Injector Sizing: REALLY dumb question

[Bruce Plecan]

-----Original Message-----
From: Greg Hermann <bearbvd at sni.net>
To: diy_efi at efi332.eng.ohio-state.edu <diy_efi at efi332.eng.ohio-state.edu>
Date: Wednesday, August 26, 1998 3:41 PM
Subject: Re: Injector Sizing: REALLY dumb question


>>In several very recent threads, it has been stated that drivability
>>problems (usually at idle IIRC) preclude the use of large injectors for
>>TPI. Yet, TBI uses one or two large injectors with generally no mention of
>>such problems. Would someone please tell a dumb sparky exactly what is
>>causing problems with high flow injectors in TPI and why that is not a
>>problem with TBI? Back to lurking in the shadows......with my 332
>>controlled and fully instrumented CSH low over my eyes and hopefully ready
>>for the impending  walls of flame.
>
>Mike--
>
>No flames here. I f you have a TPI V-8, you've got eight injectors, say
>20#. Same engine with TBI has two injectors, say 80#, keeping  the numbers
>easy and round. If you have the two engines set up with the same idle speed
>and same parasitic load at idle, you will have the same % duty cycle on the
>injectors at idle, and the same turndown ratio.
>
>Differences:
>1. I think a lot of TBI set-ups use peak and hold injectors, but don't know
>for sure. Anybody else???

All the gm TBI injectors I've dealt with have been peak+hold

>These injectors will get down to a shorter pulse time without going
>unstable as to amount of fuel delivered.

It's been said P+H run best at 1-5 msec, and saturated ones 2-10

>
>2. A TBI setup is not required to fire each injector once per intake
>stroke, so conceivably, they could go to a longer pulse at longer intervals
>to take care of idle and real light loads. (Again, I don't know--do they,
>anybody??

>From 225-4000 rpm on my bench ecms they fire (meaning each)
fire at one half the ingition pulse rate.
>
>3. I don't know of anybody building a motor hot enough to get into this
>kind of a problem and still using TBI on it. Perhaps the TB limits the
>total air flow to the point that the injectors are not (and don't need to
>be) big enough to get into the problem of too short a pulse at idle .

I'm at an ez 325 HP with 90's
>
>4. Take a Pro-Jection 4 barrel for another example. It will flow enough air
>for big power, but it has four injectors. But they are staged. Only two of
>them fire at idle. So it has twice the turndown ratio, just as sixteen
>staged injectors would on a TPI V-8.
Cheers
Bruce          NEVER Spook a cat jumping over bench ecms,
                    Cat+Bench fine, me well I never was all here
                    anyway.
>
>Regards, Greg
>
>
>