Lotus Elan and Lotus Elan Plus 2 Forums

Thanks everyone for your comments.

I did not realise that some engines will not turn past the compression stroke at all.

When I was a student and prioritised beer more than batteries, I had an old car with a starting handle. For the first start of the day I would turn the engine over gently with the handle until it jerked back at me. This was my indication that I had fuel and a spark. I would then swing the engine as hard and fast as I could. It usually started, and by the time I reached college the battery was good enough for the rest of the day. This experience of hand cranking an engine is what has caused me to think there is a need for the engine to turn rapidly (above some critical speed) in order to start.

I am also concerned about the number of teeth on the starter pinion. Gears are a bit like screw threads, they are often defined by their diametral pitch (a bit like threads per inch) or module (a bit like the pitch of a metric thread). For gears to work correctly both the pinion and the ring gear should have the same module. Again like screw threads some incorrect combinations will go together, but strength and longevity are not what they should be.

Could this be why some starters are noisy and reluctant to turn the engine. Our cars are 40 to 50 years old and may have had previous owners who did not appreciate them, were strapped for cash and didn't use the correct components. In other words the starter pinion and or ring gear may have been changed for the wrong thing.

What do you think?

Richard Hawkins

just purchased powerlite starter model rac 472 after seeing it at nec classic show,got it for a good price,it also has facility to use its trigger/solenoid,the two staters i good with project +2 had wrong 3 hole model with 10 tooth instead of proper 9 tooth one .bendix on one was mangled other one showed signs of going the same way

High compression race engines may actually have less compression at cranking speed than a standard engine with good ring sealing, this is because of the cam duration and overlap, the piston will be a lot further up the stroke before the inlet valve closes and the compression starts than on the cooking engine, at high revs the inertia of the charge mixture takes affect.

What I am trying but probably failing to explain is that a highly tuned engine may present less cranking compression resistance than a standard one although the valve springs load will be higher.

years ago I used to use a geometric formula to calculate the effective compression ratio (taking in to account the cam duration of the standard engine) then use the cam duration figures of my race cam to calculate just how high I could go with compression ratio on standard fuel, the info came in a book called practical gas flow, using this I often ran a higher compression than others using the same cam and never had detonation problems.

Going back to basics & talking about the different Lucas starter pinions, namely 9 or 10 tooth variants for the Lotus Twin Cam engine.

As I have had little to do with these parts in the past I have never stopped to give it much thought but there is something very strange that some people who apparently knew what they were doing with the definition of those pinions.
Firstly the centre distance between the flywheel (starter ring) and the starter motor is fixed & determined by the Ford bell-housing geometry.

So far as I know, nobody has ever discussed different sized flywheel starter rings so how is it that given everything is "fixed" that either a 9 or a 10 tooth starter pinion can be used with supposedly no damage to the teeth occurring.
Is there a hidden logic in there?
Or are the tolerances so wide that the tooth clearances between the starter & the starter ring could possibly be "optimised" by such selective assembly

...and also the 10% difference in gear ratio.

John

For what its worth - My assumption has always been that Lotus discovered the Lucas standard 10 tooth starter was marginal on torque for the twin cam so they reduced the gearing by changing to a 9 tooth starter which appears to have been an option Lucas offered in the design. At the high level of speed reduction and with the loose fit and tolerances for starter gears you can cut an acceptable profile for both 9 or 10 teeth without changing the flywheel ring gear at the same pitch circle locations for the gears it appears. The bigger pitch spacing between the pinion teeth in compensated for somehow so it still meshes OK - maybe the teeth themselves are thinner so the overall mesh spacing is maintained. I have never bothered to try to figure it out - I have always just stayed with the Lotus 9 tooth pinion standard as it works for me.

cheers
Rohan

rgh0 wrote:For what its worth - My assumption has always been that Lotus discovered the Lucas standard 10 tooth starter was marginal on torque for the twin cam so they reduced the gearing by changing to a 9 tooth starter which appears to have been an option Lucas offered in the design. At the high level of speed reduction and with the loose fit and tolerances for starter gears you can cut an acceptable profile for both 9 or 10 teeth without changing the flywheel ring gear at the same pitch circle locations for the gears it appears. The bigger pitch spacing between the pinion teeth in compensated for somehow so it still meshes OK - maybe the teeth themselves are thinner so the overall mesh spacing is maintained. I have never bothered to try to figure it out - I have always just stayed with the Lotus 9 tooth pinion standard as it works for me.

cheers
Rohan

That gave me a little chuckle Rohan; seems that we both said at some time,
"What will be, will be!"

