After months of being stupid and trying to diagnose this thing on my own I finally found a shop that did something besides scratch their heads and say "I dunno". The shop seems to think my high rpm miss comes from my stock injectors not being able to put out enough fuel for my motor and my HP. So I am making one last post before I buy a bigger set of injectors. For my power and mods(listed in sig), do you think it is a good possibility that my injectors can't keep up? Any suggestions or thoughts appreciated as always.


Chris

 

It's possible. Try it since you already got them.

 

With the power levels you are at, your car may definitely be in need of some bigger injectors. Put them in, get it tuned, and see what happens.

 

Did you not have a/f tuning while the car was on the dyno? Do you have the dyno graph with a/f that you can post? If the injectors weren't big enough, it would have been apparent. How do the plugs look? Are you running lean? I'm going to go out on a limb and say that the misfire is not because the injectors are too small. If it was running lean enough to cause misfires, you'd probably have melted a piston by now. Oh, and please tell me that you're not having the shop install the injectors.

 

Just doing a rationality check, 380rwHP with an M6 is about 437 flywheelHP (and flywheel HP determines the injector size). Assuming a very efficient tune (0.45 #/HR/HP), you would be right at 100% duty cycle with the stock injectors (24.9#/HR in the PCM). As injectors approach 100% DC, the quality of the spray pattern, and the flow rate becomes irregular. You probably should have gone to at least 32's a long time ago.

 

I have done 2 more mods since my last dyno. The car ran fine when it put down the numbers that it did but now I should be right at th 400rwhp mark. It only misfires at 5600-6100. The car is not my daily driver so it doesnt get that many miles on it. If its not the injectors then i don't know what because throughout the course of owning this car i have replaced every single aspect of the ignition from a MSD opti to the ignition switch and everything in between. Coil is new and shows good, it has new plugs and wires, wires are not arching, new ICM(also tested good), walbro 255 pump, and thats all i can think of right now. They are gonna hook it up to the dyno today or tomorrow and make sure that I am running lean up top before I do the injectors. Also fuel pressure is good(at shrader valve). I guess my question was, at what HP do stock injectors not perform adequately?

 

Thats the exact answer I was looking for. I really hope this is the problem so I can get her back on the road, she's been down with this high rpm miss now for 3 months.... I was looking at the Delphi/Racetronix 37# injectors. I have heard good things and I have the Racetronix fuel setup on the rest of the car. Are these good? Also I plan on doing a GM847 immediately after I fix this miss so I wanna allow i bigger injector for future mods.

 

OK.... at 400rwHP, you're at 104% DC. An injector can't provide more than its rated capacity at 100% DC, and that's "iffy". You could raise you fuel pressure to build headroom on the injectors. Running at 58psi would reduce your DC to 90%.

 

Yeah I have heard to try and stay away from messing with the fuel pressure as much as possible with our cars. Plus then I would be dropping more cash on an adjustable fuel pressure regulator. Lastly these guys that are in there now are about to be 12 years old so I guess I am asking alot of my injectors to run at 100% being so old...

 

If you're going with the gm847 cam, then for sure go with 37# or 42# injectors...

 

I'm putting out about 410 hp at the flywheel, yuo can figure for an auto or manual what that would be at the tires, and running abot 91% duty on the injectors worst case WOT at max. I would guess that your over that, and thus your problem lies there

 

You absolutly need to upgrade your injectors if your at the stock PSI
It makes perfect sense that at high rpms your missing.

 

http://www.witchhunter.com/injectorcalc1.php4

Be sure to estimate your flywheel hp. Even with your 340 RWHP your injectors are going to be over 80% duty cycle.

 

Are the injector calculators fixed for a 13:1 a/f?

 

So if I plan on doing the GM847 in the near future, would it be more wise to go with the 42# over the 37#? Cause if I go by these calculators I'm gonna need to either up my fuel pressure or go bigger after the 847. Why is it that I have heard not to mess with your fuel pressure on this board? Or is that just bs?