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I need to design an automatic shifting system, for a FSAE car. Clutch and shifts are electronically engaged and disengaged using a Pi Innovo engine control unit (ECU).

The issue I am worried about is downshifts, I would like to rev-match, for quick, clean downshifts and clutch life. There is no possibility of my team using an electronic throttle, therefore I can not "blip" the throttle electronically.

My idea is when a downshift is needed, I send a signal to FULLY disengage the clutch electronically, then when the engine RPM reaches what is consider matched, I send a signal to shift.

Would allowing my engine to rev while the clutch is fully disengaged damage the clutch? Also, how is important is rev-matching? We already replace our clutch after a week of racing, would not rev-matching be that much of an issue?

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  • $\begingroup$ Maybe my lack of knowledge on this area makes this comment not helpful at all, but I'd like to share my thoughts / understand the question better. Is your car that much different from a common car? Reving while having the clutch disengaged shouldn't harm it as far as I know. What about the use of a synchronization mesh? All in all you would extend the lifetime of your clutch with your undertaking, not sure if this is worth it at all if you replace the clutch every week anyway. $\endgroup$
    – idkfa
    Jul 22 '15 at 8:55
  • $\begingroup$ Body wise, much different, but the engine we use is a honda CBR600rr motor. A standard motorcycle engine. $\endgroup$
    – Adam
    Jul 22 '15 at 13:13
  • $\begingroup$ So, yes, everything that relates to this problem I'm having relates to a normal car. $\endgroup$
    – Adam
    Jul 22 '15 at 19:45
  • $\begingroup$ What fun!!! :-). Driving "normal" cars experience only. At one stage I got reasonably good at clutchless up and down changes with a manual box using "double declutching" but without a clutch :-). (I also had a MK1 MR2 but it was auto :-( ). | If you fully declutch and rematch when revs are "about right - probably fully home at just under correct revs, so that there is minimal jerk or adjustment from the system I'd expect it to have low impact on the clutch. Properly done I'd expect to get far more than a week from the clutch so your weekly clutch changes should easily accommodate it. ... $\endgroup$ Jul 23 '15 at 10:07
  • $\begingroup$ ... Shifter response time will matter and you probably have to lead the expcted engagement point - but sounds easy enough [tm] $\endgroup$ Jul 23 '15 at 10:08
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Overview:
I wouldn't be worrying about clutch life. From my experience with FSAE events they're not long enough to actually wear down a clutch.

However(!!):
One of the main reason racers actually blip the throttle/rev match is to avoid compression lockup. This is when the engine braking caused by downshifting (and resultantly having to bring the engine up to a higher RPM to match the wheel speed) causes your wheels start rotating slower which causes the car to oversteer.

As per your other questions:
Allowing the engine to rev while the clutch is disengaged will not damage the clutch. Your proposed solution of allowing the engine to decelerate freely and only engaging the clutch at a preset level could very well work from a clutch-life point of view but it will probably result in slow downchanges with an unpredictable application of power/traction (suddenly the engine will be connected to the rear wheels when a split second before they had just been free-wheeling). This would be pretty disastrous/hard-to-compensate-for in corners.

Alternative solutions:
Usually when these types of systems (sequential dog-boxes with some type of smart shifting mechanism but no throttle control) are employed the driver will manually downshift (such as in V8 race cars in Australia) allowing them to rev match and whatnot.

Or you could look at installing a slipper clutch on your engine/transmission which would then completely remove the need to rev-match at all and make the downshifting system just as easy as the upshifting one (without introducing additional clutch-wear or compression lockup). This would be by far the easiest solution (but unfortunately quite costly).

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