The 'E' Type Forum

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

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.

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

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!

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

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

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

Mark