Compression test....SBC 350

[Sand Dawg]

Did a compression test on a 350 Mag the other day in my boat. 160 to 155 all
the way around until # 3 went 50lbs. and # 5 went 45lbs.
What is my problem?????

[cruser]

My guess: Head gasket

[Moneypitt]

3 and 4 are across the block from each other, so it would have to be 2 head gaskets, if that is the problem. Check the rockers for movement compared to the others, it is possible a flat lobe(s) could starve the cylinders of air, showing less on the gauge. You didn't say how old the engine was, but it could also be bad valve sealing in those cylinders.........MP

[obnoxious001]

3 and 4 are across the block from each other, so it would have to be 2 head gaskets, if that is the problem. Check the rockers for movement compared to the others, it is possible a flat lobe(s) could starve the cylinders of air, showing less on the gauge. You didn't say how old the engine was, but it could also be bad valve sealing in those cylinders.........MP
His post shows #3 and #5, adjacent cylinders.

[Ralph Brunt]

do a leakdown test will tell the hole story my .02 ralph

[HP350SC]

I'll vote head gaskets as well. Either way you are yankin' the heads. Get wrenching :hammerhea

[rerfert]

Either way you are yankin' the heads.
Yes but wouldn't you like to know what the problem is first??
Do the leak down test.

[SmokinLowriderSS]

I'll bet #3 leaks down into #5 really fast. My $$ goes on the head gasket too. Make sure the head isn't warped before it goes back on, or a new one won't last long.
I would also check the rockers for tension tho as well, quick & easy check. I would think 2 flat intake lobes would do such, but I think gaskets are more likely.

[Sand Dawg]

thanks guys

[Sand Dawg]

If I wanted to do a leak down test,
how do go about doing it?

[Moneypitt]

You're right, 3 and 5 would be a head gasket between the cylinders. The next question is why??????? Along with the flatness of the head/block, don't over look the fasteners, head bolts do die and loose their tensil strength. I've seen it too many times, new gaskets, same problem. Also check the block threads, especially BBCs that are live to water..........MP

[BUSBY]

If I wanted to do a leak down test,
how do go about doing it?
Get one of these ans screw it into your spark plug hole while piston is at TDC ... connect it to your compressor & calibrate (set the air via it's regualtor) to 100 psi. It will show you how much air is leaking (shown as a percentage) 5% is normal leakage for a good motor ...
http://store.summitracing.com/default.asp?Ntt=leakdown&Ntk=KeywordSearch&DDS=1&searchinresults=false&N=0&target=egnsearch.asp
Good luck ...
Brian