The logic of British manufacturing back in the dark ages heh!

Cheers
John

Hi
I just have saw this thread, well, I had the same trouble with my starter motor and I bought an Hi-torque one from this seller in the USA : http://www.britishstarters.com/Lotus.html
It works fine and it is the less expensive I found despite postage cost to France and customs duties
Franck

Starter pinions are such ridiculous bastard gears that they cannot hardly be calculated with gear formulas. Adding teeth losing teeth changing diameter are all fudged.

It is my understanding that All original lucas starters have nine teeth. The ten tooth pinions started showing up on Lucas remans in the 70's for unknown reasons.Could be another myth

I have personally never seen an original starter with ten teeth.

I am fairly certain Lotus has nothing to do with tooth count. Lots of people like to invent ways that Lotus was being clever, when almost always it was cheap.IE S2 seven chassis has less tubes than S1, not for adding lightness but for adding heaviness to Colin's wallet

gus wrote:Starter pinions are such ridiculous bastard gears that they cannot hardly be calculated with gear formulas. Adding teeth losing teeth changing diameter are all fudged.

That's pretty sweeping statement there Gus

Cheers
John

gus wrote:
It is my understanding that All original lucas starters have nine teeth. The ten tooth pinions started showing up on Lucas remans in the 70's for unknown reasons.Could be another myth

I have personally never seen an original starter with ten teeth.

My experience down under quite the opposite. Have always found ten tooth pinions to be the norm on standard cortina and escort models with 9 tooth on sporting models like escort twincams. Like rohan I always thought it had something to do with providing lower gearing (ie more "oomph") on high compression motors.

I do recall now I used to own a twin cam escort whose 9 tooth pinion had a tendency to jam.....but whether jamming is more prevalent on 9 tooth pinions I don't know. Maybe it was just worn.

I have a stash of both 9 and 10 toothed pinions in my parts boxes, but will always fit a 9 if I have the choice.....

look in the machinerys handbook, and see gear calculations. For a 29 mm gear the smallest normal gear is probably 20 teeth[not looking it up right now]

Everything with fewer teeth needs to be adjusted[read bodged] to account for the fact that it is running in too small a circle for the teeth to get out of their own way. The teeth get shorter and pointier. As a result you can 'fit' a pretty wide variety of teeth in a spot if you are willing to put up with the noise and wear

The efficiency of torque transfer is pretty low, as the gear spends much of its energy sliding sideways on its mating tooth rather than pushing.

robertverhey wrote:

gus wrote:


My experience down under quite the opposite. Have always found ten tooth pinions to be the norm on standard cortina and escort models with 9 tooth on sporting models like escort twincams. Like rohan I always thought it had something to do with providing lower gearing (ie more "oomph") on high compression motors.

.

Again I would have to see period evidence. Every single original starter I have seen is 9 teeth, All sprite early MGB, mini starters are 9 teeth. Pretty much every car designed after ~1963 uses 9 tooth pinions of either 29 mm or 25mm diameter.

Cortina GT, and later Caterham used an 11 tooth 29 mm pinion on the Kent block

The mechanical engineer in me is now starting to think I should sit down and do an analysis of the 10 versus 9 tooth designs to see exactly what Lucas and Ford have done. i have avoided doing it for years but maybe now is the time. I have Just got about another 50 loads of stuff to move to my new house over the next week or so and then maybe i will have some time to add another project to my always expanding list of things to do Its been 40 years since I did any gear designing but I can remember the basic and look up the rest

cheers
Rohan

Rohan

I think we'll all look forward to it.....and when you've done there is always the oil/grease in trunnions debate...

John