I’m assuming that you want to lower the compession ratio to allow the use of lower octane fuel. The easiest way is to make a spacer the same shape as the head gasket and install it “sandwiched” between two gaskets. Remember to allow for the thickness of the second gasket when calculating how thick the shim stock should be.

First of all, it depends on if you want to increase or decrease compression. Both could be as easy as removing the head and doing some work, but replacing piston heads is also a good way of doing it, best IMO. I assume you plan on forced induction, correct? That also depends on the model of the engine, but for most 8.0:1 is a pretty turbo and supercharger friendly number. Also, depending on the model car you want to do this with, that actually might already be the compression.

If you’re just looking to save money on gasoline like @stranger_in_a_strange_land assumed than your best bet might be what he said. If you’re looking to increase power by forced induction, you’ve got your homework cut out for you. To change over from N/A to forced, I would highly suggest replacing internal components. I.E. Crank, main bearings, rods, and pistons, also depending on the application sleeves. So, if you’re are planing on forced induction, your best bet would be replacing the piston heads. You could get the same effect using @stranger_in_a_strange_land suggestion on forced induction, but I wouldn’t recommend it and the engine won’t take the abuse as well as if you upgraded the internals. Good Luck with what ever you decide to do though. :)

As @Axemusica said, going the higher-compression route will require strengthening the entire internal system of the engine, involving heavier duty crankshaft, rods, valve train components, etc. as well as bluprinting all moving components. All of the things that builders of racing engines do.The longevity and reliability of your engine will be reduced. Just adding a single “bolt-on” component is usually a recipe for a trashed engine in very short order.

Oh yea thought of something while in the shower.

Also note that changing compression ratios often change air fuel mixture too. Run too lean and it could be “bye bye engine”, run too rich and wasted gasoline and sticky valves could be in your future. So, if it were as simple as @stranger_in_a_strange_land suggested by adding the “sandwich” I’d still get and air/fuel ratio gauge to watch the levels in real time until you’re sure it’s safe. Otherwise fuel injectors or jets (carburated engines) may have to be adjusted or even replaced. Although, plenty of air is never a bad thing unless, you have weak injectors / carburetor. Then you’ll run lean and these effects can be damaging to an engine.

Thanks very much for the advice, why is running on lean mixtures harmful? I wouldn’t have thought excess air is harmful

@jahono More air is always a good thing, unless of course it’s forced, then by which if it’s forced than the injectors should be replaced to handle the higher pressure build up. Too Lean isn’t necessary extremely bad in *most cases, but it does tend to make the engine run hotter and thus breaking down friction and eventually wearing down the engine quicker. When doing most modification, one should think about keeping the engine in good working order while upgrading. Most of these effect are damaging over time, so it’s just things to think about when upgrading.

Largely true but there are a couple of things that you neglect.

1) Compared to older American engines, nearly every engine made in Japan is already (effectively) blueprinted from the factory.

2) Some engines are over-engineered right off the bat. For instance, the RB26DETT is known to exceed 600HP (more than double it’s stock rating) with no work on the internals; stock crank, rods, block…
However, that is not a universal thing. There is a reason you don’t see too many D-series engines cranking our more than 200HP while B-series regularly crank out far more, and that reason is that “some” does not mean “all”.