Ignition issues

[kent olsen]

The Mallory MSD in my 2.7L seems to work fine. I asked the board for comments on my stock coil running the MSD and I'm still not sure how much difference it will really make on a street/autox/track car.

BUT, this weekend I ran over to an autox with the Tampa club BMW/Porcshe. First run was easy to memorise the course. Down to the turn around 2nd gear about 6500 rpm. The return was more open and I hit the rev limiter twice in 2nd.

Then one more spot I hit the rev limiter with a big bang from the engine and black smoke. I put the clutch in and the engine quit.

I coasted back to the pit area and when I didn't find any oil leaks felt relieved. After some trouble shooting I found the rotor had shorted out and possibliy arched to a plug out of sequence.

This was a stock rotor with centrifical limiting. The Mallory MSD also has a rev limiter which was set to the same rpm.

Another Porsche owner (912) had a rotor that he lent me. When I put it in it started up and ran just fine. This 912 rotor didn't have the centrifical limiter, it just looked like a normal rotor.

I ran two more runs keeping the rpms down to around 6000 just to feel for any problems. Everything felt fine but just to be on the safe side, since I had an hours drive home, I left early.

Everything felt fine on the way home and I'm now thinking I'll stick with the rotor without the limiter.

When the car talks to you listen, then question what's she saying.

[RoninLB]

Originally Posted by kent olsen

Everything felt fine on the way home and I'm now thinking I'll stick with the rotor without the limiter.

it's what I only use because of my MSD

[T77911S]

did it mess up the rotor?

i have a 914 rotor in mine. comes in at 5500. i hit it last night and had kinda the same thing. i think it was just unburned fuel backfiring. you know, like you use to do way back when by turning off the ignition of dad's car and then back on so it would backfire

[lucittm]

Kent,
I had a rev limiter in my 2.7 911S and it saved me a few times.

Allow me to propose a theory depending on how long the engine was over-revved. You said you hit the rev limiter (which one MSD or rotor?) at 7000 or 7300 rpm (because they also can be 5800 or 6500, and you said you were OK at 6500).

The first two times you got lucky. The third time when the centrifugal weight over came the spring and touched the grounding link on the rotor (or the MSD limiter kicked in), the spark had no where to go so it jumped across the cap and hit a terminal leading to a plug wire connected to a cylinder that had just completed its intake cycle. What you heard and felt was the charge going off back up through the intake and it shut down the engine.
Alternatively, the rev limiter shorted to ground long enough for a substantial charge of fuel/air to build up in the exhaust system and when it fired, a small fuel-air bomb went off and it shut down the engine.

Did you try to start the engine with your rotor still installed? It might still be good.

If the revs are building slowly, the rev limiter is very momentary. If you really mash it and there is much inertia as the revs climb, the engine can coast (with WOT) for a second or two. It's like turning off the key for a second, then Bang! C'mon I know you did that as a kid, we all did. I split a muffler on my first car that way, it sounded so cool afterward.

Now, just a suggestion...

The way you are driving, I would replace the rotor WITH a 7300 rpm rev limiter rotor. Unless you have beefed up the lower end and installed stronger valves, springs, and pistons; your engine may not like a steady diet of 7500+ rpm with an occasional trip to 8000 rpm. It will be much better to replace a blown intake manifold or muffler that to replace the connecting rod that exited your right rear fender at speed.

I would also test/calibrate your MSD rev limiter set to a much more conservative level like 6000 rpm and go up from there, but not much.

Good Luck,
Mark

[RoninLB]

Originally Posted by T77911S

did it mess up the rotor?

http://forums.pelicanparts.com/porsche-911-technical-forum/213066-rotor-button-melting-why-2.html#post1832405

Rotor button melting!! Why?

Originally Posted by mpdevelopment

An MSD 6 working with an MSD blaster 2 coil will produce about 130ma of peak secondary current. If that supressor resistor in the rotor is 1000 ohm the peak power dissipated in the resistor is 17watts this can build up some heat and the resistor may not be rated for that kind of power. On the bench an MSD 6 working with a blaster HVC II coil will make 400ma secondary current this will heat up a 1000ohm coil wire a very noticable amount.

my non rev limit rotor is Bos-04038

there are pelicanhead pictures of MSD toasting rev limiting rotors but I couldn't find them

[J P Stein]

I have no idea what a Mallory MSD is but an MSD6AL has an adjustable (using a "pill") soft touch rev limiter. I use it set at 7100rpms along with an MSD shift light set at 6800rpms (another "pill"). The soft touch allows one to ride the limiter as long as your guts or common sense allows.....tung,tung,tung...I use it a lot at AX. My rotor is the non rev limiting type. I have never had an issue with it or "popped" the motor. I don't have a tachometer in the car.

BTW, the hi vibration MSD coil is a good thing.

[RoninLB]

[kent olsen]

I have the 7100 rpm rotor for this 2.7 (E) motor. I have frequently hit the rev limiter during an autox but now I don't remember if I have ever done that with the MSD installed. I installed the Mallory MSD 6AL last spring.

Interesting! I think now with the normal rotor I'll take it out and ease up to the 7000 rpm I set in the MSD and see what happens.