SR running 15 PSI Stock Fuel system?

[DammitBobby]

Sorry for posting this on the KAT forum but I like this forum better! Can someone explain how that could work? I was talking to someone the other day who is installing a SR swap in a few weeks. His motor is in but waiting on someone who is working from home to finish another project car. Only thing he is going to install for the fuel system is a walbro fuel pump. Stock fuel rail, stock injectors and stock FPR. If memory serves me right stock SR runs 8 PSI. How is his fuel system going to compensate for additional 8 PSI?

[Pyro]

Can be done with use of a FMU. A FMU increases fuel pressure with boost at adjustable ratios, (2:1, 3:1, 4:1, ect.).

I use 15 psi of boost on my completely stock 76 280Z fuel system. But require 100 psi of fuel pressure at 15 psi of boost in order to deliver enough fuel with the stock 190cc injectors.

There is an injector flow formula: (new pressure/ old pressure)sqrt x old flow= new flow.

So (100/36)sqrt x 190 = 316. So at 100psi, a 190cc injector can flow like a 316 cc injector at 36 psi.

FYI, this takes a lot of pump to produce enough fuel volume at high pressure. For a fuel pump, as pumping pressures go up, flow goes down.

I use the Bell Engineering FMU and three fuel pumps to get it done.

I don't have a 240sx yet but I'm working on it!!!

[:: orion ::]

FMU has nothing to with it in this situation...

The Nissan engine managment systems use a MAF to read the airflow coming into the engine...as the ECU reads more airflow, it tells the injectors to dump more fuel.

This is only limited by the amount the air the MAF can accurately measure (max of 5.11 volts, stock S13/S14 SR MAF can only read ~250rwhp worth of air) and the max of the stock injectors, which is also around ~250rwhp, maybe a little less.

Very simple and effective.

Now, with respoect to FMUs...the stock fuel pressuire regulator is a 1:1 ratio FMU of sorts...but this is only to keep the fuel flow constant. A 1:1 FPR keeps the pressure differential across the tip of the injector constant, so the flow is ALWAYS measured at 3bar (43.5psi) fuel pressure.

Same reason why the fuel pressure is lowered under vacuum (idle)...~20in vacuum is about -9psi, so the fuel pressure is lowered the same amount to keep the flow constant.

- Brian

(BTW, 15psi is BAD on a stock turbo for an S13 SR...the T25 is blowing a TON of hot air in that efficiency range....most keep the max PSI of a T25 on an SR to 14psi or less)

[nissanfanatic]

This is also a good example of why PSI is not a good way to measure or approximate HP. The T25 will only make about 250whp at 14-15psi where a nice sized T3/T4 could make 250whp at 9-10psi depending on wheel trim and tuning.