Shorter stroke vs longer stroke and bore questions

[m671054]

So what your saying about lifting the cylinder on a stroker is like running a spacer plate on a 4 mil. Seems like most frown on that. Anyone read the old post on ps about crankcase compression theory? Seems like this is what ru is getting at.

Also lets try and keep this banshee specific. Not really interested in hondas or kart engines. No unicorn engines either. Lets use examples that people can actually build.

[registered user]

increasing the stroke doesnt necesarrily meen you would raise the cylinder, like a stock stroke engine with 4mil crank for example. although i suppose you could use a spacer. but as the stroke increases, at some point, ideally you would want to increase the rod length. the longer rod would position the cylinder higher, assuming you were keeping deck heights and wristpin locations the same.

 

ya alot of crankcase compression theories out there im sure. seems like back in the day the hot thing to do was try and force the mixture into the cylinder. now i think ideas have shifted towards letting the pipe do more of the work pulling it in

 

 

there is nothing posted about honda vs yamaha or kart vs atv. it was stroke vs bore last i checked. i dont know how else to explain this besides a oversimplified pic. since there is no exact cc comparison between the cp cylinders, we will use the closest match possible, which still should help you make sense of it. if we compare the 68x68 cub (494cc) vs 72x58 dm (472cc), hypothetically we set the timing on both engines to 130* and width of 20% of bore diamter. you would end up with something like the pic. if you took the time to do the math in a real scenario you would see the black port (68x68 cub) is larger in area.

 

 

[Deadbeat]

lol did you break out the crayons for that ken

[B.b.p builders]

Lol, I like it, nice and easy to understand.

[Larry's Shee]

lol did you break out the crayons for that ken

[poopstick]

if you go BB you need to increase the port size by the same percentage, on stroke you need to measure piston speed by feet traveled per minute, so if you run a big stroke you need to back down the RPM's, on the port measuring there you are on to something but most don't bother, a sweeping transfer (Honda) is ideal not the 90 our banshees have but Yamaha did not intend for this motor to get this out of hand, port timing back angles mean so much and everytime I see that kfnife edged intake I wanna throw up! so can a good builder take a BB stk stroke and just kill a 4 mil stroker you betcha

[Coupelx]

I have a couple random thoughts to add.

 

When it comes to CC vs HP- While the the gains in horsepower are not huge, the TQ and

Useable power to be had can be especially when increasing stroke to get those CCs. The

100hp 350cc setups you hear about are mostly not rider friendly.

 

The preference to keep an engine square to slightly over square has to do

With not only combustion but most importantly scavenging which I hear is

The secret to power.

[poopstick]

yes getting a perfect pipe to help this is hard, pipes are made by alot of people to make money, now I know Matt does alot of work to get his pipes to have a better power across the band, to say look my bike made 90 HP on the dyno and then just fell off is useless,