compression test question
My guage reads a bit low. Even on my freshly built yamaha xs650 engines I get 100 or so. LLeak down passes with flying colors on the guage. My slant has a slight rough idle and I could only get 100 psi or so on the guage. But with a leak down it was more than acceptable. So could be the reman. Holley 1920 on it. It's not real bad for roughness so I just kind of ignore it for now. Also have black plug on 6. Clean for half of the porcelain but black for the other half to the threads. Car doesn't seem to use oil. No smoke while dad follows me...float height is good. Kind of a mystery I haven't figured out yet that may also be related to my slight idle quirk.
So....I would leak down test anyways.
So what do you consider acceptable?
I'll tell you my acceptable;on my guage. zero to 4% maximum.
You say; What! Hey I don't make stuff up. This is straight off the pages of Smokey Yunichs Power Secrets. A brilliant mechanic and racer of yesteryear.
At 8% that cylinder is toast. At 6% she is no longer performing anywhere near her potential.And bear in mind that a leakdown test is a snapshot of leakage at TDC only. From a few thou down,the cylinder could be split wide open, or the wrist-pin could have left a deep gash in the wall,and the gauge would still read A-OK. But at least you would know that the valves are closed. Likewise, the guides could be wore right out, or not even in there, but at least you would know that the valves are closed. The LD test is a snapshot. It's up to the tech to figure out the rest of the story.
EDIT
When I say toast, or rather when Smokey says toast, it doesn't mean the engine is junk; it just means the guy in the next lane with the exact same car but less cylinder pressure leakage, Is gonna out-run you. Since the engine converts pressure to performance, or fuel to heat energy, and the % of energy that actually reaches the rear wheels is so pitiful, cylinder leakage is the front line. With about 1/3 of the heat going into the cooling system, and another 1/3 going into the exhaust, that leaves just 1/3 for motorvating the car. Then you subtract another 15 to 20% for powertrain losses, Man!.there is not much left over. Who can afford cylinder leakage?
