Combustion chamber carbon build-up : an odd pattern?

[dtype38]

Due to the demise of a thrust bearing I had to change the crank on my race engine this week. It's a full strip-down job as the head has to come off to get the timing chain off the crank. No biggy, but when I looked at the chambers and pistons the carbon pattern wasn't anything I'd seen before. Now I've run this engine for years and it likes to run rich (like 5% CO rich - its just the way it wants it!) so when I take the head off I usually expect to find 6 black cambers and 6 black pistons. But last time I rebuilt it I put some different pistons in and this time they weren't all black. The chambers are, but I've never seen completely clean sections of piston before. Anybody know if its something I should be concerned about?

[GORDON STREETER]

It normally means oil contamination during combustion IMHO

[VIVA GT]

In my experience this is usually the early stages of detonation! (But you've been lucky, it's only taken the carbon, and not the aluminium of the piston or the head. Some may say that this shows you've got it all spot on, just on the right side of serious detonation...

[graham bahr]

are the pistons slightly taller than the old ones? if you run really tight sqwish clearances you get sections of the head/piston where no combustion takes place, when i pull race pintos and m10 bmw engines i've built ( 0.030" swish) apart they always have clean bits on the piston where the piston comes close to the head/valves

[dtype38]

I'd be surprised if it was oil contamination as its only done four or five hours running since a bore hone and new rings. Having said that, Venolia do specify very large piston clearances so it's possible.

Detonation wise, I'm actually running more conservatively on this set of pistons, down from 10.8:1 to 10.5:1 and backed off from 34deg max advance to 32deg. There hasn't been any hint of pinking so I'm hoping that isn't an issue.

Indeed the piston crowns are a lot taller than the old ones (although not the height at the edges) but being a lazy arse I couldn't be bothered to take them all out and machine the tops off so I just fitted a thicker gasket. This has kept the compression ratio similar, but ironically its massively increased the squish around the edge from the 30" I'd normally use way up to something like 120".