RPMulli

who let this guy into the thread?

Torontoworker

Then there are the guy's going up an on ramp at 45mph in 996/997 and everyone backed up behind him in Civic's and Yoda's gonin, 'Who allowed this guy a Porsche - he should have bought a smart car...'

Edgy01

Keep in mind that peak power is reached before you get above 6500 rpm, so to push the engine up there is actually not doing to anything as far as power production is concerned. But when you do run it up higher, it helps you to stay at a higher level of rpms after the shift--the engine would not drop as much as it would if you shifted early of course and wound up with the engine down at 3500 or so--you want the engine around 4500 after the shift to maintain power and torque.

The problem with these engines is that they are very free revving engines. When you shift at 7200 rpm the momentum of all that engine mass causes the engine to actually go to 7500. Not a great thing when your redline is what,--7200 or 7300?

Enjoy! I am not timid, either!

aadrew10

Doesn't revving the engine up higher when shifting only happen if you let off the gas slightly after pushing in the clutch? Momentum cannot make objects accelerate, it keeps them from accelerating.

Ahmet

Technically that's not accurate, the phenomenon of an object in acceleration remaining such even after accelerative force is removed is accurate, though not quite as pronounced as Dan's suggesting.

There's a buffer between fuel cut RPM and the stage 2 over-rev registration in the DME. I want to say the difference is around 100rpm, but I don't recall exactly. In any case, you're not going to log any stage 2 over-revs without mis shifting into the wrong gear (or releasing the clutch early when braking/downshifting on track), and that's really the only way to do mechanical damage to the engine from over revving.

aadrew10

F = m * a. If there's no force, there's no acceleration. I would be interested in seeing the phenomenon you are referring to in legitimate scientific print somewhere if you can find it. Nobody debates Newton's second law....

Ahmet

I may be wrong with that post, and I didn't quite say what I wanted to anyway, so bear with me. Here's what I meant:

When the force is removed, acceleration will slow down, but absolute RPMs will rise somewhat above that point. If we still disagree, I'll try to dig something up. I think the disconnect is caused by the vagueness of the word "acceleration" in this context. That or the fact that it's 3am.

Pac996

I did not say the rev limiter protects the engine during downshifts which explains why it makes no sense to think I said so. Sooooo OK. In case you are saying downshifting into a revlimiter situation is the same as accelerating into one then I think that's wrong mostly because it isn't that way and never will be

RPM limits made easy with red meaning stop when it should be a sign like $16,000 in red is when you run a chance of engine failure. The cams cut along with the springs and valves are going to approach floating and the crank with rod and pistons could take a hike through the sylinder walls. Increasing rpms upwards with a load on the engine will not have a tramatic rpm change compaired to downshifting into a rev limiter state of affairs. There is some major banging going on in the cylinders at high rpms. The downshifting into redline is more of a hit on the engine thus a better chance of blowing a gasket.

BruceP

I think there's a clue in the fact that your ECU stores overrev conditions, and distinguishes between hitting the limiter while accelerating (Type 1) and hitting it because of a downshift, a so-called mechanical overrev (Type 2). When they're adjudicating warranty issues, for example, the first kind is usually overlooked, but the second kind is not.

I would never, ever mechanically overrev an engine. A smooth downshift into the meat of the torque curve on this car shouldn't require risking it. My .02...

Janusz

Interesting discussion...
Let me mention that even if you see 7500RPM on the gauge for a split second (how I did) that does not mean that your engine is revving that high in reality. I am sure gauge reads always somewhat high in a similar way like on my other cars and motorcycles.

On my track bike by the way I can easily change the gearing by changing the sprocket for the very purpose of not having to hit rev limiter on the important turn at the particular track. Can't be done on Porsche so I will see how it goes if I stay in 3-rd for the turn..

aadrew10

The engine accelerating is angular acceleration. Angular acceleration occurs when the engine RPM is increased OR decreased. The car accelerating on a straight road is straight line acceleration. Releasing the gas pedal is effectively removing the force that is acting on the engine to turn it. (expanding gasses in the cylinder). I suspect the observed split second the engine raises in rpm is caused from the load being taken off the engine by pushing in the clutch, and a small amount of unburnt fuel in the cylinders that has just enough energy left to raise the rpms with the clutch in. It may also be because the operator pushed in the clutch before completely letting off the gas.

Try this: get up to 60mph, and some gear that sets the engine at a high rpm. Let off the gas, and let the engine slow the car for 3 seconds. Then quickly push in the clutch. You won't see the engine rpm rise the split second you push in the clutch. It will only drop.

Ahmet

I'm only interested in the theory behind it, I have very little interest in over revving a modern fuel injected engine by hitting redline then pushing the clutch in, as I don't see that as a practical possibility. If I can gather the cognitive ability, I'll send you a PM.

aadrew10

We must be talking about different things. I'm lost.