Pelican Parts Forums - This is what piston looks like from an overrev

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- - This is what piston looks like from an overrev (http://forums.pelicanparts.com/911-engine-rebuilding-forum/977014-what-piston-looks-like-overrev.html)

Quote:

Originally Posted by Trakrat (Post 9816793)

It's my understanding that your rev limiter just keeps your throttle from accelerating past redline.

There is no device that prevents you from manually shifting the gears to whatever gear you want that I'm aware of.
I would guess that you would have to have some kind of electronic controlled transmission to keep you from moving that shift lever.

Correct. RPM limiter cuts off ignition at a set rpm. You can still blow a shift. BUT...

Lets make it simple and exaggerate the situation. Lets take two identical Porsche motors, both with redline at 6500-6700 rpm, and they grenade at 9000 rpm.

You shift in a HPDE at 6500. You miss a shift, based on fixed gear ratios your RPM jumps up 3000 rpm if you go 3-2 instead of 3-4. You just hit 9500 RPM. Kiss your motor good bye. These numbers are for example only, I have no idea what RPM valves hit cylinders...

Now, lets say you did that exact same shift at 6000 rpm and missed. Motor jumps up the same amount of RPM (its a fixed ratio set by the trans gears)... you go 6000 to 9000 rpm (or less).

See the difference? The lower shift point buys you 700 revs in insurance.

The rev-limiter doesn't prevent an over rev from a missed shift. But the shift point determines how high an RPM you achieve IF you over rev.

Imagine doing every shift at 3000 RPM. You could miss a shift each and every time, and nothing would happen. Max rpm= 6000... The lower you set your shift point via the rev limiter, the more room for error you have. Of course, the lower the rpm, the slower the car. For me, shifting 700 rpm sooner has no drawbacks...

Bo

Quote:

Originally Posted by bpu699 (Post 9816825)

Now, lets say you did that exact same shift at 6000 rpm and missed. Motor jumps up the same amount of RPM (its a fixed ratio set by the trans gears)... you go 6000 to 9000 rpm (or less).

See the difference? The lower shift point buys you 700 revs in insurance.

The rev-limiter doesn't prevent an over rev from a missed shift. But the shift point determines how high an RPM you achieve IF you over rev.

Bo

Well... I don't think anyone wants to lower their redline... most everyone I talk to are finding ways to raise their redline.

Buuuuuttt.... one of the things I am going to look at it with my rebuild is to find a setup that reaches power peak sooner and faster. I don't want to have to constantly be at 5000+ rpms to be hitting peak power.
I plan on driving my car on the street and racing it on short tracks and autocross so I need the power curve to happen sooner.

If my power curve happens earlier in the RPM range, then I won't need to constantly wind out my gears to keep peak HP

Quote:

Buuuuuttt.... one of the things I am going to look at it with my rebuild is to find a setup that reaches power peak sooner and faster. I don't want to have to constantly be at 5000+ rpms to be hitting peak power.

Advance your cam timing to reach peak power at lower RPM. The best way to find the sweet spot takes a few iterations and some seat time on a dyno.