traviss

Just tore apart the might mouse and found another piston broken like the one I found in july. Don't know how long it has ran this way, but it has run a while like this.. before july I was running 89 octane and switched to 93 after piston problem. This second piston could have broke before that instance or not. Final timing was 32 degrees and a 9.9:1 comp ratio

Daddy has built motors for over 30 yrs and he has never seen this sorta disaster..

So far we think its pre igntion.

Building all new for spring.... and NO I am not telling what I am building just wondered what your opinoins were for what could have caused this.



Thanks

Travis

traviss

another

Vinny P

If it is detonation, the question is why did this happen with 9.9 compression, 93 octane and 32* timing?? The piston does not appear to have been run lean. When you tore up the piston the first time, did you go through the entire motor? From your post, it wasn't clear if you went through the whole motor the first time or just replaced the broken piston.

US1 Fountain

iTrader: (1)

seized top ring. not enough end gap

mopower

Trav,
Interestingly enough , a friend and I just pulled down his 406SB to find exactly the same thing in #8. Reason was , had some blow by and comp. was down about 50 lbs in #8. All the pistons looked fine , not like a lean condition.
How was the back side of yours? This one was dark yellow to brown on the under side , indicating a whole lotta heat .
Engine was dynoed prior to installation and egt's were fine .
This also had a single plane intake and holley with 1" spacer , how about your's. As a matter of fact , the engine builder left this engine a little fat on the dyno just to be on the safe side because he knows how this guy tends to run his boat
One other thing , engine only had maybe 50 hours

traviss

We think it was damaged when I was running 89 octane. I think this is the whole reason they were damaged. Cheap 89 octane. After the problem I changed to 93. When I repaired the piston in july Thats the only thing I fixed, pulled the head, oil pan and popped in the piston, was in a rush cause it was day before 4th july and we didn't have no reason to tear anything else apart.


Travis

traviss

Naa. I just checked my notes. That cylinder had a top ring gap of .021". Minimum in my mind was .016" if you go by ( .004" x 4.030") and the chevy hi per. book said minimum is .018". I was alittle bigger then both those gaps.



Travis

traviss

The back side of my pistons were all clean, no coloring at all. They had only been in the motor 98 hrs ( 3 months) They all looked brand new except for the crowns. I was running a 750 holley on the rich side with a dual plane edelbrock air gap intake, stock thunderbolt 4 ignition at around 5300 rpm.. and it spent most of the 98 hrs above 4500 rpm

WETTE VETTE

Detonation!!! 9.9 to 1 compression with iron heads is gettin up there. Unless you are running a long cam and a fat A/F mixture my guess is even 93 octane is borderline. 32 degrees is fine if not a little low as far as ignition. What are your cranking cylinder pressures? Did you see any signs of small aluminum beads on the porcelins of your spark plugs when you were dialing in your motor? What are you running for spark plugs? On that motor MR43T plugs will burn real clean, but are really a little on the hot side which can lead to the big "D". My dad's 11:1 compression 355 was getting into light detonation on 110 octane fuel and we went to NGK R5673-8 spark plugs and haven't seen an aluminum bead on the porcelins since. If you still have the plugs take them to a good race shop and have them look at them umder some magnification to further analyze them. My guess is detonation.
It happens and thats racing!!!

mopower

OK, The light just went on . My buddy DID detonate a piston last year about an hour after we first took it out .
Like you Trav , he only changed ONE piston ...I'll bet you this one was damaged but not visible . The bad one was #4 last time so we could see it (same bank) when we had the head off. Maybe it would be a good idea to change the rest of them just in case