351M compression readings

[TNT89Bronco]

I tried to find the values dong a search, but couldn't find anything. I wanted to check the "health" of my engine and did a compression test on each cylinder. Almost all of them came in around 90...one came in at 110.

Does anyone have their engine readings to compare to?

[UTfball]

Don't have my 'official' notes, but I wanna say on fresh 400's I'm 140ish. I wouldn't call 90psi bad though. Typical rule of thumb is you want all cylinders to be within 10% of eachother...but that's usually when you have 1 cylinder significantly LESS than the other 7.

[American Thunder]

With a stock/mild cam and 9:1 compression ratio, you'll get 140-150 psi. If I recall, I think a stock '75 302 with 8:1 CR I tested awhile back had 118psi. Like UT mentioned, consistency is more important than peak psi. It's an indicator of a healthy engine; that all cylinders, rings and pistons as well as valves/seats are wearing fairly evenly.
90 psi is on the low side, especially considering the other one is 20 psi higher.. But it may be leaky valves causing it, rather than rings.

Are the spark plugs oily when you took them out? If so, that's an indicator of ring issues.
Have you wet tested the compression yet? (squirt a couple pumps from an oil can into each cylinder before checking compression) And then compare the results to the dry tests. If there is a drastic difference, the rings are shot, or the bore is out of round or just worn too large for the rings to seal properly.
If the wet and dry tests are faily close in psi, you can check the valves and seats by bringing each cylinder to TDC of compression stroke, and then applying an air compressor hose to the cylinder, with a special fitting that threads into the spark plug hole. Once the cylinder is pressured up, you can listen for air leaking from the valves.

[TNT89Bronco]

I don't remember the spark plugs being oily. They were dark, but I don't think wet. I will go and see. I will try the "wet" test and see. I also need to go back and try the test again, because I know my battery was low, and I had to recharge my battery to continue the test.

The engine runs fine and it passed smog, but I was just curious more than anything. I am guessing that you take standard air pressure and multiply it by the compression ratio to get the target compression value?

[American Thunder]

It's difficult to calculate the compression psi mathmatically, since there are variables like cylinder filling %(volumetric efficiency) and all that fun stuff.

p.s. remove all the spark plugs before testing the compression too

[TNT89Bronco]

Oh. I guess I was doing it wrong then. I will remove al of them and give that a shot. Thanks!

[78-U10]

is the 351M even worth putting money into? Mine seems to run fine -- haven 't driven it yet to doing suspension and other mods. If it should die-- how hard is it to just put a windsor based engine in it?

[speedyweasel]

Good thread and better timing. I should've done a compression test two weeks ago but haven't managed to get it done yet. These figures and suggestions are useful... I love this forum.

Originally Posted by 78-U10

how hard is it to just put a windsor based engine in it?

Ew.

[Alvin in AZ]

Originally Posted by 78-U10

is the 351M even worth putting money into?
...how hard is it to just put a windsor based engine in it?

Bellhousings don't match between the two engine types.
The 429/460 bellhousing pattern is the same as 351m/400 tho. :)

A 351m can be woke-up with a cam/cam-timing change. :)
That plus a crank from a 400 would make it more than worth while. ;)

Did the cam-change-thing with a 360FE, makes a whoppin' difference!
More power over a wider band and better gas mileage too, beat that. ;)

Alvin in AZ