I've read a thousand posts here saying that increasing injector flow is unecessary until you really increase HP.

However, my car is running, but not well. I added very high flow heads (285 cfm), intake, and headers. Is it possible that I'm going to need to go to 30# injectors?

 

Not sure what cam you have. You can look at my signature and see what I have. I still have stock injectors but they are maxed out. Also I am running a little lean on top end. Alvin, www.pcmforless.com suggested I go to 30's which I will eventually do and get him to retune for them. Bottom line, though is get what your tuner suggest.

 

What is it doing that is making it "not well"? If you have access to a dyno or a wideband O2 that would really help in determining if you need more injector

 

Cam is 226 duration, .574/.571 lift.

Alvin did my chip as well, don't know why it didn't occur to me to call him.

 

93ragtop and I are running on stock 23.8 lb injectors but you have more cam than we do (my cam choice is driven by strict emissions tests--and i don't want any hassle whatsoever passing). For Road racing I think you are past the limit of 24 lb injectors even with my cam.

LT4 28# will work if you can find them. Also, FMS 24# might with the right fuel pressure.

Best advice, already given, is ask Alvin what he wants you to use if you upsize (and I think you have to).

 

Spoke with him this morning, he thought I could use an upgrade to keep up.

The local speed shop has FMS 30# in stock. Will my stock fuel pump handle this increase? I'm sure they have the 24# also, but it may not be enough. I might do that today, but I want to figure out my poor run condition first. Although...if the injectors are some how contributing it might be wise to do them now.

Small injectors wouldn't cause rough idle and running, would they? Just lean mixture at high RPM?

 

Chances are you probally won't notice it going lean because it will be over 6000 rpm. I woudl focus on the misfiring first.

 

Going lean at 6000 rpm or more--Does the engine compensate for this with timing? If not, are the stock 24#ers going to hurt his engine from a too lean condition up high?

 

Likely with his combo he will not get dangerously lean.. just leaner than a good safe target.

 

TPIS is using the 19 lb. injectors from a LG4 305 on there stroker 415's with pretty good results, I would tread very carefully going to larger injectors and get as many expert opinions as I could before I start throwing 30 lb. injectors around.

 

And they used how much fuel pressure?

You guys are too afraid to put properly sized injectors in a car.. I've got 60/65lbs motrons in my car with a factory ECU. And heres a challenge... I welcome anyone to come for a ride, suck on the tail pipe, whatever you want to do just to prove to you that large injectors can be tuned in easily without problems controlling them.

 

I have heard these things before as well, But I have to ask one question. Why did GM go from a 24 to a 28LB when they developed the LT4 motor?
Why does Holley run a 24lb injector on their stelth ram setup for a 300-385 flywheel hp. but for 385-480 they use 30lb injectors?
Both of these are systems that are actually engineered and both holley and GM see a need for the bigger injector.
I know you can crank pressure up and exceed the duty cycle but is it the safe or wise thing to do?

 

That would be an LB9 305...the LG4 was TBI.

 

To Morley: You are right sir, I stand corrected. I looked back in the book and the TPI 305 is the LB-9. To Alvin: Call Myron Cottrell, maybe he remembers how much fuel pressure he used. I don't know, couldn't find the spec in his books.

There is always more than one way to skin a cat. With proper tuning and attention to detail you I'm sure you can make these things work, but how much did you pay for the 60/65 lb. injectors? Small fortune I'm sure. But why make somebody pay a truck load of money on something that may not work, meanwhile the factory stuff is working with the proper tuning and attention to detail. Afraid of big injectors? You bet! Because they're way to expensive if you guess wrong. They make the price of the fuel pressure regulators and computers seem pretty cheap.

 

For what its worth - ran 24# injectors @42psi on our 406 CI motors that ran 11.0s/10.9s.

Have a 434 running 24# injectors, 50psi - running consistant <10.5s and dipping down to 10.2s in the better weather - probably has over 200 runs this season alone - not to mention last year, and not to mention the same set of injectors for the past 5 years.

