I recently upgraded my injectors from 24# to 32# ones in anticipation of the head & cam I'm going with over the winter. After dialing in the new size of the injectors the computer tops out the left BLM at 160, and the right BLM at 145. I've had a split BLM problem for a while, that was present before the injector change so that's probably unrelated.

Do injectors typically not flow exactly at what they're rated at? Should I be tweaking the injector size setting in the PCM to bring the BLMs back down to around the area they were before?

 

Yes exactly.

Try to find a injector constant that gets you back into the ball park of the old injectors.

 

Alvin helped me out a ton yesterday on this. I didn't get a good idle (BLMS in the 126-128 area) with Siemens 57lb/hr injectors until I cranked the injector constant down to 45lb/hr.

From there, tune away.

 

Why not use the correct injector constant for the injectors and fuel pressure that you are running? Then tweak a little up or down from there to adjust the entire fuel curve a little bit.

 

If all he changed was the injectors than why should he change more than the injector constant to get the car back where it was?

Get a injector constant that pulls the car back where it needs to be and then use the MAF tables or VEtables (small changes at that point) to get the car back 100%

 

PS.

I'm in mechanical engineering and I'll tell you the difference between a good engineer and a bad one is realizing that math and equations gets you reasonable numbers from which to grown on but it is by no means written in stone as your "correct" injector constant implies.

 

Again, everyone has their way of doing things. And I guess since you're in mechanical engineering that your way is better than mine.
Just offereing an alternate suggestion.

 

Will you do me a favor and quit posting rolling eyes to jsut about every post if i have here.

If you don't agree with me than thats fine.. Just quit with the smart remarks.

 

At the "correct" injector constant, it was lean across the board. So lean in fact, that the car wouldn't idle. I thought I was decent at tuning my own stuff, but I started to get worried when I added 30% more fuel to get the thing to somewhat idle (yet BLMs were still maxed at 160).

I saw Alvin was around and PM'd him for a little help. He gave me the advice to tune the constant first. I went back to my original tune and lowered the constant down to 52. Things were better, but still lean. Did the same with 48. At 45lb/hr, the BLMs across the board were no lower than 108 and no higher than 160. Idle was beautiful.

I did have to do some tuning to get the BLMs steady in the mid to high 120s, but it's proven that this method works, and works well. In my opinion, it worked a hell of a lot better (and saved a ton of time) over setting the "correct" constant and tuning around it.

 

I like to tell the ecm the true injector size and tune in the voltage off set table for the injector. This will correct the "reaction" of the new injector and get it in line. never a fan of lie'n to the ecm.

 

Roadie,
Was the only thing that you changed the injectors?
If so, can you tell me what injector you were running previously and what injector constant you were running with them?
Also, are you still running the same static fuel pressure as you were before?

And guys, I'm not trying to start anything here. Just trying to figure out why a 57lb injector would need a 45lb constant?

Also, you said that you still had to do some tuning (I assume to the maf or ve tables) to get the blms steady in the mid to high 120's.
If so, didn't you just fudge the injector constant and then tune around your fudging? Or did I misunderstand?

 

i agrree with dan on this one

if the injector is a 57 lbs injector at 43 psi. (our stock pressur i think) then when you put it in the car it should need no change, if it does need change becuase of the pressure it is flowed at. if its flow rated at 40 psi then you say 43/40=1.075*57= 61.27 this what it would be in our care and you put the new injector constant in the table and that it. this accounts for the different fuel pressures you made need to fine tune it with the maf or ve tables.

but i dont see why you would put 57 lbs injectors in and need to tell the pcm that they are 45s. that would be saying that the 57 lbs injectors are flow rated at 54.46 lbs and they are only a 45 lbs injector in our car because of the lower pressure

 

Engineer or not ( ) its worth asking the question, why is the constant he set so far off? If he put in a 32# injector, and his left BLM is 160, the injector is only flowing 80% of its rated value, or roughly 25.3#/HR (he indicated his flow constant was set at 31.6 in another post on another board). That's not a whole lot different than a stock injector (24.9) Even allowing for differences in the offset values, this would seem to be an extreme discrepancy. I'm not sure I'd just jump to the conclusion that the specified injector constant of off by 20% and accept it blindly.

97Z-M6:

You forgot to take the square root of the pressure ratio when recalculating the flow constant.... its:

43/40 = 1.075
1.075^0.5 = 1.037

 

Funny numbers or not you still can't argue for changing anything but the constant (or the offset tables) if all you change hardware wize is the injectors.

Of course a clean up is nessiary after getting a good constant, but at that point it should be marginal.. not 20% like he mentioned.

 

Yes.
Stock and stock '95 M6 tune.
Yes. The fuel pressure gauge in the pillar still says 39 at idle, 43 at 0 vacuum, and rises 1:1 with boost (it's a stock regulator).

I have a blower on the car and I'm getting ready for a motor swap to a bigger cube motor over the winter and wanted to get these injectors in and close enough to start tuning the new motor. If I were to break something on the stock motor as a result of bad tuning, it wouldn't break my heart.

I went with that big of injector because I'm planning to ditch the Carroll Superfueller and do all the fuel delivery within the PCM.

The injectors are definitely 57lb/hr. They are flow-matched from Racetronix.

It was close. Like I said, none of the BLMs were over 160 or under 108 with the constant set to 45lb/hr. I just made a few small changes to the MAF tables to get the BLMs back perfect.