Hit the Rev Limiter

[gRed04]

Originally Posted by MNBoxster

I agree with you re. Upshift vs. Downshift. But want to point out that upon downshifting improperly, it is possible to exceed Red Line RPMs, there is no Mechanical safeguard in place to prevent this from happening.

Please help uneducated here. I don't understand why downshifting allows you to over rev? I thought the limiter was rpm dependent.

[MNBoxster]

Originally Posted by gRed04

Please help uneducated here. I don't understand why downshifting allows you to over rev? I thought the limiter was rpm dependent.

Hi,

A Rev Limiter prevents the Engine from making too high revs, but it doesn't prevent it from actually spinning that high.

A Rev Limiter usually works in one of two ways. It can completely cut out the Ignition Spark and/or Fuel Pump (this is referred to as a Hard Limiter) and the engine suddenly stops making power and subsequently very quickly drops below Max RPM). Or, the ECU (DME in Porsche Parlence) will selectively cutout every other cylinder (referred to as a Soft Limiter) which again, drops the RPMs back into the safe range.

But, if you are downshifting, the speed of the Rear Wheels can be exceeding the RPM range for the Engine. When you let out the clutch, you can cause the Crankshaft to turn at the same speed as the Transmission's Primary Shaft. Since the Engine itself isn't doing this, cutting the Spark and Fuel will have no effect, and the Engine can spin higher than Max allowable RPMs. But, you'll probably tear up the Clutch, Throwout Bearing and such, so you'll have a plateful of issues to deal with. Hope this helps...

Happy Motoring!...Jim'99