I autotapped my car the other day and I came up with a duty cycle of 130%. You can see that topic at https://ls1tech.com/ubb/cgi-bin/ulti...c&f=1&t=003377

Anyway, I was thinking about this and I thought of something. If a set of injectors are rated at 30lbs/hr, that means 30lbs/hr at 100% duty cycle right? That's independent of any engine speed, just means the injectors are always on. But when I was doing my calculation, I was assuming that 100%duty cycle was just exactly half the engine stroke time. In equation format, it would be something like 2/[(revolution/minute)*(1min/60000millisec)]. This would give me exactly half the engine stroke time. It seems that's what everyone else thinks it should be, judging from the comments I got.

But if you think about it, this duty cycle and the duty cycle that the injector is rated at are totally different. 100% duty cycle for half the engine stroke is just 50% duty cycle for the injector. So even though I said I was at 130% duty cycle at 6600rpms, that's really probably 65% duty cycle for the injector. So when people say the duty cycle should never go past 80%, which duty cycle are they talking about? The injector duty cycle or this calculated duty cycle based on engine speed? <img src="images/icons/confused.gif" border="0"> <img src="images/icons/confused.gif" border="0">

 

The flow rating on an injectors has nothing directly to do w/ duty cycle (DC).

It's simply a flow rating for a given pressure and static operation. But static operation does equate to 100% DC

To actually calculate DC you need an on time for the injector and a time frame to use for 100% DC.

For any given RPM you have 2 revloutions of the engine between valve events on the same cylider.

Determine DC like this

DC = InjPW/{2*[60/RPM])

or

DC = InjPW*RPM/120

muplity by 100 to get percent

so for 24ms at 6400 that's .024*6400/120=128%

you need bigger injectors

 

[quote]Originally posted by y2khawk:
<strong>The flow rating on an injectors has nothing directly to do w/ duty cycle (DC).

It's simply a flow rating for a given pressure and static operation. But static operation does equate to 100% DC

To actually calculate DC you need an on time for the injector and a time frame to use for 100% DC.

For any given RPM you have 2 revloutions of the engine between valve events on the same cylider.

Determine DC like this

DC = InjPW/{2*[60/RPM])

or

DC = InjPW*RPM/120

muplity by 100 to get percent

so for 24ms at 6400 that's .024*6400/120=128%

you need bigger injectors</strong><hr></blockquote>

Ok, so you're basically agreeing with my initial calculation. But at this point I'm not entirely convinced that 130% is the correct duty cycle, it appears to be 65% because that's the true duty cycle of the injector.

This comment is interesting : "For any given RPM you have 2 revloutions of the engine between valve events on the same cylider." Can you go into this in more detail? By valve events, I assume you are talking about the same valve. To reword it, "For any given RPM you have 2 revloutions of the engine between the same valve event on the same cylider." If that is the case, don't I have the entire revolution time for injecting fuel?

Anyone else have any comments? All these people on this board adjusting their PCM fuel maps must be able to comment on this. What do you guys typically see on your injector PW?

 

[quote]If that is the case, don't I have the entire revolution time for injecting fuel?
<hr></blockquote>

You should only be injecting fuel when the intake valve is open for that cycle or else fuel will puddle when the valve is closed.

[ February 28, 2002: Message edited by: Godspeed ]</p>

 

Your autotap logs will show what the computer commanded the injectors to do they will only flow 100% obviously. Actually you should try too keep them to 80% duty cycle if possible. So you need a lot more injector.

Gary

 

[quote]Originally posted by Godspeed:
<strong>

You should only be injecting fuel when the intake valve is open for that cycle or else fuel will puddle when the valve is closed.

[ February 28, 2002: Message edited by: Godspeed ]</strong><hr></blockquote>


At speed, there is so much going on that it will harldy puddle. Most will vaporize when it hits the hot valve anyway.

If we tried to inject all the fuel only when the valve was open we'd never get anywhere on stock injectors. <img src="gr_stretch.gif" border="0">

The same valve, in the same cylinder, will open every other revolution. So in theory you have 2 revolutions worth of time to inject the needed fuel for the next intake valve opening.

Yes, you'll be spraying the back of the valve for most of that time. But the fuel is there, and will get pulled in when the valve opens.

and yes, you're at 130%. No doubt about it. 20ms is 100% at 6000 RPM.

Trust me, i watch my duty cycles very carefully
<img src="graemlins/gr_judge.gif" border="0" alt="[judgement]" />

[ February 28, 2002: Message edited by: y2khawk ]</p>

 

[quote]Originally posted by y2khawk:
<strong>
At speed, there is so much going on that it will harldy puddle. Most will vaporize when it hits the hot valve anyway.

If we tried to inject all the fuel only when the valve was open we'd never get anywhere on stock injectors. <img src="gr_stretch.gif" border="0">

The same valve, in the same cylinder, will open every other revolution. So in theory you have 2 revolutions worth of time to inject the needed fuel for the next intake valve opening.

Yes, you'll be spraying the back of the valve for most of that time. But the fuel is there, and will get pulled in when the valve opens.

and yes, you're at 130%. No doubt about it. 20ms is 100% at 6000 RPM.

