leftoverz28

I had posted this in the engine tech a while back and just got a couple responses so I figured I'd repost and see what more of you had to say. My best guess so far is that i need somewhere between 30 and 42 lb injectors...

Finally getting ready to get my car on the road after a bit of a tear down and was wondering if anyone on here had any idea what kind of power a 383 with 8.5:1 compression, ported Trickflows, intake, and 58 mil. throttle body and a blower cam might put down N/A. I need to size the injectors and am not exactly sure what to expect. The car will eventually get a set of twins and 72lbers, but its been off the road for about two years and absolutely needs to be driven this summer.
Any help/advice would be awesome.
-Mike

96capricemgr

put it together with stock injectors and see if the AFR stays good.

mrmint69

I put 42's in my car, so I would have some room to grow. I was just maxing out the stock injectors and didn't want to have to worry about needing to upgrade to higher so I went to the max, I figured I would need down the road. I used my datalog to see what my injectors were doing, if you use that program. Of course a tune is important for all the mods and I wanted the injectors so I could do the tune once. It won't hurt to go a little bigger as far as the injectors are concerned and not that much more. I went Bosch after problems from a no name brand I got a little cheaper.

96capricemgr

Some of the "faster than they should be" guys are arguing for small as possible injectors lately. Some interesting debates on LS1tech about it.

leftoverz28

Anyone know the limitations of the stock injectors?

mrmint69

I never maxed out mine with my mods, but came close.

Injuneer

You'll probably hit 85% duty cycle at about 350-360 flywheel HP. You can try pushing it higher. Theoretically, you could reach 400HP at 100% DC. The question becomes how reliable are the injectors at that level?

leftoverz28

So its really just a matter of how much power this thing ends up making...

96capricemgr

Far as reliability of injectors at high duty cycle IMO people missunderstand the term. It is not a rating of what they can handle, it is more a rating of how long they are open.

If they can bench test them at 100% DC for a minute to measure flow rating it is not terribly logical to think that the few seconds they would be there during a 11-12second dragstrip pass is going to kill them especially when there are bare minimum MINUTES of time between those blasts even if hot lapping at a near empty dragstrip. The reason I say minutes of time between those blasts is because I believe the theory is that you will overheat them if they are held static too long. Then again there is good fuel flow through them to cool them.

Injuneer

Your assumption appears to be that the culprit is "overheating". That's not correct. The issue is at what point does the spray pattern and flow become erratic.

RC Engineering used to have some really great high speed photos of what happens to injectors when they are cycled too rapidly. That's what happens as they approach 100% DC. The coil loses control of the pintle (or ball, or disc). Flow and spray pattern become erratic, and flow capacity may actually be reduced. Newer designs are raising the bar on duty cycle, but I don't think it's been eliminated as a valid design consideration.

And yes, I've read your posts on ls1tech about ignorant people like myself.

leftoverz28

So lets say it ends up making about 400hp. 42#?

Injuneer

Flywheel or rear wheel? Flywheel HP determines injector requirement. 400 flywheel HP can use 28's. 400 rwHP equals 460 flywheel thru an M6...... 480 flywheel thru a 4L60E. Might squeeze by with 32's with an M6. Go with 36's with an A4.

Add a "signature". Makes it easier for people to help you.

leftoverz28

Unfortunately there's not very many people running compression as low as mine N/A with a motor that'l flow what mine will, so its pretty hard to tell exactly what it'll make. I'v done a little research and it looks like the motor should make somewhere in the ballpark of 400 to 430hp at the flywheel. Is there any chance I can get away with the factory injectors if I'm correct?

mrmint69

If you make what you guess, then no. If you ran a datalog and saw what the percentage of maxing out was, you could see. Most over estimate there hp a lot. I would not want to risk it on my car though. Leaning out at wot is never good!

Injuneer

If you want to try and make 430 flywheel HP on stock injectors just install them and run it with a wide-band O2 sensor. Maybe your BSFC will be very low, allowing the engine to reach that figure. It's a numbers game.

Let's say it works - 430 STD corrected flywheel HP on 24.9#/HR injectors = injectors that are happy at 100%DC and an engine that is so well configured and set up to achieve a 0.45#/HR/HP BSFC. What happens when you have colder than 60* air, a barometer above 30"Hg and no humidity, and the engine makes more than 430 flywheel HP? What happens if you system voltage drops and your fuel pump can't hold 43.5psi? Gonna run lean. Hopefully you will still have the knock sensor functional.

It's your engine. Do what you want with it.