Yep NGK BR7FS or even BR8FS plugs.

Like was said, 220* is overheated. Water temp should be in the 150* range.

I run 10:1 static compression in my 598ci N/A on 93 octane with no issues. It can be done with the right setup with no problems. Plenty of guys running close to or at 10:1 and on pump 93 octane, aluminum heads, and a good tune.

I am .005 in the hole with flat top pistons and a .030 Cometic. Compression is actually about 9.95:1, but close enough to 10:1 I guess.

ans also camshaft design can effect the static compression ratio...they make them to "fool" the engine into thinking it has more or less compression...Rob

I was talking to an old boat racer at Jomar Performance and he told me that they cut the arm of the spark plug back the the edge of the center electrode to prevent detonation. I just noticed that the Denso racing plugs use the same technique. They have an Iridium racing plug with a very short arm, I'm putting those in this spring, cheaper then a new motor. Your LSA can have a major affect on cylinder pressure, I believe tighter makes it higher. I'm running 112 in my 522. By the way Jomar makes a four hole tapered carb spacer that increases throttle response like you wouldn't believe. My daughter even noticed the difference.

exhaust valve seat is gone.

Any other damage? Things look ok from the pic but its hard to tell. Sucking water back in from a bad exhaust seat will cause detonation too.....

The spark plug ground electrode cut back to the center of the insulator electrode was first used by Champion spark plug company D-9-J for example. Johnson Outboard Motor Company developed it in the late 1920's because it was easier for the spark to transfer off of the edge of the ground electrode to the center electrode. J designation was for Johnson who invented it but the short electrode could be used across the board on all the plugs for everyones product. Early outboards ran magnito ignition that produced more electrical power the faster they ran - so the performance increase of the J electrode was mostly to make starting easier.

Modern spark plugs are a miracle of engineering compared to the old days. The ceramic used in the insulators is one of the biggest changes. Early plugs were gloss baked porcelain and if sand blasted were ruined. They also had a remarkable tendency to develop carbon short circuits. Steel alloys used in the center and ground electrodes today hardly erode at all. Some plugs have a "booster gap" inside the center electrode this is to help fire the plug if the plug gets wet fouled or shorted. Copper core plugs were an idea, as were resistor center plugs for elimination of radio interference with metalic ignition wires - the legitimate gimicks are endless.

I think Harley Davidson was offering a plug with muliple ground electrodes a few years ago, and then there is whole "surface gap" spark plug issue that was developed by Kawasaki in the 1970's for the new Capacitive Discharge ignition. This plug had no heat range. The plug looked like a small button electrode which had no negative electrode in the contemporary sense.

Then of course the Jap plugs NGK seem to out perform their American counterparts.

Some engine builders believe that indexing spark plug gaps to place them exactly the same in every cylinder will produce more power.

The old Chrysler Racing Hemi engine of the 1970's to 1980's use duel ignition with two plugs per cylinder firing them at different ignition timing - they won races with it. And of course some aircraft engine use duel plugs and duel ignition for safety reason.

Some people collect old spark plugs and the prices of old plugs can reach astronomical numbers for some early plugs. Many brands are gone now - Edison made plugs, Globe was another, and every tire company had them like Firestone. These were unique products not jus re-label of Champ or AC.

Plugs is a world of science unto itself.

While you give some very good information in your posts SeaHorse, do you have to sound like a textbook with EVERY post you make??

I love it. What a wealth of information I never knew. Carryon seahorse

just looking at the pistons, you had to much compression for a normal marine engine...it detonated itself apart, block looks iffy now..what bore ? need flat top forged pistons...or cam 2 fuel...