Hitchcocks Forum

The recent posts re compression have had my very simple mind posing a question. If the compression ratio is increased by reducing the barrel length (gasket) or fitting a piston with a raised crown, does that not reduce the CC of the bike. Sorry if this is a very basic question, but what is the point of increasing the CR if the CC is reduced ? This is purely a question for my head as i am happy with the performance of my C5 (more or less)

Aah thanks. I thought it was the volume of the bore at the bottom of the stroke thus raising the piston crown would reduce the volume.

A video says a thousand words:-

Hi guys-------nice one there PapaSmurf, that's the basics in a nutshell..........Hi ANDY Mc}--The thing to note is that CR is not actually an existing, physical thing but a comparison of the two volumes that exist within the cylinder/head assembly............Skim the head, skim the top of the barrel, skim the base of the barrel, fit a piston with a higher crown...or even a thinner head gasket, each of these will increase the CR to a greater or lesser extent....

An increased CR means therefore there is LESS overall volume for the piston to compress the same amount of fuel/air mixture into .......and from our experiences with the old bicycle pump we all know that means the temperatures above the piston will become greater............It's at this stage that a poor quality fuel (or poorly mixed fuel ) will start to break down with small pockets within the mixture igniting by themselves.......This is called PRE-IGNITION or "Pinking" and it sounds just like tinkling and occurs BEFORE the ignition spark.........So, if you go from a basic 500cc Enfield with approx 6 to 1 CR and it's been very happy running on Tesco's 80 RON cheapo petrol, to a rip-snorting 500cc Enfield with a 10 to 1 conversion then do yourself AND the bike a favour and fill up with the best quality petrol available on the forecourt....Don't forget that high-lift, long-dwell cams will have the same effect ie}--- more air being crammed into the same, original volume........Later comes Turbos and Blowers where we actually LOWER the CR to cram it all in.......Ah, time for my Pink pills I see !!

I hate it when I can see the logic of both sides of a discussion. Reducing the barrel height will increase the CR because a flat topped piston will have to squeeze the same volume into a smaller space, fine so far but surely a domed piston will slightly reduce the internal volume so,for instance it is compressing 495cc instead of 500 ,like those annoying plastic tubs of cheese or whatever that look big on the outside but you find half the internal space is taken up with a convex base. Perhaps it will get clearer to me after another ginger wine.

Hi HUBB-------Here we go then}----- Clearance volume is defined as that volume remaining ABOVE THE PISTON when the piston is at Top Dead Centre........So as the stroke has remained CONSTANT then a taller piston will increase the CR, and a shorter piston will reduce it .. Height being that distance from the gudgeon pin ( Wrist pin ) centre line to the top of the piston...

PS}---- I love Ginger Wine... I also like to stroke a Ginger Pussy !!!

To put it simply, the cc of a cylinder refers to the swept volume, ie the amount of air the piston would displace moving from bottom to top, not the amount of space inside.

Ha-ha nice one Alan After much contemplation the fog finally cleared and I now understand about swept volume and capacity thanks to you and Rev. ATB John

---- for some strange reason my mind turned to this }-------------- Happy Christmas to pussy-lovers everywhere !!

That makes sense, but doesn't the fitting of a crowned piston reduce the volume?