Measuring compression ratios
To have any understanding of compression ratios (CR) in a twostroke there’s a need to know the current CR before any machining gets under way. For the purposes if this article we’re only looking at the uncorrected CR i.e. the volumes of the cylinder/head at top dead centre and bottom dead centre. The commonly heard term ‘corrected CR’ refers to measurements taken after the exhaust port has been closed by the piston. Without going into the maths there are four key volumes that need to be confirmed;
1. The volume of the piston crown. 2. The volume of the cylinder head. 3. The volume of the cylinder head gasket. 4. The volume of the cylinder taking into account any oversized pistons/bores.
From these and the volume of the cylinder(s) a compression ratio can be calculated using all that O-level maths you never thought you’d need again. With multi-cylindered engines each bore/head combination needs to be individually assessed; there’s never any guarantee they’ll all be the same! As a rule of thumb for a road going stroker an uncorrected compression ratio of around 10:1 should be a safe bet with standard pump petrol but if you’d like a little more of a safety net then the premium grades of super-green fuel will give you and your engine a little more wriggle room.
A/ With the piston at TDC a piece of thick steel is temporarily stuck to the head with grease. Using a burette, paraffin is carefully measured into the cylinder. When both drilled holes are filled the volume of the piston crown will be the paraffin used minus the volume of the two holes. B/ Using a similar technique a clear sheet of thick plastic is used to seal the cylinder head; the volume can then be ascertained. It’s crucial here to ensure that the correct grade of spark plug has been fitted or the reading will be wrong. C/ Using a micrometer to measure the thickness and Vernier to measure the diameter the volume of the head gasket can be calculated. It might not appear to be significant but on the 180cc twin each copper gasket yields 1.1 cc of volume.