided by unplugging the PCM after each pass, clearing the block learn cells. Kind of crude, but under most conditions it will work. If the air is very good, it will slow you down by making it too lean.

Rich