Forgive me if this is a stupid question but...

Our car has a rev limiter to protect the engine from over-revving. However, I have read that it does not work if you miss a shift (i.e. intend to downshift to 4th and end up in 2nd instead). I understand this is a leading cause of blown S2000 motors. My question is, why doesn't it work? I assume there is software that says, "RPM = 9,000 - cut off fuel". Why doesn't it work on a downshift?

 

All of the software in the world can't stop the engine from going into the red if the car is going fast enough.

At xx speed in y gear, the rpm will be zzzz.

It's mechanical. You'll probably notice the tires chirping trying to slow the car down.

 

I was told by my mechanic and Honda that it doesn't engage quick enough. There was a Si Civic in service because that is exactly what he did (miss a down shift) and blammo, blown engine, torn up front fender, blown front tire and two bent wheels- all the last damage due to the following accident after the engine let go. The rev limiter works, but can't engage quick enough on a missed down shift.

 

It seems that if you're going fast enough, say 75 mph, you want to downshift into 4th and hit 2nd, 2nd gear at 75 mph = 10,218 RPM with or without gas going to the engine. Thus, the rev limiter can't limit the RPM by cutting off the fuel.

 

Let's try this...

Physically push a boulder up a hill. Now, let's assume that your legs can only move sooo fast while pushing that boulder up the hill...cuz that's *it* for you. The shuffling of your legs might be considered your "rev limiter"...your body is designed in such a way as to be able to "max out" at pushing that boulder up that hill at so much of a speed.

Now...with the boulder at the top of the hill...send it back down and try to stop it with your legs. You will "run like crazy", trip, fall and get squished.

Okay...kinda a goofy analogy, but maybe there's some light of truth here. Fun imagery anyway.

 

I knew the answer. I was just testing you guys. . j/k. Thanks for the explanation.

 

[QUOTE]Originally posted by w1ngman
[B]Let's try this...

Physically push a boulder up a hill.

 

The rev limiter, which actually kicks in at 9200 rpm if I'm not mistaken, only helps you during acceleration. It prevents you from over-revving the engine if you forget to shift or don't shift soon enough. However, the rev limiter can't physically help you if you miss a downshift because the rev limiter can only cut the flow of fuel to the engine. When you miss a downshift, say drop it to 2nd instead of 4th, the engine will spin above the redline due to the speed that the vehicle is travelling. Cutting off the fuel won't slow the engine very quickly, only disengaging the engine from the drivetrain will.

This happened to me once. I missed a downshift from 6th to 4th and dropped it into 2nd at 70-75 mph. For a split second (which seemed like an eternity when it happened), the engine over-revved, the rear tires broke loose, the tach flashed (twice), and the rev limiter tried to do its job. I immediately slammed the clutch to the floor and let of the gas at the same time until my speed was well below 60 mph. I was lucky that nothing broke.

 

You could be going 90 mph per hour on the freeway turn off your car pull the keys from the ignition and place them on the passenger seat, depress clutch put the car in second and let off and it will blow your motor, the momentum of the car in combination with the lower gear will spin the motor too fast and blow it.

 

[QUOTE]Originally posted by gomarlins3
[B]I was told by my mechanic and Honda that it doesn't engage quick enough.