VB COMPRESSION RATIO (ALUMINIUM HEAD)
What ho, chums!
Using two aluminium heads I’ve measured the CR of my engine, and (to save you reading any further) the Compression Ratios are 5.62 to 6.16 depending on the choice of head and the gasket thickness. (Draganfly give a CR of 7.0 – how did they get that?)
To measure their combustion chamber volumes the two heads were fitted with spark plugs, inverted and using scraps of wood for packing beneath them their faces were set dead horizontal using a spirit level. Then using a good measuring cylinder they were very carefully filled with kerosene (i.e. central heating oil) which having very little meniscus enabled a good flat liquid surface to be obtained in line with the surface of the cylinder heads. The volumes of the two heads were 100 & 106 cc (repeated to check). See the lower picture below.
The heads, of course, sit on gaskets whose large size and distinct thicknesses provides a considerable additional volume to the combustion chamber. Two composite gaskets, one new and one used were found to be 1.58 to 1.59mm thick (use didn’t seem to cause noticeable compression) and two unused solid copper gaskets (from Draganfly) were 0.91mm thick. The tricky bit is, of course, measuring the area of the distinctive sidevalve combustion chamber shape that the gasket embraces. My preferred method is to take advantage of the areal density of printing paper, 90 gsm (grammes per square metre) in my case. (The areal densities of printing paper are well controlled and reliable. Using a denser photographic paper, typically 120g/square metre, would be better and I see that Lidl at the moment have “Premium Photo Paper” at 280gsm – better still!) The gaskets were placed over the paper, a pencil run round inside the gasket and the drawn shape carefully cut out and weighed using a sensitive balance. The cut-out weighs 1.02g and so at 90gsm gives an area within the gasket of 113.3 square centimetres. So, multiplying by the thicknesses given above a composite gasket adds 18 cc to the combustion chamber and a solid copper gasket adds 10.3cc.
In my engine the piston top at TDC is 50thou down the barrel so I have a further volume to add to the combustion chamber. This gives on my engine (at 30thou rebore) a further addition volume of 7.58cc.
So now we can put this altogether using the formula for CR of (Swept vol + combustion chamber vol) divided by (combustion chamber vol). My engine’s swept vol is 608.6cc (30thou rebore remember!) and with a 106cc head and a composite gasket we have a total head volume of 131.6cc and so a CR of 5.62 But using the 100cc head and a copper gasket I’ll have a CR of 6.16
So the best I’ll get is 6.16, and the volume caused by the 50thou gap above the piston at TDC is costing me 0.41 in CR, no mean disadvantage.
A further point is that the squish area between the piston and head is 32.2 square centimetres (determined as above) giving a volume of 2.93cc. Now, it’s my experience that squish areas always fill up with soot and carbon and so it may be eliminated from the combustion chamber. This deletion would raise my best CR to 6.29!
Well, I’m glad I’ve got that lot off my chest, MikeN.
PS The balance used was an Ohaus "Dial-O-Gram" which can be obtained quite cheaply secondhand. They're robust and surprisingly accurate. They handle up to 310g. See the upper picture below.