downunder
Posts: 3015
Joined: 10/10/2002
From: Adelaide, AUSTRALIA
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This is actually an answer to Daniel from PNG in another thread (the poll on nitro %) but I figured I'd make it a thread on it's own as a "how to" guide.
First up, I've never found an engine yet that won't work perfectly well with 80/20 straight out of the box. All engines come ready to use nitro so their compressions are fairly low (around 9:1) so what most do is find a fuel with enough nitro to suit. I worked the other way round and found a compression that suited no nitro.
Basically, raising the compression is just a matter of making the entire combustion chamber volume smaller. The thing here is that this volume includes the volume underneath the cylinder head and the piston at TDC. That's the squish volume and can be around 20% of the total combustion chamber volume. Taking out a shim reduces this squish volume so it raises the compression a little but not by much so more drastic measures are needed. By drastic I mean machining the mating surface of the head so it goes further down the bore and closer to the piston.
But before you do anything at all you need to do some quite careful measuring. First you have to find the exact swept volume of the engine by measuring the bore and stroke. A ".46" won't necessarily be an exact .46 for instance.
Then you have to measure from the top of the liner down to the piston crown at exact TDC and measure the combustion chamber protrusion (with or without any shims but measure the shims too). The difference between these figures tells you what the squish clearance is (the gap between the head and the piston). Usually this is quite large, maybe .020" to .030" or more, and I have a self imposed minimum clearance of about .010" to be on the safe side. So if you found a figure of say .034" then you know you can safely machine off .024" if necessary.
Now comes the really hard part  . You have to find the exact volume of the combustion chamber in the cylinder head. For a .46 size engine this will be somewhere around .8cc and the only way I've found to measure accurately enough is to invert the head (with a plug fitted of course) and slowly fill it with turps (kero) until it's dead level with the outer edge of the squish band. I can measure the volume down to about .01cc but it needs a lot of care and a steady hand . But that's the kind of accuracy needed.
Once you've done everything above you can now work out the compression of the standard engine. You have the swept volume of the engine so call it A. Add together the volume of the combustion chamber to the volume of the squish clearance to get the total volume (call it B). It's compression is worked out by (A+B)/B.
From here on everything is just maths and I do it by making up a spreadsheet so I can just enter numbers such as what happens if I machine off a bit from the head or take out a shim and the final compression just pops up. You could do it the other way round and make a spreadsheet where you put in the compression you want and the result will be how much to machine off the head.
Finally, by experimenting I found the best compression for zero nitro was 13.5:1 but anything around 12:1 is worthwhile. Coincidentally, another guy in the club had been experimenting with 4 strokes and he found 13.5 was ideal for them too.
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Raising compression for zero nitro - 
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