Hi,

Id like to know exactly what fuel pressure returnless NB's have.

I understand that they are pressure referenced, Id like to work out what I can make with maxxed 725's on E85.

I understand that it is 60 psi, can somebosy please confirm? I have an NB8B which is fuel limited at 230whp with RX8 yellows, which seems very low... I should buy a fuel pressure test rig for my dyno.

Thanks,
Dann

 

Nominally 60 PSIG, at the tank.

 

But does anyone have data to show its actually really 60psi under boost in the real world?

Wait, can I do a pull on gasoline, check the duty cycle, add 30% and find my new duty cycle, Im not sure thats accurate enough.

Dann

 

its not 60psi under boost. its 60psi fixed. it goes down with boost and up top as you near the limits of the pump
basically its only 60psi under perfect conditions (like just idling or normal driving) and just goes down from that

 

Its still 60 psi under boost, its just that the boost pressure opposes it, its actual pressure should still be 60, but its relative pressure should be lower.

Or am I retarded?

Dann

 

different way of looking at it but yeah same conclusion

*edit: although I think someone tested this and the pressure was still diving up top even on an n/a car

its easy to check though, just put a pressure gauge/sensor at the fuel rail

 

This is the OP question, what happens to it in the real world?

 

(edit -- I did my math wrong, look down at post 17)
It is 60-ish psi gauge (I measured mine at 62), and the regulator is relative to the pressure inside the fuel tank, which might or might not be ambient. Since it's pressure relative to the manifold that matters for fuel delivery, this means you have roughly 57-58 psi pushing the fuel out at idle, 47 psi at WOT NA, and 32 psi at 15 pounds of boost.

If you upgrade the pump to a high-flow one, you probably will overwhelm the stock regulator at idle/low load. I measured mine as high as 80 psi with a walbro 255, but the value depends on the system voltage, condition of the pump, and a bunch of other things.

--Ian

 

Great answer. Sounds like 300whp on ID725s is about it.

Thankyou for the data.

Dann

 

I have a in car electric fuel pressure gauge with the sender on the FM dbl inlet fuel rail on my 99 and read 60 to 62 psi pretty much consistently no matter if in boost or not with a 255hp walbro.

 

What is the easiest way to do this on the stock rail? Was there a fitting where a sensor could be screwed in?

 

You can probably fit one in place of one of the fuel dampers

 

550cc/min injectors did 270 rwhp for mw with high 70s of duty cycle, so 725s should be plenty of injector for that, especially since you can run IDs at higher duty cycles than the older tech RCs I had. OTOH, 300 who is probably more than the stock fuel pump can flow, mine ran out around 250-260, although that depends on what manifold pressure you're running.

--Ian

 

You can, but I don't recommend it. The dampers are there for a reason, and losing one makes for significant pressure waves in the rail.

--Ian

 

correct. this would be for testing purposes, not long term

 

Its making 300 on pump, but I would like to know what it can make on E85, I should just check the DC I think.

Thanks for everyones replies.

Dann

 

I realize that Dann, has his answer, but I question the above information. At idle, Manifold is at -10psig for 30kPa absolute. Therefore, differential across injectors is 70 psi, not 58. Similarly, at MAP=100kPa, differential would be exactly gauge or 60 (less pressure drop due to fuel flow).

As stated above, this assumes tank is vented to atmosphere or has no pressure or vacuum.

 

Duh, you're right, sorry.

I don't know what tank pressure normally is. It hisses when you open it at a gas station, so it's not exactly ambient, but I don't know how big the differential can get before the relief valve(s) open.

--Ian

 

My ~630cc FlowForce EV14s at 9-10psi (~205whp gas) on an NB2 fuel system with a DW200 need about 55%DC on gasoline and 80%DC on E85.

 

I have the 610cc flowforce ev14s from Trackspeed. Car (1999) has stock fuel system. Hydra ecu. Gt2560r w/ 3 inch straight thru exhaust. 10psi.

Is 610cc big enough to use e85 with an aftermarket fuel pump (dw300 maybe) and an otherwise stock '99 fuel system?