Okay so i think I narrowed my high rpm miss down to a fuel problem so i ordered the Racetronix fuel system kit and I want to replace my injectors too... I have mods in sig and hp numbers... Soon i am going with even a bigger cam so should I just replace them with OEM ones or should I go with like 30#'s... Oh and what is stock, 28#? Car right now has stock fuel pump and stock injectors(both 11 years old)....

Chris

 

Stock injectors are a nominal 24 #/HR. They are programmed in the PCM as 24.896 #/HR

A rough guideline that gets you in the ball park...

Flywheel HP x 0.07 = #/HR injector size

That assumes BSFC of 0.48 and a duty cycle of 85%. Yes, a good tune will have a lower BSFC and you can push them closer to 100%, but why risk problems with the injectors due to excessive duty cycle.

So... right now you're at about 428 flywheelHP (assume a 13% M6 drive train loss). If you plan to add another 50HP or so, buy injectors that will support 480 flywheel HP.

480 x 0.07 = 33.6#/HR

Yes, you could get by with your stockers at your present HP level, assuming you have a very efficient tune and push them to 100% duty cycle. I wouldn't recommend it. But you're going to find there are people here who will tell you to keep the stockers.

And don't forget to correct the injectors flow rating, if necessary, for the fuel pressure you are running.

 

thanks injuneer. That helps alot. If i get new injectors say... The Racetronix/delphi 37#ers... Then I would have to retune my car right?? I don't really have the money for that right now... But I won't need a tune for just the 255 fuel pump I am putting in right?

 

I'm using the 42lb racetronix injectors and my engine is closer to stock than yours. I bought those injectors specifically to accomodate a dry nitrous system.

The fact that they run just fine in my application, with of course a proper tune (from Bryan H in this case), is proof that they will work fine in yours as well.

Changing a tune is easy and cheap. Changing injectors is a wee bit more labor intensive and somewhat pricier.

If you're going to change your injectors, you may as well get something that is big enough to meet your needs both now and in the future.

 

If you change the fuel pump, you won't change anything (like the fuel pressure), so you don't need a tune. If you change the injectors, you have to put the new injectors flow rate and offsets into the PCM.

I run 78's. The engine idles smooth at silk. It makes 800HP with a 300-shot, and the injectors are still only at 70% duty cycle. When you keep the duty cycle low, you can start adjusting the injector timing (requires an aftermarket ECU) to optimize the amount of time they are spraying with the intake valve open. Worth a little bit of mid-range torque.

 

No argument there, but that's not the point I was making.

Judging by the posts I have read here, there are many people on this site who change their injectors more than once, each time going successively larger as they increase their power levels.

It appeared to me that the original poster was debating size choices in injectors.

I was simply making the point that when choosing between injector sizes that you choose the larger injector. If you have a larger injector than you need, you can use programming to adapt it to your needs. If you have an injector that's too small, your only choices are to either increase fuel pressure (which only works up to a point) or to replace the injector.

Therefore, going bigger (within certain limitations) is generally better than trying to cut things too close and finding out afterwards that you cut them TOO close.

I apologize if my first post on this thread created any accidental confusion.

 

By the way, that is the other part of the reason that I went with larger injectors. I am not taking advantage of it NOW, but I know that I will in the future and I prefer to buy parts ONCE.

 

This is getting ridiculous.... I wasn't even answering your post. I was answering the one above it:

I'm really getting to the point where I think some of these discussions don't even belong on boards, because it all comes down to a bunch of nit-picking and misunderstanding. I give up No more "how big an injector" threads for me.

 

Sorry for the frustration guys, but thanks alot for the help. I really appreciate it.

 

You know, my first instinct was to apologize for butting in or misunderstanding who you were talking to.

Then it occurred to me that it was entirely reasonable for me to have assumed you were talking to me.

It also occurred to me that I have lost track of the number of time I have watched you go from 0 - pissy faster than an 8-month pregnant woman in the middle of an Alabama summer.


Yeah, I get it...
You've fogotten more about cars than most of us will ever know. And I'm sure I speak for everyone when I say that we appreciate the fact that you're willing to share that knowledge with us.

Just... get the meds adjusted or something, OK?