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Posted by: 914werke Oct 4 2018, 05:51 PM

how many still run the OE (& expensive!) rev limiting rotor?

Posted by: JeffBowlsby Oct 4 2018, 06:32 PM

Rich, I recall that the service and training manual lists the rev limiting rotor as only for the 1975-76 2.0L 49-state cars.

Posted by: Martin Baker Oct 4 2018, 07:03 PM

I have a 6500 in my 2.0, the engine has JE forged pistons and lighter pins. Works great!! Tested several times, OH WHAT FUN!!! I have several different RPM limits in cars I have parted out over the years...7300 in my 914/6 2.4 S Spec engine...it functions correctly as well! I thought they all came with a 5800 in the 4 cylinder types. I have a pile of the 5800 ones.


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Posted by: 914Sixer Oct 4 2018, 08:01 PM

Personally, I like them. It saved an engine for me one time. I had a throttle cable get hung open when the return spring got bent. Kept the engine and me from going to fast.

Posted by: Krieger Oct 4 2018, 08:27 PM

I have for the last 16 years of autocrossing my four cylinder. I just drive the snot out of it and never look at the tach.

Posted by: ThePaintedMan Oct 5 2018, 02:23 PM

Got one in my track car after I got tired of trying to figure out how to get the MSD to work. Have used it several times and it kicked in perfectly. Though, the motor pretty much falls flat on it's face after 5000 anyways, lol.

Posted by: Dave_Darling Oct 5 2018, 05:46 PM

I'm using it. If I didn't, my wife would zing the motor any time she autoXed the car. She hates the gear shift, so she just leaves it in 2nd and rides the limiter....

--DD

Posted by: Mark Henry Oct 6 2018, 07:14 AM

Using the rev limiter as a shift light is bad practice. Loading and unloading the engine this way is very hard on the rod bearings.

Really the rev limiter is the lesser of two evils and is just a safety device that should be used sparingly.
Most T4 engines the redline is about 400-500 rpm past the peak HP output.

Posted by: Dave_Darling Oct 6 2018, 10:35 AM

Yes, I know all of that. It's also hard on the exhaust because fuel loads up in there and lights off!! But I can't get her to shift the damned thing, so that's what happens. Fortunately she doesn't autoX it often, and doesn't usually run out of 2nd gear when she does, so it's not exactly frequent.

--DD

Posted by: Jamie Oct 6 2018, 01:02 PM

Point of information here! I didn't know there was such a rotor, and I assume the original on my 73 has one? How does a rotor limit RPM at a certain redline?

Posted by: Krieger Oct 6 2018, 01:44 PM

A spring in the rotor stretches and grounds out the spark from the rotor at the appropriate rpm

Posted by: saigon71 Oct 6 2018, 01:59 PM

I still use one.

Posted by: Martin Baker Oct 6 2018, 04:14 PM

To answer the question; yes they are marked. Each one of them that I've ever seen has the RPM limit stamped into the rotor itself. It's not hard to find or hard to read. It's very obvious.

Posted by: Dave_Darling Oct 6 2018, 06:03 PM

Here's a photo of one:




There is a weight mounted to a horizontal slider. A spring holds the weight in toward the center of the rotor, and centrifugal force pulls it out toward the outside. When the slide moves far enough, the metal contact on it touches the path that the spark would take, and instead shorts it to ground. (To the distributor drive shaft, actually.) This means the spark does not get sent out to a plug wire.

Different weights or different springs will give you different RPMs that the spark gets shorted out.

--DD

Posted by: 914werke Oct 7 2018, 11:40 AM

Martin I assume you are referring to a /6 application as I was unaware there was more than one "limit" for /4 cars.
Is it possible to McGyver a /4 rotor with diff springs/weights, any BTDT?

Like many I often use an aftermarket non-rev rotor, but recently tried to apply OE Rev limit rotor when installing a Pertronix unit
For the application you often have to "trim" the bottom of non-OE rotors to get everything to fit just right but that's not possible on the the rev-limit version due to its construction.

Posted by: Martin Baker Oct 7 2018, 04:23 PM

Here it it, it is a 911, 914/6 rotor but it works in the DJet distributor. I was told by a well known 914/4 road racer, who is also the builder of my engine, 6500 RPM was fine, as long as it was not sustained. The man is an expert. I trust him emphatically.

I am not sure how many different RPM limit rotors there are. But I can assure you there is nothing MacGyver about this rotor.

Like I mentioned in my earlier post I have a box full of different ones. You can clearly see the RPM stamp of 6500 on this one.


MB


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Posted by: Dave_Darling Oct 7 2018, 08:51 PM

You can, but why bother when you can buy them in a good variety of limits?

--DD

Posted by: 914werke Oct 8 2018, 09:18 AM

You wouldnt, it didn't occur to me that 911's might use the same (or similar) part & have multiple vers.
Good tip, thanks Martin

Posted by: Martin Baker Oct 8 2018, 09:26 AM

You are very Welcome! I will dig out my box if you like to see what I have. I KNOW I have a few of them.

I just took for granted they would work and threw it in there, you know VW/PORSCHE interchangeability is good very often!

MB

Posted by: IronHillRestorations Oct 8 2018, 09:47 AM

FWIW, both of my '74 1.8's, one purchased in '78 and the other in '79 had this kind of rotor. At 4 and 5 years old (when I got them), I know it doesn't mean they were delivered with them.

Posted by: PlaysWithCars Oct 8 2018, 10:10 PM

My 1974 2.0 had a 5800 limiter in it. Handy at the Autocross for those stretches that the gears aren’t quite tall enough to reach. Just bounce the rev limiter a couple of times then dive into the corner.

I’ve been using these on various cars for more than 20 years and never had a problem until last week. The car was running fine but I picked up a tach bounce at cruise so I assumed it must be time for a tune up. Pulled the cap and found out that the part of the rotor that contained the rev limiter cracked and let the bits from the limiter escape into the distributor. The ground strap scrapped a bunch of material off of the inside of the cap and sent powder all over the inside of the distributor. No harm done. Just had to pull the distributor, clean it out and put a new rotor in there.