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.....

cut back plug electrode
 

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??:party-smiley-004:

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

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...

I think he said he was running aviation fuel in some percentage. We would need to know the cc of the heads to know if the domes were too much. I run -12cc pistons with blowers.

I think I need flat top pistons as well. Sucks the engines were just assembled. THis is a lesson learned from appliying car engine know how to marine application, two different worlds. I can run aviation gas 50% or 100% since its $4 gal. but this is not practical in the marine world. Problem is I have 2 engines that will have to be torn down. Piston dome is -17cc

I think you can make them live as is, you will just need to tune appropriately. Richer 12afr, less timing 31-32 and make sure the cylinder heads stay cool as possible. Alum heads would also be a benefit if you do not already run them. What is the current comp ratio?

Those look like little domes for closed chamber heads. I believe those run with open chamber heads give you around 10.5:1 compression. With the right tune up you should be able to work with that with 93 octane.

more pics. Heads have gone to the machine shop that installed the valve seats.

That one piston is gone, bummer. How does the cylinder wall look?

on another note, i would like to thank all of you for your valuable opinions and insight. all of the information and commentaries have been helpfull and educational for the future. hopefully I can get this engine toghether, tuned right and running smooth.''

again, Thanks

cylinder is scratched, will need honing or a jacket maybe. The carp in the cylinder is the residue from the valve seat

check fuel pressure, low pressure and or volume can lean a motor which will get the combustion chamber so hot the valve seats will fall out.
I had a carb float stick which caused needle to stay closed, motor was then only recieving half the fuel from the other side of the carb. pop! my engine looked exactly the same

Did the valve seats drop out or get beat into the head?

forget the av gas...does NOT work well the specific gravity is way wrong for automotive/marine engines...will help some but not as much as expected...at this point would make no sense to me to try to run this compression ratio...short of changing the pistons the only head you can get away with would be the extra large chamber 71-72 454 365oval port or 425 rect port..if you can find them...and they are not a very good head either..but would get the compression down to a managable amount...

Man it looks like those cylinders were running lean and hot, there is a whole lot of clean area on those pistons.

what should be the fuel preassure? the pistons are brand new, have only a few hours on the, break in basically. Im seriously considering changing the pistons. Im still waiting for the machine shop to get back to me, talked to them todday and they said the valves were perffect.
bore is 4.530 stroke 4.00

MILL the piston tops.

I thought you said the valve seats where gone. That is why I am interested in what happened.

Fuel pressure should be at 6 or 7 psi . I am curious as to what heads you are running . Only because they sure look like closed chamber piston's and if they are open chamber heads compression is not the problem. After a closer look at the pic's I'm going to say you used the wrong head gasket's . Blocked off the water passage's to the heads , hens 220* and a dropped valve seat.

Ditto everything Jeff said. Plus check the sea pump. Cooling was probably the issue.

Fuel preassure was arround 9 psi.

The exhaust valve seat droped and was pressed into the piston, then disintegrated mostly. The other head , the intake valve seat was also coming loose, hence the drop in compression. The machine shop said all the valves looked ok, nothing else was really wrong except the seats.

the heads are stock 502 heads, im trying to get the casting number to make sure.

Calculate the compression ratio accurately and provide the camshaft specifications advertised, and at .050 lift.

Based on the cc of domes, heads stated (116cc?) and head gaskets I'd estimate a static comrpression of 9.75-10:1. This assumes the deck height at .010".

Cam?
 

CafeRacer - You haven't mentioned cams - most automotive cams won't work in marine - this is because marine uses wet exhaust with water sprayed into the tailpipe of the marine engine to cool it.

Some guys do get away with automotive cams if they use dry stack exhaust of a complete marine system with full length water jacket right out the transom - no internal water spray to silence the exhaust noise.

Marine cams close the exhaust valve early at Top Dead Center TDC of the exhaust stroke. This allows some pressure to remain in the engine cylinder or at least prevents the intake stroke from drawing water back into the cylinder from the exhaust pipe.

Take the Merc 500EFI for example this is a Crane cam with high overlap and durration in the full context of automotive design except for one change - exhaust event closes at TDC.

If you are using an automotive cam you will pull water back into the operating engine. Sometimes it will rust everything, sometimes makes steam in the cylinders which will create more cylinder pressure than burning gasoline. Get enough water and you can hydraulic lock the engine which will bend connecting rods etc.

They will bend just with the starter motor turning against a water filled cylinder.

Sea Horse. The engine was built by Bullet Engines, for marine applications. During the rebuild the cam was nos changed. The engines ran beautifully when they came from Bullet, and im sure they installed a marine cam. Other than that, the cilinders were dry, no water , and the oil is clean.

The machine shop is picking up the engine today to investigate, they are tearing the engine down. I asked them about milling the pistons and said it could be done if need be.

max on 93 oct with steel heads is 9.5 :1 compression.
with aluminum u can get away with 10.5 :1 everything above needs 110+ octane fuel.

timing max on 93 should be not more then 32 degree.

u can get a thicker head gasket but still need to mill the pistons down a bit.
IMO

GOOD LUCK

I never saw an actual cc measurement of the cylinder head, get that volume and piston deck height then calculate the actual compression ratio. I'm suggesting this because once you know where you are exactly, there are options other than changing pistons. Milling the pistons requires a re-balance of the rotating assembly, as does changing the pistons. I would suggest enlarging the combustion chamber of the head. As a guide if you add 10cc of volume to the combustion chamber, you lower the static compression ratio by one point. The combustion chambers should be smoothed out and sharp radius's removed anyway, it is relatively easy to add 5 - 6 cc of volume and you get the benefit of smooth combustion chambers. Post the camshaft specs as requested, I'm sure the engine builder was good but allow us to complete this investigation process. If I were you, I would have a goal of this engine running safely on 93 Octane, then you can add better fuel as a precaution if desired. With iron heads 9.2 -9.3 static compression is acceptable with polished combustion chambers and the correct camshaft. The camshaft is important because it controls the amount of cylinder pressure the combination will see, thus dictating possible detonation issues.

I don't have camshaft specs, can they be meassured once the engine is torn down?

Yes they can be measured with the cam still in the shortblock by degree wheel, or there may be a grind number stamped into the core. If there is a grind number we can probably ID the specs for you. I would highly recommend checking the details of the cam. You've gone this far in your investigation, might as well finish the journey. Good Luck.

If his combo ran well prior to the rebuild for many hours, the issue is most likely something along the line of Jeff31s head gasket thought, other overheating prob or something in the tune related to timing or fuel afr. Most likely unrelated to engine combo unless fuel quality changed.

I agree, but if its a higher compression engine it would run for a while with low octane fuel at mild driving.

I think its a compression related detonation.

I think he said it ran for many hrs on the original build then only breakin till this.

I think these engines could be made to run fine if the owner has access to good pump premium. The compression works out to be about 9.5-9.75. Look at your HP500 pistons. They're small dome. The engine temp problem needs to be fixed first. At most, 140 degree thermostats need to be used. I use 120 degree thermostats from Arizona Speed and Marine for an added margin of safety. -6 size hoses should be added to the intakes to connect the rear water ports to the front water crossover. This helps prevent steam pockets from forming at the back of the cylinder heads and creating hot spots. The carb jetting seems lean. The pistons and sparkplugs look too clean for a marine engine. Go up 2-4 sizes on primary and secondary and see what the plugs look like. Last, set ignition at 33-34 degrees. Good luck.