What the subject says?
36#?
42#?

 

The #30 were to small for me too. I use the Delphi 38lbs now and they do the trick 12.8 AFR through the entire rpm band without leaning out any. The Duty cycle goes up to about 75% for me on those.With the #30 red tops the duty cycle would get to 80% at 5400 rpms and lean out the mixture real bad after that.

 

I got a set of matched 42#'s FMS injectors for sale, 250 Shipped. Pm if interested.

 

Here's a link someone just posted on our local message forums:
http://www.rceng.com/technical.aspx

 

Sizing injectors involves playing with a lot of numbers, some of which are guesses at best. Calculating backward from your rwHP, figuring 12.5% loss through the M6 drivetrain, and assuming your injectors flow exactly 30.0 #/HR, that your fuel pressure is exactly the pressure that the injecotrs were flow rated at, and that you are at 90% DC, your brake specific fuel consumption is about 0.460 #/HR/HP. That's a fairly efficient tune.

Assuming you maintain that efficiency, all you have to do is pick the DC you want to run, and the injector size will follow from there. Examples:

80% DC = 33.8 #/HR
75% DC = 36.1 #/HR
70% DC = 38.7 #/HR

But all that is figured based on your quoted rwHP. That is most likely STD or SAE corrected. On a really cool day, with low humidity and a high barometric pressure, the engine is going to make more HP that your dyno #'s indicate. So you want a "cushion". 10% would be a good idea.

Assuming 10% more HP than you are showing, and using an 80% DC, you would want a 37.2 #/HR injector. 36 will be "OK", but the 38 or 42 #/HR injector would give you a bit more headroom. There does not seem to be a problem running the 42's with the stock PCM. IT seems to be able to provide a pulse width low enough to provide a decent idle.

Its all a numbers game.

 

Great link. Thanks.
37# racetronix injectors on the way.

Injuneer,
What is the bad side of the injectors hitting 90% DC? What happens?

 

Flow rate can be reduced and spray pattern can deteriorate. Excessive heat generated by the injectors trying to stay open all the time helps wear them out. The RC Engineering site used to have an excellent writeup on the effects of excessive duty cycle, but it disappeared several years ago. The had spray pattern photos, etc.

The sizing formula shown on that site (like most others) is "generic". 0.500 #/HR/HP BSFC and 80% DC is very conservative. A well tuned high performance engine can see BSFC's as low as 0.44. Most injectors will continue to function above 80%.... but the exact point where they start to flow and spray pattern is hard to specifiy. That's why a site like RC Engineering recommends a very conservative approach, like 0.500 and 80%. In effect, they are incorporating the "cushion" for you.

I've found that a general sizing guideline is 0.07 X flywheel HP (NA engines only). That reflects a BSFC of 0.476 and a 85% DC. To me, that gives you the minimum size injector you should consider for any application. MSD has an injector sizing calculator that you can download. I've used it and it agrees almost exactly with my 0.07 factor.

 

Thanks for the detailed explanation.