Morris Minor Owners Club

Many years ago, when unleaded petrol took over from 4 star, I was told by an old classic car enthusiast to statically set the timing and then retard the ignition by 30 clicks on the adjuster wheel to make up for the unleaded petrol 'burning hotter' than 4 star.

Does this make sense to anyone?
I am not very good at dwell angles etc.

Thanks, John.

There is a lot of ill-informed nonsense bandied about in respect of old cars. On the contrary, you want to advance the ignition as far as practicable to get the best performance from the engine. Retarding it too far actually makes the engine run hotter.

Thanks Basil.

Would you make any changes to timing if changing from unleaded petrol to super unleaded?

stuffedpike20 wrote: ↑Wed Sep 01, 2021 8:06 am Thanks Basil.

Would you make any changes to timing if changing from unleaded petrol to super unleaded?

It is possible that using higher Octane fuel will permit a slight advance in timing. This can result in improved performance and economy. The other (highly topical) advantage of premium grade fuels is that some brands will continue to maintain Ethanol content at just 5%.

Thanks Edward.

Yes, today is the day that unleaded becomes E10.

Edward1949 wrote: ↑Wed Sep 01, 2021 11:54 am

stuffedpike20 wrote: ↑Wed Sep 01, 2021 8:06 am Thanks Basil.

Would you make any changes to timing if changing from unleaded petrol to super unleaded?

It is possible that using higher Octane fuel will permit a slight advance in timing. This can result in improved performance and economy. The other (highly topical) advantage of premium grade fuels is that some brands will continue to maintain Ethanol content at just 5%.

Just to endorse this - absolutely right. Using the higher octane fuel means that you should be able to get away with a little more advance due to the higher octane rating, without risk of pre-ignition or pinking.

Regarding adjusting the timing to suit unleaded, I remember that that was done to cars in the late 80s when unleaded came out - my dad's Mk II Cavalier went in for a timing 'adjustment' to ready it for running on unleaded, as did many other newish cars of the time. I suspect it was just retarding the timing a little to compensate for the fact that the unleaded is has an octane rating of 95, whereas the 4 star it essentially replaced had an octane rating of 97.

If you advance the timing too much, is it possible to ruin the engine by getting it too near to top dead centre?

Sorry if this is an obvious question; as I have said, I am not good on this subject.

stuffedpike20 wrote: ↑Wed Sep 01, 2021 4:09 pm If you advance the timing too much, is it possible to ruin the engine by getting it too near to top dead centre?

You've got it backwards I'm afraid. Advancing the timing causes the spark to occur further away from top dead centre; retarding it causes it to happen closer to TDC.

I was thinking that advancing meant advancing the spark.

Told you I didn't know much about it.

Yes, it does mean advancing the spark; causing it to happen earlier in the combustion cycle, ie. while the piston is further away during its trip towards top dead centre.

If you advance the spark too much, it will cause 'pinking'. Advance it more and it will start knocking - and eventually cause engine damage.

stuffedpike20 if you're still a bit mystified by it all this clip should help clear the fog:-

https://www.youtube.com/watch?v=Pu7g3uIG6Zo

Thank you MM and everyone.

To my brain the whole thing is counter-intuitive. Purely mechanically speaking, if ignition occurs before TDC, surely the engine runs backwards?

Lets leave it all alone now.....I'll get me coat.

As described many times before by the long departed .BMC ECCOSS

Slacken the dizzy clamp bolt so that it just holds but can be turned (Just) by hand. Take the car out and drive it,advance the timing untill on hard acceleration or a moderate uphill ,the engine pinks,stop , and progressively retard the timing,drive and stop and retard untill the pinking stops. Tighten the bolt. The engine is now timed to the best you can taking into account any wear and tear in dizzy, points, engine,petrol etc Simples..

Can't run backwards because the starter motor spins it in the forwards direction,if too far advanced the engine will just bump and stop the rotation.

stuffedpike20 - don’t get your coat! It’s because there’s a delay between the spark and the explosive effect, during which delay the piston continues its travel towards TDC … thus it’s (all being well) in the right place when it becomes subject to the explosive force .

Yes. I understand that now.

Thanks, John.