16 replies to this topic

[Iain L]

Im watching a 1275 engine of ebay to replace my 998 and just reading through the description an thought that the compression seemed a low number?

So anybody know if that would be ok?

Thanks Iain.

[Ethel]

Perfectly driveable 8.8 was standard for the 1275GT. You can always skim the head to get higher.

Im watching a 1275 engine of ebay to replace my 998 and just reading through the description an thought that the compression seemed a low number?

So anybody know if that would be ok?

Thanks Iain.

[Cooperman]

8.8:1 is a bit on the low side if you are looking for best performane. It's easy to increase it a bit by skimming the head.
Around 10:1 is good for performance, but then you really do need to use 97+ octane petrol.

[j-b_2009]

8.8:1 is a bit on the low side if you are looking for best performane. It's easy to increase it a bit by skimming the head.
Around 10:1 is good for performance, but then you really do need to use 97+ octane petrol.

i dont know if this is a dumb question but here goes,,,

why when you have high compression (like mine which is at about 10.4:1) do you need high octane petrol?

[samsfern]

to prevent detonation / pinking which destroys the engine, and can do in a matter of seconds...

currently im running 11.5:1 comp ratio, i use bp ultimate, expensive stuff! and octane booster to raise it a bit more.

Edited by samsernie, 01 May 2010 - 06:37 PM.

[Ethel]

Octane is a measure of the fuel's knock resistance. Knock is where the fuel ignites due to temperature and pressure, like a diesel. If that happens earlier than the spark would have ignited it, then instead of pushing the piston back down the bore after it's passed top dead center, it will push against the engine rotation and reach much higher pressure and temperature around tdc. Hot enough to melt the pistons.

[1293sleeper]

i will be running 10.9-1 so will be using high octain n octain boaster, going to be a bit expensive i think

[Ethel]

Swings 'n roundabouts, A high CR gives much better part throttle economy, because the combustion chambers will be less full.

i will be running 10.9-1 so will be using high octain n octain boaster, going to be a bit expensive i think

[bmcecosse]

MORE full I think you mean!

[samsfern]

theyll have the same amount, but itll be compressed more, and give a bigger bang..

[Ethel]

Erm, I meant less full than at open throttle, but yes fuller than a lower compression engine running at the same manifold depression. I guess your way is closer to the point - Higher combustion pressures = more efficient combustion.

BMC is right: the cylinders will draw in about the same charge but the high compression engine will squeeze it in to a smaller combustion chamber and burn it better.

[Cooperman]

Once you get over about 10.3:1 you really need re-curved distributor if you ae to get the best out of an engine.
With over 11:1 a custom-built one an be necessary. I know it was with my 'S' when it went to 11.2:1. The Aldon 'Red' was short on mehanical advance by about 4 degrees and Aldon re-curved one with the necessary additional available advance. With the 'Red' set to optimum advance at 6400 rpm, it would not turn over on the starter when warm.
With the custom one it was and still is fantastic.

[samsfern]

at work one of the racing midgets ( a series engine ) is running 12.5:1 comp ratio, we have to set the timing to 26 degree advance at 4k, the dizzy has been modded as well, also we use ngk bp8es spark plugs in that

Edited by samsernie, 02 May 2010 - 02:51 PM.

[l_bratch]

Sorry to hijack this thread, but what fuel should I be using for a 10.1:1 SPI Cooper engine?

I've been using standard 95 RON petrol for years, but Cooperman's comment - "Around 10:1 is good for performance, but then you really do need to use 97+ octane petrol" - made me think otherwise!

[Cooperman]

With an injected car you will be OK. It's the cars with older type ignition systems where the distributor advance is controlled by centrifugal weights which can cause a problem due to detonation with high compression and lower octane petrol. When a points-type dizzy is set for max power at the top end, the advance curve can be too much too soon and detonation, as Ethel says, can break rings or even melt pistons.