Breaking up in upper RPM
Breaking up in upper RPM Tech gurus, I am trying to sort out a problem where my GTO breaks up above 6200ish RPM. It recently started doing this when I went from 12 to 18 psi on an F1a-94 procharger. It's a bit intermitent in that it pulls clean sometimes while others it just wants to hesitate and miss. Here is the setup:
Forged 370
9.6:1 CR, 243 heads
232/242 116 LSA .600 lift cam
Stock LS2 intake
FIC 127lb/hr (1200cc @ 3 bar) injectors
Triple AEM 340's
Mallory boost referenced regulator, 55psi base pressure (recently raised from 43psi because I thought I as running out of injector)
F1A-94 procharger that hits 18psi at 6500 RPM
DSX 2.5 bar MAP sensor
8an feed, 6an return
Running E70ish from the pump
11.2-11.5 AFR and 14 degrees timing
IATs in the 130-140 range at the end of a pull (timing not pulled until 150)
I first saw the issue on the dyno and it would break up above 6000 RPM. It looked like it was going lean (12 AFR) so I upped the base fuel pressure (43 to 55psi) and redid my fuel system from a 200&450LPH pump to triple 340's. Its hard to tell if the lean condition on the dyno was actually lean or if the miss was making it appear that way. I feel like I am pushing these injectors so that may be the open and shut case right there. I still want to get a fuel pressure sender so I can log FP. Other than that, any other troubleshooting ideas? Thanks in advance.
Forged 370
9.6:1 CR, 243 heads
232/242 116 LSA .600 lift cam
Stock LS2 intake
FIC 127lb/hr (1200cc @ 3 bar) injectors
Triple AEM 340's
Mallory boost referenced regulator, 55psi base pressure (recently raised from 43psi because I thought I as running out of injector)
F1A-94 procharger that hits 18psi at 6500 RPM
DSX 2.5 bar MAP sensor
8an feed, 6an return
Running E70ish from the pump
11.2-11.5 AFR and 14 degrees timing
IATs in the 130-140 range at the end of a pull (timing not pulled until 150)
I first saw the issue on the dyno and it would break up above 6000 RPM. It looked like it was going lean (12 AFR) so I upped the base fuel pressure (43 to 55psi) and redid my fuel system from a 200&450LPH pump to triple 340's. Its hard to tell if the lean condition on the dyno was actually lean or if the miss was making it appear that way. I feel like I am pushing these injectors so that may be the open and shut case right there. I still want to get a fuel pressure sender so I can log FP. Other than that, any other troubleshooting ideas? Thanks in advance.
Haven't logged injector DC; good suggestion. The dyno was spitting out 670 RWHP when it started breaking up so between driveline loss and feeding the procharger 125HP I was probably around 900-950 crank on E70 in terms of injector load.
Running NGK B9EFS gapped .027 (stock gap), I recently changed them out to see if it would help and it didn't. Anyone have any issues with non-resistor plugs? I haven't had any issues in the past running these plugs but they may be slowly hurting my electronics.
Running NGK B9EFS gapped .027 (stock gap), I recently changed them out to see if it would help and it didn't. Anyone have any issues with non-resistor plugs? I haven't had any issues in the past running these plugs but they may be slowly hurting my electronics.
They are cold but I will also run a 125ish nitrous shot on top like I did with the 12 psi setup so it should be around 900WHP when all is said and done. Need to get things under control boost only first though. I didn't read the plugs because I didn't make a clean pass, that was probably a mistake. I have clean 7's laying around that I could try.
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Verdad Gallardo Weird stuff happens without a resistor in the plug. I know plenty of people run without them, but you never know. NGK and other sites say that too cold of a plug can cause a misfire and the plug will foul quicker and not self-clean.
Yeah I am on board with that. They definitely don't self clean when in DD mode, fouled to **** after 3000 miles even running mostly alcohol.
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Joined: Feb 2015
Posts: 1,080
Likes: 178
You could start with a resisted plug 7 heat range at same gap to see if it is primarily EMF interference from ignition. Could also change gap to.022" and see how it responds. How old are coils? Could be a coil issue. Hard to locate which one(s) without a scope or swapping in a known good set.
Check your valve springs. I had this happen one time on my 408 GTO, the big outer spring broke but the inner spring stayed put and would break up like I hit fuel cut around 6k rpm. Ran fine otherwise
I'll start with the plugs narrowed down a bit. As far as springs, I am asking them to do more at 18psi. Running PSI 1511ML's that are 380lb spring pressure. They have about 13000 miles on them so they may be worth checking. Great suggestions guys, I got some work to do and will report back in a week when I am actually able to do it.
Could be as simple as a bad plug wire.... that happened to me a year ago. Also just for comparison i ran some ID1000s on E85 (88%E) to 795/800 dyno jet, idc was around 94%. You should have room to grow with your bigger injectors and lower ethanol %.
Last edited by red fury; May 27, 2020 at 04:11 PM.
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Likes: 1,086
I would run a B8EFS even with spray. Tighten the plug gap to .018"-.020". I had a similar issue on the dyno. 18psi and .027" gap it was fine. Turned it up to 24ish psi and it would break up after 6000rpms. Tightened the gap to .018" and it went away.
Wanted to circle back on the thread as I finally got around to swapping out the plugs and wires. I screwed some 7's in there gapped at 0.02 and also put some new OEM wires on. She now pulls clean right to 6700 although it felt slow because it was a million degrees and humid out today (IAT hit 150...). Thanks for the help guys, glad it was something simple although I am starting to think my battery/alternator might be the real cause of the issue and the tighter gap is just helping it along.