Which compression ratio pistons to buy for my 4.2?

[paulsco]

Hi,

I am in the process of re building the engine on my 4.2 series One E-Type and I am going to need to buy new pistons.
With UK fuel, would you recommend going for 8:1 or 9:1 pistons and is there likely to be much difference in the power output?
The head is skimmed by 12thou (I am pretty sure it has never been skimmed before) and the block is likely to need a clean up skim of a few thou.
I am thinking of going with Hepolite short skirt pistons, would this be a good choice or would you stick with originals?

Thanks,

Paul

[daverawle]

Paul,

If you go with 9:1 pistons in an engine with a skimmed head/block it's a simple matter to measure the cr before final assembly and fit a suitable thickness gasket. I have an article and spread sheet detailing how it is done and making the calculation of gasket thickness if you are interested.

Dave

[paulsco]

Hi Dave,

Thanks for your response and offer.

I would be interested to see the spread sheet you mentioned.

I am giving serious thought to going with the 8:1 pistons; I have read that the difference between the 8:1and 9:1 in power is about 10-15 BHP at maximum output. I am thinking that with todays fuels, it would be sensible not to go too high with CR.

Paul

[daverawle]

Paul,

There are 2 documents, 1 pdf and 1 xls
PM me with your email address and I'll send them.

Dave

[christopher storey]

I hope I have not inadvertently posted this somewhere else, as this is my second attempt. I do not recommend 9:1 now, because a. they are harder to find than 7.8 or 8:1 pistons these days, and more particularly b. on a skimmed and decked engine, which many are, even with 8:1 the actual ratio will be close to 9:1 and may go well beyond it , and c. unless you commit yourself always to Super unleaded you are likely to get pinking, bearing in mind that the 9:1 engines were intended to be run on 100 octane, which is no longer available

[ChrisC]

Personally I rebuilt my engine with 8:1 but also have high lift cams and fully balanced components. I think the 8:1 is most suitable to the fuel available and I doubt you will notice much of a difference in road performance

I would not bother changing the pistons to a more sporty type unless you also throw away the con rods and put some cosworth (or similar) versions in. Its the weight of the conrods that you need to get down rather than the pistons. But DO get everything balanced. Jaguar were pretty good in their attempts but not great...