Offshoreonly.com - Compression test

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Quote:

Originally Posted by SB (Post 4939387)

My brain chat (it talks/mumbles to itself :) ) older person real experience says165-175 across the board is healthy 8.75:1 502 cid motor. :)

That Ai referred to above doesn’t know any better that to get 200-210 cranking compression needs a solid 10.5:1 compression ratio to get there.

I tend to agree with the above comments. I had a 496 Mag (not HO) it cranked around 170 - 180 psi on a good Snap On gauge. Years ago I had a 468 BBC in a drag car and it had 13:1 compression but it also had a 274* / 278* @ 0.050" roller in it which bleeds off a lot of cranking compression. It cranked about 185 - 195 psi with a 3.75:1 gear reduction starter. If I put my 4.25 gear reduction starter in it, it only cranked about 165 psi due to the lower cranking speed. I could retard or advance the cam and make 20 psi difference. I chased my tail one day because it only cranked about 170 psi, until I finally borrowed another another gauge and found out my gauge was out 20 psi. So there are a lot of variables and I was doing it on a cold engine. If the readings were all within 10% and it runs the same speed as it always did, I wouldn't worry about it. If you are really concerned about wear, I would trust a leakdown test long before a compression test to tell me if the rings were worn or the valves not seating.

That sounds like a the way, thank you. My biggest concern is knocking.

I do not have the time to do that before the toy run so will fill it with 93 and then after do these tests. I plan on pulling motors in Jan and getting rid of silent choice, Putting dry tails on, adding sea strainers and most likely pull all off transom and reseal. It is clean but still a 24 year old boat. Should have it ready for the lift by late Feb.

Quote:

Originally Posted by articfriends (Post 4939404)

I wouldnt worry so much about your "compression". most 500s test about 150 to 160. It sounds like your concern is what OCTANE you can get away with. Hers a REALLY simple test, verify base timing is at 8 degrees in service mode, throw timing gun and distributor wrench in the boat, then fill boat up with 89, plug a pair of rinda tech mates in, go out at watch for KR thruout the throttle range, deliberately undertrim the boat so it sorta "lugs", if you have NO KR, your golden on 89 and up. IF you want to "find" the threshold of KR, bump base timing up to 10 degrees which essentially "overtimes it" by 2 degrees, about how much timing I like to be from KR if my dyno and boat LIKES the timing. You start seeing traces of KR here and there, your close, if not you could go a step further and try 87. Your boat should NEVER show constant KR at wot, IF it does you need MORE octane, less timing or injectpr service etc.