Here is picture from a turbo motor that I melted down. You can see the scuff marks on the side of piston from rocking back & forth. Since detonation is not a smooth burn it cause the piston to put extra pressure on the piston skirts & cyl. walls. Thus the scratches.

This piston was result of not having a new motor dialed in & letting my son go out with some friends in boat with out me.

From the pictures it looks like those heads got real hot-- You may have a cooling problem also. Check to see if you can rock the pistons in the bore by pushing on the top and tring to rock it on the wrist pin.

Turbo, Great picture!

Jim,
Those look like domed pistons to me. I thought 9 to 1 would be flat tops, anybody?
Some times a peice of crap in the float bowl can become lodged in a carb jet and cause a lean out on cylinders it supplys.
I might want to know what happened to this engine, why was it rebuilt? Has this happened to it before?
How is the other motor holding up? What is different? How long did this one run, and did you get to full throttle?

Regards, John

Another idea. Intake vacum leak ? How was the idle of this motor ? Also , might wanna ck. the # 6 rod brg. while on the stand. Detonation can pound out a brg.

John B. I thought the domes were a little unusual for 9:1 also. The only explanation I can think of is these are tall decks with longer rods and I think the pistons are a little different. The numbers are from the specs I got from the guy that re-built the port engine the last time. Apparently the riser failed on the port engine causing extensive salt water damage to some cylinder walls and all the valve train. The block was replaced, all the valve train was replaced including the cam and lifters. I grilled him about the parts in each engine being identical and he convinced me that he was well aware of the importance of that.

The 830 cfm Holley's had #78 jets front and rear. This engine came from Hawk with 1050 cfm Holley's. I think the jetting is too lean. The 830's are sitting on an 2" adapter and the manifold had spacers under it.

Interesting note: The #3 and #6 cylinders were the hardest hit. They are also the cylinders that have the sharpest corner to make in the old intake manifold. #6 also had the furthest to go.

Thanks for the tip Donzi. That's why motors are coming out.

Compression ratio- The compression I had in 454 with piston in picture I think was 7.5-7.75 with 119CC OC heads. I now have a 540 with 119cc OC heads & the dish on the pistons looks like a soup bowl. I am told I have 8.00 now. I though that flat tops in that cu. in. engine was about 8.75. With those domes compression may be more that 9.00 & that would not be helping your detonation problem.

On Fuel- does carb have power valves in it? If so what sizes? Both primary & secondary? I think the jets in primarys are OK. (No surging while low throttle cruise) Secondary's ?? depends on power valve questions. I know there have been a lot of posts on jetting. Might need to do search & see what other people were using. Remember jetting has to do with Cu. In., carb size, cam & other things. Going from a big carb to smaller carb jets sizes are not necessary the same. If I remember correctly the smaller the carb w/ same engine jet size goes down since signal is stronger.

The #'s on the power valves tell when they open not how much they flow, that is determined by the channel restriction, (the passage which allows the fuel to by pass the jet). Jets #78 sound about right for a car, ( box stock carb ). Open headers, cool air and that open plunum spacer would require much larger jets. If it were mine, I wouldn't run less than #84's all around.

Since all the carb barrels are the same size(?), when you are wide open they all should feed the same amount to the cylinders they feed. I would keep them all equal, like they are now.

You are lucky on the pistons, checking the rod bearings is a good idea. Check pictures of big block pistons to see what you have likecompression ratio.

You probably will need high test (91 or 93). Most dyno tests I have seen on modified BBC's make max power between 29 and 32 degrees total advance.

Check the carb on the other motor. Who put the carbs on?

Looks like reversion to me. Washed appearance of chamber, detonation, center cylinders pounded. Water takes up combustion space which increases compression astronomically. I would take a hard look at the exhaust. BTDT

My .02

BT

I don't think it's the exhaust. It's got Gil manifolds that don't leak and dry risers.

Interesting thought, if they leak it would most likely occur between the manifold and the tailpipe. It wouldn't hurt to check this out. The center cylinders look like someone cleaned them!
Did someone get water in your gasahol. Reversion?, what cam are you running?