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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?
@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:
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" :-)
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?
I wouldn't pay too much attention to the answers above, here's my thought process.
The third gen Rx7's are for the most part turbocharged, and turbocharged rx7's will create a lot of backfire heat when thrown into neutral. The waste gate maybe hasn't activated and released any heat.
Since engine rev is high- the motor no longer uses the turbo to exhaust and vent the heat produced so it creates backfire- and this heat will heat is and can be trapped in between the turbo downpipe exhaust and the catalytic converter and can maybe cause the cat. converter to melt. Now, the waste gate is suppose to do that job- but in reality it doesn't and the car can overheat due to exhaust.
Rx7's produce a lot of energy already, but when turbocharged it's next level heat and therefore a lot of heat. The oil flow is certainly an issue but this model has an electronic oil-meetering pump that circulates oil throughout injectors and inside rotor housing so without an issue- making this an exhaust issue not oil issue. The issue isn't oil in this car, it's too much trapped exhaust heat not ventilating out of the catalytic converter.
