Re: Injector Duty cycle calculations

[Roger Heflin <rah@horizon.hit.net>]

On Sun, 23 Jul 2000 timsiford@hushmail.com wrote:

> FELP
> 
> I bought a new set of injectors (24lb SVOs) to replace my stock 22lb injectors. 
>  The car is a 1990 IROC 350 with the 730ECM.  It has a 52mm TB, MiniRam,
>  AFR heads, 218/224cam, 2800 stall, and 3.73s.  I switched to 24lb injectors 
> because I was calculating 100+% Duty Cycles with the 22lb injectors.  Now 
> that I have the 24lbers installed I am still seeing high duty cycles (90% 
> at 6500rpms).  I am running a PROM where I used an injector flow constant 
> of 28lb/hr and have the pressure cranked up to 50psi.
> 
> Diacom shows an injector pulse width value of 8.3ms at 6500RPMs.  Is this 
> a 90% Duty Cycle?  I am using the following formula:
> 
> RPMS / 60(sec/min) / 1000 (ms/sec) * Pulse Width (ms)  * 100% = Injector 
> Duty Cycle

You have 9.26ms at that rpm.   So 8.3/9.26 = 1.04 ms off.   From
everything I have heard Duty Cycle is not the isue, minimum off time
is the issue.   Somewhere around 1.0-1.5ms is as close as you want to
cut it.  Going from 22lb -> 24lb injectos is really not enough to
improve things.  I went from 22->30lb ones.

And also given higher pressure that raises the size of the injector.

 > 
> Since the 730 is batch fire this equation appears to be correct.  However,
>  I viewed some Diacom run from a '97 LT1 and saw injector pulse widths of 
> 10ms at 6500rpms.  Given that that ECM is sequential, I don't understand 
> how those duty cycles could be so much less than mine?!  This car is even 
> more seriously modded than my car (224/236 cam, AFR heads, manual, 58mm 
> TB, 4.10s).  

What rpm are you measuring on the 97?   And what fuel pressure is he
running?   If that cam is a 224/236-114, it has run out of power a
couple of hundred rpm before 6500.   The above setup should have peak
hp about 5700-5800 rpm.

 > 
> Am I missing something here?  Is Diacom reporting wrongly?  Any help is 
> seriously appreciated because I am getting ready to sell the 24lb injectors 
> that I just installed (this past friday - two days ago!) in order to buy 
> 30lb injectors.

What were the O2 sensors reading in both above cases, I have found out
there can be quite a bit of fuel variation even at the same O2
reading.  While tuning mine I figured out aht +-15% fuel in the
correct operation point of the O2 sensor could make little or no
difference in the O2 reading.

			Roger

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