Bill Ball

Tony:

You hit 100% duty cycle at 5K RPM and 80% at 4K RPM. So, according to the information given on duty cycles you are flirting with danger over 4K RPM. Hmmm...

John Speake

Hi,
Andy is correct in that the point at which the injectors go 100% is mainly due to MAF voltage -airflow into the engine. The mapping also has an rpm parameter as well, of ocurse.

Essentially, as soon as the MAF voltage rises above 5.6v, then the injectors are on 100%. So from there upwards you are relying on fuel pressure rise and injector size to try and mainatain a reasonable A/F

GoRideSno

Something else everyone needs to realize is every 928 that is modified to make more power wether it be stroker, SCed or other will max out the injectors. IIRC Anderson uses the stock fuel mgt system and his car makes more power than most of the SCed cars. Maybe someone could do a poll to find out how how many 928 owners have had problems due to this.

Andy K

James-man

With the concern over 100% duty cycle, how prevalent this may actually be, it would be interesting to do a bench test to see how long it takes an injector at 100% to start acting up and burn out.

I haven't looked carefully at an injector, but it would seem that it is in a hostile environment (heat-wise) in the first place, and the incoming fuel would be at a much lower temperature than the injector body itself. Does the fuel help cool the injector?? Maybe not relevant, but to fully simulate at the bench, maybe it should be put in a toaster oven and hooked up to a room-temperature water source, maybe not?

Anybody want to play Mr Science with this? I am interested in the results.

Thanks.

James-man

BTW my bets would be on a very considerable amount of time before failure.

GoRideSno

James,
The temp around the injector would only be around 140-160F, like the air on a warm Arizona day.

Someone should just e-mail Mark Anderson and ask him what he does about maxing his injector duty cycle. I am sure whatever the most experienced 928 racer in existence does about this will work just fine for everyone else.


Andy K

drnick

andy, im sure that info is proprietary - can you imagine how much its worth in used engines??

GoRideSno

LOL, Nick. It's really rethorical on my part. My guess is that he does nothing.


BSFC of an SCed engine is .55 and the injector "size" has to be re calculated given fuel pressure. Injectors are sized at 43.5psi. If you are running 70psi the a 30# injector becomes about a 38# injector.


Andy K

mark kibort

Tony, Just saw the post. Great job at the "strip".

by the way, whats the hp you are putting to the ground again?

as far as the injector issue. didnt see if the data was calcuated or measured from the injectors. However, Stock injectors work well for up to 335hp to the wheels on an s4, and on an 84, those stock injectors work well to 290hp to the rears. (sub 90% duty cycle). I dont know the difference in the injectors as far as flow rate, but ive heard that they both are 18lb injectors (anyone know?)
by the way, i was running 52psi on my stock 84 fuel pressure at 293rear wheel hp with near perfect ratios for A/F.

Anderson and others just use a 30lb injector and back off the fuel pressure to support 420+ hp to the rears.

Tony

Only danger is concerning duty cycle and injector reliability, not AFR. And as stated by John that once anyones MAF volts go to appx 5.6 v it will max the injectors out no matter if they are 10# or 100# pound. Once they are maxed out its the fuel pressure runnign through them that determines the the amount of fuel and the resulting AFR. As long as i have my fuel pressure set so as the rpm passes 5000rpm, that magicaly "maxed out MAF/injector" point, i dont go lean at 6800rpm, Im fine. Uh.....I think

Again, good discussion though.


In a nut shell then if i had 19, 24,30, 36, 42 or 60 lb injectors in my car the duty cycle would max out at 5000rpm (ref the data i have) no matter what, Correct?

Mark, my last dyno in the fall was 375rwhp and 372rwftlbs. That was with a few air leaks as well. Im pretty sure im in the 380s now, sure feels strong. One of these days ill go back again.

sharkmeister85

Once again I'm enjoying another report on Tony's Supermodel. Keep up the work! Being somewhat technically challenged, I'm trying to grasp this whole injector size/duty cycle thing. Does the question here boil down to are the injectors really running at 100% of their d/c when the MAF hits 5.6 v, or are they maybe really only running at 80-85% d/c because that is what the fuel management computer interprets 100% to be?
Glenn

GoRideSno

The S4 has 19# injectors but idle fuel pressure is 57 v/s 43.5 (at which injectors are measured) so they act like 22# injectors. 80-84 cars have 24# injectors (low Z though). I can't recall the pressure off the top of my head.

Thanks Mark, exactly what I thought. And exactly what I suggest for setups that are making 400rwhp and below for S4. Above that need to have the BEGI2025 reg.

Andy K

Tony

Yeah, what Glenn said. Thats what im thinking. The signal for my duty cycle comes from the #1 injector wire at the connector. The signal i get is what the injector sees. Best way to answer would be to hook up the logging to an NA car. Simple to do, but i think the Wideband O2 sensor with my LM1/Aux Box has to be properly in placed for the "whole system" to work.

Bill Ball

The scary claim is that the injectors are not designed to run open 100% or even 90%. I'm not sure why that would be. Having a solenoid valve stay open for extended periods doesn't sound particularly odd, but the claim that they arc, fibrillate and die needs to be substantiated or refuted.

Jim_H

Probably any EFI book will substantiate it. There are 2 different discussions on it in an EMS book I am reading. Hopefully they aren't making it up.