rob6773

While I have heads off my boat I want to look into the cause of the damage. It is a 21 ft. stingray with a bravo I and the Chevy 502/502 hp automotive engine. It has the aluminum bowtie heads that came on the engine, I think the cam is the 110 overlap cam that comes in that engine and it has tubular headers on it. (I think the headers are Eddie Marine.) The flappers are missing from inside the exhaust exits.
At mid season we changed the head gaskets. At the end of the season cylinder #8 had dropped to 40 psi. (it actually did 67 w/the miss) I pulled the r/s head off last week and brought it to a pretty good machinist/engine builder. He pointed out to me that the head gasket was getting ready to go again and the gasket had left an imprint in the head.(It needs to be milled) He thinks it was a timing problem and detonation caused the issue. I only had audible detonation one day when my dad had the drive pushing up the bow and the trim tabs pushing the bow down. That caused a ping, which then caused me the move the distributor a half dozen times. The machinist thinks it would have taken a lot of detonation to cause this, not just detonation on one ride.
What do you find when a engine has reversion? Would it cause the type of head damage I have?

Vinny P

I have a simple question.. where is your total timing set to? You should not have to move it a half dozen times. Set it between 32 and 36 total ( depending on your set up and fuel ) and forget it.
Reversion will destroy an engine by water intrusion into the oil and cylinders. It can cause valves to become brittle and snap off. I have never seen it eat headgaskets like your engine. You will need to figure out exactly what cam profile you have, then call you exhaust manufacturer to see what they say about the posibility of reversion. 110 overlap doesnt sound correct.
Detonation will eat headgaskets, but will also cause other wear, IE: spark plugs tips may be melted off, the top of the pistons may be pitted or burned, the upper rod bearing will wear prematurely, burned cylinders or heads, etc...

stevesxm

vinny is correct about the timing for sure. if all else is even close to correct, then you should be able to set it somewhere around 8 to 10 btdc at idle and see the total advance numbers he is talking about at revs. and if the fuel is any good and the compression ratio isn't something crazy then it ought to be fine right there. set and forget. what i don't see in your description is anything that would have made the compression that low. you suggest the hg was getting ready to fail but at that sort of compression number it would have had to have already failed catastrophically and obvious to see ( along w/ other symptoms). i'm just guessing here but ill bet that if you made a little cover plate for that cylinder and leaked it down with the head off you are going to find that the detonation broke the top ring . of course it could be a bent valve as well but that sort of compression number smells of some sort of hard mechanical sealing issue... and if you say you had audible ping, im betting to broke a ring or two.

either way... i would leak that cylinder to get an idea before i went much furthur.

as mentioned... reversion puts water in the cylinders and ruins your day but that would tend to be across the board and you would see evidence in all the cylinders... not just one. when you put the al cylinder heads on , did you cc them and confirm the compression ratio ? some combinations will give you well over 10.5 to 1 and will ping like hell and die on regular gas.

lake speed

if you hear detonation then it has been detonating long before that. most of the time you dont hear it.

wannabe

Reversion does not cause detonation. But you should have the flappers replaced. It sounds like you have a timing issue.

Wannabe

Back4More

The CR on that crate motor is 9.6....32* max advance would be the most I would run at, and thats with premium fuel.
Are you using a 140 stat? What water temp do you see?

rob6773

Here is info from GM's site about the cam and compression-

Camshaft duration (@.050 in):
224 degree intake / 234 degree exhaust


Camshaft lift (in):
.527 intake / .544 exhaust

Lobe centerline is 110 degrees


Camshaft Part Number:
12366543


Camshaft type:
Hydraulic roller


Compression ratio:
9.6:1

We used 93 but not a great brand, I used to argue w/my dad that we should of been going further down the road to Sunoco to get their 93.
I think the timing was where it belonged until the day my Dad had the drive up and the tabs down, that created a ping due to massive friction. I know we had problems timing this thing because it has an MSD system and my dad had the digital snap-on timing light.(They don't work well together) The engine guy that I'm having fix the head will set me up with timing tape to set it at total advance.
The boat usually ran around 140 on the temp. gauge but I think dad my have put in an automotive 160 thermostat. I did not do a leak down test because I was concerned that the plug threads are damaged on that cylinder. I'm having that fixed also. If my machinest does not find either valve damaged on that cylinder the engine will need to come out. I took off the other head yesterday and found it looking better. The right bank had corrosion especially on the spark plugs, the left bank appears fine. The signs of detonation were also not as bad on the left head.

stevesxm

you don't need the head on to leak down that cylinder... you just make a square plate of al or steel just big enough to cover that hole and tap a hole in the middle for the fitting and drill the approriate holes for where the head bolts would go and seal it with a gasket or o ring and just do that hole. at that low a compression you will see the answer right away. it is a very useful tool and tells you a lot right away. it is a real time saver in the long run for situations just like this when you are scratching for direction.

Vinny P

I dont claim to be a cam expert. My .02 says, while your duration is not huge, that centerline is too tight for marine wet exhaust. Most prefer 112-114 range. That cam may revert water regardless of whose exhaust you run. This is one of the many problems you run into when dropping in a GM crate engine, designed for a car, into a boat. It usually doesnt work too well.
Forget about timing tape. That wont last more than a few minutes before it blows off. A good Snap On inductive light should have an advance system built into it. Just dial in your desired advance, bring up the RPM's and turn the distributor until the balancer says 0*. You cant miss that way.

Sean H

You are supposed to use an inductive light not at digital one with the MSD boxes, the digitals are too sensitive to the multiple sparks and throw the timing off.