Possible over rev with rev limiter?

Tim L · 06-04-2002, 05:27 AM

Last week I was at the track and under full throttle, the transmission popped out of third gear at 5,000 rpm. Is it possible that the stock rev limiter didn't catch the revs before some damage was done?
Should I do a leak down test?
84 3.2

Thanks,

Tim

Joeaksa · 06-04-2002, 05:37 AM

Tim,

Several situations where the cars rev limiter may or will not stop its going over the limit.

1. selecting the wrong gear in a downshift (ie 2nd instead of 4th)
2. a transient where the motor comes out of gear while at full throttle

The limiter is built in the DME computer and shuts off the power to the fuel pump. In both of these cases the motor is not held back by a lack of fuel and could go over the rpm limit.

Should you do a test? How does the motor sound, feel, oil pressure and temp? Usually the damage done in a over-rev is either the rod bolts coming loose (3.2 motors) or the pistons hitting the valves or such.

If your motor runs fine and all else seems Ok then would not worry too much about it. If you are going to race it you might want to get the tranny fixed as this happening on a regular basis will find the weak link in your motor and then you will not have to look for it... it will find you!

Joe

Tim L · 06-04-2002, 06:21 AM

I had the transmission rebuilt by Gary Fairbanks, It's back in and The car runs fine, but when I had the engine out I adjusted the valves and they seemed looser than normal nothing real excessive but I'm thinking I hit some valves, so I guess I should go ahead with a leak down. Does the rev limiter really work by shutting off the fuel pump I wouldn't think that would be quick enough to stop an over rev?

Thanks,

Tim

Jim Sims · 06-04-2002, 08:16 AM

If the inertia of the engine or car is the cause of the overrev (as in the case of popping out of gear or a missed shift) then the only way to prevent it is a fast acting brake in the system. This would likely mean complexity, weight and shock loading of another kind. Jim

Tim L · 06-04-2002, 10:25 AM

If it was engine inertia than the engine would stay at the same rpm. Inertia keep objects at the same speed it takes force to change the speed.

Tim

Joeaksa · 06-04-2002, 10:36 AM

Tim,

The rpm limiter on the DME cars is really the fuel pump. It simply shuts off the fuel supply to the motor and this works fine under normal situations.

The old style limiter was a "finger" on the rotor that would ground out the spark at a certain rpm.

Neither of these will "save" a motor should it come out of gear under acceleration or downshifting incorrectly.

Joe

Leland Pate · 06-04-2002, 10:49 AM

You just sent chills down my spine.

When I over-reved my engine (79 SC) at a hill climb a few weeks ago, the only unusual noise I was hearing afterwards was what sounded to be a loose valve. I kept telling myself that it was my paranoia acting up... so that weekend I re-adjusted the valves (only about 1000 miles since old valve adjustment) and I found one or two that just seemed a little too loose.

.....oh man...

Now I'm concerned again.

Tim L · 06-04-2002, 11:02 AM

Leland,

What caused the over rev situation?

That's what I found. I had about the same miles since the last adjustment but they were mostly track miles. I've been trying to convince myself this is normal and nothing got bent.

Tim

Jim Sims · 06-04-2002, 11:21 AM

Quote:

Originally posted by Tim L
If it was engine inertia than the engine would stay at the same rpm. Inertia keep objects at the same speed it takes force to change the speed.

Tim

I'm using inertia in the sense of inertial energy store like a flywheel. If not from the inertia of the car/engine where else would one get the force to speed up the engine to the over rev state? There is no fuel or in some cases no spark hence no power generation in the engine. The natural state of the rotating engine with the given throttle position without the load from the transmission is the over rev speed. That is the condition its inertia demands; the vehicle loads are the forces pulling down the engine speed. Remove the loads and the inertia of the engine's moving parts takes it back to the over rev speed. Missing a gear is slightly different; in that case the forward momentum of the car (again this is the inertia of the car) is used to accelerate the engine to over rev speed through the large mechanical advantage of too low of a gear.

Tim L · 06-04-2002, 11:33 AM

Jim

Inertia acts on the moving parts of a motor, flywheel included, the same as other moving objects. You can't accelerate the motor (increase rpms) without force, gas, spark, or the force of slowing down the car as in a missed shift. I think the problem is there is enough fuel pressure in the system to accelerate the motor after the load from the car is removed and the rev limiter kicks in.

Tim

Jim Sims · 06-04-2002, 12:01 PM

Yep, you're right. E=I omega squared. Loose the connection to the drive train and there isn't anymore E. Must be the residual fuel as you say. Cutting the spark was better but at the risk of dumping a lot of fuel downstream into a catalytic converter or power turbine? Cheers, Jim

Leland Pate · 06-04-2002, 12:08 PM

I was topping out in 2nd gear comming out of a turn... I went to grab 3rd and somehow ended up in 1st. It only lasted for a split second, but it definately over-reved. My girlfriend caught it on tape... It is just a very loud barking "VroOOOM"...

After the valve adjustment, the slight ticking noise I thought I was hearing was gone. And one of my intake rockers (#3 or #4) was just a tad loose.

Hmmmm... Time for one of those fancy gate shifters...

Superman · 06-04-2002, 01:09 PM

Of course I can't say whether your engines are damaged, but I can suggest you quit worrying about it. IF you think a valve is bent, do compression and leakdown tests. If they were bent, I think your engines would not be running as well as before. As for rod bearings, time will tell. You can 1) Quit worrying and drive the car or 2) tear it down to the rod bolts and rebuild.

There is one thing I recommend you NOT do. Don't worry about it while you're doing nothing but driving the car. That's your worst option. Do nothing, drive the car and worry.

Tim L · 06-05-2002, 06:08 AM

Superman, you sure do take all the fun out of it! I surely will do a compression test and probably a leak down as well. Since my motor was out due to the fact my transmission needed to be rebuilt I didn't think it was possible to do these tests. I'm really not worried, once the tests are dun and I know what's up I'll fix what needs fixing. I'm a little confused on what I should or should not do.

Thanks

Tim