electrical and ignition Ignition Rotor

[FrictionTape]

Took off the distributor cap tonight and found this rather large Ignition distributor rotor. I know absolutely zero about this, but it's obviously not the original. I am replacing the rotor and cap, and I'm wondering if there is any benefit to keeping this or should I just go back to the original.

[visionaut]

yeah, it's likely the original (style-wise anyway) rev-limiter rotor.  you can use it (they used to come in different rpm variants) or the simpler non-limiter style.

[OriginalOwner]

I'm thinking that is a centrifugal rotor, with a spring-resisted radial slider ??  It is supposed to be calibrated to short the ignition at max rpm.  My '74tii had one, and it did just that.

 

I decided many years ago I didn't want to afford it, so I began using the much less expensive rotor for a 320i.

 

Cheers,

 

Carl

[Mike Self]

Yep, that's a rev-limiting rotor--was OEM on my '73, not sure what other years came with 'em, but not your '68.  It used the plain jane rotor (same as the contemporary VW Beetle.)  So long as it's working properly and not shorting out below the car's redline, keep using it.  Won't harm a thing.  

 

mike

Edited June 3, 2015 by mike

[FrictionTape]

Awesome, thanks for the insights!

[Conserv]

Yep, that's a rev-limiting rotor--was OEM on my '73, not sure what other years came with 'em, but not your '68. It used the plain jane rotor (same as the contemporary VW Beetle.) So long as it's working properly and not shorting out below the car's redline, keep using it. Won't harm a thing.

mike

Mike,

I've seen them on '72s (e.g., VIN 2762204, July 10, 1972, came with one) and '73s. And it sounds like Carl (above) had one originally on his '74tii. I've never seen an original one on a '75 or '76. I've seen them much more often on tii's, which made me think, back in the day, that they might be standard on tii's, dealer-installed on non-tii's. But today I have nothing to support that old theory!

Might still be available, alongside the standard rotor:

http://www.realoem.com/bmw/enUS/showparts?id=2583-USA-10-1972-114-BMW-2002tii&diagId=12_0997

There was also discussion back in the day about the cut-off point for these rotors (there was only one version available in 1974 when I installed one on my '70). Somehow, from some source -- I have no recollection whether it was the Roundel, a factory publication, or just "some guy" -- I was told it had a 6,200 rpm cut-off so that the engine would, indeed, not exceed the 6,400 redline. I recall very specifically, however, that the rotor's cut-off was NOT 6,400. And to a youthful version of me, that seemed very significant! Assuming it was true!

Regards,

Steve

Edited June 3, 2015 by Conserv

[Roland]

Subject was recently addressed here >>http://www.bmw2002faq.com/topic/166190-car-bogging-troubleshooting-stumped/?p=1042755

 

Rev-limiting rotors came in many variations, each with different rev limits.  To the best of my knowledge, the max speed was imprinted on the bottom of the rotor.  As Mike noted, the rotor depends upon spring tension to short out the ignition when the distributor spins at a certain pre-defined speed.  Thus, after a few seasons, the rotor's internal spring tension may change and reduce the rotor's cut-out speed.  Testing is simple with a (stroboscopic) distributor testing machine, if you have access to one, or similar.

 

In the absence of testing equipment, standard maintenance used to call for cap and rotor replacement at certain intervals (mostly dependent upon mileage).  The rev-limiting rotor is no different.  Not unlike other conductive ignition parts under the distributor cap, rev-limiting rotors can suffer from the same carbonization, corrosion and wear.  If the working parts of this style rotor are corroded or carbonized, the rotor "may" continue to function, but the rev limiting feature may be less-than-reliable.  On a related note, it's probably a prudent idea to test each rotor with an ohmmeter to determine resistance.  (Like so many things, the procedure is in the shop manual)

Edited June 3, 2015 by Roland

[Conserv]

Roland,

This is a great link. I missed it.

But in 1974, under Hoffman Motors, there was only one rev-limiting rotor available through my local BMW dealership!

Regards,

Steve

[OriginalOwner]

yes, my '74tii had the rev-limiter installed as original equipment.

 

ebay has everything ...... of special note:  the description says,  "6600 redline" ..... hmmmm, that's a problem.

   http://www.ebay.com/itm/321228577475

 

Cheers,

 

Carl

Edited June 4, 2015 by OriginalOwner

[gaius]

i have always found my right foot to be adequate as a rev-limiter.

used with my ear, and brain of corse. 

Edited June 4, 2015 by gaius

[peterschop]

i have always found my right foot to be adequate as a rev-limiter.

used with my ear, and brain of corse. 

 

Yes, shifting by ear is very easy in my 2002. I always shift well before 6K. But when I had my 528e I used to hit the rev limiter quite often. The M20 six was a low rev engine with the torque range low in the RPM's. I think the red line on the tachometer was about 4500 RPM and the rev limiter was set at about 4800 RPM.

[gaius]

that is nice to know as i just picked up an 86 325i, a project for down the road. got it for 400.00 from an old friend. 

[peterschop]

that is nice to know as i just picked up an 86 325i, a project for down the road. got it for 400.00 from an old friend. 

 

The 325i had the same M20 engine as the "e" with a shorter stroke therefore giving it a higher RPM and more hp at the expense of fuel economy. Sounds like a good project.

[gaius]

hmmmm, thats interesting. i wonder if i could swap the i for an e 

[peterschop]

hmmmm, thats interesting. i wonder if i could swap the i for an e 

 

Well I suppose you could but I think most people would want to go the other way. The "e" M20 motor only had 121 hp but it had loads of torque, 171 lb·ft making it very drivable. I got rid of mine after the trans self destructed (leaky seals, lack of fluid) at 220K miles but the engine still ran like new. Wish I kept it.

Edited June 5, 2015 by peterschop