[Guest]

Well, I think it's bad news. I just did a compression test on my 37k mile boss 351. The results: (1) 180: (2)95: (3) 175: (4) 175: (5) 170: (6) 175: (7) 100: (8) 160

My observations after owning this car for six months (and not driving it too much, as it's mostly disassembled).
* low on power. The cleveland has not impressed me in this car. Just doesn't pull strong like I thought a boss should
* no oil on the spark plugs (very minor if any), and not burning oil as far as I can tell
* good oil pressure

I was planning to pull the engine in a couple months to detail the engine compartment and detail the block, so I'll do whatever I have to get this thing running the way it's supposed to.

Also, a question. As three or four of my cylinders read around 175 (as my maximun compression reading), I would've thought my highest cylinder would've read over 200 psi as the boss has 11.7:1 compression?

Could a head gasket be causing these wacky readings? A couple of notes. I did this on a cold, cold engine with the throttle closed and all the spark plugs removed. The low cylinders I checked several times (as I was heartbroken) - and removed the screw-in tester and reinstalled it. Also, on the lowest cylinder, #2, I pulled the valve cover and checked the valve adjustment. It was fine (just like the other cylinders).

Maybe I've got a broken ring. I didn't try squirting any oil in the hole....maybe that's the next step. That or a leakdown test.

I would greatly appreciate any advice. Thanks and best regards.

 

[Guest]

Do the leak down test on 2 and 7 with the oil squirt. If the figures improve you need to rebuild the low end as you've got ring troubles or worse. If the numbers do not improve, have a complete valve job done.

 

[mstngjoe]

Are you getting any smoke from the tail pipes at any time? At idle or upon acceleration from a stop?

 

[Guest]

Joe - No oil smoke at all. Not at idle, not at acceleration. I'm hoping I've got a couple bad valves. Or maybe my cam is worn. I would think the leakdown test will tell me the truth.

Do you think I should do the test on a warmed-up engine? Logic tells me the readings across the board may be higher, but the two low cylinders will still be low. Thanks guys.

 

[2ndNatr]

Your suspicions are correct - follow Pony67's advice. See my webpage for wet/dry compression test results, for a 289. #2 and #7 are out of whack; the others look OK. Maybe low for a 351 Boss (considering the compression ratio). Again, a wet leakdown test will provide details.

Good luck!

 

[Art]

A leakdown test is a far better diagnostic procedure than a compression test. A leakdown test will pinpoint the problem, whether it be an intake valve, exhaust valve, or worn rings.

Sounds like an overhaul may be in order... 37K miles is hi-mileage for a Boss motor... Most didn't make it that long.

 

[cam65fb]

On the subject of PSI rating and your high (for a street car) compression ratio. If you’re running a very mild cam with duration in the high 190 to low 200 ranges then your cylinder pressure should be around 172. But it is difficult to build cylinder pressure when the valves are open. Just one if the reason why radical cams can make a "street" motor slower than with the stock cam. BTW, I got to 172 by the following: (14.7 * 11.7 = 171.99) where 14.7 = atmospheric pressure and 11.7 = your compressions ratio. Caveat, I’m doing this by memory, so…. Good luck on your diagnoses, sounds like you’re on the right track.

 

[camachinist]

Do the leakdown warm....

And check a known good cylinder for comparison...

Any other operational signs like oil which contaminates quickly or excessive blow-by when PCV disconnected?

Get back to us with the results..