318 compression test results

OP
I don't care what the Service limit is;
90psi is low, even for a lawnmower, and
A 40 psi deviance is drag on the next cylinder.
Those 90/40 specs, if correct, are, IMO, conceived to fill a warranty agenda;
The same deviance can be had as a lobe falls off your cam, and who among us ignores that when it happens? Oh yeah great engine! it only has two cylinders at 120psi, compared to all the others at 160psi, so Ill just wait until they all drop to 90psi; Com'on; that's , dare I say it, retarded.
The 318 at full 135/140 is already a lo-po slug, but at 90psi, it is beyond my imagination to conceive that the factory spec could be that generous.
Just cuz an engine like that still runs is meaningless; the power and fuel consumption are already pathetic, not to mention oil-consumption if the rings are the cause of it. And if it might happen that an intake valve has gone down, then you have an engine-fire waiting to happen.

That's partly how the 318 went from being a strong runner before 1973, @150/155psi, to a pos bag-of-chit by the mid 80s, 130/135psi. Same cam-specs I might add. And that situation probably ushered in the Magnums, which with their new head designs, and slightly bigger cam, allowed the pressure to return, bringing back the power/economy . A Magnum is, at it's most basic, just a higher-compression 318 with better heads.

By the way; here is a list of cylinder pressures, to compression ratio equivalency, for a 318 operating at 800ft with an Ica of 48* . All C/R numbers are approximate.
at 90psi, it is 5.9 to 1 .............. :(
at 100 it is 6.4........ at 110 it is 6.9 ....... at 120 it is 7.3
at 130 it is 7.75 ..... at 135 it is 8.0 ..... at 140 it is 8.2
at 150 it is 8.6 ....... at 155 it is 8.85 ... at 160 it is 9.1

Now you know why the early 318s were so peppy, namely; close to or better than 155 psi
And at 8.0 they are slugs.
and at 90psi, they are only a little more powerful than a fresh 170 slanty.

Just so you know; my 1969FSM (the only one I have);
for the 9.2 rated engines;
the pressure is listed as 125 to 155, and
the maximum variation as 25psi
I'm assuming that the range of these numbers is partly to account for the elevation at which the test was done; the manual does not say, in addition to the usual factors such as cranking speed and degree of dryness on the ring-to-cylinder wall face.

end rant