Detonation on one cylinder?
 

Any experience on this, no noticable misfires or anything. Both shells wont even hold in to the rod. Upper is the one that has the wear in the middle. #8 bearings looked great so dont feel like its an oil issue. Crank was freshly machined, no other changes from last year when i broke a rod. This was about 15 passes. New Molnar rods

What cylinder is this where talking about and what type of intake manifold and heads are you running?

Also are the heads ported?

And what Carb are you running?

Not that this is nessesarlly your case, but I have heard of issues with Holley vacuum secondary Carbs where the control spring used was allowing the air valve to modulate back and forth at full throttle to such a degree and rate that it leaded out a few cylinders enough to take out a few cylinders / spin rod bearings on two consecutive motors before this issue was found!

Only a run recorder on the guys tack helped him review things and solve why this was taking place!

High Port heads 380 cfm version. victor intake. 1050 Dominator. the only thing I noticed is the center divider of the intake gaskets moved off to the side a little for all 4 pairs. the the intake side chamber in the head were black on the outside end ports but clean on the inner ports if that makes sense. But still only had the issue with #7. Nothing had really changed since I went back thru the engine after the rod broke except I but a smaller cam in. .

Pics

I'd check the dimensions of the crank/rods, deck height, etc.
Possibly one or more are not 'correct' enough for that cylinder?
Rod too long/short, deck for that cyl off, cam timing off for that cylinder, etc.

The piston looks like it wasn't firing properly. Mixture or from something else?

The shiny spot at top of piston contacting head?

Pics of all the pistons may help.

This cam may not be what the engine wants for your combo?

:confused:

the edge may have got a little cleaned off from the water taking the head off and sitting in the cylinder? Yes the cam isn't set up for these heads and combo, just something temporary that's why its for sale lol.

All cylinders have a slightly different burn rate, because of many factors. If you're putting a lot of timing in it, you can push one cylinder over the edge before the others join in.

Did you keep the same timing after the cam change? Cam might have affected cylinder pressure and burn rate?

Eric

I actually dropped timing. I picked up a .1 in the 1/8 going from 30 to 28 total. Msd distributor is locked.

Pic of Crank, 7 is to the left and will get polished before it goes back in. sorry not the greatest pic

Cylinders 5 and 7 Fire consecutively and as such rob fuel from each other too much if things are on the lean side especially as rpm / firing rates go up.

Shapes and area changes in intake manifold runners can make a good amount of the fuel drop out of the air stream and just coat the port walls before it makes it into the chamber.

This all adds up to rod pounding detonation as you have experienced.

When you get things back together I would jet up that driver's side rear of the Carb atleast 3 jet sizes or whatever jet increases are needed until you can get the plugs in 5 and 7 to look a tad fat, also a Carb sheer spacer like the Wilson one might help if the big combined back flow pulses from 5 and 7 are playing havoc with cylinder 7.

I would also consider getting probe into 7s Exh tube to confirm that cylinders running temp less you want to be going through this whole rebuild again.

Also please post up close up pictures of all of your plugs noting which cylinders they came from if you would?

I didn't know i had an issue so didn't pay attention to what cylinder they came out of.

Maybe that cylinder rings didn't seat well. It could have been burning a little oil causing detonation.

It's very hard to tell where the color change over point is on the ground strap of you plugs, your going to need to read this I am also posting up and start off with new plugs I would say.

Have had intake manifolds where the port work steered fuel away from a particular cylinder.Firing order influences intake traffic.Weld o2 bungs in the suspected primary pipes .Best test is to dyno with 8 o2 sensors.Bill C.

If 5/7 burn consecutively, is a 4/7 swap beneficial?

The one plug got smoked in the head when the valve broke, but yes its getting new plugs. Had ngk 8s in there

They look very rusty. Were you getting water in the cylinders?

I had some valve seats leaking water. Have you ever had them pressure checked for that ?

The castings are brand new. Maybe since i had a hole in the piston, it over pressurized the crankcase. It blew the water and oil out the dipstick and all over the side of the car, and also out the breathers with the header evacs. I had oil coming out the collectors.

So thinking the extra moisture went up into the cylinders.

So that piston in the picture of your second post had a hole in it, where?

#4 had a hole in the top from a broken valve