red line rev limiter

[david911]

In my autocross experiences, I have been too chicken to actually hit the rev limiter, so today I thought I'd try on the on ramp, just to see what it felt like.

I got off the gas once the tach showed 6800 (red line is 6300)!

What do you guys think is up with this? Could someone have eliminated the limiter (scary thought)? Could the tach be off?


David

ps I have a 1988 coup

[marcesq]

David:

It is possible that the car has been chipped to exceed the stock redline, or it may be tach error.

If you want to make sure, remove the DME and look at the tabs on the bottom to see it they have been opened and if so chances are it has been chipped.

FWIW

[Steve W]

Stock limit is set at 6540. Have you chipped your box? The rev limit is programmed in the Motronic chip at location 130h, at which point, power to the fuel pump is cut off. Some chip companies raise the rev limiter to 6840, 7040, and I've seen as high as 7320.

[david911]

I will check the DME box next time I have the seat out.

How do they figure out what a safe redline is?

David

[marcesq]

David:

Chip companies don't care because there is a disclaimer that goes along with the product.

Porsche theoretically has knowledge of the product, and it's limits over time.

There have been documented cases of rod bolts breaking on our engines, and the appearent reason is continued over reving. At the stock redline the engine is within "it safe zone"

Chipped DMEs, amoung other things, specifically raise the redline. The 3.2 can probably take a higher redline than established by Porsche, but IMHO it is there for a reason. There will be many people that will disagree with me, but the alternitive is not pretty.

Chipping will provide safe additional rpms when a 3.2 recieves modifications like, ARP rod bolts, cams, stronger valve springs, and the other goodies that go along with a top end job.

My 1 cent.

[Steve W]

OK, marc just corrected me - thanks The rpm limiter does not cut off the fuel pump, but cuts out the fuel injectors once every other cycle, effectively cutting fuel flow in half, and cycles at the limit in what feels like a surge, and deceleration with a +/- 80 rpm window.

[ChrisBennet]

Originally posted by Steve W
OK, marc just corrected me - thanks The rpm limiter does not cut off the fuel pump, but cuts out the fuel injectors once every other cycle, effectively cutting fuel flow in half, and cycles at the limit in what feels like a surge, and deceleration with a +/- 80 rpm window.

Steve & Marc,
I'm curious, if the fuel is stopped but the same amount of air is going in the engine, a lean fuel mixture would result. Does the DME stop the spark at the same time?
TIA,
Chris

[A Quiet Boom]

Chris, yes it can cause a dangerous lean condition which is why companiess like MSD use "Soft" rev-limiting to pull sparks out in order and soften the strain on the engine. Drag-racers launch on a starting-line rev-limiter called a "stutter-box" which is in effect a preset launch rpm and they are against that limiter with full-throttle! No engine damage! I've seen aftermarket supercharged cars burn pistons by leaning out against the factory rev-limiter which doesn't happen with a "Soft" rev-limiter. Example I leave the line with my 410 motor at 6000rpms full-throttle against my stutter-box. I like them so much I put an MSD 6al in the P-car and if I miss an upshift or just overrev I get that "stutter" to remind me I'm an idiot