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The manual for the 3rd generation Mazda rx7 says not to roll downhill without the engine engaged, because the exhaust can overheat. Unfortunately, the manual does not explain the mechanism by which the overheating occurs, so I have no idea whether it can affect other cars.

What could cause the overheating, and could it be a problem with other turbocharged manual transmission cars? Or just twin-turbo rotaries?

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without the engine engaged Is that exactly what it says? I can understand if it said the transmission but I am not sure what they mean by the engine engaged. – Larry♦ May 6 '11 at 20:33
I haven't owned an fd3s for four years, so I don't remember the exact wording. But since the transmission is what engages the engine to the wheels, just read it however it makes sense to you. – user502 May 7 '11 at 2:33

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@dude's answer reminded me to come back and look at this question. If it hadn't been for the RX-7 I would have argued pretty strongly that all manual cars can coast quite happily downhill in neutral, however I have learned from my mother in law, who has an RX-7, that rotary engines cars have some very odd problems:

  • Running the engine for less than a couple of minutes can flood the engine so badly it needs to be towed for 5 minutes just to clear the engine.
  • Oil flow is so marginal and leaky that bad weather can make the difference between it overheating or just failing to start.

Plus other issues, so I reckon your question is almost the wrong way round. It could be "is the RX7 the only car you can't coast downhill with" :-)

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Those damn apex seals – crasic May 13 '11 at 9:23
Heh, my Toyota MR2 has that flooding issue if you shutdown without having let the engine run for a few minutes! Not just a rotary issue! :-) – Brian Knoblauch Jun 2 '11 at 18:02
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You know, I can understand why they would say this but I'm not sure I could fully describe it in words haha.

I think it has to do with how oil is circulated through the rotary motor. If you're going downhill the oil pick up may not be doing enough to circulate the oil through the engine when you're not in gear. I suppose that if you kept it in gear, the oil pick up would be operating at high rev's since you're cruising downhill anyway.

I'm not really sure how this would affect exhaust temps...perhaps oil starvation is the reason why it does increase the temps?

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Oh, and I doubt that this would apply to anything else but a rotary engine. – Dude318is May 11 '11 at 17:02

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