Thread: Series compression ratio.

you tell by the serial number of the engine, theres some other to look for things as well but unless your a rover nutter or have a low and a high comp engine side by side they arent glaringly obvious. some have a compression ratio stamped onto the block/head but I think thats an after market thing.

the compression gain is all in the head and is achievable by skimming if your game enough to.

the 7:1 works reasonabley on LPG but you'll loose a lot of torque at the bottom end.

Assuming you are talking about a 2.25 petrol, the heads that were cast to allow 8:1 had a 1/2" square boss on top of the head, next to the rocker cover, alongside the carburetter. If it is 7:1 it has the number 7 stamped on it (or sometimes no number), if it is 8:1 it has 8. If there is no boss, the casting was not designed to allow 8:1, and it is 7:1.

A small amount of grinding is allowable on the head in both cases but if there is no boss the compression cannot be safely raised as high as 8:1.

Of course, there is the possibility that the compression has been raised and the stamping not changed - after all, these engines were last installed in new cars over 25 years ago.

I can't confirm, but I suspect all five bearing engines, very rare in Australia, are 8:1. Apart from these, Australian ones seem to be almost all 7:1, although the square boss heads were fitted from about 2a suffix G for 7:1, although much earlier for 8:1. (I'm away from home at present and have no references.

John

I will back on all 5 mains bearings being 8:1 and I thought that all series III engines were 8:1 as well...

IF all goes well and you want one, depending on location I might just have sIII petrol engine in running but yet untested condition for sale as a complete unit.

Thanks for the post Dave. From what John said I definately have the later head, although it is 7:1 (7) stamped on it. I will try taking my head of and see what can be done, I may as well do rings etc and start afresh with the gas. I am a bit far from you regards the engine, I am in central Victoria.
Rgeards Andy.

Just be cautious about the possibility that the compression has already been raised, without being relabelled. Any competent engineering shop should be able to get the height of the head to confirm this. I have no direct experience, but I have heard this later head can be safely raised to about 9:1, although it is likely to give problems on standard unleaded at that compression, but should be fine on gas.

John

Thanks for the advise re compression.
Does anyone know how much must be shaved of to raise the compression ratio to 8:1?
And what is the standard cyl head dimension. (I dont think the head has ever been off, but I would like to be sure before I start planing metal.
Thanks
Andy