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255lph rwhp

Old 09-05-2009, 08:19 PM
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well i have a walbro 255 pump with stock fuel rails with 42lb injectors. how do i tell what pressure my fuel rails are running? is that something that my tuner is going to have to tell me, or is there a guage i can put on the rail. because from what you guys are saying my pump should be putting out more fuel and not limiting me at 350whp and 380wtq. my a/f is 11.5 throughout the whole power band also if that matters.
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Old 09-05-2009, 08:20 PM
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i'm looking for around 420-450whp even only 400whp if i can get it without going with a BAP or dual pumps.
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Old 09-05-2009, 09:29 PM
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With the Moderator's blessing, here's Bik003GT's PM question and my response:

Originally Posted by Blk003GT
What fuel rail pressure are you running, does the FRP stay steady at WOT and peak power?

Knowing the actual FRP, at WOT and peak power, will quickly highlight whether or not

i'm running stock fuel rails. so how do i tell my fuel rail pressure? would i have to ask my tuner this? i know i'm running a steady 11.5 a/f throught the power band but they say i'm maxing my fuel out only running 350 and 380. its a 255 walbro. so i'm not sure whats going wrong here bucause correct me if i'm wrong but i should be able to squeeze more out of that pump.
If the pump can supply and hold an 11.5:1 AFR then there's nothing wrong with the pump's capacity, as the first sign of maxing out the pump would be the fuel rail pressure dropping and the mix going lean. So I don't know upon what they are basing their conclusion that the pump is pegged--unless they tried to push it richer and the juice wasn't there, or they just needed to come up with a good story as to why the performance is lacking.

The fuel rail pressure can be logged through the OBD2 port using a number of tools, Delta Forces' OBD2DataScout is one--there are others--it can also be observed by installing a pressure gauge on the rail.

Note that the pressure logged by most apps (and displayed by a mechanical gauge) will be the pressure needed to make the pressure differential across the injectors 40 psi.

The EEC-V system maintains a constant pressure differential of 40 psi across the injector--so if the manifold vacuum were 20 inHg (-9.8 psi) the rail pressure will read 40 - 9.8 = 30.2 psi. If you were running 8 psi of boost the rail pressure would be 48 psi, to keep a constant differential of 40 psi.

I suspect the tuners are playing it safe, and have not pushed the timing as far as they could, which can get into dangerous territory--or that, depending on boost levels, you need a mix richer than 11.5:1 (that could not be achieved 'cause the pump wussed out) to hit peak performance.

What octane fuel are you running, do you know what the spark advance at WOT, near peak HP, is?

With regard to the rails themselves, I have no idea where their flow rate would max out, but my gut tells me it is far in excess of 600 HP--think about it, and look at the tiny hole in the injector's nozzle (even 96 lb/h injectors) as compared to the size of the rail and fuel lines.

It is fashionable to install "high flow" fuel rails, however I suspect that the OEM rails can support any n/a application and most boosted apps as well.
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