A Friend of mine has 42lbers for sale that i can get for a good deal.Would these be ok for me to run or are they to big.My combo is 383lt-1,11.0 comp,mildly ported lt4 heads,stock lt4 intake,230/236 cam 1.6 pro mags.SLP shorties,stock TB.I will be getting LT's and TB in the next few months.I put down 365rwhp 370rwtq so with LT's and bigger TB i should be closer to 400rwhp,just want to make sure these can be tuned right before i buy them.Thanks Rob

 

As long as you can get your pcm tuned for them, they should work.

 

I have LT-edit,I just want to make sure,they are not overkill,and will work even though my motor my not need such a big injector.

 

I'm using 42's on a mild 355 setup just so I'll have room for improvement and they were a good deal. From what I understand, too big is fine as long as it's tuned. It's when they're too small is when you run into problems.

 

From my understanding running an oversized injector
like that even when chip tuning is not the way to go.
Spray pattern, Atomization,Fuel pressure and duty cycle
are the dictating factors.
Have read where 22#-24# injectors quite capable of
handling setups and cubic engines alot more radical than
listed above with acceptable duty cycles.
larger injectors, lower fuel pressure,less Atomization, or
fuel spray, less efficient.
This is from people who test and tune injectors and not
people who sell them

Just my two centavos

later

 

Do you have any articles or proof of this?

 

Researched this for about a week before i bought my
injectors and TPIS was one of them. This was about a
year ago.
There were 3 or 4 others and think even car magazine
had article on the subject. Printed some of the articles out
and will post when I come across.

I could have gotten larger injectors for my project which
will probably be upgraded as time goes by but ended up
with the SVO #30s.
Was also told that a good pump is key to this.
later

 

I love this site but the more i read,the more i second guess everything,I need more info......

 

Why would using a "large" injector affect atomization? The atomization is affected by the fuel pressure, not the pulse width. Just because you run a shorter pulse width on a larger injector, the spray pattern and the atomization are identical, once the injector opens. The ONLY place you might see a minimal affect on atomization is at idle, where the short pulse width may be a problem. But numerous people have managed build an engine that idles smoothly, using 42#/HR in jectors. Mine idles smoothly with 78#/HR injectors.

Why would a larger injector cause "low fuel pressure"? The fuel pressure is dependant on the fuel pressure regulator's ability to control the pressure. A 24# injector with a long pulse width will use the same amount of fuel as a 42# injector with a much shorter pulse width. Fuel consumption is based on brake specific fuel consumption (BSFC) and that does not change with the size of the injectors.

A 22# injector, running on an engine with a moderately efficient tune (0.47 #/HR/HP BSFC) is good for about 375 flywheel HP at 100% duty cycle. You don't run an injector at 100% duty cycle..... it eventually overheats and wears out the injector, and the fuel spray pattern, the fuel atomization and the fuel flow rate are all adversely affected, exactly the same thing you attribute to a "large" injector. Running that 22 #/HR injector at a more reasonable 90% DC limits them to 337 flywheel HP. Are you seriously suggesting that the engine described in the original post is not capable of making more than 337 flywheel HP?

His chassis dyno figure indicate he is making 440 flywheel HP. And, he inidcates he will be making future mods, increasing his HP. Let's say he's looking at 480 flywheel HP. He definitely needs larger than "22#-24#". My rule of thumb is flywheel HP X 0.07 to get you in the ballpark on injector size. Following that, he needs at least:

480 flywheel HP X 0.07 = 33.6 #/HR

That would yield an 85% DC, assuming a tune capable of producing a 0.47 BSFC. Assuming a VERY efficient tune (0.44 BSFC) and running the injectors to 90% DC would yield 29.4 #/HR.

If it was my engine, I'd run AT LEAST a 36 #/HR injector. There is absolutely no way he could run a 22 - 24 #/HR injector. At 0.44 BSFC, it would take a 106% DC, and we all know the injector can not operate at more than 100% DC.

As for his 42 #/HR injectors, with a normal tune, he will be running about a 67% DC, and everything will be fine. There is an advantage to running a very low DC... it means the injector is firing primarily when the intake valve is open, and not spending a lot of time spraying fuel against a closed valve, which is what happens when you use a "small" injector. I run less than 70% DC at 800 flywheel HP, supplying the needs of a 500HP stroker with a 300 HP DRY nitrous system. And that allows me to adjust the injector timing using my aftermarket ECU, to optimize HP based on injector timing.

 

Thanks Injuneer.