Weird high RPM cut-out, Pertronix or MSD coil at fault?

[Bear911]

I have a retrospective tech question. My 76 with OEM blue Permatune (yes it came out that way!) was cutting out hard at 6000 RPM. I took the car to a respected Porsche mechanic who removed the pertronix I and MSD high vibration blaster coil I had sourced from Pelican and fitted. I thought I was doing preventative maintenance while in there, pfft... He replaced them with the stock points and a new OEM Bosch coil. Problem solved and the car pulls as hard as ever.

So my question is, has anyone experienced either the pertronix or an msd coil to cause such a high RPM cut? The $$$ have been spent and the car is now fixed, but I'm still confused why my setup didn't work.

[dicklague]

Are you still using the Permatune?

By removing the Pertronix and the coil it is tough to say which of the 2 where defective. I am sure it was not both. I hate trouble shooting where you replace a bunch of parts.

Which one was bad?

I would suspect the coil.

did he tell how he troubleshot the Pertronix? You could hook a dwell meter or a scope, or a frequency meter to see what the Pertronix output looks like. That is what I would do and rev the engine to see what happens.

I would bet on the coil being defective.

If it were me I would bench test the components with a square wave generator and push it to 5000 and then slowly build up speed to duplicate the problem. Once you have duplicated it then put in a different coil and see what happens.

I have run Pertonix in my 911 for years and love it. If your mechanic can't tell you why he think it is defective, I would put it back in.

[Uncle Enzo]

Is it possible that at 6k the built in rev limiter on the button (assuming you have one) was kicking out and shutting it down?

[dicklague]

Originally Posted by Uncle Enzo

Is it possible that at 6k the built in rev limiter on the button (assuming you have one) was kicking out and shutting it down?

was the rotor replaced?

[gsjohnson]

I would suspicion the rotor as well. They will melt down with an MSD when there is too much ignition resistance. That's a whole 'nother chase...

[supdaf]

I had a pertronix II that did not have a hard cut-out at 6000 but did have a miss at that rpm. I also had a couple of other issues that went away with re-installing the points. No other components were changed and the car has the original bosch cdi unit.

My vote is the pertronix.

[Bear911]

Thanks for the inputs. The matter has definitely been confused with multiple parts. I bought the MSD coil as the old Bosch coil's one pole snapped off when I removed it to test it and see if it is still within spec, doh! Should just have left it alone. The pertronix was done before that and ran fine but I didn't try take the car past 6k. The issue only presented after I had done both and attempted a 'spirited drive.' Bugger. I guess that is what they call fiddle fiddle fu..

The rotor is newish and is still working fine currently. The cutout was very hard and definitely not like a rev limiter. I guess I really want to blame the pertronix, but perhaps the old permatune only wants the exact bits it's been used to for almost 40 years. Or maybe the distributor's retard wasn't happy at max rpm with my installation of the module.

I'll hang on to the MSD and pertronix for one day when I feel brave again. Square wave generators etc are beyond my level of expertise

Thanks again.

[T77911S]

i think the stock points system is great. its reliable

[Bob Kontak]

I do remember my secondary wire where it came out of the coil would arc to the small primary terminals on the side of the coil at higher RPM's. That was with permatune box but other components were Bosch and Beru.

Can't remember why this happened but I did have similar symptoms to you. May be as simple as your fat plug wire was not seated all the way.

[spuggy]

Originally Posted by Bear911

The cutout was very hard and definitely not like a rev limiter.

Not like a "soft", electronic rev limiter? Because the mechanical rotor rev limiter is VERY hard - on my 930, it cuts ignition abruptly and completely, so it's a "steering wheel in the face" event, followed immediately by very loud backfires and huge flameballs out the exhaust as it lights off the unburnt fuel when the ignition comes back on again with the RPM drop...

I thought my 930 was running fine; I don't often hit the rev limiter, but I had a few times in the weeks running up to going to the dyno to fine-tune something. I just scolded myself for getting sloppy/not paying attention - 400HP 930 on full chat with a 5-speed in a light car doesn't take long to hit red-line in almost any gear..

On the dyno, car had a hard cutout - almost identical to the mech rev limiter - @ 6,000 RPM. But my mechanical rotor limiter (6 pin Bosch from a '77 does not have a soft limiter like the 930 or SC ones do) doesn't kick in until well over 7,000.

When examined, the Bosch coil was weeping dialectic. Replaced, car would rev freely over 6,000 RPM again. Not that there's any good reason to rev almost any 930 that high, other than the noise it makes. Which is truly glorious.

So I think it could have been the coil. They don't have to leak to break down internally.

[Bear911]

Thanks Bob and Spuggy. I did actually replace the coil plug wire as well, without change. Interesting that the mechanical limiter is that hard on the 930. The event you describe is exactly what it felt like. I almost needed a new pair of shorts! Anyway, the rotor that was present while the issue existed is still the car now that it is running properly. I have no intention trying to take it past redline now to experience what the rotor limiter feels like.

If no one has ever experienced a new MSD coil giving trouble at high RPM with a permatune box, I have to assume that it had something to do with pertronix installation. Probably not the unit itself, but something I did maybe. High vibration can do funny things to old wiring.

Oh and I also like the fact that points won't potentially fry themselves if the ignition accidentally remains on without the engine running. So yes, +1 on points I guess.

[wjfk32]

INMOP- The most cost efficient/reliable way is to go with an MSD6AL-& MSD EPOXY-BLACK COIL. And never look back. Its a hit or miss with permatune in 911/930 TURBO/COMMUNITY

[wjfk32]

[timmy2]

Originally Posted by Bear911

Probably not the unit itself, but something I did maybe. High vibration can do funny things to old wiring.

38 year old engine and ignition wiring can be troublesome.

If you want a new engine and ignition harness that is pre wired to make the Pertronix a plug and play option, I have built a few of them configured in that manner.

You can go back to points at any time as well.