Over Rev

Technical and Repair Discussions

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PostPosted: Tue Jul 06, 2021 5:30 pm
I just bought a car and was looking at AIM data. My understanding is that max RPM is 6800. A certain amount of overshoot would be expected. Does anyone know what RPM range would cause a long term issue or failure in the near future? Any symptoms to look for? I don’t have any symptom but just curious what other may have seen in their data before or after an engine failure.
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595
PostPosted: Wed Jul 07, 2021 9:48 am
The rev limiter kicks in at 6,750, but the engine is capable of surviving above that level Rev's over 8,000 could result in bolt stretching at the crank and, at one point, Enterprises had a program to replace them (would require sending back to Enterprises and I don't recall the cost.). If it's really short duration, it may be worth running as-is if the max not much over 8,000 (I've seen up to the mid-8000 range survive).

Over 9,000 is generally fatal or about to be. This is usually caused by a missed up-shift 3rd to 4th, getting 2nd instead.

I would suggest pulling the log data from the ECU using the PE3 software, as it should contain at least one hour of history. Assuming the over-rev happened relatively recently, you should be able to get a better indicator of the peak and duration of any over-revs.

You may want to run a leakdown test, in any event, for any early indicators of a potential future failure.
Bob Breton - SRF 51 - San Francisco Region
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PostPosted: Wed Jul 07, 2021 3:46 pm
" Over 9,000 is generally fatal or about to be. This is usually caused by a missed up-shift 3rd to 4th, getting 2nd instead." Yes, the money shift!!! :shock: Unfortunately I am familiar with that situation.
Jerry Aplass SRF #204
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"Straights are for fast cars. Turns are for fast drivers." - Colin McRae
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PostPosted: Wed Jul 07, 2021 6:29 pm
BIG money (but not enough to justify a sequential gearbox!)
Bob Breton - SRF 51 - San Francisco Region

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PostPosted: Fri Jul 09, 2021 12:50 pm
Thanks for the help. I did have a look at the data. The engine only has 25 hrs on it. I used the AIM software to see the peak RPM in the 84 log entries. There was 1 peak at 8280 rpm. It was literally a spike and didtn't look sustained. The engine starts and runs fine and I ran it at a test day yesterday. It didn't seem low on power compared to other cars on the straits. Ill do the leak down check next.
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PostPosted: Fri Jul 09, 2021 5:42 pm
8,280 sounds survivable. 25 hours is about the break-in time. I still suggest pulling the PE3 logs if the data from the AIM is less than an hour old. Worth getting comfortable with the PE3 software as it can help diagnose issues that won't be recorded on the AIM side (e.g. chasing crank sensor issues...)
Bob Breton - SRF 51 - San Francisco Region

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PostPosted: Fri Jul 09, 2021 7:24 pm
Confession time although I might regret it.

About 30 engine hours ago, I sent my motor back to Enterprises for a bottom end rebuild after I had two overrevs (the infamous 3-2 upshift). The events were to 8,800 RPM, very brief and didn't feel any ill effects afterwards. I sent the motor back for the rebuild on advice and out of an abundance of caution. Enterprises did not find any problems. My con rod bolts were still tight.

This year I made a 5-2 downshift going into South Bend at VIR instead of a 5-4. The motor spun to 9,800 RPM, total duration of 0.9 sec above 6,800. I've chosen to roll the dice and the motor has been running well for 6 1/2 hours over two race weekends since the event. I suspect the motor is a grenade waiting to go off. We'll see.

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