6 replies to this topic

[valve bounce]

right then im building up my 1275 engine and im trying to work out my compression ratio
volume in head 21cc
volume in piston 10cc
volume in head gasket 2cc
33cc

the block has been decked so the pistons are abosoutly dead level with the top of the block

so 1275 devided by 4 =318.75

so 318.75 + 33cc = 351.75 divided by 33cc = 10.65:1 is that right?
i tryed sevral diffrent CR calculators on the net and i got 10.11:1 to 10.89:1

also i am using 21251 pistons what CR can i go upto before i hit problems? the car is going to be a fast road with odd bit of auto test, 12 rallies etc, i have gone for 276 kent cam, 28/36 weber, maniflow lcb with twin box system, aldon dissy, 940 head with 35mm ins and 29 outs. the car is running 10ich wheels and a 3.44 diff.
any suggestion of spec alterations welcome

please help

thanks robert

[Ethel]

The maths looks ok, you could maybe chuck in a CC for the ring land. It'd be well worth reading up the relevant bits of the yellow bible (Vizard says it's when the inlet closes that matters, as compression doesn't start until the valves are shut) - I think you could get away with a bit more compression with decent ignition and fuel. I wouldn't worry about the pistons - no piston will like detonation and avoiding that will limit your CR

[Cooperman]

Looks correct to me.
Don't think it would be wise to go any higher than about 10.6:1 with the 21251 pistons as the oil slot is very big.

[valve bounce]

thank you, i shall leave everything as is at the moment and continue the build

does anybody have any sugestions as to which aldon distributor to use, its a A+ block by the way

[Cooperman]

thank you, i shall leave everything as is at the moment and continue the build

does anybody have any sugestions as to which aldon distributor to use, its a A+ block by the way

Give Aldon a call and ask for Rob. They are very helpful and with the engine spec they'll advise.

[miniobsessed]

right then im building up my 1275 engine and im trying to work out my compression ratio
volume in head 21cc
volume in piston 10cc
volume in head gasket 2cc
33cc

the block has been decked so the pistons are abosoutly dead level with the top of the block

so 1275 devided by 4 =318.75

so 318.75 + 33cc = 351.75 divided by 33cc = 10.65:1 is that right?
i tryed sevral diffrent CR calculators on the net and i got 10.11:1 to 10.89:1

also i am using 21251 pistons what CR can i go upto before i hit problems? the car is going to be a fast road with odd bit of auto test, 12 rallies etc, i have gone for 276 kent cam, 28/36 weber, maniflow lcb with twin box system, aldon dissy, 940 head with 35mm ins and 29 outs. the car is running 10ich wheels and a 3.44 diff.
any suggestion of spec alterations welcome

please help

thanks robert

I think you should re-check your numbers

Have you CC'd the head? Standard is 21.4cc but even if yours is standard there are 'production tolerances'.
21251 Pistons have a stated dish volume of 8.8cc.
Most 1275 head gaskets I've come across have a compressed volume of around 4.3cc.
Have you measured the bore, stroke, deck height, etc with micrometer gauges? A few thou here and there will affect you CR...

All of this means little if you are only looking for a 'safe' CR but if you are looking to get the best from your combination of bits then it all has to be spot on.

Crunching the numbers... If you go through and accurately measure all of these components you should be aiming for a static (measured) CR of 10.1:1 to 10.5:1 to get the best out of your engine while avoiding detonation (due to CR). If you are using rough numbers to get 'pretty close' then I wouldn't exceed 9.5:1 or be prepared to buy some octane booster...

[valve bounce]

the head has been played with abit and then skimmed so the volume wont be the smae as standard when i meassured it with a piece of perspex and surynge it was 22cc. i checked the piston to top of bore with my dti gauge and from top of piston to top of block there was 0.002mm.
i was miss informed on the head gasket volume so thanks. ive rechecked my pistons and your right there 9cc with my measuring equipment so i got that wrong

so 1275/4=318.75

318.75+35.1=353.85

353.85/35.1=10.0811:1

im happy at that my old 998 use to run 12.82:1 and that was using shell optimax and loads of advance it only lasted 4000miles before it ate the bores and piston rings.

thanks, robert