After reading some stories about overrevving, mechanical overreving and money shift I have a question I cannot find an answer to for the moment...

Let's assume we have a missed downshift (from 6th to 3rd instead of 6th to 5th for example) while trying to brake on the engine instead of the brakes themselves and you are not on the throttle.

Will the rev limiter prevent the engine from overrevving? I assume it won't because the wheels are turning fast, you engage a small gear and the engine will be forced to pick up much revs... Eventually the rear tires will spin a bit as a result I presume...

But will the rev limiter light on the dashboard at 9000rpm start blinking if it is overrevving this way?

And if you did not hit the limiter but instead the revs only went up very quickly this way to lets say 8500rpm, can this do any harm?

Thx for any reply's on yet another overrev question!

 

The rev limiter will not prevent a mechanical overrev from going into too low of a gear at a specific speed, say 2nd gear at 90mph. The rev limiter light should start blinking in this situation, but it's not maintaining the rev limiter rpm as it can't immediately slow the car down to the specific speed that translates into the rev limit at that particular gear.

I did ask, in a previous thread, about whether or not you would feel the engine cutting in and out due to the fuel cutoff if you were WAY over the rev limiter. A good friend told me no, that the only way you would feel the engine cutting in and out due to the fuel cutoff is if you were on the rev limiter. No one here specifically confirmed that. The reason I asked this question was if my friend is accurate, then I didn't overrev the engine to the extent that I thought I did in my previous thread because I felt the engine cutting in and out as it bouced back and forth off the rev limiter. (8200rpm versus 10,000rpm+ that I originally thought).

 

the answer to your question about feeling it is this.

if you throw the car into 2nd at 90mph, you will simply feel the car slowing down at a dramatic rate.

your friend made it sound like if you were at the limiter, you would feel the limiter kick in. however, he's wrong. the only time you'll actually FEEL the limiter kick in is if you are accelerating in a gear and hit the limiter. your acceleration would suddenly be stopped and it would feel like you hit a wall.

this is not the case on an overrev. if you let the clutch out into a mechanical overrev, the limiter would not kick in unless you actually had your foot on the gas. even then, since you were shifting gears and now are way overreved, you will feel as if the car is simply slowing down, no matter how far you are over the rev limiter or not.

the only way to truly know what revs you hit is to use a gear calculator using your speed and the gear you went into.

 

If you're in mechanically overrev, the limiter isn't going to be cutting gas on and off - it's going to be off, period. You won't feel anything, except some warmth in your pants.

That doesn't mean the tach won't flash. In fact, if it doesn't, you probably haven't actually overrevved (not sure here, as I don't know exactly how responsive the tach is).

 

yes the tach will flash if you overrev

 

Thx 4 the replies, guys! Just wanting to be sure that a mechanical overrev would be signalled by the rev limiter flashing. Never thought of it really, but it is a nasty thing to happen as nothing will prevent the engine going F1 rpm

When nearly mechanical overrevving this way, lets say from a steady 2500 rpm right up to 8500 rpm, would the sudden increase in engine rpm be bad for the engine or any other parts? Or does a big rev in neutral or a WOT start will give an even more drastic increase in rpm?

 

Speaking from experience?

 

Not really, most of the wear going on your synchros. and since you aren't going over 9000 it's not a mechanical overrev. the danger of a mechanical overrev is valve to piston contact, since the valves can't keep up with the speed. otherwise if you are going in say 6th gear at 45mph and drop to 2nd and go, that's not as bad, and really puts most of the wear on your synchros, but in no way should it be damaging if done correctly (ie, you floor it to match the rev/speed)

 

 

The tach WILL blink if you mechanically overrev the engine.

The tach's 'redline' isnt there sensing if your foot is on the gas or not.
(inducing the fuel cutoff) It is simply programmed to blink if the revs hit the max on your tach.

In the case of an overrev, the tachometers revs will max out, and therefor it will blink at you.

However, I sure hope you arnt paying attention to the blinky lights if you hit second at 90MPH.

EDIT> And to answer the other question: No, a downshift resulting in high RPM (not overrev) will not hurt the engine.

But, a 2 to 1, or 3 to 2 downshift on an AP1 resulting in 8500+ RPM isnt worth the time. The car would have most likely been faster staying in the current gear.