Motorcycle engine sizing

[JimW]

All that wording in the rule book about ?displacement must exceed the max allowed for the next lowest class? just leaves me wondering about what class what size fits into.

For engine size class 350cc.  Is this 350cc and smaller (down to the next class), or 350 and larger (up to the next class)?  Is it inclusive of the upper limit?

So class 350 cc is:
350cc up to but not including 500 cc? (this is what I think)
350 cc up to and including 500cc?
250 cc up to but not including 350cc?
250 cc up to and including 350cc?

And the stuff about 0.020 overbore.  Does this mean if you were 349.999?. and a 20 thou overbore put you at 351, you would still run in the 250cc class? (assuming I understand the classes ? which I don?t).

Thanks,
Jim.

[JohnR]

the class size is up to the limit. so a 350cc class takes bikes from 251cc up to 350cc. The .050 overbore is ONLY allowed on vintage engines.

I THINK that a 350cc class means "less than 351cc" so 350.999cc would be OK.

[MattS]

(although this is a little loose, since a Honda GP 6-cylinder 250cc engine, with a 0.020" overbore might be, say... 270cc's) up to 350.000cc's (except that a Honda 6-cylinder GP 350cc bike with a 0.020" overbore might be, say...375cc's)


Those little 6 cylinder Honda's had a bore & stroke of 39mm x 34.5mm. Since they are not pre '55, the most they could bore it out to is 39.5mm. Calculating that out, the max displacement is 253.7cc's.  That's a little less than 1 1/2%, not too big of a jump. A 350 would see even less of a percentage increase. So even on a multi-cylinder, tiny piston engine, the over-bore isn't very drastic. On my 500 vintage, even a .050" oversize bore amounts to less than 14cc's. But I'm only at .010" over, which makes it 503cc's.

Still too slow.....


matt

[MattS]

You are of course, probably correct with the math.  I just find it interesting that the "oversize" applies ONLY to the bore, and not the stroke.  The result is still that you could have two visually identical engines, both having the SAME displacement, and the "stroker" would run in the 350cc class, and the "bored out" motor would run with the 250cc bikes.

Is there a logic here that I am not seeing?

The only logic I could see would be in the vintage class. The rules require you to use OEM barrels, which might be worn in the 50+ years of use. That would allow you to clean up the bore on those rare, hard to get parts.... though most could be re-sleeved. The crank can be re-conditioned without having to increase the stroke. As for the non-vintage .020" over-size allowance, beats me. I guess it could be the way engine classes are defined. The motorcycle engine classes run along with the motorcycle manufacturers standard engine sizes.... IE  125cc, 250cc, 350cc, 500cc... they are all nominal engine sizes. Any bore clean-up would put them over the class limit. The car engine classes, because of the huge number of different displacement in US made engines, are not standardized in their size. A 327 V8  or a 350 V8 can be built up till it's 372 cubes and still be in the same class.  Maybe the motorcycle engine classes should have split the nominal engine sizes, like 150cc, 275cc, 375cc, 525cc.... then we wouldn't have this problem.... no, I don't think they should change them now. Messing with already established classes and records is a bad idea. Hindsight is 20/20.

It's just all a guess be me, I'm a new kid.


Matt

[MattS]

Interesting.  The SCTA used to also have 200cc, 600cc, 883cc and 1500cc classes.

Now, where did THOSE records go?........

Like I said, I'm new to LSR, this being only my second year racing. I figured that all records were something to be respected & cherished. I don't know when, or how many, class changes there have been, or why. I understand making rule changes for clarifications, new technologies and safety reasons, but making changes like the after-market case rule for A & M bikes had a large & immediate effect that surprised a bunch of people. I have a feeling it's not the first time its happened, nor will it be the last.

But I'm an optimist, so I think it can only get better.....



Matt

[Nortonist 592]

Thats what you get for thinking.  Bad idea.  Leave it to the experts.

[T.RexRacing]

Sooooooooo..............if my RZ350 needs to go .020 to clean it up and ends up at 351CC's I can still run the 350 class?I hope so 'cause it's ported and the guy who did the work is no longer porting.Lemme know please Scott so this winter isn't a waste of time.

[Nortonist 592]

You'll still be legal.  Just don't stroke it and get 351cc.  Then you're in the 500cc class.  Makes sense, doesn't it?

[T.RexRacing]

it actually makes sense 'cause a two-stroke responds to a stroke increase more than bore.It's like installing a longer duration cam in a four-stroke.