I am running a bog standard 2 litre Black Top on bike carbs and Megajolt. How much more bang will I gain by having 1mm taken off the cylinder head,.Thanks.

You will gain 1/2 of sod all and loose the money you spend. With the increase in CR you may need the ignition map modifying which will cost you some more cash.

Modifications need doing on a proven route, a CR increase on its own is totally pointless. You need cams and probably some porting to take advantage of the increase but then you may need the pistons pocketing for clearance which will lower the CR slightly. Most of the increase will be at higher revs thus the car will be less relaxing to drive.

If it runs well leave well alone.

The engine is giving 156bhp @ 6.5k.at
the rear wheels It is used for hill climbs and is never driven less that flat out except between runs. We can alter the ignition map using the lap top, I have a good free flowing exhaust manifold, afr is spot on, just need a bit more power but difficult without blowing my pension. I understand that the valves will be close but at this stage havent done any measuring.....

[Edited on 6/12/15 by redturner]

What is the standard compression ratio?
How much will 1mm reduce chamber volume?
How close will the valves be to Pistons with head skim?
You could use a thinner head gasket to reduce combustion volume below the chamber this can achieve similar results, Cometic do gaskets from thicker for turbo applications to very thin 0.25mm I think.
You can use a compression ratio calculator and input the standard gasket thickness then use thinner values to see the difference

Get some cams, mate of mine had a 2.0 silvertip on bike carbs, some Kent (not sure which ones) cams and cleaned the inlet up with a dremel.

Made 204 bhp on the rollers.

Cat cams do a nice setup as well.

These days raising the compression is mainly a choice used in conjunction with wilder cams.
It was different in the 1950s and 1960s low compressin engines where quite a lot of power could be made by raising the CR, once the CR is over 9:1 the gains for each 0.1 rise of CR start to fall off. I don't know but Mondeo compressiojn ratio but most modern 4 valve heads alloy heads are around 10.5:1 going to over 11:1 according to both theory and practice won't show much gain perhaps even a loss if the ignition has to be retarded however fitting wild cams bigger valves and throttle bodies work better with higher compression ratios.

This a link to a graph showing the maximum effect of raising the compression ratio on an engine running on the Otto Cycle.
This graph is governed by the laws of thermodynamics the real world effect of raising the compression ratio is much lower.
link to graph

Interesting, as most engines run 9 to 11 to 1 the increase produces zip on that chart however when using cams with more duration the increased compression helps the engine at the lower revs effectively fill in the torque gap before the engine comes on cam.

I can only talk about Pinto,s as that's what I have built and they like compression, adding ½ to 1 ratio makes a difference if matched to a cam.

Like all tuning it's not about 1 thing but the collection of many tuning tricks that add up to a noticeable difference.
I now fit crank scrapers as for £25 it's easy cheap and can add 2-3 bhp

[Edited on 7/12/15 by snapper]