I have 434, 30# injectors at 45 psi, turning over 7k rpms, running 10.1s@133+ pulling 1.37 60' times, monitor A/F through my Innovate WB, can make it run 10.5 A/F ratio with a simple command of the ECM, indicating that my injectors are enough to drown a cat sitting near my exhaust - and have well over 250 passes down the track this season alone.

Have Hooked ups car running 30pound injectors running high 9s weekend after weekend, pulling wheel stands that could dazzle the most experienced racers.

It boils down to you need two things to make good reliable horsepower. Air and fuel - all of these above setup run well above average and do it weekend after weekend, run after run - dialing into the nearest hundreths of a second.

Not to mention there are a few other dozen cars here that perform WELL above the curve with similiar setups and 24# injectors........

Choice is yours - if there was more power to be made or a need for more injector, all these cars would have them on it.

 

First dial in the air fuel ratio.

Second take a look at your injector duty cycle. If it's above ~85% in the rpm range you intend to spend most of your time at then you need to do one of two things....Bump up the fuel pressure and then retune accordingly or swap in larger injectors and retune.

Drag racing spends so little time at peak duty cycle it's not a valid data point.

When road racing you spend 15+ minutes at or near peak duty cycle every session that you're on the track.

Summary...Focus on the AFR and duty cycle as they relate to your intended usage. If you need to lower the duty cycle you can get there via more fuel pressure or via bigger injectors. Both approaches require proper tuning.

 

Matchlight, Alvins car is supercharged. Also, I believe he eventually plans to run around 15lbs boost, so he will need big injectors. I believe he was simply trying to make a point that with proper tuning big injectors can be tuned. The reason I say this is Alvin and I have talked back and forth via e-mail about injector sizes.
Back to the orig. poster, he has a 93 LT1. 30lb injectors (ford) can be had for $219.88. The LT1 pressure regulator is $154.95 These prices are from summit. So as you can see there is very little difference in price.
I was told to go to the 30lb injectors on mine (by Alvin) The ford 24's may get me by, but the 30's will leave room to grow.
Again I will say, IMO ask your tuner and follow his advise.

 

The only way to determine (without dyno time / precision air flow meter) is to:
1) get a scan tool that is able to see the fuel injector duty cycle like data master. Just over $200 buck for cable and s/w license. You should strive for 80% duty cycle
2) get a WB meter like innovate to determine what specifcally you are running for an AFR.

I currently have the miniram 383 with full exhaust and I'm using FMS 30#s. My heads flow a modest 265 cfm. At full boogie, I am 60% duty cycle and 12.5 AFR (too much gas). I have my fuel pressure set at max. Once I started tuning I just set the AFR and forgot about it.

 

No certainty in the answer but here are the results:

Spoke with Alvin twice today and after this consultation decided to try the 30# injectors for three reasons: wasn't going to hurt the car, would support the planned 383, and I might get lucky and it was the source of the problem.

Bought 30# FMS injectors and installed them. Car now runs like a raped ape. (that's a good thing)

There are two possible reasons for this being the fix, and probably a combination is the real answer.

1) injectors were too small
2) one of the old ones was plugged/bad/whatever

Both are entirely possible. Although the car ran fine prior to the top end rebuild, the injectors sat out for two weeks while the motor got worked. So it's quite possible that a piece of oil dry fell in and clogged one...

Or, I listened to my tuner and now the car runs great, sounds mean, and will kick the crap out of some 911's at the track tomorrow. We'll see in 24 hours.

All I can say is thanks to Alvin.

If I blow a motor due to too much fuel this weekend, then we can further discuss on Monday.

 

Oh, and to quiet those who would reprimand me for not checking codes first, I did try. I have access to a tech tool but could not get the ECM to communicate so I was forced to use barbaric means and replace parts that might not be bad (the last chance for track time is TOMORROW).

Luckily I started with the injectors.