I know when doing mpfi on a /6, the injectors used are usually rated 19lb/hr give or take a bit. How does this convert over to TBI injector rating? I am using a dual injector throttle body with a GM computer. The computer doesn't fire both injectors off at the same time, it alternates between the two.

Pierre,

The total lbs/hr for all injectors is what you're after. 19lb/hr X 6 = 114lb/hr total, so if you have 2 TBI injectors they should be 57 lbs/hr each.

FYI, I tried running my MegaSquirted Holley TBI with alternating pulses, but it idled a little better with simultaneous pulses. Not sure why, and I haven't reinvestigated it after dialing in my fuel map. I am running 85lb/hr injectors on my built '64 motor and they work fine.

Lou

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Home of Slant6-powered fun machines since 1988

Lets say 114lb/hr is the target. If the two injectors are being pulsed alternatively, wouldn't each injector have to be rated at 114lb/hr since only one is on at any given moment?

You wouldn't happen to know what the yellow/blue injectors are rated at, would you? I think the gm/rochester tbi's use the same parts as the holley. I think I might need smaller injectors to get my idle steady. I have tried pinching the incoming fuel hose a bit but it seems that the computer opens the injectors longer and pressure returns to where it was before pinching those. I want to make sure howell gave me appropriate injectors/body for my application before I change the gm computer for something else.

Do you think the 500cfm throttle body and 114lb/hr total injectors is a good fit for a stock-rebuilt motor (other then clifford intake and dual shorty headers with single 2.25" exhaust to rear) ?

Pierre,

I think it's unlikely your injectors are too big. My motor will not use that much more fuel than a stock one at idle, and the 85lb/hr injectors idle just fine. My bet is it's a faulty sensor or something like that, or the computer still needs recalibration.

The timing of the injectors relative to each other (alternate or simultaneous) will not affect the total lbs/hr needed because this is an average over time.

I think the 500 CFM 2bbl TB should be just about right for a stock Slant. Certainly it's OK for idle, and the only thing I might worry about is bogging off the line. Generally, you can get away with a little more CFM rating with EFI than w/ a carb.

Lou

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Home of Slant6-powered fun machines since 1988

I'm running 36 # injectors which I've had up to 65% duty cycle at 5psi boost. That works out to about 140 #'s per hour for the engine with theoretic max of 216 #'s/hr.

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Cheers!
Al T

I have a feeling its just the calibration thats way off. Lou, what is your fuel pump/regulator setup like? The gm throttle body has its own internal regulator that can't be adjusted. It is pre set at ~13psi, that is the pressure all the gm injectors are rated at.

Also, you are not using the IAC are you? I know the megasquirt can't control an IAC (out of the box, at least), just a solenoid to set fast or slow idle. You just keep yours fully shut?

Al - I still need to check my datalogs and see what my max duty cycle is, but I think it's approaching 80% with my 170 lb total feed injectors.

Pierre - I use that adj fuel pressure regulator on the Holley TB I'm using (aftermarket), and I have it set around 15psi. I bet you can retrofit an adjustable regulator to yours using ProJection parts (see Holley website). No IAC, just a solenoid with one step. I can adjust what temp it comes off at and that's it.

Lou

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Home of Slant6-powered fun machines since 1988