RZ/RD 350 & Misc. 2-Stroke Tech BBS

Your talking trenching the lower case, nas pa?

LC Cnd wrote:Your talking trenching the lower case, nas pa?

you only need to trench the cases if you are going more than +4mm stroke.
to fit a rz stock crank the cases sometimes need side cutting on one side of the crank case to clear the crank .
geny side i believe.
mark

Hi , I know of a motorcycle engineer who built a big bore lc for one of his customers some 30 years ago. he said he machined the cylinders out and relined them with bultaco 250 liners. he also fitted rd400 rods to the crank shaft . what he did say was the transfer tunnels got narrow because of the bigger liners. he also said the guy who owned the bike rode it like it was his last day on earth and it would blow larger bikes into the weeds sounds like fun but would think it would be terrifying .

LC suffer from the same inefficiencies due to too small transfer ports as RZ? suboptimal to say the least. Bashee cylinders cure this as well.

The rd400 rods ive heard of before as well back in the day, something like 400cc on close to standard bore with those or something, or maybe it was a +4 throw, I forget it was a long time ago... to make a fast LC you should fit an RZ bottom end and then look at the fast banshee stuff, thats how we do it now...

Hooligan wrote:I'm still a bit fuzzy on why stroking a crank works? The exhaust port height is still at the same level and so no compression is realized until the piston closes off the port. Crank stroke doesn't change that factor. Thus the compressible volume does not change regardless of crank stroke.

The only thing I can surmise is that stroking works by increasing the suck and blow volume which increases the air-fuel to residual exhaust ratio?

There are two potential downsides to stroking the crank. Reduced rpm redline and slower rev generation.

not many serious people work with corrected compression ratio's, as they only matter at starting. when running, the compression ratio differs all the time, in no small part due to the amount of mixture that is "pressed in" by the pipe.

what REALY matters (and why strokers are best as square as possible) is time-area : the time a given port is open.
let's say you have a square engine : 54x54. now we exaggerate and make the bore twice as big (108mm). now all your ports can be twice as wide. but assume you still want to compete in the 125cc class, you need to have a stroke of only 13.5mm. so your ports will also be only 1/4th of their original height. now a simple calculation of port area and the conclusion is that your total port area is only half as big as it was , which can not be a good thing .
yes, the shorter stroke can in theory rev higher. but since it does not have the port area to support the higher rev's (smaller ports with even less time to flow due to the higher rev's), it will not produce more power. it will even produce only about half the power of the original because power comes from flow and flow comes from port area : half a port can only flow half the amount of mixture ( (assuming the "state of tune", i.e. the quality of cylindre filling is the same).

so if you want to increase your capacity for more horsepower, you need to go to a "as square as possible" engine. if your engine is oversquare like an RD350, your best option is to increase the stroke.
going oversquare by enlarging the bore gives some gain in bottom/midrange , but no more top power