4 replies to this topic

[oliver122]

Hi guys, I was wondering if raising the compression from 9.5 to 10.5 make a difference. I've got a Kent 276 cam installed in a 1275 and was advised before to run it at 10.5, at the time I put the engine together I never checked the cc of the head, now I decided to measure the volumes and work out that my compression is at 9.5. I was wondering if raising it up to 10.5 would make any differnce. My Chambers in my head are at 21.5cc average, I've worked out it would need go down to 17.7cr to reach the 10.5cr, how much would need to be skimmed of the head

Cheers in advance

[absx2]

Just remember you will have to use super unleaded with an octane booster when running high CR`s these days as pump fuel is only getting worse.

The dynamic compression ratio is what you really need to know to get the best out of your cam choice so hopefully someone will come along and work that out for you or even better explain to us all how to do it.

[DeadSquare]

Hi guys, I was wondering if raising the compression from 9.5 to 10.5 make a difference. I've got a Kent 276 cam installed in a 1275 and was advised before to run it at 10.5, at the time I put the engine together I never checked the cc of the head, now I decided to measure the volumes and work out that my compression is at 9.5. I was wondering if raising it up to 10.5 would make any differnce. My Chambers in my head are at 21.5cc average, I've worked out it would need go down to 17.7cr to reach the 10.5cr, how much would need to be skimmed of the head

Cheers in advance

Congratulations on your 1,000th post.

 

Unless you are going 'Motor Racing', you won't really see enough difference to make the effort worth while, compared to a big re-bore, or better still, supercharging.

[Steve220]

Just remember you will have to use super unleaded with an octane booster when running high CR`s these days as pump fuel is only getting worse.

The dynamic compression ratio is what you really need to know to get the best out of your cam choice so hopefully someone will come along and work that out for you or even better explain to us all how to do it.

Cooper works MPi C/R is 10.5:1 and doesn't need super unleaded. I see where you're coming from, but i would say once you're in the 11's, thats when you should start to think about detination.

 

To the OP - if you're desperate to lower it, consider having the head skimmed, but watch your valve to piston heights.

[Cooperman]

I run my 1964 Cooper 'S' at 11.1:1 and use premium unleaded with Valvemaster Plus additive. It has a special custom built distributor running with points in order to comply with the historic motor-sport regulations. My 1990 Cooper road-rally car is at around 10:1, but I may increase it to around 10.3:1.

On road cars, with the correct ignition advance curve you can use current premium unleaded up to around 10.5:1. You will need to have the advance carefully set up.

Valves hitting the pistons is not an issue with the A-series engines and even if the timing chain breaks the valves will not normally hit the pistons even with a head which has been skimmed quite a bit.

There is no doubt that our old A-series engines do like a high-ish CR and it improves torque a lot.

The problem is modern petrol. With the old 101 RON leaded petrol which we used to get in the 1960's a very high CR was possible so long as the distributor was modified to suit. Now we have nasty petrol and an additive is recommended for any petrol when using aything over about 10.5:1 or slightly lower.