To stroke or not to stroke: that is the question

[jcs003]

how about de-stroking?

john

[rablack21]

Sound good info in this thread guys!

[rsss396]

rsss396
cruching some quick numbers....the 76mmm stroke with a 130 rod gives a 1.71 ratio.

stock ratio -130.3 is 1.80
125 short rod is 1.73

adding a +4 stroke with a 130 rod makes the ratio worse than the short rod at 1.71

IMO this would make for a very high piston speed (F.P.S)

If I am wrong please correct me
Thanks

your numbers are correct and here is a little more to expand on those

calculated @ 8700rpm

125.5mm x 72mm = 1.743 rod ratio w/ avg piston speed of 20.9 meters per sec and a peak of 76.3 mps
130.3mm x 72mm = 1.81 rod ratio w/ avg piston speed of 20.9 meters per sec and a peak of 75.5 mps
130.3mm x 76mm = 1.71 rod ratio w/ avg piston speed of 22.0 meters per sec and a peak of 81.0 mps
132.0mm x 76mm = 1.74 rod ratio w/ avg piston speed of 22.0 meters per sec and a peak of 80.7 mps

my Liger
144.0mm x 83mm = 1.73 rod ratio w/ avg piston speed of 24.1 meters per sec and a peak of 88.1 mps (present setup)
144.0mm x 85mm = 1.60 rod ratio w/ avg piston speed of 24.7 meters per sec and a peak of 90.9 mps

[85drag250r]

your numbers are correct and here is a little more to expand on those

calculated @ 8700rpm

125.5mm x 72mm = 1.743 rod ratio w/ avg piston speed of 20.9 meters per sec and a peak of 76.3 mps
130.3mm x 72mm = 1.81 rod ratio w/ avg piston speed of 20.9 meters per sec and a peak of 75.5 mps
130.3mm x 76mm = 1.71 rod ratio w/ avg piston speed of 22.0 meters per sec and a peak of 81.0 mps
132.0mm x 76mm = 1.74 rod ratio w/ avg piston speed of 22.0 meters per sec and a peak of 80.7 mps

my Liger
144.0mm x 83mm = 1.73 rod ratio w/ avg piston speed of 24.1 meters per sec and a peak of 88.1 mps (present setup)
144.0mm x 85mm = 1.60 rod ratio w/ avg piston speed of 24.7 meters per sec and a peak of 90.9 mps

I am still very new to engine building / porting on my own stuff, so I am always trying to learn as much as I can.

Can you explain why you have used a 1.6 rod ratio before? To me it seems like that ratio would put an extreme side load on the piston
Thanks

[rsss396]

1.6 is not a ideal rod ratio for sure, and the very reason I shortened my stroke on my bike.
Wear on the stock cr500 cylinder was very bad but then so was the stock 79 stroke due mainly to the large intake opening and half moon cut away on the pistons. Wear in the bridged intake Liger cylinder was much better but still more than I liked.

The problem with 85 stroke is the rod choices, the cr500 144 rod is the longest rod with smaller sized crank pin.
 Arctic cat has a 146 and I think even a 156 but have very large crank pins and then the crank physically runs out of metal when you start pushing the stroke
I think Pete at Hybrid did a cr500 liger with a stroked crank and AC rod and had to do all kinds of welding and moving bolt holes on the cases because of the rod big end size, then stating he would never do it again.

sometimes you build things in compromising ways simply because of resources but I will say while the +6 stroke crank was hard on pistons it lasted 4 hard seasons and is still together.

[udontknowme]

got a spacer plate and +10 rod ready to go in a 300 when i get around to it. thought of using the cr500 rod but i dont think i will

[wilkin250r]

Many moons ago when racing was more popular, the max displacement for the 250cc class was 265cc (to allow for overbores).

However, many of the top racers used to STROKE their engines to 265cc rather than bore them, even though boring would easily be the cheaper option.

[etccb]

Stroke it ryan!!!
There are some killer kits right now with it all including the case bore mod required for the 330/350.