Usually use the pressure tester to look for a large discrepancy in pressure from one or more cylinders to the rest. If you have 240 psi in six cylinders and 180 on the remaining two there is definitely a problem.
Compression ratio is a different matter, if you have two engines exactly the same except one has dished pistons and one has flat top pistons, the flat top has a higher compression ratio and will have more psi on a compression test. Now if you change the camshaft on the higher compression ratio engine, put in a camshaft that closes the intake valves later than the original camshaft you will lower dynamic compression ratio and the cylinder pressure in that engine. It will still have the same static compression ratio but will have less cylinder pressure than it had with the original camshaft. Now take two engines that are exactly the same, engine A and engine B, they have the same compression ratio and the same cylinder pressure. Now change the camshaft in engine A. The new camshaft for engine A closes the intake valves sooner than the camshaft in engin B. They both have the same static compression ratio but now the valves in engine A closes the valves sooner than the valves in engine B. Engine A now has a higher dynamic compression ratio and a higher cylinder pressure, but they both have the same compression ratio rating.
A leakdown tester or cylinder differential pressure tester will put air into the cylinder at a specific psi, 'used to use 140 psi on piston engine propeller equipped aircraft. One gauge of the tester tells you psi of the air going into the cylinder, the other guage tells you the pressure the cylinder is maintaining and thus the pressure loss. If there is a large pressure differential in the cylinder it can indicate a problem in the cylinder such as burned valves or worn rings/cylinder walls. Listening for and feeling air escaping out the crankcase breather MAY [MAY] indicate a ring or cylinder wall problem, air leaking out the carb would indicate an intake valve leakage and air out the exhaust would indicate a leakage past the exhaust valves
Sorry I missed the part about how to interpret the results of the compression test. I wouldn't worry about the 110 in # 8 on a used engine. Number 8 could be higher due to a bit of carbon build up or maybe the engine cranked a bit more for that cylinder. I doubt it is intake lobe wear.How long ago, how many miles, since the test? Check it every once in a while, if it keeps going higher while the rest stay the same it may be cause for concern.
That is aweful high compression. Last motor I ran that had that much compression ended up busting ring lands out of two pistons(they were cheap cast pistons) and wasted the rod bearings. I never heard any detonation but apparently it was happening. These days I shoot for around 175-185. No more. I agree with compression that high I would be running some 110 mix untilI could do something about it. Think of it like this... when the piston travels up on the compression stroke the higher the compression is, the more it's trying to push the piston back down against the pressure buildup. The rod bearings(mostly the rod side) are the wear parts that take all the heat of the action.
I'd say it depends on the shape of the combustion chamber and the amount of quench. We don't know what the build is so we can't really tell if there is a danger of failure. I think 200 should be ok with 93 octane but about five pounds lower with every one point drop in octane. Fuel quality is another problem. Premium gas where I live will hardly keep the engine running as its so old. It cost so much no one buys it and it's now junk. I run 120-125 cranking psi. but I also have aluminium heads and a heart shaped combustion chamber,zero deck with lots of quench. Like Firebob and Bigchief, I would want the best gas I can get without knowing whats inside your motor. Better safe than sorry
Well that sucks, Man I sure miss those days when I used to pay around $0.50 per gallon for 100 octane leaded gasoline. Only drawback was cleaning the lead off the plugs once in a while.... Sigh! Now they make fuel out of food, crappy fuel...sigh!
Well time moves on for better or worse. I can get 94 octane at Duncan, Vancouver Island. There are a lot of racers and gear heads over there so the gas is always fresh. Not so on Pender Island where I live, no gear heads and gas so bad I put regular in and an octane booster to get me off the island.
But then none of this deals with tbtx2010's question. I hope we helped more than muddled.
