The value of 11mm (7/16”) BTDC for ignition is a nominal value where optimum performance was found to occur for a nominal set of conditions when M20s were first produced. It is a function of engine speed and rate of fuel burn.

Rate of fuel burn in turn is a function of engine compression, air temperature, grade of fuel, humidity and altitude, so treating this value as a precise number and making micro adjustments to the timing gear position seems futile.

My view is that I need to know where the nominal 11mm (plus or minus 1mm) position is on the timing control level. It needs to be in the second half of the travel to ensure that I can retard a little when I need to. I make a small paint mark on the lever at this position. I then find a hill that I can climb in top gear at ¾ throttle and adjust the lever to find the best position. Due to the improvement in the rate of fuel burn for modern fuels, I find best performance occurs later, possibly 7mm to 8mm BTDC. This is the position I then set control lever for most riding.

I did some measurements and drawing board construction and found the following:
• 11mm BTDC equates to 37 deg BTDC
• 7.5mm BTDC equates to 30 deg BTDC
• The MO1 mag has an adjustment of 22 to 23 deg. This is equivalent to 45 deg of crank rotation.
• The timing gear has 36 teeth, so adjustments can be made in 10 deg steps without removing the gear. This is equivalent to 20 deg of crank rotation.
It follows that the nominal 11mm position can be found for 2 teeth positions within the range of the timing control lever. I select the tooth position that gives the most advance and start from there. I do not remove the timing gear.

email (option): pvlietstra@gmail.com

Thanks for the info Peter.
That is very interesting.

The lowest octane I can get nowadays is 91 RON,
So clearly it is much higher than what was around 70 years ago (70sh' 80sh RON?)
and must have an impact on timing..


Thanks,
Noam.

email (option): noam10@gmail.com

Hi Peter, I agree with your opening statement and have mentioned the differences in modern fuels previously and the possibility that this may require alterations to the original settings..

Beyond that I don't really agree with your conclusions regarding the (possible) optimum position.

Firstly you give a figure of 7-8mm btdc as the optimum...this is not a precise figure and isn't in fact a clear 'optimum'.

Secondly you are relying on your impressions of how well the bike is running to determine the engines performance. This is not an accurate measure. It is accepted in tuning circles that a power improvement of less than 10% is not really quantifiable on the road with any degree of certainty. Additionally ,the affects of changes to both power and torque figures need to be determined..
The only accurate way to assess the performance of the engine with reference to power and torque figures is to use a dynamometer and then experiment with standard and alternative settings.
However, having concluded that 7-8mm BTDC is the optimium timing position why don't you just set it accurately to that using normal timing methods and retain the 'normal' use of the lever?...

In conclusion I agree with your original premise and think that there may be some benefit in experimentation to determine whether there is a 'better' timing setting that differs from the original, but could be applied to all M20s running modern fuels....Personally though, I don't think your methods are accurate enough to provide a definitive answer to the question of what that figure is. I also think that whatever the figure may be the timing should be set accurately to that, if for no other reason than to maintain consistency in the settings..a key requirement when trying to diagnose problems.....Ian

email (option): ian@wright52.plus.com

Any alteration to the ignition timing as described by Peter above will result in it being possible to over advance or over retard the timing using the a/r lever. This is really not desirable, causing pinking, overheating, holes in pistons etc. Stick to the book settings, BSA knew best.

Thanks for the input. It is interesting to note that up to the early thirties, BSA always specified timing at TDC with control fully retarded. This equates to 45 deg when fully advanced. Perhaps they changed for the reasons given by Keith!

One point I am trying to make though is that it is pointless being precise about something that is inherently fuzzy. I am 96.57% sure about this.

The other point is that the 11mm (37 deg) is based on low grade fuel and is a little too early for modern fuels. I am certainly more comfortable with a 7 - 8mm advance, equivalent to abouit 30 deg. It may be my imagination, but it also feels better.

email (option): pvlietstra@gmail.com

whoosh!! a lot of this talk about timing went over my head peter but are you saying that 7-8 mm would be the optimum setting I have found that that anything other than 11mm doesn't work

email (option): roger.beck@node6.com

The 7/16" figure is based on the worst load conditions: full throttle at low RPM, which will produce destructive knock if too far advanced.
However: at part throttle (steady 40 MPH cruise, etc.) cylinder pressure is lower and will tolerate more lead, perhaps another 10°. This also reduces engine temperature, and improves mileage.
Simply avoiding use of full throttle at lower speed also allows slightly more lead, but of course makes for less spirited performance.

email (option): sales@victorylibrary.com

"I am 96.57% sure about this."



Thanks, I needed that!
nt

email (option): nicktogATgmailDOTcom

Any timing setting will be a compromise to a degree because of the lack of an accurate advance/retard mechanism. The manually operated adv/rtd set up is at best a crude and inaccurate method of adjustment. (Bear in mind that a modern electronic ignition system has infinitely variable timing throughout the rev range..)
The question really, and the point raised by Peter, is whether there is a 'base' setting more suited to modern fuels than the original 7/16" BTDC setting...
As the engine is running for the greater majority of the time at the 'fully advanced' setting could this setting usefully be changed?
I think it is entirely possible there may be a better setting than the standard one for use with modern fuels but the problem is, unless you can test that idea in a measured way and accurately identify any point at which an improvement occurs there seems to be little point in changing from the original setting.
It is the case that although we suspect fuel characteristics are different these days we don't know what the original fuel was like (and bear in mind no change to the timing settings were made despite the change from pre war fuel to wartime 'pool' petrol and then to improved post war fuel).
In fact it may well be the case the old 100 octane fuel was further from 'pool petrol' than the current offerings from a combustion point of view.
Another consideration is that what would be the optimum setting in one country may not be the same in another country due to the widely varying specifications of fuels and other factors...It gets more complicated..
I think until someone gets a good M20 onto a dyno for comparison tests any conclusions will be basically a 'shot in the dark' or a 'best guess' and not sufficiently proven for my taste..Ian

email (option): ian@wright52.plus.com

An M20 flat out on a dyno!!! I'd pay good money to see that.

I suspect that the quench clearance may vary enough between individual engines that knock suppression between 2 bikes may be several degrees apart.
I'd like to see .035", but I don't think too many were built that close; the H-D flatheads were all larger than their "official" figures (3/64"-1/16"), some by 1/32".
In actual road tests, differences as small as a worn chain, low tire pressure, weak mixture (correct jet, but minor air leak etc.) and especially 1 tooth too many on the engine or gearbox will make a safe ignition setting on 1 machine knock easily on another.

Does anyone know - was this built under actual Ricardo license?

email (option): sales@victorylibrary.com