Lowering Compression Ratio

within the micra comunity we use 2 oe head gaskets and have been proven to work well with 160+ hp and it works fine.... but if you can get a copper gasket made up, i think they can be done in virtually any thickness iirc!

don't count me on this but it works, theres are other methods and try thinking using different combinations and use an online compression ratio calculator
www.csgnetwork.com/compcalc.html

Also need to know if they're flat top pistons or is there a combustion chamber in the head or is that flat too ?
Either way, you need to know the volume of that

Bore area on 1 cylinder is - 51.53cm2
Volume of one cylinder is - 432.85cc
Overall capacity is - 2597.11cc

Pistons are flat top.

You head gasket needs to be 4.71/51.53 cm thicker. That is roughly 0.9 mm.

more often than not yes, which would be rather more preferable than two gaskets.

or just stick to the standard compression and get it mapped properly, more responsive engine then

This is a fair point, what are you wanting to lower the compression ratio to achieve? 9.25 isn't exactly high, dependent on the engine you could get away with decent boost on that with proper management, especially if it's a 4 valve head.

What's the engine this is being done to?[/quote]

^ whs
need more details really,as raising the head by fitting a spacer/more gaskets could cause more problems than it's worth!
an OHC engine will have the cam timing slightly altered for example,or longer/adjustable pushrods needed on an OHV ect

9.25 is low enough for anything really
What engine is it?
There is no need to go lower than 9:1 ever, unless you're lazy and can't be arsed to do it properly.
Properly specced, set up and mapped high CR engines will take shitloads of boost, and be responsive and fuel efficient in vacuum.

Lowering CR allows you to run more boost as it raises the knock threshold, but will not convert the air/fuel mix into as much power as a high cr engine. So you need to cram more air/fuel in there in the first place.

Cool charge raises knock threshold - ie efficient intercooler.
A turbo working inside it's max efficiency area of a compressor map will provide cooler charge than one that is being over-worked.

A good exhaust manifold design (especially divided manifolds) reduce 'reversion', increasing the knock threshold - as less spent gas is contaminating the fresh air/fuel mix.

Good fuel will raise the knock threshold,
Water injection does amazingly well too, doing several jobs at once, letting you run more boost.

Cam timing also makes a huge difference to the engine once positive manifold pressure is introduced - cams can 'spit' a little bit of mix back out into the inlet as they don't shut the inlet valve in time for the compression stroke - this is the 'dynamic' compression ratio which is the actual reality version of the theoretical compression ratio that you work out.

The way the boost is introduced is major key - too much too early in the revs is what kills a lot of high-cr forced induction engines. If you have this much boost that early, then you need a larger turbo as the hot side is too small and is strangling it.

Basically, employ as many of those things as you can and not worry about it

I realise most of these things are turbo-related, but similar principles apply to supercharging.

I ran 20psi on a stock vauxhall 20seh engine (2l 8v, 10:1 CR) for about a year till it got upgraded, no det, knock or anything. It was brilliant, I was a fool to change it!
Sorry for the essay, hope it's helpful!

You will also have to consider the possibility of not having such an strong over-engineered engine. I for one have a 1.8 turbo which runs 0,7bar stock and blows ring lands at 1,1bar, and that's at 7,5:1 CR. And that's an all stock engine, so sadly, not all engines take to boost very well.

If you want to do it properly, you'll need to have a proper look at the bottom end design and decide if you need stronger pistons, thicker rods, lower CR, etc or none of the above.

If parts (engines) are cheaply enough and you just fancy a go, then by all means have a go

the rolling road operator when i got the granny dyno'd said that would be fine for boosting, and my engines at 10:1 cr ;D

its all down to keeping the timing in check at the end of the day. and getting decomp plates made up is a bit of a compromise really. two headgaskets is just asking for trouble on a cast iron straight six surely? ;D

bung the turbos on there with the standard lump, then if it lunches something worry about it then