So difficult a question
 

the gurus on Pelican don't seem to have an answer. So I'm posting it here in the hopes we have people with a deeper understanding of all parts of a Porsche.

http://forums.pelicanparts.com/911-e...ting-79sc.html


Anybody have a donor tach from a 78-9 SC to donate to answering this question. No old wifes tales will do. Want to know, want a schematic, want to know if it can be tuned and changed.

:)

ARF

I can let you borrow one. Specs in the Bentley

There is also a 79SC in Boyds that the gauge can be borrowed from. I haven't removed it yet, but it's on my short list of things to prep for the engine swap.

Wanna be a little more specific.

ARF

Don't want to do anything until I'm sure there is something to do. Opening an instrument ruins the seal and unless you have the proper equipment, you really can't get it back together again properly.

Thank, will keep it in mind.

:)

ARF

Yeah.. This gauge would only be available to borrow in its entirety to test. Not to be opened/mangled as I'll need it for the 3.0 engine.

My 83SC does rev limit by cutting the fuel pump off. Your schematic on pelican looks right. There is a speed limiter relay w/ all the 'brains'. I've never physically located it, but there aren't anywhere near enough connections on the back of the tach for it to be physically located inside.

Find the fuel pump feed wire at the fusebox (16 in your diag) and follow it (eventually disappears down near the steering rack tunnel to the pump)? It has to be one of those black box relays in the front of the car somewhere.

John, the schematic you posted on Pelican is for a speed limiter, not a rev limiter. Looks like it just cuts the fuel pump off at a certain speed (say 130 mph). That's why it takes a signal from the speedo, not the tach. It's designed to stop you from going too fast, but it doesn't give a shit how fast your engine turns. It's not going to shut your engine off at 7000rpm.

I guess you could rev limit by cutting the fuel pump, but that seems like a bad way of doing it - even after you cut the pump off, residual pressure in the lines is going to feed the engine for a while, esp in an injection system like CIS. A rev limiter to work well needs to be pretty instantaneous, and turning off the fuel pump would not be.

MSD boxes have a rev limiter that works by cutting spark (the proper way to do this). If you need a limiter, then get one of those. Tunable and changeable. Not sure if the original CDI boxes on the CIS engines have the same, but if there's going to be a rev limiter anywhere, it'd be there or in the distributor rotor.

I think I remember hearing that the distributor rotor controlled the rev limits.

http://www.opelgt.com/forums/opel-ti...tor-links.html

It's at least true for the older cars. Not sure about the SC though.

An Update:

For the early cars there were two types of ignition rotors available, one with a rev limiter, and one without. The rev limiter protects your engine from overzealous drivers. A small floating weight on the rotor grounds out the coil at the RPM limit preventing valve train and other engine damage. In 1978 and later, rev-limiting is controlled by the CD-box.

http://www.pelicanparts.com/catalog/...BAStun_pg1.htm