Stephen Max

iTrader: (59)

There is an equation I found in a book on supercharging (Supercharged ) that gives the following equation for estimating the new fuel pressure needed when boosting to a given level, and adjusted for AFR as well:

Required fuel pressure = SFP * ( 14.7/AFR * PR)^2 + BP

where: SFP = stock fuel pressure
AFR = desired air/fuel ratio at boost
PR = pressure ratio, i.e. (14.7 + boost pressure)/14.7
BP = boost pressure

I am assuming stock fuel pressure is referring to the base fuel pressure, i.e. 34 psi.

So a V1 SC with a 3.25" pulley producing 9 psi of boost (maybe on a good day, since some of you seem to be getting a little less) and a desired AFR of, let's say, 13, requires a fuel pressure of:

RFP = 34 * (1.13 * 1.61)^2 + 9
RFP = 122 psi

That seems like way too much, and even an 8:1 disk in the fmu will only give me 89 psi (using Vortech's formula) at 9 psi. I don't think any of you guys are running 122 psi fuel pressure for 9 psi of boost, am I right?

If you assume a stock fuel pressure of 43 psi (fuel pressure at WOT) and want the AFR to remain the same, then you get:

RFP = 43 * (1 * 1.61)^2 + 9
RFP = 120 psi

Still seems like too much.

I'm wondering if there is a typo in the book and the exponent in the equation should be 1/2 instead of 2 (i.e. the fuel pressure required is dependent on the square root of the pressure ratio, not the square). In that case then the calculated fuel pressures are a more reasonable 55 psi for the first case and 64 psi for the second case. But those seem a little low, actually.

Any comments?

Stephen Max

iTrader: (59)

Hmmm. Only 10 views and no replies. Too theoretical for you guys?

delio

iTrader: (11)

I got some math to do...

RBa

iTrader: (2)

My stock FP is 32psi at idle. Boost pressure is 18 on vaccum . Someone do the equation for me... I dunno my AFR & PR?? No way to read it now.. I'd do it myself but I didn't dyno my car yet.. And those reading are when I am at idle..

And I am running lean so my fuel pump and FPR are being overworked..
Is it normal if you hear your fuel pump??

Stephen Max

iTrader: (59)

Your boost pressure is 18 at idle? Wow, it must be around 30 at redline!! j/k. You don't make boost at idle, 18 is the amount of vacuum you have at idle, measured in inches of Hg (mercury).

The equation is for estimating the amount of fuel pressure needed at maximum boost, i.e. at redline for a SC'ed car, you can't apply it at any other condition.

I've never heard my fuel pump - if yours is noisy, maybe you'd better keep an eye on fuel pressure just in case it's on its way out.

ToYLeT902

iTrader: (12)

He said 18psi on vacuum...

hlh0501

iTrader: (1)

this seems to be a good formula, I just wonder if it is correct,

and my biggest question is - wouldn't it be different for each dif. size/trim supercharger or in my case turbo?? b/c a T3 @6psi will require far less fuel than a T78 @6psi ....

I want a greddy a/f wideband O2 sensor, and EGT.. too bad greddy a/f is 320$ and egt is another 150 or something.. then i wouldn't need formulas haha

according to your formula at 12pounds of boost, I need 167psi fp, and with vortech 12:1 (what i was running until recently) I would have been feeding it 161psi , SCARY! I have now leaned it out some and it still runs fine, with 12:1 I was off-the-charts rich on the dyno...

delio

iTrader: (11)

I always wondered why you went with such a big disk... what did you drop it down to? 8:1 would appear to be perfect for a setup like yours, 7-10 PSI of boost. 6:1 may work as well too but I would want to be on a dyno to see it for myself.