I believe the "correct" pressure to run on multi point fuel injection is 43.5PSI with the vacuum line disconnected. I have an adjustable regulator and have used 50PSI to get a little more flow when I had smaller injectors. Now that I have 36# I think I should lower it back down.

Has anyone tried running lower than 43.5? It seems that the fuel pump would have a better chance of keeping up if the pressure were lower. Just alter the fuel curve to deliver enough fuel right?

Anyway it got me thinking about what pressure other people may be running and what your reason is for that pressure.

BTW this is on a 414 Dart/220AFR HSR combo with a 236/242 roller cam and Bosch 36# (white) injectors. This combo runs with ~12Hg vacuum @800RPM instead of 18 or so for stock. It seems this would effectively raise the fuel pressure when idling since the regulator is lowering it less. Should this be taken into consideration?

I run 36lb injectors.
I run about 30psi n/a. Reason why I do this is because I have a dry nitrous kit.
I want more room for psi when I spray.

Rule of thumb, always use the smallest injector possible with the most pressure possible.
If you don't have enough pump for that then you'll need to use less pressure with a larger injector.

I typically use about 60psi. Remember most all the new stuff runs at 58.

My Bosch III's are spec'd at 39 lbs FP from FIC.

There was a very strong debate back years ago when I joined here in 2002 as it pertained to psi vs flow ratings and different manufacturers etc.

The guys that started using the Ford injectors stated that the Ford fuel pressure ratings and injector ratings were specified with 38psi instead of the GM 43psi standard.

When I did the rebuild of my 383 in 2003 I had the chip burned for 30lb injectors and it was still running slightly rich. If taken into account a Ford 30lb injector running in a GM vehicle with TPI at 43psi will be actually right at a 32lb injector due to the 5psi GM/Ford difference.

I don't know if they ever changed the flow ratings specs for pressure over the years but I will say one thing, the Ford racing injectors are loud as hell!

Anyway, good luck with ya project man! I hope ya nothing but success!

Adam

Yeah the whole "at what PSI injectors are rated" issue/debate will go on. I used Bosch injectors on this build and used their specs. Also rated at 39 PSI but Bob R calculated the batt offsets for 43 PSI. The Bosch also ran rich for me so I have tweeked the batt offsets a little as well as using 42PSI pressure. So far it seems to be working well.

Ford Injectors loud? The stock Multitecs were loud too. Sounded like a solid lift cam. The Bosch seem quiet although my Comp rollers with short travel lifters are pretty loud at idle/low rpm. Sounds like a sewing machine lol.

I used to jack up the pressure when I had smaller injectors but prefer to keep the pressure low so the pump has a better chance keeping up. One interesting note, my Holley AFPR wouldn't allow me to set the pressure lower than about 38. Could be a regulator issue or possibly a return line restriction.

Thanks. It is running great now. Getting the final tune as good as I can before heading to the dyno. Looking forward to that trip!

Big pump and 50psi or so.

Lol I guess that is the best approach. What pump are you running? I'm using the "stock" TTA replacement but I don't know if it's really stronger than the original one.

Any comment on the Holley AFPR not going below ~38PSI? I'm wondering if it's a regulator issue or a return line restriction. It does drop below 38 when at high vacuum so I suppose that answers the return line issue.

I use racetronix pumps in my cars, one has twin pumps.
Also take a look at what the fuel looks like coming out of the inj at different PSIs. The finer the mist, the better burn you get.