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Old 10-02-2012, 10:43 AM   #1
06AWD
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Default First track day. Power cut at 100 mph.

Yesterday I took a day off from work to participate in a Hot Laps event at New Jersey Motorsports Park. Had an incredible time and was impressed with the car's capabilities.

2011 STi Sedan
Cobb Stage 1
Nameless cat-back
K&N drop-in filter
All else stock

Traction Control turned off (held button in until TC light turned yellow. My cousin's GT-R to the left of my car.



However, I experienced an intermittent problem during 2 of my 4 sessions on the track. Coming off the last turn before the straight, the car would cut power right at 100 mph with a violent jerk, as if I had knocked the car out of gear, or let off the accelerator quickly. At first I assumed I was just at the top of 5th gear and had touched redline, and tried shifting into 6th on the next lap, but the same thing happened.

At 100 mph in 4th gear, the car was at 5K rpms so I knew I still had a bit of room at the top of that gear.

This happened 4 or 5 times during my second session.



During the 3rd session, I kept the TC set to it's "green" mode where TC is off but the ABS is still active. During this session, I did not experience the problem. In the 4th session, I used the same setting as the previous session but had the problem occur 2 more times. Again, I watched the speedo climb to 100 mph... and bam.. power cut.

After the last session was over, the only difference i could see between the 4 sessions was that sometimes I would come out of the last corner with my foot on the floor, and sometimes I would come out pushing a little less. The power cuts tended to happen when my foot was on the floor.

I'd appreciate insight from anyone who may have experienced this as well. Everyone at the track seemed to be perplexed why this was happening. The car did not give me any lights on the dash and is running fine otherwise.

Thanks for reading. Average MPG was 8.1 for the day. Ha.

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Last edited by 06AWD; 10-02-2012 at 11:42 AM.
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Old 10-02-2012, 11:17 AM   #2
WRXt4cy
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I did my first track day a few weeks ago and didn't have any issues like this. I kept the TC/VSC completely off (orange). To my knowledge, this doesn't disable ABS. I don't think ABS can be disabled.

The only time I've seen other Subarus have issues like this on the track, it was either caused by overboosting and hitting fuel cut or by not having enough fuel in the tank and starving the pump because the fuel moves to the back of the tank under a long stretch of acceleration.

Since you are stage 1, I highly doubt it was boost fuel cut. How much fuel did you keep in the tank?
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Old 10-02-2012, 11:20 AM   #3
the suicidal eggroll
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How much gas was in the tank?

I would also pull that drop-in filter back out. It doesn't help you in any way, and many tuners have shown that even drop-in panel filters will screw up the AFR on the newer models.
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Old 10-02-2012, 11:33 AM   #4
06AWD
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Thanks for the replies. Fuel was added with about 1/3 left in the tank, in between the 2nd and 3rd runs.
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Old 10-02-2012, 11:36 AM   #5
Mike Costin
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Fuel cut from overboosting feels more violent than the onset of low tank level, at least when I've experienced it. Should be hiccuping a bit before it starts bucking. ECU fuel cut would have triggered a blinking CEL for overboost condition. Same question about fuel level.

Also, wet track? I don't have TC on my Scooby, hoe does the control deal with an inside wheel spinning at speed?
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Old 10-02-2012, 11:38 AM   #6
06AWD
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Quote:
Originally Posted by Mike Costin View Post
Also, wet track? I don't have TC on my Scooby, hoe does the control deal with an inside wheel spinning?
Track was slightly damp in the morning and dry for the rest of the day. The temp outside was around 71 degrees. Not sure exactly how the TC functions.


Quote:
Originally Posted by Mike Costin View Post
ECU fuel cut would have triggered a blinking CEL for overboost condition. Same question about fuel level.
Yeah. I figured a CEL would have popped up after I felt it, but it didn't. During other laps, I was able to pull out of the last corner and touch 110 in 5th with no problems. I worked the car pretty tough the entire day, keeping the revs around 5-6K, mainly in 3rd gear for the majority of the track with the exception of the straight where I got into 5th. And as I said, it ran perfectly fine the rest of the day. I drove the car home over 2 hours and to work this morning. Still feels fine. No CEL's.

Last edited by 06AWD; 10-02-2012 at 11:43 AM.
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Old 10-02-2012, 12:12 PM   #7
WRXt4cy
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Quote:
Originally Posted by 06AWD View Post
Thanks for the replies. Fuel was added with about 1/3 left in the tank, in between the 2nd and 3rd runs.
I'm thinking it could have been fuel starvation. I've seen it happen as low as 1/3 tank because the fuel pick up is in the front of the tank.

I try to not to let my fuel level get too much below one two ticks below half.

The people I've talked to that experienced fuel starvation described it as being pretty quick and violent much like boost fuel cut.
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Old 10-02-2012, 12:49 PM   #8
06AWD
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Not that I know what I'm talking about, but wouldn't fuel cut be more like a stuttering before the actual cut? I assume fuel would gradually be starved from the engine as I accelerate, and then slop back into it as I brake, right? The issue I had was an immediate cut of power, as if someone tuned off a light switch. Then the power came back at the next corner, as if the switch was turned back on.
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Old 10-02-2012, 01:01 PM   #9
the suicidal eggroll
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Quote:
Originally Posted by 06AWD View Post
Not that I know what I'm talking about, but wouldn't fuel cut be more like a stuttering before the actual cut? I assume fuel would gradually be starved from the engine as I accelerate, and then slop back into it as I brake, right?
The fuel will slosh to the side and starve the pickup. When it does, fuel pressure will instantly drop to 0 and combustion will cease.

It will only stutter if the pickup still has access to some fuel so it intermittently pressurizes the system.
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Old 10-02-2012, 01:08 PM   #10
06AWD
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I wish I had connected my Cobb AP during the runs, or had at least one helpful gauge in the car to try and help diagnose the problem. Oh well. I'll consider that next time I go to the track to be more prepared, including some proper brake pads and tires because I was slipping all over the place towards the end of the day.

Thanks again for the replies.
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Old 10-02-2012, 01:20 PM   #11
WRXt4cy
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Quote:
Originally Posted by the suicidal eggroll View Post
The fuel will slosh to the side and starve the pickup. When it does, fuel pressure will instantly drop to 0 and combustion will cease.

It will only stutter if the pickup still has access to some fuel so it intermittently pressurizes the system.
Bingo


An AP wouldn't have anything helpful to diagnose fuel starvation that I can think of.

A wideband o2 gauge might help and is a pretty good gauge to have if you track your car but again, but like eggroll said, it happens pretty dramatically... which is good because you don't spend much time in a lean condition which would be very bad at WOT.

I would suggest keeping more fuel in the tank for track days and see if you avoid this issue that way.
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Old 10-02-2012, 01:29 PM   #12
06AWD
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Cool, thank you. I'll keep an eye on that next time I'm at the track, which may be sooner than later. It's addictive!
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