So you may have heard of Mahle Jet Ignition that's used in F1, and Honda CVCC from the past. They ignite a rich prechamber mix to then shoot jets of flame out into the main chamber instead of relying on the flame front propagating from the spark plug.
Lean burn or charge dilution tolerance isn't the only efficiency benefit of fast combustion though, as we've seen Toyota and Mazda demonstrate. Since mechanical compression and thus expansion ratio are quite limited on a gasoline engine, fast combustion gets you as much effective expansion as possible.
They are obviously not retrofittable since they involve adding a direct injection system and a prechamber.
However I noticed these:
https://prometheus-at.com/prechamber-spark-plugs/
They're used on large natural gas engines which would be a little different from a gasoline engine since natural gas would diffuse into the prechamber faster than gasoline vapors, but it should still work, because fuel/air mix gets forced into the chamber on the compression stroke. The prechamber will have slightly more residual exhaust lowering its EGR tolerance, but it'll also be a little hotter which might negate that. The smaller the prechamber volume, the less residual exhaust will remain in it.
What if you were to fabricate a cap that goes over the spark plug forming a small prechamber? Typical charge temperature at max compression is something like 800K, and the flame burns at something like 3000K, so you can kind of guesstimate the jet orfice size. It would need to withstand something around 800psi and operate at high temperature so stainless steel seems like a good material.
E.g. 0.5cc chamber on a 500cc cylinder gets you about 1% of the total fuel air mix into the prechamber, seems approximately on the same order of magnitude as what the properly designed stuff has. Copy the 6 hole pattern, copy what looks to be approximately 1mm diameter jets.
Since it's a passive prechamber it won't help very much with increased EGR or lean burn, but it should be able to facilitate increased compression ratio. Getting a MPFI engine up to say 13:1 compression ratio on pump gas like the Toyota A25A-FKS would be a really big deal, that would be a massive fuel efficiency gain!
I fully intend on trying this someday when I have time but wanted to put it out there in case someone else thinks this is easy to do. How hard can it be?
