GM cars, for example.

Flat foot one in neutral and see what it does. Some stopped revving around 3500, I believe. Come to think of it, I never tried it in park, just neutral. If I were designing it the limiter would be in both park and neutral, but they didn't ask me.

Hey, I'm getting old. Maybe I'm wrong....

I was once.

 

I heard a while ago that when you free rev the engine that the rod pulls the piston down, instead of the piston being forced into the rod. That change in load for the piston can cause wear on the backside of the rod and crank.

Didn't say why it would break things, just what happens and its bad.

 

The shop that built my engine had no concerns pushing the engine to the rev limit unloaded, to demonstrate the rev limiter (7,200rpm) settings in the MoTeC ECU. It offers choice of - cut fuel random cylinders, cut ignition random cylinders, combination fuel/ignition, and all driven by a "seed" for a random number generator. The sound the engine makes hitting the rev limiter varies unbelievably depending on which "seed" number you enter. I asked them if there was any risk in doing that, and they said no. It was enough to make me cringe, but they weren't concerned at all. Engine is still going strong 7 years later.

 

The rod pulls the piston down unloaded, under high vacuum conditions, every other stroke. This is called the intake stroke, it is not bad - it's good - because if there were no intake stroke our cars would not be very fun to drive at all.

So how is free revving any different?

What are the technical reasons it is "bad"?

Is the idea free revving = bad just an old fart's fable?

 

Thats just what i was told. I've never been able to figure it out either. Doesn't make mechanical sense that it would be bad. I think its more of a fable than anything cause if you wind the crap out of a dying motor, they tend to come apart.

 

I never do it. I may rev the engine, but not to redline. Unless you have something specific you are doing/testing (like the case that Injuneer posted), I can't think of why you would (no, someone revving their engine to redline does not impress me). IMO, not doing it would be better for the engine than doing it.

 

At RPM the tension on the rod gets pretty high, the high vacuum experianced at high load would only serve to increase that tension, so IMO it is harder on at least some parts. Certainly many engines can handle this extra load, like Fred's for instance, but because of this I would be leary of doing it with a stock shortblock.

Without load I suspect the interia of just the engine would serve as enough of a flywheel to smooth out any valvetrain or spark issues so I don't think it will help you diagnose them.

 

Centrifugal force. Load on the engine counteracts on piston reciprocation which keeps the piston from blasting out of the head and/or stretching the rod.

 

Beware of ficticious forces. *

Correct, under load the compression stroke helps the rod in it's job decelerating the piston, and the power stroke helps accelerate the piston. So, under load the rod and crank see lower inertia forces during half the revolutions than they do at the same rpm under no/light load.






* "Centrifugal force" is a pet peeve of mine.

 

Just rev the frigging thing to the moon and tell us what happens.You will either be lucky or not.

 

high rpms and little to no load is a perfect recipe for spun bearings. its also why people blow their motors doing burnouts.

 

centrifugal force is not a real force, it is the name for the perceived force felt by rotation, high school physis.
http://en.wikipedia.org/wiki/Centrifugal_force

 

The greatest tension load on a rod is during the intake stroke, so how will free revving increase it?

 

Can you give me a technical explaination of why this would be true?

 

According to who/what/when/why/where/how?

What he said...