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[stinkwheel]

Still need to measure the deck height to find the compression ratio. I've got the cases back and (hopefully) ready after new bearings, cam relief and general de-burring. Just a case of setting aside the time to get them together without being disturbed by either work or domestic issues.

[Alan R]

Hi Guys----------Hi Vince,..Good old Horace Batchelor, eh ??...now that IS a blast-from-the-past !!....."spelt}-- K....E....Y.....N......S.....H.....A.......M "...

Hi Stinkwheel..oh dear matey, in a bit of a pickle I'd say.......OK, as we are dealing with Volumes then let's use the standard "cc" ( Cubic capacity ) designation..........Compression Ratio is given as}----- Swept Volume(SV) + Clearance Volume(CV) divided by Clearance Volume (CV)............SV = pi x r x r x L where pi = 3.1425, r = Bore divided by 2 ( 87mm divided by 2 = 43.5 ) and L = Length of stroke = 103mm...This gives an SV of}-- 612.38cc..........Now at this stage and for a rough guide only, I'm going to assume that the top of the piston invades the Cylinder Head volume by 50%....Thus the CV = 145.6 divided by 2 = 72.8cc...........So, by applying these figures back into the Compression Ratio formula we get}---- SV (612.38cc ) + CV (72.8cc ) divided by CV (72.8c ) = 9.411 : 1

If we now also allow for the small reducing effect of the spigot volume I'd estimate your Compression Ratio will be approx. 8 : 1 --- which should give you a reasonably peppy but tractable engine--all other things considered...Please remember that this is only a rough estimate but should give you an indication of what to expect....

[Bullet Whisperer]

The only way to find the true compression ratio is to assemble the top end on the engine with any gaskets, spacers etc you intend to use in place, park the piston, with top ring fitted at least, at TDC with the valves shut and measure the amount of oil required to fill the combustion chamber and cover a couple of threads of the plug and decompressor holes. This amount, in cc's is what you need to add to your swept volume and then divide that total by it to get your true C.R.

[ric]

That's the method I opted for used (obviously with the plug hole at the highest point) when I raised my CR to just over 9:1.
It's also a great method for finding TDC if you don't have access to accurate equipment. Just a minute amount of movement at the crank will cause the oil level to visibly rise or fall at the bottom of the plug hole.

[stinkwheel]

Well, my idea was to keep a note of all the "fixed" measurements now, while all the parts are new and not fitted which will then allow me to directly calculate the CR with varying thicknesses of compression plate/gaskets without having to physically change them and measure the effect. The only one I'm missing now is the baseline deck height (edge of piston crown below/above the liner). There are then some really funky calculator tools that take account of conrod angles/piston dwell etc that you can plug all the figures into and find what effect varying them has (like a thicker base gasket will have a slightly different effect to a thicker head gasket).



That's the plan anyway. I can always sanity check my calculations by measuring once it's built up.

[Bullet Whisperer]

A simple way to work out what various spacers, gaskets etc will do to the C.R. would be to take the cc of the engine, in your case 612 and divide it by the stroke, in your case 103mm = 5.94cc [let's call it 6cc]. Now, as a for instance let's assume you may or may not use a head gasket of 1 mm thickness - the swept volume will stay the same, but your combustion chamber volume will rise or fall by 6cc. 2 mm barrel spacer?, that will be 12 cc and so on.

[Alan R]

Hi Guys-----------Spot-on, Paul !!......mathematically speaking dividing the Swept Volume (SV) in cc's by the stroke (L) in mm we get a rating of}-- "cc's per mm ".... ( that is cc's displaced per mm of piston travel ).. And as Paul says that can be equally applied to a cylinder base gasket thickness....