top end hesitation (misfire)

[GSTSpyder916]

I turned up my boost to 20psi on my setup and noticed on the dyno that I was getting a dip several times once I got to around 5000 - 7000rpms. It almost feels like a misfire in the car and on the dyno charts it is a dip then back to the normal curve then dips again several times to redline. I noticed when I turn the boost down to 15psi it runs fine.

My question is, is this the limitation of the ECU like I think it is and its just running too much boost for the stock ECU to compensate or what could I possibly be looking at here? My mods are listed in my profile (my A/F ratio is around 11:1 from 5000 - 7000rpms when using a wideband on the dyno) Any help would be appreciated, also if you look at my dyno #s they seemed very low to me for my mods....any ideas on being able to get it up above 300whp or what I might be overlooking here? (I dont hit full boost until around 4200 - 4500 which seems very high to me)

 

[ELC 4g63]

i think your problem is the spark plugs in your car. this sounds like the same problem i had so all you need to do is get yourself some BPR7ES spark plugs and those should do the trick.

 

[xojaycavox]

i had that problem .. and changed my spark plug wires.. and it went away.. when you misfire.. do you end up running rich? or black smoke? if so.. its your sparks..

 

[ddavisaf]

You don't have an aftermarket fuel pressure regulator in your mods. That could be part of your problem.
Also have you checked for boost leaks?
And then spark plugs would be my next suggestion. What kind are you running and what are they gapped at?

 

[larsenracing]

My guess is plugs and wires will do the trick (get some performance wires) on your plug wires look at the part of the wire the drops down into the head and look for white carbon tracks running around on them if they do they definitely need to be replaced other wise that is a place to start. Wires will run about $65 to 100 and the ngk bpr7es plugs are a stage colder plug like mentioned above and they are about $1.68 a peace.

PS> my car did the same thing about 1 year ago and this solved it

thanks

 

[Tevenor]

Most high-end hesitation is due to airflow and spark gap being too big. Make sure whatever spark plugs you run, the gap is correct. run between .028 and .030 for normal boost. Maybe smaller if you are the king and are running massive airflow/25+ lbs of boost.

 

[andymoraitis]

I think ddavis has a very vaild point. Not having an AFPR on a 255 pump is asking to run pig rich. Also, your late spool issue should be pointing you towards a boost leak test throughout the intake tract. Your dyno numbers are low considering the setup and I think it has more to do with the state of tune than anything else. So you can ease your mind, you aren't even close to maxing out the stock ECU.

I also noticed that your injectors are a bit out of the safe zone for what an AFC can handle. The advice you've received above is very sound, but I would also recommend that you read your plugs to see what they tell you in terms of being rich or lean. If they show carbon fouling, re-check the gap and replace the wires unless you've already done so. Do your boost leak test and also consider either an EPROM ECU with a chip to control the 750's or go with DSMLink instead of the AFC.

It's all about the tune!

Andy

 

[GSTSpyder916]

Ok, sorry, I guess I left that out but I do have brand new NGK BPR7ES plugs and NGK wires in the car (in fact he put brand new ones in while it was on the dyno - stock gap though) and I do have a B&M fuel pressure regulator....forgot to put it in my mods list (its set for stock fuel pressure though)

also andymoraitis: I agree with you on the injector/AFC issue but at this point its about the money....DSMlink or AEM is next on my list but for now my AF ratio seems decent, its around 13:1 down low and goes to 11:1 from 5000rpm+ (the AFC and MAFT is pretty much leaned out to the max)

as far as a boost leak I've done the test and there are no boost leaks

thanks for the help guys, Im going to regap my plugs and adjust the fuel pressure a little.....I'll post up with the results

Bullseye power was a great help on this too, they offered to swap out my compressor for a V-trim or 57 trim if I cant get the spool/HP issue worked out on the H3 compressor due to my lack of cams and airflow to the turbo.....they're awsome over there

 

[andymoraitis]

Aha! If you run a B&M Command Flo Modifier, that will only allow you to turn the fuel pressure UP and not down. If you're running stock fuel pressure on that unit, you're running into injector overrun and she's rich. I would invest in a proper AFPR and get back on the dyno to tweak fuel pressure down in small increments. You should be getting great timing with the AFC and large injectors, but if she's rich, you're still losing something.

Nevertheless, I'd still read the plugs and even post a picture if you're not sure how they should look, but the re-gap is still a great idea as suggested above.

Cheers!

Andy

 

[Tevenor]

To reiterate what Andy said, the fuel pressure issue will be a problem. Wether it's the root cause of this issue or not. Do the spark plug gap first, then the FPR issue.

No sense in trying to figure out why your horse is slow if you ignore the fact that he's tracking 3 tons of stone behind him. Or if you prefer stripper terms, make sure you got the 20 before you worry about keeping your cool in the back room for your private lap dance. Or if you prefer cooking terms, don't worry about how spoiled the milk might be if you don't even have an oven. Or how about just plain old DSM terms, phase 0 is always where you start. Plug gap is phase 0. FPR regulator is phase 2-3.

 

[hellotbone]

Most missing backfiring studdering is plugs/wires/coils/tuning related.

 

[ddavisaf]

Phase 0

 

[GSTSpyder916]

I regapped my plugs to .029 and the sputtering has stopped now and is getting up to 22psi w/o any indication of knock or misfire so it looks like you guys were right once again.

Now Im looking at issues with my HP #s....I have a dyno tuning session setup for next Thursday to try and mess with the FPR and AFC more. I was also going to adjust the wastegate actuator to make it tougher to open the flapper (hopefully that will help my spooling issue a little)

Thanks everyone