Lowering Cubic capacity?

[mjc85]

The 141cc that motovert is selling in their Pro is exactly 140.2cc, how could i lower it to below 138cc to race in outlaw class? The stock compression is 9.5:1 so i would also like to raise compression to 12:1 or 13:1

Cheers
Mitch

 

[Mack]

Just don't tell anyone about your .2cc advantage...

 

[binky77]

sneaky sneaky

 

[mjc85]

Mack, i need to lower it 2.2cc to be under 138cc, i'm sure the officials will be aware of this motovert engine that is 140.2cc, if i got a Higher compression piston say 13:1 from 9.5:1 would this lower my compression 2.2cc ?

 

[Mack]

Compression has nothing to do with cubic capacity, so no it won't help.
The only way to alter cc is changing the length of stroke or the bore dia.

I think they will allow your motor no worries, just go race.

 

[Linc]

Compression has nothing to do with cubic capacity, so no it won't help.
The only way to alter cc is changing the length of stroke or the bore dia.

I think they will allow your motor no worries, just go race.

The first thing you need to do is check that the rule book doesn't have a +/- capacity of 5%.

My race class is 110cc with 5% allowance making it 115.5cc max capacity but still called 110 class.

 

[sixeven]

As a measurer of fifties through MA we don't run exactly with the rules as far as tolerance goes. We have NO tolerance for oversize because of the low cost of replacing cylinders if worn, so if your engine is over capacity by even 1% then you are illegal. Sorry guys. thats how it is.

 

[AGRO]

to answer your question you need to bore out your cylinder as much a possible then get a sleeve and matching piston to make it smaller... to be honest sounds like a waste of time...

 

[Bones]

why do you need to bore it out, couldnt you get a custom made sleeve and accomidating piston? any of your local machine shops could do it but then how do you fasten down the sleeve so its not floating around in the cylinder?

 

[AGRO]

you bore it out so that your sleeve is alot thicker... this helps with strength and heat disapation..

 

[eichkay]

I think you'l be stick in the OPEN class.

 

[motodad43]

HHHMMMMM
Just a thought if you had a 140.2 standard say with a flat top piston and you wanted to raise the compression (as in your case to 13:1) you would be fitting a dome top or kicker piston which technically would decrease your cc's that is as long as your high compression piston is the same diameter as your original one, that is because in reality the displacement would decrease by the volume of the dome on your high compression piston and to measure this you would just push the dome of your high comp piston into plasticine up to the top lip and then just fill the depression in the plas with water and measure it 1 cc=1ml.
cheers motodad

 

[Mack]

Sorry motodad, that won't work. The dome makes the same shape at BDC as TDC...

 

[motodad43]

yeh of course. but not the same as the original flat top and that makes the difference in the displacement. i may still be wrong but let me put this to you
If you had two cylinders both being the same height and diameter but one having a domed base which would have the smaller internal volume or in this case dissplacment?
my money is on the domed one

 

[Mack]

Your half way there...first you were only considering the shape change at the top of the bore. Now your only considering the shape at the bottom of the bore. You must remember whatever shape the piston is doesn't affect the volume because it makes that same shape at both the top and bottom.

 

[andy600rr]

Motodad43, Mack is correct. The easiest way to think about it is.....

Displacement is a component of bore x stroke (we all know that) and the combustion chamber volume of the haed plays no part in this.

IF in fact the head volume WAS a component of displacement THEN you would be correct.

 

[motodad43]

Ok ive been polite up untill now lol ....andy600rr i dont know what post you are reading but i never said any thing about a head. and here is the true definition of displacment
Engine displacement is defined as the total volume of air/fuel mixture an engine can draw in during one complete engine cycle; it is normally stated in cubic centimetres, litres or cubic inches. In a piston engine, this is the volume that is swept as the pistons are moved from top dead centre to bottom dead centre. so thats the true definition but cc is allways loosly just defined by borexstroke that is fine and acurate only if the piston top is flat.
Mack i agree with what you are saying to a point, and that point is that mjc85 was saying that the motor was 140.2cc at 9.5:1 and he said he wanted to take it to 12:1 or 13:1 . i did say technically a kicker piston would reduce displacement compaired to a flat top.

 

[noidea]

engine volume = cylinder = area of bore x height of stroke.

There is no consideration of the compression or piston/head shapes.

 

[Mack]

Flat, concave, convex, doesn't matter. All exactly the same swept volume. Have a close look at my diagram, imagine this is the shape made in the bore by the piston as it travels the length of its stroke. It is more easily thought about from the bottom of the stroke rising up. It is accurately measured with oil while the piston is at BDC. Then at TDC the remaining space is measured and minused to give the volume which is displaced by the piston in its upward travel. Compression ratio is just how much in volume is crammed into how little a space. Now this is affected by piston shape, but displacement is not.

EDIT- The three shapes I drew are just an example of three totally different cylinders (even though they're only 2d), which do you think has the greater volume?

 

[motodad43]

all three have the same volume but none outline the changes that i spoke about in the past.
But these ones do, the first is the standard 140.2 with flat top piston and the second is with hi comp piston fitted
ill concede that the swept volume maybe the same but the internal volume of the later is less