Compression question....502 Mag

[kevnmcd]

Anyone know what the stock compression should be in a 502 Mag motor? Lost quite a few rpms over the last few trips and I might know why now. :frown: The range was 75 - 120 psi....I am no motor guru but I know enough to know that this isn't good. :rolleyes: What do you guys think....time to refresh? :idea:

[ECeptor]

If you ran your compression test right and got 75psi on one cylinder, sorry, it doesn't look good.

[wsm9808]

Yea, that is too much spred from low to high readings, and 75 is too low. I dont know specs on your specific engine, but I would guess you should have compression in the 120# range minimum and they should be within 10% max spred.
If you do not have oil mist spraying out the valve covers/breathers or oil in the air cleaner, the rings and pistons may still be OK. If it was mine, I would pull the heads first and check the valves. We see alot of boat heads that "suck" the valves(the intake valve heads cup inward and appear to be getting sucked into the head). If that turns out to be the problem, you may need to drive the boat different. Pulling wake boarders/skiers at 1/2 plane lugs the engine and can cause your valves to cup.
Or do a leakdown test first, it should tell you if its the rings or the valves are leaking. Good luck.

[quiet riot]

squirt a little oil in the cyl's with lower #'s and repeat test. If it raises then rings are worn/sticking or bores worn. If it stays low might be head/valves and maybe avoid going any deeper then pulling the heads (unless its a hole in piston.)
Remember to do the test when engine is warm and with throttle open, but with that spread somethin is going on. Was just one cyl at 75 and most around 120? Could just be a valve sticking (guides or seat problem.)
jd

[kevnmcd]

Was just one cyl at 75 and most around 120? Could just be a valve sticking (guides or seat problem.)
jd
Nope...here were the numbers.
#1 - 115 psi
#2 - 100 psi
#3 - 110 psi
#4 - 75 psi
#5 - 100 psi
#6 - 85 psi
#7 - 95 psi
#8 - 120 psi
I may have not have done exactly the way an expert would have but I was consistent with each cylinder. I also did it 3 times in each cylinder to make sure it was giving me an accurate reading.

[cyclone]

Anyone know what the stock compression should be in a 502 Mag motor? Lost quite a few rpms over the last few trips and I might know why now. :frown: The range was 75 - 120 psi....I am no motor guru but I know enough to know that this isn't good. :rolleyes: What do you guys think....time to refresh? :idea:
i believe the stock compression is around 9.2:1... Dont quote me cause its been awhile since i worked on one.

[SmokinLowriderSS]

I believe you SHOULD see pressures of 150 to 175 lbs if your motor was healthy. Looks like it's a bit sick to me. Do the oil squirt and retest. if pressures rise, bad ring seal, if not, valve seal problems. How many hours on the engine? Also, your spread of pressures is too wide, 10% is accepted as normal from hi to lo. Looks like you are in the market ro build it again. TIME FOR UPGRADE PARTS!!!!! :D :D :D

[kevnmcd]

I believe you SHOULD see pressures of 150 to 175 lbs if your motor was healthy. Looks like it's a bit sick to me. Do the oil squirt and retest. if pressures rise, bad ring seal, if not, valve seal problems. How many hours on the engine? Also, your spread of pressures is too wide, 10% is accepted as normal from hi to lo. Looks like you are in the market ro build it again. TIME FOR UPGRADE PARTS!!!!! :D :D :D
That is what I thought too...pressures should be higher than I am seeing. I currently have 416 hours on the motor. Looks like I may try and make it through the season and then pull it out this winter for a refresh. I am thinking, since it is out and apart I might as well take this opportunity to do a little upgrading! :D

[ECeptor]

Have the valve springs ever been replaced? I've stock Merc 502 springs last only about 200hrs.

[kevnmcd]

Have the valve springs ever been replaced? I've stock Merc 502 springs last only about 200hrs.
Not that I know of. I have put the last 116 hours on it and never done the springs. Not sure if they were done prior to that.

[ECeptor]

Not that I know of. I have put the last 116 hours on it and never done the springs. Not sure if they were done prior to that.
Would be a cheap and easy place to start. Also would be something you could fix quick and have the power back for the end of the season!

[Thunderbutt]

I just checked mine and had 150 on all of the holes ( HP 500 ). If I were you I'd would also do a leak down test.

[Beer-30]

Nope...here were the numbers.
#1 - 115 psi
#2 - 100 psi
#3 - 110 psi
#4 - 75 psi
#5 - 100 psi
#6 - 85 psi
#7 - 95 psi
#8 - 120 psi
I may have not have done exactly the way an expert would have but I was consistent with each cylinder. I also did it 3 times in each cylinder to make sure it was giving me an accurate reading.
Just confirm that you did the compression check with the throttle wide-open? If not, that will hinder your readings.

[jstwkd]

I just checked mine and had 150 on all of the holes ( HP 500 ). If I were you I'd would also do a leak down test.
Hey do you know what your compression ratio is???

[kevnmcd]

Just confirm that you did the compression check with the throttle wide-open? If not, that will hinder your readings.
I didn't have the throttle wide-open, oops :hammerhea but I don't think that would have accounted for the wide range of numbers. Do you?

[ECeptor]

"dry" compression test is run on a warmed up engine with throttle wide open, right?
"wet" compression test is the same, but with a tablespoon of engine oil put into the plug hole prior to cranking. If it is the same as dry, valves are the problem. If higher than dry, it's the shortblock.
both should have three readings per plug.

[Beer-30]

I didn't have the throttle wide-open, oops :hammerhea but I don't think that would have accounted for the wide range of numbers. Do you?
I would hope not, but ya never know. Try it again w/wide-open and see what it does.
I have never done the oil-squirt thing, although I see no reason why it wouldn't help diagnose.

[SmokinLowriderSS]

Yes, run the test again with the throttle wide open. It could very easily cause those pressures to be so low. You want as low a manifold restriction as you can get for a compression test.
The oil squirt seals the rings temporarilly increasing compression if blowby is a problem. It will have no effect if rings seal good but valve seats leak.
Depending on what becomes of this, an upgrade would be the only "right(to me haha)" way to do a rebuild. Taller cam, new springs coming with it, all the other goodies there too. Exhaust improvements.
It's ONLY money$$$$. :D :D

[kevnmcd]

Depending on what becomes of this, an upgrade would be the only "right(to me haha)" way to do a rebuild. Taller cam, new springs coming with it, all the other goodies there too. Exhaust improvements.
It's ONLY money$$$$. :D :D
Definately a taller cam, some porting, and new exhaust! :) BTW...what is money again? :hammerhea

[SmokinLowriderSS]

I own a late 70's Big Block Chevy Jet Boat :crossx: :crossx: (and I pay for 2 ex-wives :argue: ) ... THUS ...... here's the equasion. :wink:
Money= something I have very little of. :eek:

[SmokinLowriderSS]

I'm gonna find enough $$$$ this winter though for a set of Lightning headers and 4" slash-cut outlets (thru-transom shorties). Will be 2K. :eat: