I just got paid and it's time to order some injectors and a fuel pump. I don't know what size injectors I should go with though. I assume that if you go too big on the injector size you won't be able to go low enough on the pulse width at idle and as a result the car will run rich, is this correct? Here's the combination-

Forged/balanced bottom end
Ported heads- about 250/180
236/248 .608/.578 111 LSA
All bolt ons
Auto trans
Nitrous use later on, but will be using a wet shot

28, 32 or 38 lb injectors? I'll be ordering the ones on the Racetronix site unless anyone knows a better place to get them from.

 

If you want 32# Racetronics, some one had a new set for sale a few days ago in the forsale section. (I did a search, but couldn't find them) sorry

 

Assuming 450flywheelHP, an 0.48#/HR/HP BSFC and an 85% duty cycle, you will need at least a 32#/HR injector (flow rated at 43.5psi).

I don't think anyone is having problems with idle with larger injectors on as stock PCM. With that cam, your idle's going to be pretty high, and its not going to run real clean anyway.

 

Not directly related to the post..but at what point is it a good idea to change injectors because they are to small or not sufficient?......i'm at about 320 rwhp with na motor......

 

When the duty cycle exceeds 85-90%.

You have to base your calculations on flywheel HP, not rear wheel HP.

 

Again, I'm not much into the clasic automotive math stuff as every car seems a bit different and it never seemed work exactly for me, but very much into real world dyno stuff.

What I have found......... If you go above 390rwhp @ 6000 rpm you max out the injector cycle on 24lb injectors using a 44psi fuel pressure. I've seen a car make 430rwhp with 28 lb injectors but was running 60 psi fuel pressure to do so.

If you go above 450rwhp and 6200 rpm you max out 30 lb injectors running 43 psi pressure. We didn't bother to check the injector cycle, we maxed out the fuel cure on LT1 edit and it still went lean.

Currently have 42lb injectors and I'm hoping to hell I can post where they run out of power @ 60 psi.

 

Thanks for the calculations, Fred. I'm going with the 38 lb/hr injectors for some breathing room in there.

 

You will most likely never need anything over a 30 lb injector unless you decide to get real, real serious and have a big back pocket to back it up. It takes real good heads, (read expensive) like 450 rwhp+ serious before you need larger..... which ain't easy to do with a LTx motor regardless of what is posted on this list as it's the internet and everyone has a 1000 hp and knows everything.

I've seen literally thousands of dyno runs and your combo with max tuning will have diffuculty breaking 400 rwhp due mostly to restirictive head flow. You've got a very good cam for the setup if you wind the car up high, but your still limited by the heads and the frosting. Do you have the frosting?.......... Long tubes, true dual exhaust, electric waterpump, adjustable fuel pressure reg, strong ignition, added welded and ported intake, larger TB, etc.,etc.?

A smaller injector will always have a better spray pattern over a larger injector and that means better efficiency. If a bigger injector is always better, why do they even make smaller ones?? If you have enough gas, you have enough gas!

Again..... throw the abstract math out the window and give me real world results anyday. Maybe I'm just plain not smart enough but put the car on the dyno and see if it goes lean, if not your pissing away money as larger injectors won't help a bit. Not one Iota. Dyno's with with wideband 02 sensors don't lie and proper A/f is a real deal.

The 390 rwhp stuff I posted regarding 24 lb injectors was work done using my old 97 MTI headed, 306 cammed, long tube, frosted SS Camaro with every bolt on in the world and hundreds of dyno runs. The information posted was done by Haltech to develope a new engine managment system in which my car was the test mule. Not some back yard redneck.

Put your hard earned money into fine tuning what you already have and you will be way ahead of the game.

 

Denny-

With some homeported heads and a HOTcam I was hitting 91-92% duty cycle with the stock injectors. I have quite a bit more porting on the heads now, and as you can see a cam much healthier than the HOTcam. Basically, I had to upgrade the injectors anyway, and since Fred said rich idling wasn't a concern I decided to get some larger than what I would hypothetically need to eliminate them as a possible bottleneck down the road. It really cost me about $40 to go from 28 lb to 38 lb injectors which is worth it for my piece of mind.

As far as the "frost" goes..... The car will have Hooker long tubes, will not have true dual exhaust (at least at first), I will not chance another motor to an electric waterpump so I'm running a new stocker, do not have an adjustable fuel pressure regulator but could get one easily if it's absolutely necesary, haven't decided on ignition yet, I have ported the intake some, and it will have a 52mm throttle body. It's going to be running just north of 11:1 compression, although I'm not sure exactly what it will end up being since the deck height has not been measured yet, nor have the heads been CC'd.

 

Not sure why you have to make denigrating comments about my attempts to help someone, who provided very little info up front. It was a simple ballpark estimate, based on assumptions of:

-estimated flywheel HP
-estimated BSFC
-a rational goal for DC
-stock fuel pressure

Where possible, I estimated on the side that would prevent him from selecting an injector that was too small. Yes, its questionable as to what the HP is going to be (I "erred" on the high side). Yes, you can do much better than 0.480 BSFC on a good dyno tune (I "erred" on the high side). Yes, you can push past 85% DC, but I personally wouldn't. I'm running 70% DC on a 800HP setup, including a 300-shot of dry nitrous. BSFC is 0.053, injectors are 64# Bosch, running at 58psi. All numbers are based on an engine dyno tune, measuring actual BSFC, and "real world" HP.

Based on the numbers, a 32#/HR makes sense, and there is no real cost savings or "plus" in trying to buy a 30# injector and trying to squeeze by.

You seem to assume I've never seen a dyno..... you are wrong.
Simply checking the "clasic (sic) automotive math" would have told you that would happen.... assuming you know the BSFC. You could have saved a dyno pull.

 

Fred is absolutely right.

To answer the original question though, I would go with 32# injectors at this point. (they're actually 31.6# just like Ford SVO 30#)

 

My stock njectors had 113% duty cycle at 383rwhp/353rwtq at stock fuel pressure...

 

is this normal? Or should you change your injectors?

 

Are you using DataMaster to read your duty cycle? DM numbers for BPW and DC make no sense at all (or I don't understand the definition of "BPW"). I have seen bolt-on engine approach or even exceed 100% DC in DataMaster, while the wide-band is showing there is plenty of fuel.

I'll say it one more time..... injector size needs to be based on FLYWHEEL HP. The injectors really don't care if you are losing 12% through an M6 drivetrain or 24% through a TH400 with loose, non-locking converter. Mentioning "rwHP" without identifying at least the type of drivetrain (RealQuick indicates he has an M6, so that's clear) can be misleading.

 

Fred, I believe I used Datamaster at the time...so perhaps it is a bad reading??