Compression Ratio

[Marcus2571]

Simple question which will no doubt provoke a complex answer!

Is it possible to change the compression ratio on any XK engine from the US 8:1 to the UK 9:1? I gather the variation with the lower compression US engines was to take advantage of US lower octane fuel that was cheap and plentiful.

Is it a case of installing a differing head gasket? Or are new pistons and con rods needed?

Thanks

Mark

[Hugo]

Not that this is particularly relevant to your question, but bear in mind that the stated c.r. is only notional; the actual c.r. depends on valve timing as much as anything (especially the point at which the inlet valve closes). I have seen petrol engines with (stated) 14.5:1 ratios, whereas I used to run diesels at 14:1.

[chrisfell]

Pistons are the difference.
9:1 have a larger crown.

[cactusman]

As Chris says, the difference is the piston tops. 8:1 are dished I believe whereas the 9:1 are domed. Not a five minute job to change!

[PeterCrespin]

Smaller dome, not dished. Not even flat topped. The low compression was to cut down on NOx emissions.

Tried to quote this post but it didn't have a quote option and says there's pirated software?

Pete

[christopher storey]

I have both 8:1 and 9:1 in my Es. Both are UK spec . The 8:1 car is smooth and tractable and more than adequately powerful . The 9:1 engine has a noticeable tendency to pink ( as they did when new ) , is no more powerful , and is not as smooth - although in fairness it is a good deal older . There is little point in changing for a road car, and in any event if anyone has meddled with your engine by e.g decking the blockthere is no knowing what CR you really have

[Marcus2571]

Thanks all - very useful as ever. Will stick to the 8:1 I think...

Mark