The motor still isn't running too well - idling at 2000. But anyway I ran it for a while on the weekend until the temp needle started to lift off of cold and then did a compression test.

Cyl 1 showed 160psi twice in a row and then I did 2,3,4 and they all showed 150-152 so I then went back and re-tested cyl 1 to find that it then read 150.

Perhaps the motor cooled down enough during the compression test to vary the readings on cyl 1. I wonder if the motor was hotter would they have all read higher?

I'm very confused now. Either way it seems I'm not going to get anywhere near the 192 that people say I should get.

Is the engine new or old? Have you checked valve clearances (.014") hot on both valves.

Sorry about the second post,
Cranking pressure depends on engine condition including ring seal and valve seal.
I suspect other factors are at play here, possibly cam timing. How did you setup the cam timing?

Engine temp wouldnt vary the readings that much over a short period. The temp would only make 5% diff I would expect, say 10 Psi maximum.

Someone else set up the cam timing.

Anybody got any advice for checking cam timing on the a12?

If the engine has an after market cam, you can check the inlet valve lift at TDC.

Stock cam timing:-
Align the the crank pulley key groove on a line passing through the cam centre and crank centre. The groove should be on the side away from the cam. Match up the dots on the crank and cam pulleys with the bright links on the timing chain.

To check the timing, rotate the key groove till it is vertically above the crank centre line. The bright link should be nearly directly above the key groove. A line drawn from the crank centre should pass through the crank timing mark the cam centre and the second bright timing link on the chain.


Sorry if its a bit boring.

you can can cam timing by pulling the valve cover off and and rotate engine so that #1 cyl is at TDC on overlap adjust both valve clearance to zero lash rotate engine to TDC on compression stroke and measure clearance they should be the same if not Exhaust clearance greater is retarted cam timing, intake clearance greater cam is advanced. only works with standard grind cams (same grind on intake and exhaust lobes) remember to reset clearances to correct clearance. You can also check for a streched chain by pulling off distributer cap
rotate engine clockwise know while watching rotor turn engine counter clockwise the rotor should move at the same time if delayed shows chian is streched.

I have never seen as much as 178 Psi in a low-compression engine. 140-150 is more typical, and as long as the four cylinders are close to each other the engine is good.

britishcarfreak, your test results (150-160psi for all cylinders) sounds very good to me. The A12GX came in two compression levels, according to my parts book: standard and premium fuel.

Well I seem to be a little confused here because I read that the a12GX was a standard 10:1 and by my reckoning that's going to be a fairly high PSI but I would have thought closer to 170 would be 10:1. Anyway people are now talking about fair figures for low compression.

I thought that anything over 9.5:1 was high compression and that say 7:1 or similar was low.

What is the standard comp. ratio of an A12 GX - is it 10:1 and by peoples reactions here do people consider 10:1 to be high or low comp?

By "low compression" I mean 8.0 - 9.0. There aren't any modern naturally aspirated engines to my knowledge lower than this. Or at least not common.

Pre-emissions controls, over 9 was common. Compression was lowered more or less across the board for the US in 1971 to around 8.5:1.

Does psi relate directly to comp ration?

In other words if 10:1 = 192psi on an A12 - will 10:1 on a ford v8 also give 192psi?

I thought that cam operation had a lot to do with static comp. readings and that the actually operating compression would be a combination of effects and for example at good revs a car with extractors may actually acheive better than 100% cylinder filling and therefore have a higher comp. ratio at that instant than usual.