By how much does a rebore increase capacity?

[SPRIDDLER]

Another thread in the Forum about engine capacity had me wondering by how much an e.g. +60 rebore would increase the capacity of a nominally 498cc single. It would interesting to know for sure..............

EDIT.
Lubricated later by strong coffee and a couple of fags I've had a go at the calcs which shows that a 60 thou oversize produces 515.5 cc.

Standard Bore Diameter 3.250" + 0.060" (oversize) = 3.310"
Radius = 3.31 divided by 2 = 1.655 therefore Radius squared = 2.739
Stroke = 3.656

Volume of cylinder is.....
V= Pi x radius squared x height (in this case the height is the Stroke) therefore........

V= 3.142 x 2.739 x 3.656 = 31.463 cu in. which converts to 515.5 c.c which is an additional 17.5 c.c on a 498 c.c. 500 single ).

Is that correct? And if so, should a larger main jet be used?

[Eamonn]

The engine CC is therefore an increase in volume of 3.5% - I'd suggest that such a small difference wouldn't warrant changing the main jet. Never heard of anyone worrying about the increase capacity before...!

[Alexlib]

It is a good question, I guess at 4000 rpm (66 per second) the cylinder is filling 33 times , so the increase in volume must be met with an increase in gas flow speed through the carburettor, gas flow efficiency is impossible to calculate, it is possible that the change in flow speed could increase fuel flow . This is why wind tunnels still exist. There is no substitute for practical work .

[EddieDay]

My 2 penneth: I don't think a larger main jet would be required for a 3.5% increase in swept volume. The air / fuel ratio is determined by the speed of the air through the venturi and the diameter of the fuel jet. (As air speeds up through the venturi so its pressure falls, the Bernoulli effect, so fuel is drawn into the airstream through the jet). Owing to a slightly larger volume of air being drawn into the cylinder the amount of fuel would increase too. If the volume increase was larger, say to 700cc, we would need a larger carburettor not just a larger main jet because the friction of the air passing through the venturi would be too high. The carb choke size and the jet size should be increased roughly in proportion.

[SPRIDDLER]

My 2 penneth: I don't think a larger main jet would be required for a 3.5% increase in swept volume. The air / fuel ratio is determined by the speed of the air through the venturi and the diameter of the fuel jet.

Yes, that was also my conclusion about the main jet shortly after posting that hasty idle thought.

(I've had cabin fever today, stuck indoors waiting for a B.T. engineer who never arrived ).