Hey ppl i just registered to this site.
I have questions about fueling underboost. But first let me share my vission. So i got myself a gt28 garrett disco potato. Putting in forged pistons and rods lowering compression to about 8.5:1 and also putting in titanium valve springs because i plan to raise rpm limit to about 9000rpms. Now having said that i can say im planning on running 25psi of boost before i get falmed from everyone thinking im running that much boost on stock internals loll... i will be running megasquirt and putting in 850cc bosch injectors and ill have a 300lph fuel pump supplying fuel load. Now my question is how do i figure out how much rail pressure ill need and if ill need to upgrade my rail or regulator. In conclussion what do i need fuel pressure wise to support 25psi of boost. Im also willing to run meth injection to take care of any potential pingage. Need expert help please and thank you!!!!

 

Run the most your fuel pump can put out, use a fuellab fuel pressure regulator. I'd suggest going with ID1000s instead of the 850s.

Not sure if your GT2860 will be able to do the 25psi you're asking it to do.

But I'm just speaking from my chair.

 

rc injectors calculator should tell you pressure @ rail based on Target crank horse power https://www.rceng.com/technical.aspx

 

A GT2860 should be able to spin to 25psi fine, probably won't make much power at higher boost levels then that though.

 

Yes iv been to this site before but it doesnt have a formula for pressure @rail based on hp...

 

Play with the numbers and you should get your answer

 

Ok heres another question. Based on the calculations its telling me i need 850cc injectors at 43.5psi at rail to support 400hp. So does injector flow rate compensate for pressure at rail then?

 

your stock fuel pump should be able to flow enough up to 300whp with at least a replacement unit. I'm not 100% sure but I think its flowing enough to maintain the 43.5 psi @ the rail. Also yes using bigger injectors means you wont have to run any sort of bandaid to rail the rail pressure

 

Ok sounds good to me. I mentioned before that im upgrading the the oump to a 300lph anyway so i should be able to have enough fuel supply. I was just wondering if ill need more rail pressure them have to upgrade the rail since the stock is designed for 43.5psi. I have my whole build figured out this was the only thing i was unsure of. Thanks a lot for the help guys ☺

 

I'm on a stock fuel rail @ 75psi base pressure with an adjustable fpr. The rail will definitely take more than 43psi if you need to raise the pressure.

 

Sweet good to know pdexta. How much boost are you running? And can the stock regulator work within higher fuel pressures as well?

 

It tends to get overwhelmed with the higher pressure, but a FPR like the one I linked to earlier solves the issue.

 

Cool so all ill need is to upgrade the regulator.

 

19psi. You'll need an aftermarket regulator to raise the pressure, but they're relatively cheap considering the rest of the stuff you need for a +350hp miata build.