My engine setup is a LE1 headed 355 with a cc502 cam(218/224 .526/.536) with headers and full exhaust. I am currently running a mailorder tune, but am wondering if I would benefit from going with 30 pound injectors and retuning. I have searched to find the limitations on the stock injectors or if i would benefit, but with no avail.

 

There are people who claim to have run stock injectors up into the 450HP range, at stock fuel pressure, but that's not likely, at least without a tune that is extremely efficient (0.44 BSCF or better).... the kind of thing you only get on a dyno, at best, and running them at 100% DC.

A rough guideline that will get you in the ballpark for the smallest injector you should use is to multiply the flywheel HP by 0.07 That number assumes a decent tune, and that you don't want to exceed 85% duty cycle.

Backing up from that number, the stock 24# injectors (programmed as 24.9 in the PCM) will not be the best choice for anything over about 360 flywheelHP. Yes, you can push them closer to 100% DC, but that's not good for them, or for your engine.

If you think your engine will be under 425 flywheel HP. a 30# injector will be OK. Over that, consider a 36# injector.

 

My question is will it get me any more power? I have yet to put this engine combo on a dyno but it will be seeing its day soon enough

 

If you run your injectors to 100% duty cycle, they will not operate correctly. The spray pattern may be screwed up, the flow capacity may actually decrease, etc. End result, if the injector can't supply the required fuel, you will run lean. If you run lean you will not make the power you are expecting, and you may run into detonation.

The duty cycle will be determined by flywheel HP and the brake specific fuel consumption. No one can tell you the exact point at which the injectors can no longer supply the required amount of fuel.

 

Very interesting, Fred: I remember many posts by you regarding injector size, etc. but I never considered that I might need anything more than the stockers-----and have never heard from other than you that I should be concerned about them.

I'm running a 227 with ported stock heads, shortie headers, etc. I haven't dynoed, but suspect that I'm running 400 plus at the crank.

Only have about 3k on the engine, but it seems to run strong. So are you saying that I should upgrade to at least a 30# injector? Will this give me more power? What injectors, brand, size, etc. do you recommend? Will I have to have PCM do a retune with larger injectors?

I must sound like a newbie, but you truely are the only one who has suggested going with larger injectors even though I only have around 400 plus change at the crank.

Please advise----I value your input.

Thx.

 

This isn't rocket science... its simple math.

Injector size = (flywheel HP x brake specific fuel consumption) / (duty cycle x number of injectors)

There are several online injector size calculators, but the are EXTREMELY conservative, assuming a BSFC of 0.50 #/HR/HP, and lmiiting the injectors to no more than 80% DC. Using those overly conservative numbers, for example:

(400 flywheel HP x 0.50) / (0.80 x 8) = 31.25 #/HR

My suggestion is to use a less conservative approach, assuming 0.476 #/HR/HP and a DC limit of 0.85

(400 x 0.476) / (0.85 x 8) = 28.0 #/HR

Note that the 28#/HR injector could support 470HP if it was operating at 100% DC. But injectors can't operate RELIABLY at 100% DC. They need to stay open constantly, while the PCM is trying to turn them on and off. RLC used to have a great online guide to injectors, that shows high speed photographs of injectors operating at DC's in excess of 90%. The spray pattern and atomization went to he!!, the flow capacity actually decreased. Is this what you want happening in your engine when you are reaching peak fuel demand?

But notice that there is also another variable in the equation..... BSFC. In engine dyno testing my NA setup, we achieved a BSFC of 0.44 #/HR/HP at the peak fuel demand. That's the result of the physical configuratuion of the setup.... combustion chamber shape, squish, plug, injector type, intake manifold porting, and the amount of time you want to put into optimizing power as a function of A/F ratio, ignition timing, fuel octane, coolant temp, etc. NASCAR engines probably get the BSFC below 0.40.

Another issue... many people rely on rwHP, obtained on a chassis dyno. Test on a warm, humid day, with a poor barometer, and your engine didn't really make the 400 rwHP that the dyno indicated. It made less, but was "corrected" to standard conditions. The dyno A/F meter may have indicated that it stayed at the desired A/F ratio all the way to peak power/peak torque, but that doesn't mean it would have had the same results if the engine was actually requiered to produce the "corrected" HP, rather than something less. What happens when you run the engine at the track on a cool, dry day with high barometric pressure? Now the engine can actually exceed the "corrected" HP by a significant margin.... but will your injectors still be large enough to supply the required additional fuel? Then there's the issue of variations in fuel density.... but I think you get the point.

Since its unlikely you will ever measure your engine's BSFC, and you will never know the exact point in DC when the injector flow breaks down, you build some "fat" into the calculation. My 0.07 factor is my best guess at what will give you the correct minimum injector size, that will prevent you from getting into trouble with fuel supply.

I practice what I preach, running the equivalent of a 78 #/HR injector..... barely working (35% DC) at 500 flywheel HP NA, and still not exceeding 70% DC with a 300-dry shot (0.53 BSFC) on top of that.

 

Thanks Fred: so assuming that I have around 435 hp, you recommend I go with 30 or 36 size. Will I need a retune and what brand injector are you running/recommending?

thx

 

Bubba, I've got the same concern and I am following this thread closely. The builder/tuner of my 383 advised that larger injectors were not required for my mods even though I questioned the need for something larger. But I don't know jack about injector sizing.

Not wanting to spend more money than necessary and trusting my builder/tuner, I went along with the stockers. Now I am wondering if a larger aftermarket set would have been smarter.

 

Don: I assume that you agree with me when I said that of all the posts over the years, along with all the mods that have put people in the 400+ hp at the crank, few signatures have revealed the requirement to go with larger injectors. You'd think that it would be one of the first things one would do after installing even a baby cam with head work.

If you use the std calculation for injectors you will note that anyone with a small cam and head work should be looking a a min. of a 30# injector and probably in my case a 36# injector since I figure my hp around 425 +-.
I believe that the Lt4 even has 30# injectors and it's probably rated around 400+-.

However, given the fact that a set of 8, Accel or Bosch injectors will run between 400-500 bucks, plus you probably would need the PCM retuned (don't know about that). that most of us would put that last on the list, since the stockers "seem" to function fine for a street/strip F-Body.

I sorta feel like a dummy, but as I said, not much said about larger injectors here, and I've been on here since 97. Let me know what you decide.

bubba/jim

 

A set of 30# SVO injectors will only run ya about $300 (what i'm running in my setup, no, not in my sig... my sig would be a book if it listed everything)

Yes you'd need a retune.

 

FYI, The 3800 series II S/C engines (GTP's) run "36lb" injectors. At stock LT1 fuel pressure they flow between 33-34lbs which is perfect for ~425Fwhp.
A set of six used ones can be had for $30-50, so you buy two sets of them, have them flow tested and run the 8 best.

Here is a thread I made about them.

 

It seems you would get to a point of negative returns, particularly with a NA engine, in that the intake/heads/exhaust will only flow so much air, therefore burning only so much fuel...BUT the extra capacity isn't all that expensive once the need for the increased fuel supply a 30# injector will provide. Personally, my stroker made over 400 hp on the stock injectors, but was SERIOUSLY starved for fuel after about 5Krpm. Lean is good, in moderation. Detonation is right around the corner.
And yes, programming is required to tell the PCM that it now has larger injectors to work with...

 

Don't forget that the LT4 had 28# injectors factory installed!