STATIC (about) Injectors

[bruce]

OK, here's some numbers I thought I'd share with you, about injector sizing.
There seems to be alot of inertia about how fast you can go on what size injector, and I'm not going to get into the whys of tuning. This is just some info., as the computer *sees* things.
This is using Direct Scan and running an ecm bench.
Everything is constant between all the runs other then injector size.

To maintian a 12.5 AFR, at 255 gms/sec., at a 90% injector DC.

30#/hr injectors can only support 1,360 RPM
40#/hr injectors can support 1,460 RPM
50#/hr injectors can get you 1,510 RPM
55s get you to 1,560
BUT with 72s at a 75% DC you can maintain the same 12.5 AFR, to 5,150 RPM..

ONE BIG NOTE, most cars can't peg the MAF at that low of rpm so things aren't as bad as the appear at a glance. But I hope ya'll better grasp the **WHY** of having to rescale the MAF to get a balanced fuel flow at higher then stock HP levels. Just as a hint what my MAF, it reads 160 at full song. Then I used the PE vs RPM, and PE vs RPM to get the curve perfect at ranges other then what the MAF covers. You can just set the PE vs Temp table at what you want, but that has 0 drivibility, compared to what CORRECT is.

Oh, and 55s with a 100%DC get you to 5,150
50s at 100%DC get you to 1,560.

When you look at things as the ecm does, things do take on a new twist.

These ain't any numbers I drew out of a hat this is how the ecm calculates things. These numbers reflect the battery voltage offsets, etc..

When you out of injector, your out of injector. You want to fight detonation, blown headgaskets, that's your decision, having enough injector, and then just actually tuning is alot more fun.
HTH

Plese note these are just advertised flow rates and don't take into account the increase in flow rates as a result of increasing fuel pressure due to boost.
Hope this sheds some light on things

Oh, and if you run a WB you can verify that the ecm's commanded
AFR numbers match the the WBs, that is if you have the programming right in the ecm.

 

[Dean]

Thanks for the info. I won't pester you with questions since I don't really get it. I do NOT "grasp the **WHY** of having to rescale the MAF to get a balanced fuel flow at higher then stock HP levels". Though I did notice where you pointed out that your MAF readings 160gps @ WOT, and I recall you mentioning a recalibrated MAF sensor on another thread. Would I be correct in my thinking that you have had your MAF recalibrated so that the operating parameters are within the maximum range of the ECM, and then you did extensive modifications to the PROM programming to work with this recalibrated MAF sensor? Or, am I so lost that it is hopeless?

BTW, I find it shocking that there could be such a disparity between the 72s and the rest of the lot. Where did you see the "break point" that makes such a drastic change?

 

[Dean]

ps: Are you the one I sent my DS files to a couple months ago, showing MAF dropout? If so, I have some more, at the 18fps rate.

Let me know.


:-Dean

 

[bruce]

Originally posted by Dean
Thanks for the info. I won't pester you with questions since I don't really get it. I do NOT "grasp the **WHY** of having to rescale the MAF to get a balanced fuel flow at higher then stock HP levels". Though I did notice where you pointed out that your MAF readings 160gps @ WOT, and I recall you mentioning a recalibrated MAF sensor on another thread. Would I be correct in my thinking that you have had your MAF recalibrated so that the operating parameters are within the maximum range of the ECM, and then you did extensive modifications to the PROM programming to work with this recalibrated MAF sensor? Or, am I so lost that it is hopeless?

BTW, I find it shocking that there could be such a disparity between the 72s and the rest of the lot. Where did you see the "break point" that makes such a drastic change?

Maybe this will help.
The stock calibration is for a stock engine.
The instant you peg the MAF you run out of resolution.
The healthier the engine the quuicker it does peg the MAF.

The MAF table math is
table entry times scaler divided by 255 and the result is grm/sec..

table #5's scaler is 255 so that makes the math easy the table entries are actually grm/sec
So with a table of like

80 100 120 190 220 255 255 255

Which means (255-80= 175), that in 1/2 of one table the MAF must account for 3/4 of the airflow that the engine can use. Now with a hipo engine that flows 500 gm/sec, can you see how the oem calibration gets overwhelmed?.

Now there are ways around this.

What I've done is reassigned some values to the MAF scalers to just use the MAF for a lower flow rate and then blend in sort of a PE vs TPS, and then PE vs RPM to blend in enough fuel for WOT.


Typically a GN has a throttle that at a certain point feels like the secondaries have opened, and that's when PE is enabled. On mine it's just a linear response to throttle input. Squeeze the pedal, and just go faster. Since there are no rich or lean spots, from not having pegged the calibration, and with matching engine mods, it's much harder to get mine to detonate. Which is a good thing.

As far as the 72s, that's what the difference is from having enough fuel, to just not enough. Once you cover the fuel flow to max Torque it's all downhill from there. Remember 55s at a base of 42 with 24 PSI get you 66 PSI of fuel pressure and 68ish#/Hr. The injector sizing stuff is just something I'd stumbled on while experimenting on some other stuff. It was an eye opener to see how they did.

 

[bruce]

Originally posted by Dean
ps: Are you the one I sent my DS files to a couple months ago, showing MAF dropout? If so, I have some more, at the 18fps rate.

Maybe.
two ways around that, Translator or ME.

I've noticed some cars are really bad for that and others aren't. While a MAF might help, there is no guarantee that it will long term.

