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While pretending I was a race car driver this morning, a question occurred to me:

When you engage the clutch quickly while up-shifting at wide open throttle (i.e. accelerating as fast as possible and WOT only when clutch is engaged, not while actually shifting), there is a large speed difference between the clutch and flywheel over a short period of time, especially on new cars plagued with the dreaded rev hang. Some cars cannot drop the engine speed fast enough to adequately shift quickly (like my 2016 Subaru WRX) and will actually cause a bouncing sensation if you try, which I presume to be the clutch bouncing off the flywheel. My mechanical intuition says this is a bad thing to do. On the other hand, you can practically side-step the clutch on my 1994 Mazda Miata because the RPM drops so quickly.

If you were to shift slowly (but still WOT), the clutch is in contact with the flywheel for a greater period of time, grinding away to pull the needle to the correct RPM for the selected gear. However, this long contact time at different speeds also seems like it would cause a great deal of wear.

Question: What is the optimal clutch engagement speed to mitigate wear at wide open throttle on a car with a manual transmission and petrol engine? Could it actually harm the powertrain in any way on newer cars by shifting too quickly?

  • When changing gear you should be able to closely match the engine speed for the next gear selected so that it is not noticeable that you are changing gear. I try to achieve this but miss it sometimes. I do remember being at Monaco and listening to Prost changing as he was going up hill, perfect changes without touching the rev limiter unlike those chasing him... – Solar Mike Jul 7 '17 at 16:35
  • I try my best to closely match the engine speed as well, which often leaves me shifting slowly in the WRX and quickly in the Miata. – McGlothlin Jul 7 '17 at 16:38
  • You state "at wide open throttle" ... this seems not quite right, because most people when shifting release the gas pedal to allow the engine to spin down. Even in racing, if you leave the engine at WOT when you disengage the clutch, your engine will rev sky high, which if no rev limiter is in place, you're going to break something, or at the very least encounter valve float which is never a good thing. – Pᴀᴜʟsᴛᴇʀ2 Jul 7 '17 at 16:38
  • You're right. What I meant was really "wide open throttle before and after shifting gears, but not during". I was trying to convey accelerating as fast as possible, but I couldn't think of how to word it. I'll update the question. – McGlothlin Jul 7 '17 at 16:41
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The only noticeable clutch wear occurs when you're getting the car moving from a stop, where the weight and momentum of the engine is using the clutch to get the weight of the vehicle moving.

When you shift, regardless of the RPM, the clutch only needs to spin the engine up or down to match the vehicle speed. Since the engine internals weigh almost nothing compared to the weight of the vehicle and it's momentum, there really isn't much wear that takes place, unless you leave the clutch half engaged and/or rev the **** out of the engine.

If you're dropping the clutch into 3rd gear at 70 mph, the engine will spin up to speed almost instantly, but you'll be putting a lot of stress on the engine, transmission, and the rest of the drive train unless you rev match the engine. If you're shifting normally (aggressively counts as normal to me) there won't be any 'excessive' wear.

  • This is a good point, I didn't consider inertia of the entire vehicle vs inertia of the engine. Thanks! – McGlothlin Jul 7 '17 at 17:27
  • Thank you for accepting my answer, but you should probably give it a day or two to encourage others to submit better answers. – MooseLucifer Jul 7 '17 at 17:28
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    Would you like me to un-accept? I'm used to Stack Overflow where the question/answer pace is much faster! – McGlothlin Jul 7 '17 at 17:29
  • There aren't always defacto answers here, and people can put a lot of time and research to make sure they have the right info. For a theoretical answer like this it's a little more clear cut, but somebody could still come along with hard evidence of clutch wear rates were they so inclined. I'll leave it up to you, as I do love me some rep points c: – MooseLucifer Jul 7 '17 at 17:36
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    Thank you for being understanding. I'll come back in a few days and if yours is still the best I'd be happy to give it back! – McGlothlin Jul 7 '17 at 17:38

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