Series MM Mechanical Rev Counter

[mike.perry]

If the adhesive dial is stuck to the face of a Series MM speedo then the rev counter readings will approx correspond with the speed of the car, ie at 45mph the needle will be indicating 3000rpm. If the rev counter is driven from the back of a dynamo which was common practice then the following factors have to be taken into account :- the speed of the dynamo compared to the engine speed (the different sizes of the crankshaft and dynamo pulleys) and the drive pinion gearing of the speedo which I have not managed to remove from my spare gearbox. Possibly the reduction gearing of the rev counter drive from the dynamo.
The calculation should work out at 0 difference between the rev counter and the speedo.
Any comments or calculations welcome

[mike.perry]

[frame][/frame]

[katy]

If the adhesive dial is stuck to the face of a Series MM speedo then the rev counter readings will approx correspond with the speed of the car, ie at 45mph the needle will be indicating 3000rpm.

Only in high gear

[mike.perry]

Thanks for that, I had taken that for granted which is why I envisage the drive off the dynamo which would be a constant relation to the engine speed

[RobThomas]

If I get time over the weekend I'll give it a try on the other car (which is running). The main concern is, as you mentioned before, the relative sizes of the front pulleys since this set is designed for the A-series dynamo and crank pulley on the narrow belt. The narrow pulley can be fitted to the MM waterpump but the crank nose is metric sized and the A-series won't fit it (I've tried).


[frame][/frame]

The other option is to run a mechanical tacho from the front end of the camshaft as per some Mini kits of yesteryear?

[mike.perry]

Hi Rob,
Your photo shows the setup I had in mind. I assume that is a C39 dynamo, is there a specification or is it just a different back plate with a drive take off? I guess if the dynamo turns at say 2 x engine speed would it mean that the dynamo pulley was half the circumference of the crankshaft pulley? The difference in pulley sizes would then be corrected by a 2:1 reduction gear?
The speedo I have used for the rev counter is a Series MM with a 1600TPM so the needle movements would match the speedometer in top gear.
I see the 25D distributor in the photo, have you done any comparison tests with the MM distributor?

[RobThomas]

Hmmmm. Isn't the speedo cable run slower than crankshaft speed by virtue of the little worm & cog reduction drive that comes out of the side of the gearbox? Something like a 22 tooth cog on the end of the drive? If so, the speedo will not read the same as if driven off of the camshaft. I might need to think this through more clearly!!!

The dynamo is classed as a C39-PV2-ExtendedDrive, as fitted to the Frogeye and there are few interchangeable parts since the main shaft is longer than the standard C39 whilst the rear plate has a thread cast into the extended end. The gearbox on the back of that allows for the pulley size differences, IIRC.

That Dissy is a DM2 from a series 2 803cc but I've had great results from the 23D4 from a Cooper S, 40819 (same as my security code number at work) since it has the same baseplate max advance (18*) and it brings the advance in quite early. Trial and error seems to be the only way, as BMCEcosse says. So long as it starts and the max advance is just short of pinking then you are going to as close as necessary for a 50 year old car and just as close as a worn 50 year old dissy from the original fit. Starts first time, every time and runs well.

[mike.perry]

I mentioned the drive pinion gearing as a concideration in my first posting, I still have to extract the gear from my spare gearbox to count the teeth

[RobThomas]

Right. At 3000rpm indicated on the revcounter dial I showed 2300 on a cheapo electrical revcounter on an oscilloscope. At idle they both read 850 rpm, suggesting some slippage on the fanbelt plus a mismatch of pulley size. This tallied with the aluminium pulley being worn down and somewhat polished. I removed the revcounter dynamo to change the pulley for an iron one butgot sidetracked by a wobbly reading on the scope which turned out to be the No.4 plug lead having poor transmission of the spark due to the cheapo carbon core breaking down where it went into the cap and the screw had ripped the lead open enough for it to start slipping out. Copper leads and resistive caps (or resistive plugs) will be on the list of jobs next week.

End result is that I think the C39 PV2 dynamo method will work quite well if you have some need to 'go retro' and it ought to be possible to get a printed facia to go on the revcounter dial to match the MM style.