I've been working a little with Corbin's fuel pressure calculations and I am a little confused. Here is what I am working with...

30# fuel injectors (about 315cc).

First, to get the amount of fuel flow your engine needs per injector, you use:
HP * BSFC * 10.5 / 4

If I assume I am making around 230 hp, we have...

230 * 0.525 * 10.5 / 4 = about 316cc/min

To find our maxmimum fuel pressure, we are told to use:
((((Consumption / Duty Cycle) / Actual Injector Size)^2)*43.5)+ Boost

In my case,
(((316 * 100% / 315)^2)*43.5)+7 is really just 43.5+7 because my duty cycle is 100% since I don't use a fuel computer, therefore (316*100%/315)^2 is about 1. So 43.5+7 is 50.5.

Based on these equations, I should need about 50.5 psi of fuel to flow my engines need.

Now we would want to calculate what SFMU disc to use to achieve this. To start, we have to figure out what the static fuel pressure should be. If we know that the stock fuel injectors are flowing 276cc/min of fuel at 0 psi (manifold pressure), we can then use the same fuel pressure equation to compensate.

((276/315)^2) * 43.5 = about 33.

33 psi of fuel at static sounds fine to me. So, now we can take our estimated maximum fuel pressure, and subtract the static fuel pressure, and divide by our boost pressure to get our rise rate.

(50.5 - 33) / 7 = 2.5

This suggests a rise rate of 2.5:1 to acheive 50.5 psi of fuel at 7 psi of manifold pressure. Now, heres the question. If we calculate the approximate rise rates of the various SFMU discs, we get something like 6.6:1 for the 3.62" disc, and about 4.6:1 for the 2.72" disc.

I first tried the 2.72" disc, and found my EGT's to hit 1600F very quickly, an indication of running very lean. Assuming the 6.6:1 rise rate of that disc, and my static fuel pressure of 33, then my fuel pressure at 7 psi of boost should be about 79.2 psi.

See the conflict here? Based on the calculations, I should have been quite rich, when in fact I was lean. So, why are these numbers so far off?


1996 Eclipse RS / 5-Speed / Aerogear Body / Coated in Magnetic Red Metallic rev.2 / T3 Turbo Intercooled @ 8 psi / More at http://home.insightbb.com/~mgalyan/
Mark Galyan - Indianapolis, Indiana

i hate fuel pressure calculations. i got nowhere fast (same problems as you) with them when i was trying to tune my "new" fuel system last February. basically, i found someone with my same injectors and harassed them enough until they told me what their ballpark fuel pressures were. from there on out it's trial and error with the EGT gauge, oxygen sensor voltage readout, and the bleed valve on the S-FMU.

Since you have a T3 turbo, your fuel needs may be different than Hahn's S16G. Also, did you read your fuel pressures while boosting? That is very important. And were you boosting in closed loop (part throttle) or open (WOT)? Part throttle boosting will inevitably lead to high EGT's as the ECU is adjusting fuel flow. In my experience, the fuel calcs have lead to slightly rich mixtures.

Corbin

'95 ESI-T
HRC+FMIC+ETC...



Gimme fuel...Gimme fire...Gimme that which I desire