Cracked rings and piston lands Why?
 

After performing a compression test cylinder 7 had 45 pounds and cylinder 6 145 pounds. All others are 185 to 195 pounds. Thus the reason for excessive blow by. Pulled one head, pulled 2 pistons and discovered cracked piston lands and rings. The head gasket was fine. On cylinder seven I could pull off two pieces of ring land if I tried. What could be the cause? The only thing I can think of is some fuel boil over at heat soak. This engine has probably only revved to 6000 RPMs 10 times in it's 14,000 mile life. 38° of timing all in at 2600 RPM.

Maybe the end gap on the rings was too tight and didn't allow for expansion when hot.

Mild detonation over a long period of time can cause this.

I'd guess detonation (knocking/pinging). It's the functional equivalent of hitting the piston crown with a hammer. I've seen it break top ring lands and knock piston skirts off.

Detonation....

195 lbs pressure is quite a bit. What gas are you running?

X2 with blykins. Too high compression ratio for gas used. Advance to high for compression. It does not take high rpms to damage the engine under these conditions. With loud exhaust it makes it hard to hear it knocking. Since you have some pistons out measure or find the CCs of the heads and find or calculate piston valve relief CCs and piston height. Do some reading on Dynamic Compression ratio. If you have the cams specs you can calculate it here.
https://www.uempistons.com/index.php...bc6726aec853da
Generally, you want dynamic compression between 8 to 8.5.
Static compression does not take into consideration your cam, which is critical.

I have a friend that can't drive his Cobra on the street without race gas. Big mistake on the engine design. Good luck.

Some years ago I also lost some pistons to suspected detonation. Following a rebuild with new pistons and to new specs, I installed a knock sensor that provides a visual indication of detonation, because its impossible for me to hear it. As I recall, it was an MSD product.

Hey ....this is not a new problem. When they shifted the top ring to a higher ...hotter ...position on the piston, the ring gap needs to be around .035 inch. If you do not have enough gap the ring ends bump and the crack the top land. New pistons and gap them properly . Don't listen to any of the other crap.

Unless it's a Keith Black hypereutectic, that data is not correct.....

I buy pistons by the truck load, gap the rings at .0045/.005 per inch of bore and don't have this trouble.

To the OP:

If you pull the piston out and all the ring gaps are lined up, that's another sign of detonation. Also check out the upper rod bearings and lower main bearings. You can also have some piston/head clearance issues which will hammer on the ring lands.

Looks like some tanks of 89 out on the road in the boonies got me. And probably some of the 91 that I put in is not 91 either. Shall I back timing off to 36 when I get it back together? Btw I have a 4150 holley with 72 primaries and a 65 power valve at sea level . Malloy Diz with no vac advance. Hey ya pay to play sometimes. Summer is the best time for cobra work here. Way too hot and too Rainey now.

[quote=blykins;1358658]

To the OP:

If you pull the piston out and all the ring gaps are lined up, that's another sign of detonation. QUOTE]

Brent - curious, how does detonation cause the ring gaps to all line up? I know rings move around but trying to figure out in my mind the connection between the two.

Young1, if you make no other changes, I would at least pull some timing out. However, maybe you can take some measurements and use a little thicker head gasket, or make some other corrections to help out.

Dan,

Not sure of the science behind it, but it's one of those things that has proven to be a tell-tale....

Just checked. Ring gaps are not lined up.

Thanks - some things defy logic.

Detonation itself won't line the ring gaps up.

But if you are running in detonation you will deform the piston - which will pinch the second ring as the land (the name for the upper and lower groove surfaces) folds up, locking it in place.

The damaged second ring segment will eventually cause the top ring land to deform and repeat the failure in a different fashion - by giving the upper ring's gap a place to "catch" in the dent - hence the line up.

What brand and type of pistons were they?

You might need a bigger cam :)

OP as others have already commented you have a detonation problem caused by engine design choices you made, timing you chose to run and gas you chose to use. Some times the gas problem is unavoidable when you are on fumes. Better planning can mitigate this.

The detonation devices that are generally commercially available will only tell you after the fact that you had a problem — not particularly helpful but better than nothing. What you need is a detection and correction device that will detect the sonic signature of detonation at the onset of detonation and then in real time pull the timing for that just that one cylinder to save it before the next ignition event.

That I am aware of there is only one commercially available product that does this. It is called the J&S Safeguard system. In addition to pulling ignition timing in real time on a cylinder by cylinder basis it incrementally restores timing until it "hears" that detonation sonic signature again and then backs off just short of the detonation timing threshold.

I use the system with a coil on plug (COP) ignition 9:1 c/r, a screw blower with 18 psi of boost and 91 octane Kalifornia gas. I have no detonation. If you get the occasional bad tank of gas (as we all occasionally do) it will protect all the monies (and parts) you have in your engine.

To my knowledge it does not work with old point style distributor ignitions. It does work with more modern ignitions. This is the web site => J&S Safeguard. I just looked at the webpage and I was wrong. They do manufacture a system for distributor ignitions with points. It is their universal system.

They are not cheap (~$600 base system) but then again neither are engine parts — as you either will or already are discovering.

I persuaded a friend to put one on his twin turbo engine and his tuner was dismissive of the product until he made a mistake on the fuel and timing maps that sent the engine into detonation country.

When it happened the Safeguard lit up like a Christmas for all eight cylinders and the car immediately fell on its nose. No damage was done and when they discovered the whoops in the fuel and timing maps even the tuner became a true believer.

This device will pay for itself in broken parts you don't break — anymore.

Ed

I am working with Southern Automotive the engine builder right now. Spec sheet says that the Pistons are Keith black hypereutectic. Work is so crazy I have trouble even getting phone calls and emails written to get it all resolved. I shall keep in touch.

Admins pls delete this post, it was a mis-post