Compression - is it important ?

Posted by DavidM On 2011/2/21 6:13:38

I recently had my 1929 Packard off the road for some repairs for a few weeks. During that time I had to turn the engine over by hand so I loosened all of the spark plugs to make it easier to crank.
With work complete I took it on a test drive on our nearby "test hill" then on a 100 km club run. It ran very well, except for what sounded like the start of a failing exhaust manifold gasket.
The engine was recently fully overhauled so the manifold leak was puzzling till I recalled a few days later that I had not tightened the spark plugs.
They were all very loose, typically a couple of turns to tighten.
So I tightened them then took it back to the "test hill" expecting a noticeable improvement but it topped the hill maybe 1/2 mph faster at best. Being realistic it felt no different except the exhaust leak had gone.

So how critical is is it that valves are sealing and rings holding compression?

It reminds me of a comment recently from a friend who has been around cars all of his long life, he said that an engine reaches its peak performance when its about 75% worn out!