SEFI v MPFI

**brian89gp** · 09-10-2013

Trying to flush out of there is any reason to favor a SEFI ECM over MPFI.

SEFI fires each injector once per 720*? MPFI fires all injectors or a bank of injectors once per 360* or once per 720*? Aside from emissions is one inherently better then the other? I had a 94 3.4 DOHC (SEFI) that I had converted to the 93 '9396 (MPFI) and it idled and ran much smoother with the 94 ECM then it did with the 93 but there was also the addition of the 24x crank sensor and MAF vs MAP differences to so the better idle quality might be attributed to something else.

In the $2e code I cannot find where it switches to batch fire after a certain RPM, just under 450rpm for cranking. Am I just overlooking it or does it run SEFI all the way up the RPM range? I seem to remember that SEFI switches to MPFI at 3k RPM.

As far as emissions, the 3800 ran on a SEFI ECM from the lat 80's onward. Does anybody know of a reason why this was done when the emission standards pushing for SEFI didn't really come into full swing for at least another 5-6 years?

**RobertISaar** · 09-10-2013

i have MPFI engines that run at LEAST as smoothly as SFI engines... i think it's all in the calibration.

i've found the same end-result for 2E in that i've never found it to switch out of SFI once running, i don't have a scope to verify though.

and from what i''ve been told over the years..... the 3800s apparently NEEDED to go SFI to meet emissions standards or as GM's testbed, possibly both.

**brian89gp** · 09-10-2013

Power wise, is there any benefit to firing the injector SEFI over MPFI? Also, is MPFI fired once per 360* or once per 720*? I would imagine that even with SFI the majority of injector on time at everything other then idle or light cruise will be at the back of a closed intake valve anyway?

**EagleMark** · 09-10-2013

SFI was GMs push towards emissions compliance early on. Same as the LT1 would have passed 97 emissions in 94, although it needed crank trigger for misfire OBDII compliance in 96.

They run SFI at idle and low RPM... don't know where the change is to batch? But it is there, may just be it runs out of available time per revolution?

**RobertISaar** · 09-10-2013

shouldn't be a difference in power.... fuel is fuel, if the same amount is injected, i don't see a difference. you want to shoot against a closed valve anyways, vaporizes the fuel when it sits against a warm valve. when you talk power, you're also talking about large amounts of fuel and i don't see the double-fire fuel charge "running out" of fuel between injections when that much is needed.

MPFI is usually 1 injector per revolution, though there are some calibrations that either use or can use 1 injection per 2 revolutions, like SFI, just not sequenced.

i calculated the values for a stock 3x00 cam, i want to say that during the full 2 revolutions, the intake valve was open 27% of the time? so if you were trying to target injection to happen during the valve open time, injector DC above 27% some fuel wouldn't make it. if targeting during valve closed, beyond 73%, some fuel wouldn't make it.

i don't see the point to adding code to switch in and out of SFI.... for a given RPM, you have a set amount of time to inject all of the fuel necessary, doesn't really matter if it's sequenced or single vs double-fire. there is a slight processing time penalty to stay in SFI as RPM rises, but i imagine constantly checking RPM thresholds to see if need to jump in/out would make up for most, if not all of it.

**RobertISaar** · 09-10-2013

actually, 27% sounds too long now that i think about it....

http://60degreev6.com/forum/showthre...ce-quot-of-SFI

there we go. stock cam is apparently 262*. so 262/720 = roughly 36.4%.

**brian89gp** · 09-10-2013

Not adding code, just determining if my affinity towards SEFI is founded in fact or if it is an illogical preference then choosing an ECM based off of that determination. MPFI sure seems a lot easier.

**RobertISaar** · 09-10-2013

i meant, i don't know why GM would have added such code.

**steveo** · 09-11-2013

the question is, why the hell not go sequential if there's a choice? the benefits for low rpm driveability, idle quality, and fuel economy are real; less pooling in the manifold at very low rpm, and trims to account for low rpm flow differences in intake ports (among other things). finer control of injection timing is rarely bad

i guess when you approach big cam territory it matters less and less, since what happens in the intake manifold at idle to low rpm is just a shit show anyway, spraying gas everywhere all the time probably wouldn't make much difference, and you'd never operate in that range except for a stumbly lope-show at a stop light.

sometimes i think that you might as well just have one big fat injector in the middle of the plenum that's stuck wide open for idle fueling on a big lopey low vacuum car with a plenum, it'd probably do as much good.