Hard to see in this pic, but why do the trailing spark plug holes have such a small little orifice in them, as opposed to the full diameter, wide-open port of the leading plugs?

 

My guess is to reduce compression losses as the apex seal sweeps over it. The trailing plugs are basically there for 'cleanup' ignition, and so the smaller exposure is not a big deal.

Dave

 

I'd agree with that theory. If you look at the animation on Rotary Engine Illustrated's site, you can see that the trailing portion of the power "stroke" occurs right as the apex seal goes over the trailing plug hole.

 

There's a pressure difference between adjacent chambers, intake vs compression. When the apex seal goes over the trailing spark plug hole there's a compression leak so the hole diameter is smaller to minimize this leak.

At the leading spark plug position both adjacent chambers have the same pressure when the apex seal crosses it. Since the pressures are the same there's not a compression leak so the hole is full sized.

 

The winner of the correct answer!!!!!!!!!!!!!!!!!!!!!!

 

Excellent information guys, thanks!

 

Hmm, that's basically the same thing, except I didn't talk about the rotor position issue.

Dave

 

when people run all 9's, do they fit right in the leading holes w/ no problem then?

 

I'm interested in knowing that myself.

I've got 4 brand new NGK 9's that I've been meaning to put in, along with some thick thick Taylor Pro spark plug wires.

 

9's fit in the leading hole with no problem.