Cylinder Bore Size - does it make a difference?


(Nicholas Bruno) #1

Taking an X30 for example, standard bore size is 54mm. The largest overbore piston available from IAME is 54.27mm, which is .5% larger than 54mm. This may be a dumb question but would that make any appreciable difference in power?


(James McMahon) #2

It’s not a dumb question at all. Given that a lot of “fast but fresh” engines tend to be lower bore it doesn’t seem to make a difference on track.

The other thing to consider is that you need to calculate the difference in area, then displacement from the smaller bore.


(Jessie James) #3

The oversized pistons are for worn barrels to allow for honing. You wouldn’t oversize for power as it will devalue your engine. It could improve performance as you could get a straight barrel . There are also after market pistons from metior that go bigger but these are for practice and club uses as would be considered illegal.


(John Leah) #4

The 54.27 piston would give a 1.00 % increase in area and displacement over a 54.00 together with a marginal rise in
compression.

The potential advantage varies with engine type, with the plated bore engines having the least variation in piston size.

The old air cooled short stroke 100s had piston sizes from 50.6 to 51.4 with a cast liner, variation on displacement 3.18%.

Whether there is any real advantage is open to argument and testing but there are people who would not be put off by the ’ depreciation’ if they thought there was gain to be had.


(Christian Fox) #5

As a bore wears and gets bigger, the shape of the squish area will change. For spec engines, without the ability to alter the cylinder head shape, you will need to run a progressively tighter squish as the bore gets bigger. So any potential benefit you get from more displacement might be be offset in a loss of top end with the tighter squish/lower exhaust port height. The flip side is brand new cylinders are sometimes small enough in diameter so that even the smallest available size piston is too big to get a proper piston/wall clearance. I will hone these longer to remove more material and get the clearance to where I want it.