Fastbird93

Ok, while trying to figure out my stumble issues, I had a thought. I'm running 30# SVO injectors, and when I look at my tune on Tunercat, I noticed that the injector constant is set to 26.xx.

My thought is that this is too low for the 30# injectors, which is letting them stay open for too long and dumping the extra fuel into my mix.

If I were to bring the injector constant up, would I have to mess with any of my tables, or just leave them be and keep an eye on datamaster's Fuel trims??? I'm running a speed density system if that helps. Thanks!

TriPinTaZ

you should change your injector constants to 30.xx. you are correct this will lean out your car so if your using speed density you ma want to add a little fuel to the upper RPM range 4500-7000 to make sure you dont take away too much fuel by changing the injector constants.

your PCM is adding fuel based on the 26lb/hr injectors, so any tuning you do is going to be half wrong with the 26lb constant setting

Fastbird93

That's what I was thinking, but the guy doing my tuning swears by this tuning technique.

rskrause

It's a relatively trivial difference whihc is made even smaller when you consider that SVO's are rated using a higher fuel pressure than GM (43.5 vs. 42psi if my memory is correct). If your car is running well, leave it alone. If it is rich across the board, the change will lean it out.

Rich Krause

Dan K

Doesn't make much sense to me. If you're running between 40-45psi of fuel pressure you should find an injector constant of 30.00-32.00 ideal with those injectors.
On a 93 that's as mild as yours you should be able to run vemaster and get quite a bit of the part throttle fueling done without much of a problem. And if all you've got is a mail order program, you can get the wot stuff better than the mail order guy. JMHO.

Fastbird93

Well, the mail order guy and I have been tuning it via datamaster to the tune of 9 different revised tunes now. He doesn't use VEMaster either. Says he doesn't like it and that it's not accurate enough.

As for my car, it's not running well enough. The first time we ran the new combination, it was fine aparrently, although I never saw it myself. That was coming from Joe Prince. The opti seal blew out on the dyno, so everything had to come off. after that, the car was running pig rich, stumble with any fast throttle movement. I can't figure out what it could be. I thought this might be a fix, but I'm still pretty convinced that it's not the problem.

Injuneer

What do your long term fuel trims look like with the 26.xx injector constant?

DOOM Master

That injector constant is way too low for SVO 30# injectors. The formula for determining the correct injector constant for different fuel pressure rated injectors is (sqrt (fuelpressureyourcar/fuelpressureoriginalinj) * injectorrating). In this case, for SVO, it would be (sqrt (43.5/39) * 30) (Ford fuel pressure is at 39 psi for future reference). So the injector constant should be set to 31.68, no way it should be down at 26#. Like Injuneer said, check your long term fuel trims also. Personally, I'd look into getting a new tuner.

Fastbird93

Last time I checked they were holding in the 110-128 range.

I still have my doubts as to this being my problem though as the car was running on essentially the exact same tune less minor changes and wasn't as rich. I'm pretty convinced that it's some kind of mechanical failure, I only have to find that problem part.

DOOM Master

If you are seeing BLMs that are low like that, I'm willing to bet that if you put the injector constant to the 30.xx-32.xx range, it will go away. I know someone who had a similar problem on his 93 LT1, just as you are having. He put in new injectors and PCMforless accidently sent him the wrong file with lower injector constants (I think he said he got 30# and they tuned for 26#, but don't quote me on that). His problem sounded like the same problem you are having. He fixed it by putting in 24# SVOs he had laying around.

InjectedSS

ALL injectors have a specific rating at certain psi's.. the SVO's are rated at 30lb's per hr @ 39.2 so as said above you have to calculate that to figure what they would be with the 43.5psi which our car's put out, which would give you 31.68 lbs per hr..

Another example is accel injectors.. Their 30lb injectors are rated at 27.9 lbs per hr @ 39.2psi and 29.4 lbs per hr @ 43.5psi.... So if you had accel's 30lb injectors you would use 29.4 as the injector constant

Injuneer

Well, a BLM of 110 indicates the PCM is pulling about 14% of the fuel it arrives at with standard calculations. Lets see...

26.xx / 0.86 = 30.xx #/HR

If the shoe fits......

Alvin@pcmforless.com

whoa whoa whoa,


I'm doing sean's tune.

Last time you sent me a datalog your BLM's averaged within something like 97-98% to perfect 128 in the worst pressure vs. RPM cell.

I derive my injector constants by taking a car tuned up to a perfect 128 before the injector swap and then after the injector swap I move the injector constant till it comes back into a 128 range.

I have plenty of injector constants I derived this way and all work great. and all are very differnt from what you said

If the car was perfect with the first chip and the unorthodox injector constant but after you guys tore the front of the car down again the car is running funny wouldn't that tell you that

1. either your work is flawed
2. the bogus AFR data you guys sent me is still haunting you guys.



For all you guys who just take what you read on this site like its written in the Bible (like your injector constant ideas) I challenge you to test and experiment to find your own method.

Dan K

Then how do you find an acceptable injector constant for a car that you don't have a "perfect" tune for when someone comes to you for a program?

Why? Figuring injector constant based on an injectors rated flow and the fuel pressure that you are using works fine and just seems a bit easier for most to understand.
Do you feel there is an advantage doing it the way you described? Or is it just the way you do it?

TriPinTaZ

I agree that using the wrong injector constant may work, but any corrections your PCM does to fueling will be incorrect due to the wrong constant.

I do not see why you should have to use a incorrect injector constant.

In my car I used 42.4 injector constant for my 42.5 lb lucas injectors. and any BLM problems were corrected via the VE table. Also altering your spark advance will change your BLMs. Maybe the tune needs more timing ?