I need some help tuning the fuel in my car. I ordered a SFMU that should be here by the end of the week. The rest of my fuel system is as follows: Nippon Denso 190lph in-tank pump, Accel inline pump from HRC, 415cc injectors (40#), SAFC, and the missing link.

I would like to run somewhere around 14-16psi of boost. I'm gonna tune as close to that as I can before having timing advance issues because I do not have an ignition with timing retard yet.

So my questions are: what should my idle and max fuel pressures be? which disk/spring combo should I use?

I know you guys can't give me exact figures because every car is different, but any type of ballpark figures/suggestions would be greatly appreciated!

Thanks!!

Here's what we do, using the formulas Corbin has already setup for the board. http://www.2gnt.com/www/corbin/example.html

~300HP x 0.525 x 10.5 / 4 = 413.4375 cc/min

This is measured at 100% duty cycle though.

At 85%:

413.4 / 0.85 = 486.4 cc/min

So, 486.4 cc/min is your desired fuel output at 15 psi.


Applying your fuel injectors to the situation:

((486.4 / 415) squared x 43.5) + 15 psi = 74.8 psi

So you figure you're aiming for that amount of fuel pressure at your max boost. The difficulty is achieving that fuel pressure, whilst maintaining a good idle, and therefore desiring a low enough fuel pressure.

Assuming your idle would be somewhere in the 25-30psi range(based on other people's experiences), you'll need to make up for that 30-35 psi during your boosting to achieve the 75 psi at full 15 psi of boost. Therefore, 35/15= is 2.33. That's the ratio'ed disk you'll need in your SFMU. I believe there is a disk for about 2.5:1-3:1 that you can use. Fine tuning can be achieved with the S-AFC.

Sweet!! Thanks! That's a great starting point for me! I'll throw a datalogger on it to see what my fuel trims are doing with that setup and go from there. I can't remember.....can we view/calculate injector duty cycle with a logger?

No problem. Everyone should really try and take advantage of Corbin's website, especially when dealing with fuel calculations. They're extremely valuable.

In regards to your question, you can view fuel trims, but not duty cycle.

Patrick you have mail...

so where does this 35 come from?
"Therefore, 35/15= is 2.33"
i just wanna make sure im doing this right.
and how do we know its 30-35psi we got to make up during boosted operations?
if you guys can break it down for me?
thanks


Michael 99gs
HRC Stage 2
Stage 3 process begins gathering parts for rebuild

http://www.dreamwater.net/purplebeast

>so where does this 35 come from?
>"Therefore, 35/15= is 2.33"
>i just wanna make sure im doing this right.
>and how do we know its 30-35psi we got to make up during
>boosted operations?


The 35 is the difference in fuel pressure between his desired fuel pressure aoutput at maximum boost psi which is 15. Therefore, 75 psi(of fuel) minus 40 psi (of fuel at 0 psi) equals 35 psi. I took that number and divided it by 15 psi of boost to attain an estimated fuel to boost ratio, the same ratio used for the Vortech SFMU.

>The 35 is the difference in fuel pressure between his
>desired fuel pressure aoutput at maximum boost psi which is
>15. Therefore, 75 psi(of fuel) minus 40 psi (of fuel at 0
>psi) equals 35 psi. I took that number and divided it by 15
>psi of boost to attain an estimated fuel to boost ratio, the
>same ratio used for the Vortech SFMU.

Yeah, that makes sense, except I think Jason was a little low with his calculation. If I'm running 30psi at idle, I need to gain 45psi by full boost, not 35. That still puts me at a 3psi gain of fuel for every 1psi of boost, so I'd still use the same disk.

I was confused there for a minute too. It's because JasonESi figured it for 25-30psi of fuel at idle, and admin3 figured it for 40psi at idle. It's the same idea though.

when figuring out the difference. is it always going to be max boost fuel pressure minus 40 ( at 0psi ) or does the 0psi fuel pressure vary when doing the formula?
thanks


Michael 99gs
HRC Stage 2
Stage 3 process begins gathering parts for rebuild

http://www.dreamwater.net/purplebeast

By the way, I am "admin3." :-)

Anyways, yeah, I think I miscalculated there a bit. But the fuel pressure will go up between idle and static fuel pressure, the measure between full vacuum and 0 psi. Whether or not it rises up from 30 psi to 40 psi is difficult to say, but it does tend to rise some.

So, if you were to calculate figuring on more of a rise, say 75 minus 30, it would be 45 psi of fuel pressure rise required.

45/15 = 3 (there's your 3:1 ratio)

Or 40/15= 2.67 (still about the same disk)

In either case, you still use the same disk.

>In regards to your question, you can view fuel trims, but
>not duty cycle.

How can we view our injectors duty cycle? I want to know if they are maxed out at WOT. Anyway to know for sure?

This is the only one I've ever seen:




http://www.racerpartswholesale.com/afratio.htm

corbin is a engineer and knows all there is to know about our fuel systems i am sure he can design something to monitor duty cycle.
please corbin???

I have a digital multimeter that reads frequency and duty-cycle. I haven't tried i on the injectors yet, but it looks like it would work. I got it at sears for something like $30.


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