Forum

[Revband]

The internal volume, yes, the swept volume, no.

[Alan R]

Hi Andy-------Yes, it reduces the CLEARNCE VOL..... Looks like you have been at the Christmas sherry a tad too much, eh ??( LoL !!)...........I say this because you appear to have gone right back to your first question.....So just to re-iterate then}------SWEPT VOLUME = Area of piston x Length of Stroke expressed in Cubic Centimetres ( CC's. )...........CLEARANCE VOLUME = The volume remaining ABOVE the piston when that piston is at Top Dead Centre ( TDC.)...........COMPRESSION RATIO = The comparison of the TOTAL VOLUME within a cylinder to the CLEARANCE VOLUME and the mathematical formula for that is} SWEPT VOL. plus CLEARANCE VOL. divided by CLEARANCE VOL. and is expressed in plain numbers eg}---- 10:1,....6:1 etc.

So that's pretty much it as far as the basics are concerned........As you can see, any alteration to any of the above will have a "Knock-on" effect with those initial figures........For instance 1)..If you change a flat-topped piston to one with a raised crown then the CLEARANCE VOL. will have been reduced and the CR will have been increased.............2)..If you fit an over-sized piston as part of a standard re-bore then the bore will have been increased, albeit slightly, thus the SWEPT VOL. will have been increased ( slightly )and so the CR will be altered ( assuming that the original head is re-used.).......3}..Drastic change to bore size eg}--- converting from a 350cc engine to a 500cc engine using the original bottom end....Here you really should use the matching head and barrel in order to maintain the 500 engine's CR....

Then of course there is our friend the Diesel Engine with nice little diversions such as the bowl-in piston !!.....At this point I suggest we put on our back-to-front nice,clean white coats and watch this}-----------------