Twin plug firing one slightly after the other
 

I have heard folks set up the twin plug firing on 911 air cooled engines so that one plug fires some milliseconds before the other for the same cylinder.
Can someone tell if this is the way to do it and if so why this is done?

Cheers

It’s not the way to do it, and there’s no reason to.

Jay Leno’s mechanic did it. See UTube video of brown ‘70-71.
My RST is twin plugged and fires at the same time-I really wouldn’t know the difference as there is soooo much power.
Cheers,
Johan

Not sure other manufacturers tried it, but MB ('98) introduced their first V-6 engine (M112). Among other design features, their ignition system fires each twin plug set with sequential offset timing, also alternating which plug fires first. This was their solution for improved emissions. Twin plugs shorten the combustion process and allows higher compression ratio for more power and fuel mileage. The system also equalized spark plug electrode wear. However, advanced electronics and improved engine management systems allowed them to eventually revert to single plug engines. AFAIK.

More engine details here:
https://www.wardsauto.com/news-analysis/mercedes-benz-32l-sohc-v-6

Sherwood

I thought the point of NOT firing both plugs at the same millisecond was so the two flame fronts would meet in the optimum place (instead of the wrong place) to aid cylinder air flow arriving at the exhaust valve. The power gains were significant in some huge airplane motors, prolly not so much in a small car motors.

As I understand it , Basically ....

If you have an offset single plug on a flat piston the explosion will happen first on the side where the spark ignites it Then spread covering the piston top completely .
Not ideal as you want a massive explosion all at one covering the full piston top area to force it downwards at maximum force to create crankshaft torque .
(A 4 spark-plug set-up in each cylinder would start the splowshun at the same time .... )
If you go anything other than flat , it gets worse as the gasses escape to the other side of the dome opposite the spark plug And sit there waiting for the flame spread to catch it up .

If you imagine a 90* wedge prison going into a 90* head chamber with a spark plug on one side it will take a while for the flame to spread enough to get to the other side to ignite ...
A typical 4 valve head basically has 2 valves on either wedge and a central spark plug (gt3 etc) so the flame spread is pretty even .

If it is not even , small spark timing can be used on a twin plug to ignite the “lost” gases before the flame takes its time getting to them .

There is slightly more too it , but that is my basic understanding :D

I’m sure somebody will confirm the reason it is not more widespread is that even the best distributor manufacturers admit their timing isn’t accurate enough to compensate for any losses in order to produce a more efficient system .

Spark ignition and combustion:

https://www.innovatemotorsports.com/resources/myths.php