Trust me, i watch my duty cycles very carefully
<img src="graemlins/gr_judge.gif" border="0" alt="[judgement]" />

[ February 28, 2002: Message edited by: y2khawk ]</strong><hr></blockquote>

This is pretty interesting. So in reality, a 30lb/hr (which is measured when stuck open) injector will never inject 30lb/hr because you're limiting it to 50% of the revolution, and then artificially limiting it to 80-85% of that. So a 30lb/hr injector will only really ever inject 12lb/hr. That means 8 of these injectors can really only support about 200HP?

If I want to support 500HP, that means I need a 75lb/hr injector, based on what I just said.

Ok guys, so what's a good injector I should go with?

 

No, you're not limiting to half the revolution.

The more or less correct way to size injectors is based on power output. Then on max RPM

read my response near the end of this thread

https://ls1tech.com/ubb/cgi-bin/ulti...c&f=1&t=003348

You need to know your power output and/or desired ouput. Are you NA, forced induction, N2O?? That will give you a better idea of the BSFC ou should use.

Rule of thumb for NA

It takes .5 lb of fuel per HP per hour, .55 to .6 for blown apps.

so in my case, i use .6

My peak numbers at the flywheel are about 625 HP

625*.6= 375lb/hr
divide by 8

46.875 lb/hr per injector

I'm running SVO 42's at 4 BAR base pressure, they flow about 49 lb/hr at that pressure, at 100% duty cycle.

46.875/49=95% duty cycle to get that flow

This is exactly what i see. I hit 18ms to 19ms pulse widths by red line

.019*6000/120=0.95

So the question is, how much power are you making? How much do you want to make without changing injectors?

 

Ok, I have hit 473 on the dyno, that comes out to 557 flywheel. So, inputting those numbers into the calculation, using a .5 BSFC, I come out to needing 34.8125 lb injectors? My O2's were at 930mv on this run, how can all this be possible with stock '98 injectors?

 

At 473 rwhp I would be looking at 36lb/hr, and possibly the 42's if you want a bit more room to work with.


Harlan is correct on everything he has said - you are not "limiting" the injectors - they flow 30lb/hr when they open regardless of how long they are open (excluding the ramp up/down time). What you are limiting is the amount of time they can be open.

On a SEFI setup we look at each injector/cylinder pair. We can basically spray fuel in on the exhaust stroke and part of the intake stroke. - that gives you 2 strokes out of the 4 stroke cycle to spray fuel into the valve. The time for these to strokes is a function of rpm.

It doesn't matter if you hit the back of an intake valve - you will most of the time anyway, since the intake valve itself isn't open for very long. The fuel will vaporize instantly, and will remain suspended in the column of air.

Chris

 

[quote]Originally posted by Jax White TA:
<strong>Ok, I have hit 473 on the dyno, that comes out to 557 flywheel. So, inputting those numbers into the calculation, using a .5 BSFC, I come out to needing 34.8125 lb injectors? My O2's were at 930mv on this run, how can all this be possible with stock '98 injectors?</strong><hr></blockquote>

dynojet?? Where are you losing 85HP in the drivetrain?

what were your O2's when you hit the street? I bet they went down.

 

[quote]Originally posted by y2khawk:
<strong>No, you're not limiting to half the revolution.

The more or less correct way to size injectors is based on power output. Then on max RPM

read my response near the end of this thread

https://ls1tech.com/ubb/cgi-bin/ulti...c&f=1&t=003348

You need to know your power output and/or desired ouput. Are you NA, forced induction, N2O?? That will give you a better idea of the BSFC ou should use.

Rule of thumb for NA

It takes .5 lb of fuel per HP per hour, .55 to .6 for blown apps.

so in my case, i use .6

My peak numbers at the flywheel are about 625 HP

625*.6= 375lb/hr
divide by 8

46.875 lb/hr per injector

I'm running SVO 42's at 4 BAR base pressure, they flow about 49 lb/hr at that pressure, at 100% duty cycle.

46.875/49=95% duty cycle to get that flow

This is exactly what i see. I hit 18ms to 19ms pulse widths by red line

.019*6000/120=0.95

So the question is, how much power are you making? How much do you want to make without changing injectors?</strong><hr></blockquote>

I haven't dynoed yet, so I don't know how much power I am making, but around 450hp based on what other ARE S2 H/C cars do. So with 450hp :

450 * 0.5 = 225 lb/hr
comes out to 28lb/hr injectors

I'm running 99 injectors, which are close to 27lb/hr injectors I believe.

At 6600rpms, I see 24ms PW. Which comes out to 130% duty cycle, which means my injectors are commanded to inject 35lb/hr. This comes out to 280lb/hr of fuel total, and 560hp?

And what I don't get is, what if there was a car that redlined at 10000rpms? Then a 49lb/hr injector that was fine at 6000rpms for 625HP (19ms injector PW) can't support a car that has 625HP at 10000rpms (19ms injector PW will be 158% duty cycle). So if what that equation everyone uses is true, the engine speed can't be a factor like we're saying.

Anyway, not trying to argue with anyone here, I'm sure you all know that. I appreciate all the help. Basically, it sounds like everyone agrees with what I initially thought to be duty cyle. But after what I saw and thinking about a bunch of fuel injector scenarios, it kind of became unclear. Ok anyway, enough confusion. If I just want to support 500HP NA, what injectors are good for me?