2011-01-27

Engine blues

Explosions are cool. But when they happen in an engine cylinder at the wrong time, they are called detonations and then they're not really so cool anymore.

That's what happened in my little Miata's engine. As a result, it began using a bit of oil and also lost some compression.

On a leak-down test, we measured eight percent compression loss, and when we looked inside the cylinders with a scope, we saw scoring on walls and piston heads in cylinders One and Four.

The current theory is that detonation was caused by a temporary leaning out of the mixture after my car sucked in some gunk (it's a technical term!) through the intake. The two cylinders that are on the same cycle are both affected, while the other two are a-okay.

For now I will continue running the car as-is and see if it deteriorates any further. The engine is a Rebello, built in 2006. It is still going strong, even with the compression leak, and can pull on bone-stock Miatas in a straight.

When the time comes, this engine will come completely apart, and the block will be honed and rings redone, a valve job too--the whole nine yards.

Another idea is to build a whole new race engine while this one is nearing the end of its useful life. It wouldn't be a bad idea at all to have a spare race engine.

Luckily I got time to figure it all out, and there's no real need for immediate action.