turbo foxbody high rpm misfire.

[BryceK]

Hello all, I have a turbo foxbody here that has a high rpm misfire that I'm not sure whats causing it. The car runs great and clean all the way up to 6k and then it misses and power just drops out. Normally an ignition misfire I see the AFRs go lean but it is not doing that. The other miss I normally see is weak valve springs and the car has brand new valve springs in it. Its running around 10-11 psi but we are looking to go more. Running .031 gap now on NGK race plugs in a 7 heat range i might tighten that up to .028. or is there something I can do with the dwell settings? This is an all new build, the injectors are ID1000s at %65 dc. It makes over 530hp to the on 10-11psi but falls off hard at 6k.. The limiter is a hard ignition cut at 6500. If anyone has a recommendation I would appreciate it. I'm attaching a log and tune.

[BryceK]

log

[BryceK]

tune

[turbo conversion]

One thing that catches my eye is the dwell type is set to fixed duty.

I think for basic trigger it should be set to standard dwell, anyone?

David

[Matt Cramer]

Fixed duty is normal for TFI ignitions, as well as MSD boxes.

Some general questions:

1. Have you confirmed with a timing light that the timing is correct across the rev range? Make sure it's not retarding at high RPM.

2. What are you doing for an ignition system? You will need a relatively hot ignition to light off a V8 at 6000 RPM with that much boost.

[BryceK]

Yes it is a TFI system. using MSD TFI billet distributor and MSD blaster SS coil. I did check the timing with the light and had to do a little changes with trigger offset to get the rotor phasing in a sweet spot. I set it to fixed 20* and checked the timing with a light and then had a drilled cap so i could see where the rotor was. its now perfectly lined up with the cyl and timing is right on 20*. how can i be sure its not retarding at high rpm? leave it fixed and rev it up that high? i might try another TFI module just in case. otherwise im going to tighten the gaps up to .028 and see what happens. I noticed my spark latency is set to 10 is this normal? i did not change it.

[Matt Cramer]

Yep, set fixed timing and rev it in neutral without any load.

I'm not sure you will have adequate spark energy here without a CDI module.

[turbo conversion]

Spark latency default is 0, this needs to be set to 0 before checking the timing for retarding at high rpm.

David

[turbo conversion]

I have never used spark latency so not sure what would happen with it set to 10, try setting it to 0 and see what happens.

David

[BryceK]

gonna get back to this in a little bit but i had a question. when in fixed timing at 20* what amount of "jumping around" is acceptable. i mean for the most part its right at the 20* but it'll jump around a bit. i guess it could be the inductive pick up though. and if it was actually the timing changing what could cause this? should i put another TFI module in it just to be sure?

[billr]

Well, I want the spark "jumping" (spark scatter) to be well under 1/2 degree; but that may be difficult to ensure using a dizzy. Why not fit the engine with a missing-tooth crank wheel and wasted-spark coils? Super-duper dizzys and coils are rather antiquated now, move past all the problems inherent with a dizzy-based system.

[BryceK]

ok this thing is driving me nuts. so I regapped everything at .028" cleaned the cap. quadruple checked timing with a light. and still it hits 5800-5900rpm and the car goes to crap. the afr is perfect the timing is perfect the temps are all good. boost ramps up? is this an indication of something? To be honest the car is REALLY acting like it has weak valve springs. I know this should work because the car was tuned on factory ecu by myself with pretty much the same combo of parts but he broke the stock block which we were expecting with running 550whp on stock block. now we are at the same power level and same boost level on a BOSS363 short block and megasquirt and have a bad miss. I'm going to have to try a new TFI module and see what happens. heres a log from the last pull

[billr]

I gotta ask... have you looked in the rear-view mirror when the engine is faltering, to see if black smoke suddenly appears at the same time? I usually see that when detonation happens, which kills power just as you describe.

[BryceK]

I really don't think what I'm dealing with is detonation. That's normally way more violent and judging from the plugs and all the data that I have there is nothing that would indicate it is detonating or at least I don't think so. Here is a pull from last night and the plug immediately after that pull. Hard to see the WOT area but its maybe a tad fat and timing and heat range all look good. if anyone wants to take a look and give me any feedback its greatly appreciated. thanks everyone.

[BryceK]

idk why I'm having an issue with pictures

[ashford]

there are alot of maybes in this situation, valve float, lifter pumpup, spark energy of choking on the turbine. ill try and address these to help out troubleshooting.

does the engine misfire like a plug wire is pulled, does it feel random or just change tone and fall on its face.

1. if it is spark blowout it generally happens at max torque, i had an engine misfire around 4500 then come out of it on they dyno at higher rpm, i closed the gap to .020 and it went away.

2.with a distributor and single coil the amount of dwell time available diminishes with rpm for example it takes 10ms to make a full revolution at 6000rpm, with 4 ignition events per cycle there is 2.5ms to charge and discharge the coil. generally dwell time is 2 ms and full discharge time is 1ms so .5 ms has to be removed from one or the other. this is the shortcoming with distributors and the reason cdi ignition is popular as cdi hwell time is shorter.

3. valve float is probably not an issue here if spring seat pressure is above 130 probably will need a bit more on the exhaust side if backpressure is excessive

4. lifter pumpup, this is the most likely cause if it is not ignition related, the simplest way to troubleshoot is to readjust the rockers so there is 0 freeplay in the pushrod and no preload at all on the lifters. when the engine heats up you will loose about .002 effectivly making it a solid lifter, if the problem goes away get some comp high rev lifters. i had issues with this on almost all sbf engines i have built using oem lifters. turbo cars especially don't like them my first turbo car the more boost i gave it the lower the rpm i could achieve, a solid cam took care of that issue.

5. a too small turbine creates excessive back pressure and will act like a wall that cant get over, power simply levels off

[turbo conversion]

From what I can see your plugs have small black spots on the ceramic, that is an indication of detonation.

The only thing I saw in the data log was map/dot deceleration triggered 3 times once in boost.

I don't think that would have anything to do with your issue, but it should not be happening.

David

EDIT: word

[BryceK]

Thank you for the responses. I agree normally when I have a spark blowout issue it is at peak torque. This is not acting like that. I guess you could say it does change tone and sort of just fall on its face. The last time I had a car do something similar to this was also a boss 363 and the issue there was valve float. So I do believe we are on to something with the lifter pump up. I have them set at zero lash +1/4 turn. I'm going to throw an msd 6A on there to give a little more juice and I just finished getting a boost line on the downpipe because I was also curious about back pressure. unfortunately I do not have a pre-turbo port for checking drive pressures. Forgive my ignorance but what is the map/dot deceleration? and what could be causing detonation? You can see the temps are all in line and the AFR and timing are great, Although I am reporting a small discrepancy between my dyno wideband, the AEM guage, and what tunerstudio is reporting. The dyno and the gauge are both reporting the same so I went with those and tunerstudio reports the car being a bit leaner.

also here is a link to the turbo that is on the car. I got this from the tag on the center section saying 11329. The customer says it has an upgraded billet compressor wheel also.
https://www.vividracing.com/catalog/-p- ... oCwVvw_wcB

[ashford]

definately too small a turbo, that is something you would put on a high strung 4 banger to a stockish 302. if i were to bet the drive pressure is in the 30-40 lbs range where your trouble spot is.
taping into drive pressure is easy, get a cheap mechanical oil pressure gauge with a copper line weld a npt bung to it then drill the hole out

[BryceK]

well Ashford, you win.