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Old 05-10-2003, 03:28 PM   #1
8Complex

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Default Need injector pulse width help...

Yesterday I was out tuning with RiftsWRX and found that stock injectors on an RS-T say 100% duty cycle at 7.3ms at ~4800rpm (4-5psi boost). Now this goes a bit against what I've been reading for equation-wise. Normally the rpm/ms equation goes like this:

rpm/2/60=x
1000*(1/x)=ms available/rotation

So at 4800 rpm
4800/2/60=40
1000*(1/40)=25ms

What? how could it possibly be reading 100% duty cycle with only 7.3ms of injector on-time when there is 25ms available?

I thought it was a software glitch, but I began to believe it as we were testing and watched the A/F's [edit] rise rapidly after hitting that 100% duty cycle point.

Does this seem a little odd to anyone else? Is there a flaw in my numbers somewhere?
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Last edited by 8Complex; 05-11-2003 at 03:10 AM.
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Old 05-10-2003, 09:00 PM   #2
Jon [in CT]
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I agree with your calculations.

For any particular 4-stroke cylinder:

Revolutions per Minute / 2 (ignitions per minute) / 60 = x (ignitions per second)

1000 / x = milliseconds interval between cylinder firings.

Quote:
I thought it was a software glitch, but I began to believe it as we were testing and watched the A/F's drop rapidly after hitting that 100% duty cycle point.
If you saw your "A/Fs drop rapidly," then I presume you saw your A/Fs drop from, say 12:1, to 10:1, which I agree is a substantial drop. But I don't see a problem with this.
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Old 05-11-2003, 12:21 AM   #3
RiftsWRX
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Quote:
Originally posted by Jon [in CT]
I agree with your calculations.

For any particular 4-stroke cylinder:

Revolutions per Minute / 2 (ignitions per minute) / 60 = x (ignitions per second)

1000 / x = milliseconds interval between cylinder firings.

If you saw your "A/Fs drop rapidly," then I presume you saw your A/Fs drop from, say 12:1, to 10:1, which I agree is a substantial drop. But I don't see a problem with this.
He meant rise.

from 11.1 startup shooting up fast to the 12's and 13's

Jorge (RiftsWRX)
www.ProjectWRX.com
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Old 05-11-2003, 03:10 AM   #4
8Complex

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Yeah, my mistake. For some reason I always think it opposite direction even though I know better reading it. They rose very rapidly... within about 5-800rpm they rose from a nice 11.0:1 up to low 13.x:1's.

So if there is no error in my math, I wonder why the ECU is telling me that 7.3ms is 100% duty cycle at 4800rpm...
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Old 05-11-2003, 03:55 PM   #5
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Could be your fuel pressure trailing off...have a guage hooked up?

Ada,
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Old 05-11-2003, 04:24 PM   #6
8Complex

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Hmmm... there is a mechanical gauge underhood I might be able to extend and tape to the window, but the car has a Walbro 255 in it.

I'll check that out as soon as the weather clears up a bit (damn 40mph winds here today), though I'm still wondering why the ECU says it's gone 100% duty cycle at such a low opening time.

Re-analyzing the math here...

rpm/2/60=x
1000*(1/x)=ms available/rotation

That formula isn't correct now that I look at it. That shows amount of time of all 4 cycles for a single cylinder. Obviously you aren't going to be spraying during the latter stages of compression (depending on overlap, of course), not at all during ignition, and most of the exhaust cycle (depending on overlap as well). So realistically if we want to narrow it down to 2 of the strokes of the 4, we're talking about actual rpm's, not half. So the corrected equation would be :

rpm/60=x
1000*(1/x)=ms/true rpm

What this means is that the previous possible opening times have now been cut in half to 12.5ms@4800rpm.

So now, I suppose, if the ECU really wants to use strictly the intake stroke to fire the injectors, it will really only be seeing a 6.25ms opening window. This seems much more likely to me... I will have to re-read into the logs to see exactly when it starts to fall off as the previous 7.3ms@4800 was estimations off the top of my head and could have been another factor in odd numbers.

So now in the end, we're almost looking at a calculation like this:

rpm/60*2=x
1000*(1/x)=ms/single stroke
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Old 05-11-2003, 06:03 PM   #7
Jon [in CT]
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Quote:
Originally posted by 8Complex
I'm still wondering why the ECU says it's gone 100% duty cycle at such a low opening time.

Re-analyzing the math here...

rpm/2/60=x
1000*(1/x)=ms available/rotation

That formula isn't correct now that I look at it. That shows amount of time of all 4 cycles for a single cylinder. Obviously you aren't going to be spraying during the latter stages of compression (depending on overlap, of course), not at all during ignition, and most of the exhaust cycle (depending on overlap as well).
First, if by ECU you mean the stock ECU, then the ECU doesn't actually report "duty cycle." It does report RPM and injector pulse width for the #1 cylinder. Some software in your PC is calculating injector duty cycle from those two variables.

100% injector duty cycle does, in fact, mean that the injector is continuously open, through all 4 engine cycles.
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Old 05-11-2003, 08:13 PM   #8
8Complex

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No, it's running a Haltech E6K with staged injection, which I believe is basically sequential injection with secondaries.
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Old 05-12-2003, 12:06 PM   #9
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There's part of your problem...Haltech's only allow max IDC of 80% from what I remember.

Adam
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Old 05-12-2003, 01:37 PM   #10
8Complex

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Thats fine though, this car will never see all 830cc's of injection at the same time when the secondaries kick in. I just need to get them plumbed in and move the crossover point to starting up the secondaries lower and it should be fine. My only problem at this point is the tiny budget that I'm under... looks like it's going to be twin fuel systems. WRX pump for the stock injectors and Walbro 255 for the 550cc secondaries.
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Old 05-12-2003, 03:23 PM   #11
RiftsWRX
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Quote:
Originally posted by 8Complex
Thats fine though, this car will never see all 830cc's of injection at the same time when the secondaries kick in. I just need to get them plumbed in and move the crossover point to starting up the secondaries lower and it should be fine. My only problem at this point is the tiny budget that I'm under... looks like it's going to be twin fuel systems. WRX pump for the stock injectors and Walbro 255 for the 550cc secondaries.
Sounds like party time

You need my pump then?

Jorge (RiftsWRX)
www.ProjectWRX.com
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