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A couple of things to consider........ manufacturers quoted comp ratios are notoriously misleading. If manufacturing/casting/machining/assembly tolerances were perfectly tight, ie. no deviation, there would NEVER be a need to pursue ANY avenue of blueprinting. Ever ridden 2 identical bikes with similar km's etc and wondered why one motor seems so much stronger than the other? A part of the explanation will include the dissimilarity in, and the number and range of tolerance variation between the 2 engines in a number of critical areas INCLUDING comp ratio. In the late 90's and early 00's, we put a good number of R1 motors in speedway sidecars...... we measured the comp ratios before pulling each motor down and typically found a +/-2 compression points from highest to lowest against the manufacturers spec. This is not a Yam only thing, all motors will give you a similar variation, older motors more again as manufacturing processes were equally less finite. Measure YOUR motor empirically or your result will be totally inaccurate. Use the formulae listed and the inherent error found in certain steps can either : a) cumulatively send you on a wild goose chase or b) inadvertently get you close to the mark by the sum of over-stating some and under-stating other measurements because of this inherent error.
Another point .... comp ratio is very easily calculated using this:
BORE DIAMETER SQUARED(in cm) x STROKE (cm) x .7854(constant)/ UNSWEPT VOLUME.
This will be accurate for ONE cylinder only. Why? Again, variations in the volume of unswept space between cylinders of the same motor and the reason why you must know what that volume is before calculating an individual comp ratio. Once this is known, a simple calculation will yield the change in volume needed for the necessary comp ratio change. This will usually be expressed as a single ordinance of vertical displacement UNLESS piston compression height and/or crown footprint is changed in which case there are other things to consider.
Another thing to ponder is that the piston crown, with respect to shape and spatial abberation , has NOTHING to do with the CR if the same piston is to be re-used after the CR adjustment . The same applies to the head IF the face isn't machined or the chamber volume changed....... make sense.
Therefore, all you need to know is the unswept volume to calculate the CR changes with respect to any deck height changes. Careful location of TDC will yield this measurement with a bit of well thought improvisation. As a point of interest, modern 4 valve multis will work well with stock or modestly lowered comp and boost pressure ratios of .35-.6 depending on a number of factors that I wont go into. The beauty of this is that relatively elevated comp ratio enhances off-boost throttle response and transient throttle spool-up, someting to consider with a mid-displacement bike motor. Any idea what particular turbo you will use?......... another important choice. One last thing, try to avoid really thick headgaskets to lower the CR, they don't tend to last long due to yield complications. There are a plethora of other considerations to make before you start...... do a bit of research before you roll the dice mate
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