 

[ijames]

Remember 55s at a base of 42 with 24 PSI get you 66 PSI of fuel pressure and 68ish#/Hr. The injector sizing stuff is just something I'd stumbled on while experimenting on some other stuff. It was an eye opener to see how they did.

Um, at 24 psi boost the pressure differential across the injector is still 42 psi (66 psi fuel pressure in the rail inlet pressure - 24 psi boost outlet pressure), so the injector is still acting like it's exactly the same size it was at idle (if it was rated at 43 psi which I think is a common rating pressure, it looks like a 55*sqrt(42/43)=54 lb/hr inj).

 

[TurboDave]

Originally posted by ijames


Um, at 24 psi boost the pressure differential across the injector is still 42 psi (66 psi fuel pressure in the rail inlet pressure - 24 psi boost outlet pressure), so the injector is still acting like it's exactly the same size it was at idle (if it was rated at 43 psi which I think is a common rating pressure, it looks like a 55*sqrt(42/43)=54 lb/hr inj).

Good point, and one that most people forget. Differential pressure is always the same (whatever your base/static was set to). That's why it's always best to try and get the car/chip tuned for 42.5 lbs. fuel pressure, since that (3bar) seems to be the most common value for "flowing" injectors.

 

[CPAKCP]

Bruce,

Great post somewhat over my head but hey thats what this board is all about - a place to learn. I take it from your post that to make 500 HP / low 11 and quicker ET - you recommend to go with the 72 inj - min. size of injector??

Thanks in advance

 

[Dean]

Originally posted by bruce

Maybe this will help.
The stock calibration is for a stock engine.
The instant you peg the MAF you run out of resolution.
The healthier the engine the quuicker it does peg the MAF.

Makes perfect sense, which is one of the reasons why I went to the Translator setup.

The MAF table math is...

This is where I get lost. I have no idea of what a MAF table entry is, or a scaler, but I would like to. I could take a guess, which might be close, but I would rather know the correct answer. I kinda know the "255", but not really. This goes back to getting the "easily understandable" GM instructions you mentioned on another thread.

I can see how the OEM calibration can be overwhelmed by an engine moving more air, but that's the reasons for "custom" chips, no? I do understand how an identical same chip will perform differently on identical (bolt-on wise) cars that are in different states of tune and condition. No two cars are really the same, they can just appear to be.

Originally posted by ijames
Um, at 24 psi boost the pressure differential

I have wondered if that is totaly true. Is it the same amount of square inches being effected? (I don't know... I have never taken an injector apart.) I realize that the total sq.in. difference may be minimal, but it is the % that would do it, no?

Originally posted by TurboDave
...42.5 lbs. fuel pressure, since that (3bar)...

Isn't 1 BAR one barometric pressure, or 14.7? That would make 3 BAR 44.1. That is where I now have my FP set at.

Originally posted by CPAKCP
Great post somewhat over my head but hey thats what this board is all about...

I'm with you on that one!


:-Dean

 

[Two Lane]

Awesome Thread

Let's keep this one going!

ttt

 

[ijames]

1 bar = 0.98692 atmospheres = 14.504 psi, so 3 bar = 43.51 psi

Also, if the 55's were rated at 3 bar or 43.51 psi (instead of the 43 I assumed, sigh) then at 42 psi they act like 55*sqrt(42/43.51) = 54's (such a negligible change I never should have brought it up, except that it made me go look up a bar ).

 

[TurboDave]

Thanks Carl, I don't know why I had it in my head it was 42.5 and not 43.5

 

[Dean]

Originally posted by ijames
1 bar = 0.98692 atmospheres...

Thanks for clearing up my misinformation!

 

[Dean]

Here's a thought regarding the 36# vs. 72# by a friend of mine...

A 36lb injector will flow half the fuel in a given time that a 72lb injector will, all else being equal. If a 72# injector supports a ratio of 12.5 lbs of air to 1 lb of fuel up to 5150RPM at 75% duty cycle, doesn't it reason that a 36# would support 12.5:1 AFR up to 2575RPM at the same DC?

Obviously, I'll need the response phrased for a 6th grader.


:-Dean

 

[bruce]

Originally posted by Dean

A 36lb injector will flow half the fuel in a given time that a 72lb injector will, all else being equal. If a 72# injector supports a ratio of 12.5 lbs of air to 1 lb of fuel up to 5150RPM at 75% duty cycle, doesn't it reason that a 36# would support 12.5:1 AFR up to 2575RPM at the same DC?

For as far as the guestion goes with the disclaimer of all else being the same, yes.

BUT,

Fueling a motor is about Pulse widths, and volumetric effiecency.

 

[bruce]

I found a couple GN injectors yesterday, and just for grins put them on the ecm bench today.

At DC approaching 98% they do get erratic. And since these were just being fired dry, I could imagine the problem being much worse, when under pressure. having not been dry fired before, and with keeping the sessions short, I doubt they were to the stage of overheating, or sticking from the pintle being peened over.

There is some error from what DS reports to what my DC meter reads, while minor at 4-5%, it does exist. So I might take an indicated 104ish % DC to actually go static.

Reguardless of what I did with the programming I could not make the injector go static cleanly, it would always chatter off a few times almost randomly, as I swept the rpm up. Dunno, but having an injector just momentarily turning off, or PARCIALLY OFF just doesn't sound like a whole lotta good for an engine at WOT.

 

[Dean]

Were these new injectors?

:-Dean

 

[bruce]

Originally posted by Dean
Were these new injectors?

:-Dean

No