compression ratio

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Oct 16, 2006
I am considering using a decompression plate at the base of my cylinders, with the thought that this will make it easier to kick the thing over.

Les Emery sells one, as does Mick Hemmings.

Obviously this will alter the compression ratio (probably from '9 to 1' to '8 to 1') but will this also lower the cylinder pressure? If so, any idea how much?

Thanks for your help

Yes, it will lower cylinder pressure, how much depends on the thickness of the plate and the original cylinder volume. The pressure is also dependent on the temperature of the air in the cylinder. That is why pressure readings should be compared with each other, rather than much faith put into absolute readings.
Believe I recall hearing that these plates are NOT recommended for reducing compression. See INOA Tech Tips for a more authoritative view. I don't have my copy handy. - BrianK
thanks for the speedy response.

I looked in the INOA tech stuff but couldn't find any mention, the UK owners club mentions it but doesn't disapprove.

I think I'll try it and see what happens.

My error, apologies; these plates ARE recommended by INOA. What they recommend against, and what I was thinking of, are the thicker aluminum gaskets that Norton sold for a while to address the problem.

Sorry for the (my!) confusion... - BrianK
The problem with moving the cylinder up (plate under the cylinder, thicker head gasket) is that it disturbs the quench effect.
If the resultant compression ratio is very low (7:1) quench is no longer essential for knock suppression, and the engine may be fine.
However, if the ratio is high enough the engine will now knock with the spark and mixture settings that were correct before. If you use E85, race gas etc. you may get away with it. Retarding the spark to reduce knock reduces power even more, and increases temperature.
The safe method (unfortunately) is the correct low dome piston, or modifying your pistons by removing metal only from the dome in the open area under the chamber while leaving the circumferential OD area as-is to retain quench clearance. Yes, this may require re-balancing.
I have run an 030" plate since 2002 with Combat head giving about 9.3:1. Even without it this engine never pinked, detonated or anything even when pulling a high gear at low revs. Maybe I'm just lucky. That's on regular 95 unleaded. I don't use any gaskets just a thin smear of the dreaded silicone clear bathroom sealant. Mine still holds my weight (13.5) on the kickstart due to a slightly higher numerical primary ratio.
I wouldn't lower it too much as I did originally with two gaskets, especially on the 750. That dropped the ratio to 8.6:1 which seemed to drop the power significantly.
My point is not that it's always bad, but that there is reliable way to tell in advance whether it will work or not, other than the general rules of tall gearing, high weight, wide ratios, lean jetting, high spark advance, poor gas, high air temp, etc. are not the best candidates.
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