question about larger injectors w/o the SAFC..

using the SFMU to adjust idle, static, and max fp, is there any reason why i couldnt use 40lb or 42lb (420-440cc) injectors, w/o adding a SAFC?

i plugged the 420cc injectors into the example on Corbins page, and assuming a 100% DC since theres no SAFC, i came up with ~45psi of fuel pressure @ 9lb of boost. Does the formula still work for this size injectors @ 100% DC ? Are those numbers correct, 44.5psi @ 9lbs w/ 420cc and no SAFC?

Would the static/idle pressure be to low to allow for decent driveabilty at anything other than WOT? How do i figure out what the static pressure would need to be w/o the SAFC?

i know the ecu can only adjust for 25% bigger, but i run 315cc injectors right now just fine, and they are about 35% bigger...so if i can run these fine w/ just adjusting fuel pressure, what is the limit to how much bigger i can go....

anyone?

I'm curious too...anyone?

>question about larger injectors w/o the SAFC..
>
>using the SFMU to adjust idle, static, and max fp, is there
>any reason why i couldnt use 40lb or 42lb (420-440cc)
>injectors, w/o adding a SAFC?
>
>i plugged the 420cc injectors into the example on Corbins
>page, and assuming a 100% DC since theres no SAFC, i came up
>with ~45psi of fuel pressure @ 9lb of boost. Does the
>formula still work for this size injectors @ 100% DC ? Are
>those numbers correct, 44.5psi @ 9lbs w/ 420cc and no SAFC?

You have done this right. 44.5 psi will get you close to where you want to be.

>Would the static/idle pressure be to low to allow for decent
>driveabilty at anything other than WOT? How do i figure out
>what the static pressure would need to be w/o the SAFC?

To match stock injector flow at 0 psi (static setting), you need about 252 cc/min of fuel. It would take 15.66 psi at 0 psi manifold pressure to match that fuel flow. I don't think that is very feasible.

>i know the ecu can only adjust for 25% bigger, but i run
>315cc injectors right now just fine, and they are about 35%
>bigger...so if i can run these fine w/ just adjusting fuel
>pressure, what is the limit to how much bigger i can go....

Long term fuel trim (LTFT) can adjust 25% and so can short term (STFT). So thats a max of 50% adjustment. I go over this is my MFI page:
http://www.houstonhardware.com/~cbehnken/mfi.html
Point is that you want your short term trim free to make changes. If your injectors are 35% bigger, you use all 25% of your LTFT and 10% of your STFT. That only leaves your STFT the ability to lean by 15% if it has to. You will need every bit of that last 15% to quickly adjust for changing driving conditions. So in order to have good driveability, you must lower the fuel pressures on bigger injectors (I'm glad you are already doing this). The ideal situation is to aim for both of your fuel trims to be zero. The next best thing is to max out your LTFT and leave your STFT at zero. So if you wanted to max out your LTFT at 25% and leave STFT at 0%, running 420's, you would need 24.5 psi at 0 psi manifold pressure. Thats a little sketchy. 24.5 psi of pressure across an injector isn't much, but it may work.

Corbin

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



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

Thanks Corbin, i appreciate the detailed reply, its exactly what i was looking for. Im gonna go read over the mfi page now.

as always, your the man to go to :)

and just a fyi: right now, w/ the 315's, im running ~27psi @ idle, ~40psi static, and ~62psi at 8lbs of boost

Hey corbin, i got 440's and afc an sfmu, are you saying that under closed loop operation we should basically be tuning for 0% LT and ST fuel trim. I was playing with the settings and if i sit at idle with my pocketlogger plugged in I can basically make my LTFT go to near 0% by leaning the afc out. about 43% right now at idle with ~25psi idle fuel pressure. if this is the case should i try to tune my closed loop operation so i get near 0% LTFT across the board?
one more thing, when our car goes into open loop operation what sensors is it using to determine the correct amount of fuel?
thanks
james

0% LTFT and 0% STFT is ideal. That means, no matter what the humidity, temperature, elevation, or load...your ECU will have the best chance of injecting the right amount of fuel. You should try to get 0% trim from idle to full boost (closed loop). You can't really do this, so the next best thing is to keep LTFT reasonable (<20%). On my car, idle is 0% LTFT and STFT oscillates from -2 to +2%. At 0 psi boost LTFT is -13% and STFT fluctuates to meet whatever demands I am placing on the engine. At 6 psi boost, LTFT is locked at -13% (ECU does not adjust this when you exceed the limits of the stock fuel maps) and STFT is -5%. When I go WOT, I run a little rich with EGTs between 1400-1480F. I'm happy with that.

I can not prove this, but I bet open loop fuel control is based on the LTFT map, info from MAP, IAT, ECT, TPS, RPM, and a little extra fuel thrown in to ensure safe operation. I think the shop and tech manuals talk about this some. They don't tell you everything.


Corbin

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



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