help with lower cranking compression after rebuild, causes?
11-05-2007, 04:34 PM
#1
help with lower cranking compression after rebuild, causes? hoping someone can give me some reasons why,
engine = 2002 181 cid (3.0L) GM (mercruiser marine in a 19' boat).
bore/stroke = 4.00" x 3.60"
CR = 9.3
cam i think (from reliable sources) 204/204 deg @ 0.050", don't remember the centerline #'s.
cranking compression had been 170 psi last year on my gauge which i trust, and was getting 5000 rpm wide open. this is my baseline, and i was happy.
let family member use boat this summer, plug falls out of the pan, ran dry and siezed one rod bearing. pulled motor, had rebuilt by good shop.
had crank turned 0.010,
one new rod- the one that siezed,
4 new pistons
new cam and cam bearings
in addition, had the shop do cylinder head work, they had a flow bench and they cleaned in and around the valves. Also got 3 angle valve job, and the head was shaved just enough to clean it up. If anything I would think this would've helped compression, no?
Have about 3 hours on the engine and it runs great,
but cranking compression test only gets me at best 150psi, on average it's 140psi after 3 puffs. And my wide open rpm's at best are 4800 and it takes 3+ seconds to climb past 4600 to 4800. Prior to rebuild the motor would get right to 4800 no problem and reach 5000 in 1-2 seconds.
So I have fairly good reason to believe i'm not getting as much power as before the rebuild, and best I can tell at this point is because of reduced compression. I can pretty much rule out all other variables: same prop, clean hull, same weight in boat (just me), it's even cool weather now which should help, and I am 100% positive the carb, fuel delivery, and ignition is 100%.
Can I expect better compression #'s in the next 10-20 hours of operation, does anybody have first hand experience with this?
Will rings really seat and seal that much more as the motor breaks-in?
other than that, I can't figure what would cause the apparent loss in compression and the 200 rpm loss at top end. With all the planning going into the rebuild the head work, I would think I'd get the same level of performance right off the bat.
engine = 2002 181 cid (3.0L) GM (mercruiser marine in a 19' boat).
bore/stroke = 4.00" x 3.60"
CR = 9.3
cam i think (from reliable sources) 204/204 deg @ 0.050", don't remember the centerline #'s.
cranking compression had been 170 psi last year on my gauge which i trust, and was getting 5000 rpm wide open. this is my baseline, and i was happy.
let family member use boat this summer, plug falls out of the pan, ran dry and siezed one rod bearing. pulled motor, had rebuilt by good shop.
had crank turned 0.010,
one new rod- the one that siezed,
4 new pistons
new cam and cam bearings
in addition, had the shop do cylinder head work, they had a flow bench and they cleaned in and around the valves. Also got 3 angle valve job, and the head was shaved just enough to clean it up. If anything I would think this would've helped compression, no?
Have about 3 hours on the engine and it runs great,
but cranking compression test only gets me at best 150psi, on average it's 140psi after 3 puffs. And my wide open rpm's at best are 4800 and it takes 3+ seconds to climb past 4600 to 4800. Prior to rebuild the motor would get right to 4800 no problem and reach 5000 in 1-2 seconds.
So I have fairly good reason to believe i'm not getting as much power as before the rebuild, and best I can tell at this point is because of reduced compression. I can pretty much rule out all other variables: same prop, clean hull, same weight in boat (just me), it's even cool weather now which should help, and I am 100% positive the carb, fuel delivery, and ignition is 100%.
Can I expect better compression #'s in the next 10-20 hours of operation, does anybody have first hand experience with this?
Will rings really seat and seal that much more as the motor breaks-in?
other than that, I can't figure what would cause the apparent loss in compression and the 200 rpm loss at top end. With all the planning going into the rebuild the head work, I would think I'd get the same level of performance right off the bat.
11-05-2007, 05:06 PM
#2
Rings should seat almost right away. Dont let anyone tel you different.
Do a leakdown test. That'll tell you more about the integrity of the motor. It could be alot of stuff, you covered most with the same prop, clean hull, weight, that stuff. but it could be in the driveline, exhaust, cam timing, etc.
first thing to do is make sure the leakdown is minimal, and if not wheres it going?
Do a leakdown test. That'll tell you more about the integrity of the motor. It could be alot of stuff, you covered most with the same prop, clean hull, weight, that stuff. but it could be in the driveline, exhaust, cam timing, etc.
first thing to do is make sure the leakdown is minimal, and if not wheres it going?
11-05-2007, 05:43 PM
#3
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Did you use synthetic to break the engine in? It takes a little longer for the compression to build. Most ring manufactures do not recommend breakingan engine in with synthetic. If you used synthetic, drain it and put some conventional in and see what happens.
11-05-2007, 09:01 PM
#4
oil is mobil delvac1300 15w-40. It's a flat tappet hydraulic cam by the way.
A good amount of moly paste was used on the cam along with clevite assembly lube everywhere. So that's mixed in with the oil, but i doubt that would have any affect on ring seal. I agree, rings seat immediately, which is why i'm a little stumped. With the 3 hours runtime so far, the oil still looks like water on the dipstick.
i went and got the boat, towed it back to my dock, pulled it right then, had the motor out the following weekend. prop wasn't dinged, the drive went directly in my basement and stayed there until it went back on the boat a few weeks ago. I have all those variables covered, so i am very certain that whatever the cause might be is in the motor. And all the drivetrain is, for those who don't know, is a coupler mounted on the flywheel with the driveshaft from the outdrive fitting into it- like the driveshaft fitting into the tail end of a transmission. I put the drive on myself in 5 seconds, so I know the motor alignment is dead on.
I have a leakdown tester I made, if it's not too cold when i pull the boat in a few weeks i'll give that a try i guess.
i'm wondering if the bottom end kit had aftermarket pistons that are off and sitting lower than the originals. what's the possibility of that, i mean the block says hecho en mexico? Or maybe my new head gasket is thicker? I have no idea how to find out the head gasket thickness of the oem gasket vs the new felpro one.
A good amount of moly paste was used on the cam along with clevite assembly lube everywhere. So that's mixed in with the oil, but i doubt that would have any affect on ring seal. I agree, rings seat immediately, which is why i'm a little stumped. With the 3 hours runtime so far, the oil still looks like water on the dipstick.
i went and got the boat, towed it back to my dock, pulled it right then, had the motor out the following weekend. prop wasn't dinged, the drive went directly in my basement and stayed there until it went back on the boat a few weeks ago. I have all those variables covered, so i am very certain that whatever the cause might be is in the motor. And all the drivetrain is, for those who don't know, is a coupler mounted on the flywheel with the driveshaft from the outdrive fitting into it- like the driveshaft fitting into the tail end of a transmission. I put the drive on myself in 5 seconds, so I know the motor alignment is dead on.
I have a leakdown tester I made, if it's not too cold when i pull the boat in a few weeks i'll give that a try i guess.
i'm wondering if the bottom end kit had aftermarket pistons that are off and sitting lower than the originals. what's the possibility of that, i mean the block says hecho en mexico? Or maybe my new head gasket is thicker? I have no idea how to find out the head gasket thickness of the oem gasket vs the new felpro one.
