I've been looking over my runs from the strip on Friday and I've noticed that my injector pulsewidth starts to cut off from 10.1 ms at 3800 rpm, to 8.45 ms at 4600 rpm (my shift point), holding a duty cycle of around 64. My MAP stays the same (89 kPa), my O2 sensor voltage goes up independently(830 - 935 mv). It does this through both shifts, making 2 "X"'s on the graph where the PW goes down and the RPM goes up.

I'm sure it's just basic engine theory that I'm missing, but why is it cutting fuel at high rpm operation? Is it just a fluke that it sticks around 64 DC?

 

At 6,000 rpm you have 10 ms to fire ALL the fuel in one revolution (the way our injectors are measured). At 3,000 rpm you have 20 ms for fire ALL the fuel in one revolution.

The formula for Duty Cycle is ms*rpm/60,000. I don't have a calculator but I will let you compute the Duty Cycle.

But you are confusing the decreasing PW time as meaning less fuel getting into the engine. The PW does follow the TQ curve of an engine, but that is ONLY the amount of fuel in 1 revolution. At higher rpms that revolution takes less time. What you need to look at is the amount of fuel in a given amount of time. Therefore you need to look at ms*rpm to determine the amount of fuel delivered in 1 minute. Divide it by 60 and you will get the Total PW in 1 second.

But you need to look a the total amount of fuel in a given period of time, not the specific RPM (except for the DC computation).

And this gets to the next part - Injector sizing. As you may have noticed, as the rpms go up, you have less and less time to get all the fuel into the cylinder in 1 revolution. As I said before, at 3,000 rpm you have 20 ms. At 6,000 rpm you have 10 ms. And 12,000 rpm you have 5 ms. You cannot take longer than the 10 ms to deliver all the fuel @ 6,000 rpm. It is physically impossible.

If your injector size requires more than 10 ms to deliver all the fuel the engine requires @ 6,000 - that is when you need a larger injector that will deliver that fuel in less than 10 ms. But then you get into the next problem - the injectors also need to be opened a minimum amount of time to maintain a good idle. Too big of an injector (for the top end) and you may have a circumstance where the Injectors cannot maintain a short enough PW.

PS: It is generally not a good idea to run Duty Cycles much above 85%. If you are hitting Duty Cycles over 90% - you need larger injectors.

 

Thanks Glenn, I understand now.

My duty cycle has never gone over 70, so I think I have the injectors sized right for what I have at the moment. (Datamaster computes DC for me.) For some reason it's just hard for me to think of the timeframe that the injector has to inject fuel, maybe I just think slow.

 

Four strokes only take in a charge once per 2 revolutions. So wouldn't it be 20 milliseconds at 6000 rpm. I think you should double the time figures in the post above.
J

 

You would think so, but our systems are "double fire" and the Injector PW is the length of time each injector fires per revolution.

On the newer SEFI systems, the PW is per operating cycle (2 revs).

 

I am confused. My PW are about 12 to 13 ms at 4500 rpm. I was never really sure if the '165s and 730s used the double fire mode or single fire mode. How did you figure this out.....double fire mode?

Thanks,
Jay T.

 

I thought that is how it got the name batch fire because it fires one bank of four injectors at a time. This would have to mean that one of the pistons would be on the compression stroke while another would be on the exshaust stroke and one on the intake and so on. Every time the intake valve opens it has been hit with two injector pulses. I think this is somewhat correct, if not I will have learned a new thing.

Steve

 

Also keep in mind that max Volumetric Efficiency usually occurs at max Torque. Thus, you need the most fuel at max torque. A VE curve follows a torque curve pretty darn well. Thus, if you have a stock TPI (or even a LTR TPI setup) your injector pulse widths should drop off as RPMS go above 3800.

Tim

 

So it'd be safe to assume that if I change my cam to move my power range to higher RPMs (after changing the intake, of course).. I'd shift my VE table up as well?

Trax - I've been meaning to ask you, how did you go about choosing your custom cam specs? Why high lift and short duration? (If I'm reading the card right.)

 

I still don't buy that the engine only has 10 ms to spray the injector at 6000 rpm. It takes a little over 2 ms just to open them. On my '165 engine, all 8 injectors fire at once. Not 4 at a time.
I don't know, I still see it as 20ms at 6000 rpm.
J

 

I still see it as 20ms at 6000 rpm

The injectors are fired once per engine rev not once per 2 engine rev. 1/(6000rpm/60sec/min)= .010 sec or 10 ms.

HTH

John

 

Please explain why my PW are around 13 msec. Also, the math suggests that high power engines would idle way too rich if injectors where sized using the 10 ms at 6k rpm. Time for me to look at a hac..........
Thanks,
J

 

Oh I just got it. I see now.............you say the injectors are fired once per revolution. There is two revolutions between the intake valve open/close.....so there is really almost 20 ms avail.

Thanks,
J

 

Junk, you keep thinking "operating cycle" (2 revs per operation). GM when they designed the system, based it on 1 revolution for "batch fire systems". If you look into the "hac" for a MAF bin, you will see "Single fire vs Double Fire". By default, MAF and SD use "Double Fire" which means every engine revolution. And it's really 8 little pulses every revolution with a total time = to whats displayed on your scan tool.

The formula for the max time available per RPM = 60,000/RPM. In the example you gave, you said "13 ms @ 4500 rpm". You do the math, but it indicates a near 100% DC of your injectors. You only have 13.333 ms @ 4,500 rpm.

PS: With MAF ECMs, it WILL calculate a PW > than what is physically possible. I have a friend with a Supercharger and a MAF setup that gets 13 ms @ 6,000 rpm yet he was showing excessively lean with a WB O2 sensor...Wonder why?

The ECM will happily calculate a PW that is needed, but there is NO LOGIC in the ECM for when the PW calculated exceeds the physical maximum limitations (i.e. the ECM commands 13 ms but you only have 10 ms)...bottom line, you get 30% less fuel than you really need and run lean. My friend has since gone to larger injectors and now gets PWs around 8.9 @ 6,000 rpm and proper AF ratio.

Just remember that our ECMs are "Double Fire" and the PW is measure per revolution, not operating cycle (2 revs).

Now, SEFI DOES work on Operating Cycle "Single Fire".

 

I read David Vizard's book on Camshafts and Valvetrains, talked with a few people with similar specs and then played with figures in DD200.

Tim

 

Thanks Glenn, I see what you mean. I read what you said, and looked through the hack for the 165 ECM. I shouldn't have been so lazy in the first place and looked through the hack. Thanks for taking the time and explaining it.
That makes sense with the injectors running about 100% duty cycle with the engine they are on, too.
J