I have a leaking injector, If I go with a larger in injector do I need to change my tune?

 

Why would you want larger injectors?

 

I have plans for a Cam change in the near futher. But in the mean time I want to take care of the injectors. I know I'll need a re-tune with the Cam. But for now Im going to change the injectors and fuel pump

 

The answer is; it depends. If you keep within 10% of the stock injector flow rate the PCM can usually adjust for it. Anything more and you'll have to tune or risk running very rich...

 

The PCM can adjust for it.... IF the PCM isn't already using a good part of the maximum long term correction (LT, aka BLM) deviation to correct for some other problem. The PCM can only pull 15% using the LT's, or add 25%.

Second problem... it will run rich at WOT. When the injector is larger than the PCM is programmed for, the PCM will subtract fuel, based on the O2 sensor feedback. If the PCM is using the LT's to subtract fuel, when the PCM goes into power enrichment (PE) mode, it ignores the reduced LT's and defaults to a 128 LT correction. Hence, you have lost the correction, and the engine will run rich... absolutely the last thing the overy rich PE mode stock tuning needs.

You need a tune.

 

There is a formula that calculates the flow rate needed for a given HP. You would need to have an estimate for the HP you are aiming for. The formula is.

Injector Flow Rate (lb/hr) = (Engine HP(1) x BSFC(2)) divided by (Number of Injectors x Injector duty cycle(3))

For BSFC use 0.5 for an NA engine.
Use 0.8 for injector duty cycle.

If we re-arrange the formula to calculate the HP that a given fuel injector will support. You get this:

Fuel injectors max HP (lb/hr)* = (Injector flow rate x Number of injectors x 0.8) divided by BSFC

So a 93 which is batch fired had 22 Lb injectors and could support up to 280 HP.
94's and up had sequential fired injectors of the 24 Lb variety and could support a little over 300HP.

Since you don't have a signature, we don't know which you have. So we included both.

Formulas here:
http://www.injector.com/injectorselection.php

 

That is the typical online "textbook" formula for sizing injectors, and its very conservative. My recommendation, for a normally aspirated LT1 is to multiply the flywheel HP x 0.07 to get the minimum injector size.... e.g. 400 flywheel HP x 0.07 = 28 #/HR.

That corresponds to a brake specific fuel consumption (BSFC) of 0.476 #/HR/HP, and an 85% duty cycle.

 

Agree with the above - you need a tune if you install a larger injector and also with the advice on injector sizing. However, since you are going to need a tune anyway a larger injector than absolutely needed will have "room to grow" and is what I would advise. Get 42lb injectors and you will be good for just about anything NA as they will support in the range of 600hp. They would even be good for a mild blower setup if you ever went that way.

Or, if don't want a PCM tune you can just get an AFPR and tune for the new injectors by adjusting the fuel pressure. Injector flow rate varies with the square root of the change in fuel pressure. That is the easiest way to tune it for small changes in needed fuel flow as it does not require tuning software, cables, etc.

Rich

 

Thanks guys. I figured a re-tune was called for. I like the sound of going 42lb's, They should have no trouble for my needs even if alittle N2O is in the future aslo.

 

You don't size the injectors for nitrous unless you are running a dry system, and even then, some dry systems for the LT1 add the extra fuel for the nitrous by increasing the fuel pressure when you spray. A wet system adds the fuel through spray nozzles, a plate behind the throttle body, or direct port nozzles. The fuel for the nitrous does not pass through the injectors.

 

By my "conservative" formula. For 400HP you would need 31 # injectors. Your saying push the 28's to get there. Makes little sense to me. Why push them with a more severe duty cycle?

Seems Rich agrees with me since he's saying go large. And hope your PCM can cut the injector pulse width small enough. ????? Not sure this is the best approach either.

Let's remember guys, he only wants to put a cam in it. He's not likely to get near even 400HP. So 1 says 28's for 400 HP, I say 31's and the last says 42's. Some body here is right and the other 2 are not. So who is it?

Periodical after periodical says size them correctly. Use the "conservative" formula. Why?

Just trying to spark up conversation and not be "confrontational". FWIW, I will be changing my Vortec to sequential and a ram air intake and all the more information I can get out of this would be helpful.

 

Your opinion, and you're entitled to it. Your interpretation, and you're entitled to it. All I can do is tell him what works very well for me, and for the many others I have helped with this topic. Note that my formula recommends the MINIMUM injector size. I did not say that was the ONLY injector size.

 

Thanks again guys. Im going with 42lb's and the racetronix fuel pump and wire kit. Are there any better fuel pumps out there then there walbor's?