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Andyinchville1

Manufacturer/Dealer
Dec 26, 2007
502
1
18
Scottsville, VA
HI All,

I was just looking at an old partially disassembled engine today and was thinking of a low $$ way to increase displacement and power...

Recasting an entire cylinder would probably be expensive so I thought to myself....what about just making a spacer that fits on the bottom of the cylinder jug and make a new connecting rod (of course slightly longer studs would be needed too).....the longer stroke will add displacement and torque....how many additional cc would 1/8 more give you?....How about 1/4 inch more?...Of course hopefully there will be enough clearance inside the crankcase?

I know connecting rods can be made of aluminum so machining a rod shouldn't be too difficult....of course the balance issue comes into play BUT since we are using the same piston maybe it won't be too far off?

Minor porting for the additional displacement may be necessary but then again maybe not.....same fuel / air mix with a longer stroke should yied tons of torque...

So what do you think? Seems like it wouldn't be too expensive to get more from our engines!

Andrew
 

cityevader

New Member
May 11, 2008
170
0
0
Santa Cruiz, CA
Stroke is not relative. It is dependent on the length of stroke from TOP to BOTTOM.
If you increase the top by the same amount as you increase the bottom, you haven't increased the stroke or anything else, except a bit of metal. Now, if you say you can increase the height in which the piston travels, yet maintain the lowermost point the piston travels...now we're talking! (Of course, you'd need to modify the crankshaft and case to match the increased throw...and oh yeah...don't 2-strokes need super-precise intake/exhaust porting!
 

Andyinchville1

Manufacturer/Dealer
Dec 26, 2007
502
1
18
Scottsville, VA
Hmmmm....messing with crankshaft sounds expensive....Wasn't there something about altering an engine's power characteristics by messing with rod length and piston heights WITHOUT messing with crankshaft throws..I wonder if we added the spacer at the bottom of the cylinder, used a longer connecting rod, and made a custom piston if that could accomplish anything power wise?

Andrew
 

cityevader

New Member
May 11, 2008
170
0
0
Santa Cruiz, CA
Assuming no changes made to porting(which are critical in regards to piston positioning) the only way to increase it as you're alluding to is to increase the bore. In a car, one would bore out the cylinders and slap in wider pistons. From what I understand, these engines have just enough metal for the existing pistons. If one was knowledeable about custom manufacturing their own pistons to match their own crankcases...well, now we're talking about scratchbuilding.
 

Ilikeabikea

Active Member
Jan 27, 2008
2,322
0
36
68
Ptown, Texas
I know a lot of karting guys will use a longer rod and shorter piston to change the rod angle. They do that so they can turn higher rpm and a little less likely to make a viewing port on the side of the block.............................
 

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
7,281
1,832
113
Los Angeles, CA.
I wouldn't do that to your engine!!! The intake & exaust timing are controlled by the port heights... if you mess with rod length & piston height, you will change the timing!! (& probably not for the better!!)
I would look into boring the cylinder & finding a bigger piston,(with the same height)... but then you have to re-balance the crank!