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Post by wkamil23 on Jan 13, 2021 19:54:33 GMT 1
I'm currently building an RD350 F2 YPVS that is chasing speed record. We have a spare engine and would like to do something different to it. We are in production class so there are few restrictions we have i.e. capacity needs to be no more than 350cc, no external modifications, standard exhaust etc.
Anyway, after reading few articles it would make sense to run biggest possible piston i.e. 66.25mm and reducing the rod to 50.8mm maintaining the same capacity.
Are there any advantages or disadvantages on doing this to a RD350 2 Stroke? Will it work?
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Post by muttsnuts on Jan 13, 2021 20:38:35 GMT 1
think your biggest challenge there would be the port timing and position of the existing ports as you'll need to shorten the barrels/deck height to suit, you may not have enough material available to do it by skimming the top (water jacket etc could be a problem if too thin), conversely if you skim the base of the barrels, you may hit issues with the transfers and also the cylinder skirt, although you could recut them if necessary
Conrods, that might be an issue, although I am thinking something like a TZR250 conrod might fit and is about the length you mention, pretty sure the big end pin is the correct size, so you'd just need to measure the width of the big end and check against the YPVS big end conrod width
As with most things, anything is possible, as to whether or not it will work, you'll need to do some port work, ideally you'd do this in software first to see if it will yield BHP to start with over and above a stock motor, I am assuming you can do port work
The other issue you'll have with 66.25/66.50mm pistons is the rev's, the bigger the piston, the less you can rev it as otherwise you'll take the pistons/pins etc beyond there limits, the beauty of a short stroke is that with the right size pistons you can rev the "tits" off it with smaller pistons, clearly a square "style" engine would be ideal, but your a long way from that, your over square, so it actually might work against you
Interesting thought/idea though
HTH
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Post by loudy on Jan 18, 2021 21:05:51 GMT 1
Could the original con rods not be used?
Is it not the crank pin circle that directly affects the measured stroke?
Therefore the crank that would need a lot of attention as well as porting?🤔
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Post by steve63 on Jan 19, 2021 14:04:13 GMT 1
Could the original con rods not be used? Is it not the crank pin circle that directly affects the measured stroke? Therefore the crank that would need a lot of attention as well as porting?🤔 You are correct. Without reducing the big end pin circle diameter from 54mm to whatever reducing the length of the rods will only drop the piston down the barrel. Terry Becket I think it was used to take metal off the bottom of the YPVS cylinders. Maybe to drop the bottom edge of the ports for some reason. I never knew exactly why. You could spot them quite easily. |
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Post by reedpete on Jan 19, 2021 15:33:02 GMT 1
Shortening the stroke isn’t going to help.
Without knowing anything about your current spec, the normal rules can be applied....the more cash you can spend the more power you can put down on the back wheel, but it’s a diminishing return curve ....and ultimately the budget has to sit easy with the philosophy of the class you are in.
If you are getting really serious about this you need to do the simulations and make a plan first.
If you know the gap between target max speed and where you are now then you can establish how much more power, how much less drag you need.
What about fuel? Oxidants? Rule bending?
Always good to know what the existing record holder had...
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Post by reedpete on Jan 20, 2021 13:52:08 GMT 1
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