Piston Slap: Less Octane = More CVT?

Steve writes:

Sajeev,

My wife drives a first-generation R50 Mini base model with the dreaded CVT. This is a transmission widely reported (read: complained about on message boards) to not last well beyond 75,000 city miles. Hers is just now clearing 80,000 and it shows no signs of early struggles, even under the hellish torment of stop-and-go traffic in Houston temperatures.

Perhaps coincidentally, my wife has never put premium fuel in this car, despite it being a requirement. Premium fuel would supposedly generate 114 horsepower; without premium fuel, I would guess 7-9% lower, at, say, 105 horsepower. It is a slow car no matter what, but at least it makes up for it in urban maneuverability.

Nevermind what this is gradually doing to her engine (I’ve never tested out whether we’d get better fuel economy and save with premium fuel), is it possible that her derated engine is saving her transmission? Less power through the transmission means less heat, so we can get more miles out of the CVT, right?

Once that CVT goes, it’s crusher time, unless batteries and electric motors suddenly go on a huge sale.

Sajeev answers:

Crusher time? You and I know that Houston has so much wealth/prosperity there’s a market for your MINI. Put it on Craigslist for $1,000 with a hurt gearbox and it’ll be sold in less than a week.

That said, I reckon you’re right about low octane fuel being a (not the) reason why your CVT fares better than average. CVTs, like any moving part in a vehicle’s powertrain, like any fastener installed to a torque specification, is rated to a specific torque limit. If only we knew the MINI CVT torque limit!

Not that the MINI is a torque monster, but if your wife keeps the revs low (read the multiple comments from “simbasat” on this thread), if switching to regular fuel lowers torque from 110 lb·ft to about 10% less (let’s say it’s an even 100), then yes, she’s effectively given the CVT a longer life.

By how long is hard to know. Perhaps I can interest you in a Mercury Grand Marquis while we ponder?

[Image: Shutterstock user Pixel B]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.