StreetSweepa

Do our trucks need bigger injectors or a bigger fuel pump to run over a 100 shot of N2O, or is this just not a good idea in itself. Where can i buy bigger injectors and a bigger fuel pump for my 4.8?

<small>[ August 07, 2002, 09:21 PM: Message edited by: StreetSweepa ]</small>

BigTex

iTrader: (16)

Your stock injectors should only support about 320 rwhp at 100% duty cycle. They can flow more than 100%, but that means the injectors are fully open during the intake cycle, and partly during the exhaust cycle too. You put yourself at risk of going lean on the top end which is a no-no for nitrous.

With a wet shot, your stock injectors are fine. You could upgrade your injectors, but you will need programming for them.

parish8

iTrader: (6)

correct me if i am wrong, when you are at 100%duty cycle your injectors are open all the time. that means they are open thru all 4 strokes, they spend most of their time spraying on the back of a closed intake valve.

this brings up another question i have had for awhile, does injector offset rally matter? if your injectors are open 80% of the time does it really make much diference when they open. i could see how it may make some diference at part throttle when you duty cycle is low but not at wot.

BigTex

iTrader: (16)

100% duty cycle means the injectors are open through the intake and exhaust cycle. Fuel is hitting the back of the closed intake valve waiting for the next intake stroke. The fuel that goes in late won't have a good opportunity to mix well. This is one reason why you can make more power with larger (properly tuned) injectors.

Injector offset probably wont matter much at WOT @ higher rpms. You will notice it more during normal driving and low rpms WOT. Having the offset tuned properly will really help your air and fuel mix properly giving you a better burn. You dont want the fuel to be injected too late to mix well.

XLR8NSS

iTrader: (4)

Yup, BigTex is right about the duty cycle.

When I had the Procharger on my SS I would see a calculated duty cycle of 100% at ~4000rpm but, wouldn't start to see the O2's drop off until ~5500rpm or so.

John

BigTex

iTrader: (16)

OK I finally see the light - thanks. So it is normal then to see fuel flowing into a closed valve and possibly using the complete 2 revolutions to inject enough fuel? The part here that threw me off was that you PCM could command a pulse width LONGER than is physically possible given the two rotations to one valve opening. This said, when you go static, your injectors are open through both cycles, and still cannot deliver the required amount of fuel as ordered by the PCM before it has to start servicing the next pulse. So if we log a pulse width that is larger than physically possible, we know that injector isn't really flowing that amount, but is 100% open and are probably running lean. correct?

What really had me confused was the ability to log the pulse width @ say 130%. But that value is only what the PCM wants, not what is being delivered. That makes much more sense to me.

bgreen

So if we log a pulse width that is larger than physically possible, we know that injector isn't really flowing that amount, but is 100% open and are probably running lean. correct?

You've got it now. 130% or 190% is still just 100%. Most factory calibrations won't go past 70-80%, but as soon as you start modifying and adding airflow.. all bets are off. I'll see high duty cycles in other vehicles, especially turbo vehicles trying to get all they can out of a certain injector.

Also, you mentioned going leaner at higher RPM... your peak torque will be where you need the most fuel, as you go to higher RPM (nearer the HP peak) you will actually need less fuel.

Brian Green
89 TTA
01 Jeep TJ (5.3 going into it in the fall)

parish8

iTrader: (6)

ok, now that we all agree on duty cycle and all what about injector offset?

this brings up another question i have had for awhile, does injector offset rally matter? if your injectors are open 80% of the time does it really make much diference when they open. i could see how it may make some diference at part throttle when you duty cycle is low but not at wot.

what do you guys think, is injector offset worth trying to adjust, i kind of hope that it isn't, there is enuff to worry about without trying to dial that in too.

bgreen

No, that's not the way of it.

100% is open all the time. It's basically commanding a pulsewidth that is longer than the period between intake valve openings, so the injector should be opening again before it even closes from the last pulse, if that makes sense.

The way it is now, your injectors spray on a closed intake valve. The spray pattern of the injector is a narrow stream on most of the newer injectors. The sequential FI fires it on a hot closed intake valve, atomizing the fuel, the valve opens and fuel rushes in.

Duty cycle is pretty easy to calculate for a certain condition. Let's say at 5000 rpm. 5000 revolutions per minute, is 1 minute per 5000 revolutions, multiplied by 60 (or 60/5000) is 0.012 seconds, or 12 milliseconds. You inject once per every other rotation of the motor, so 24 milliseconds, is the time it takes for the motor to rotate twice, or one four stroke cycle (time between the intake valve openings). If your injector pulsewidth is 20 milliseconds, your duty cycle is 20/24 or 83%. This is the straight math, not taking into consideration that some injectors will go static before others.

Brian Green
89 TTA
01 Jeep TJ

bgreen

I believe the offset table is mainly to compensate for voltages. In old GM calibrations, the offset was not a timing issue, meaning it wasn't an offset referring to the actual time of injection, it was an offset that actually added to the pulsewidth calculation. So an injector offset of 1.0 millisecond actually added 1.0 ms of pulsewidth. GM has done a lot with injector calibration in recent times. In old calibrations, there was a single line table of injector offset, just versus voltage, and the injector flow rate was a set value, not varying versus voltage or load or manifold pressure.

Varying the voltage changes the injector response drastically, so compensating somehow is really necessary.

I don't think the timing is crucial at all at WOT, considering a bank to bank or batch fire injected engine makes about the same power as any sequential injected engine. If you are referring to idle/emissions - then I believe it may make a difference, but I'm not sure you can change it with LS1 edit.

Please post any corrections/info regarding the above..

Brian Green
89 TTA
01 Jeep TJ