Shorter stroke vs longer stroke and bore questions

[B.b.p builders]

Can you guys fill me in on a few things. I'm wondering what, if any are the benifits of say a +4mil stroke with a bigger bore( as in bigger then 68) like say a 4mil over 500cc motor vs a 10 mil over 500cc motor?

For example using these #'s as reference only

A 4mil 535 vs a 10mil 535?

Bigger the stroke = more bottom end torque?

Lower the stroke = more?

You can see what in trying to get at and understand here. I'm looking to better understand anything and everything to do with the bore and stroke and combinations of.

[trickedcarbine]

Well, this can definitely turn out to be a decent topic. Question is kinda loaded but I guess I'll start.

 

Given the comparison above, the short stroke motor in theory should rev faster, however it would have massive pistons swinging around on those lil rods. If this combo could physically stay together it would potentially run a higher peak number then the 10 mm motor as the RPM should be higher. There is a possibility that the piston weight is so great that it could actually slow the motor. For years I thought I would be better building a huge displacement 4mm motor that had massive bore, but after actually tinkering on one I wasn't quite as impressed as I thought I would be. It just seemed to have a peak RPM wall and once the bike hit it you got some severe shake letting off the throttle. I heard that motor later let go of the rod right under the small end bearing.

 

With the 10mm set up you would certainly have more torque and with the smaller pistons the whole assembly is actually more square then the 4mm combo. It could possibly even rev equal to or greater then the 4mm combo above. At this point in life I believe that the 10mm set up would probably be a lot less taxing on all things crank related. Now take the 10 and add equally huge pistons and all this goes out the window.

 

Just initial thoughts.

[m671054]

Didnt titan build a 4mil dmx a few years back? Wonder how it did. Also didnt jmadd build a larger bore shorter sstroke combo for the 650 class thats running well?

[trickedcarbine]

Wasn't mine, it was an old 78mm bore twister top end on 4 mm. Pre small block era, pretty much almost a T-Rex. I was gonna buy it. I rode it, tweaked on it, but after a whole day I just wasn't feeling it. This was the experience that made me finally start considering 10 mils for my set up.

 

No idea if it was balanced. I asked and he said he bought of a dune guy from Oregon on Planet sand back in like 09....

 

I just think there is a broad spectrum of workable combos from over square motors to under square motors, but if you flirt with an extreme sized piston in relation to the crank, I think you could be asking for trouble.

[sheerider11]

The higher rev of the shorter stroke would concern me. It's like spinning around in circles with a 2 pound hammer and then a 5 pound hammer.

The force of the piston tugging on that rod at high speed is probably intense.

[trickedcarbine]

Don't think that would be a fair comparison as there are a lot of variables between the two. For the sake of keeping it apples to apples i think it's best to just keep cylinder design hypothetical.

[shanYE west]

there is 54mm stroke with 73mm bore cylinders out there.  And they run pretty well!!  Baby cheetahs

 

I also wanted to try a 78x54... thought it would be neato.

[trickedcarbine]

The later of the two would probably sound awesome.

 

Is there much rod difference in rod size when you get over the 10mm stuff?

Edited August 26, 2014 by trickedcarbine

[m671054]

You mean length?

[trickedcarbine]

No, I know there are 125 rods. I'm referring to the girth or thickness of he rod.

[B.b.p builders]

Tricked, when you refer to motors as "square" or over square or under square, what do you mean by this?

[B.b.p builders]

My main reason for wanting to know about this, like many of us here I enjoy building my bike as much as I enjoy riding it. Some of you know we are trying to grow atv drag racing here in ON Canada, so I plan to build a cpl. Bikes to sell locally for basically the cost to build it..

I like doing out of the NORM, so I thought about doing a 4mil bigger bore motor. Judging by the first few responses it would be a good motor, but not as reliable as a more balanced stroked/bored motor.

I'm thinking something over 500cc, but under 600cc, with a 4 mill crank..

I have my 535 cheetah, and in a week or 2 I'm picking up a 421 cheetah(not cub) the 421 is a pv, trail bike with lots of good parts to sell, to pay for the drag parts needed.

First idea was to do the 68mm bore cylinders, have them ported and set up for drag racing. But that just seemed kinda boring.

So my thought was to sell the 68mm bore cheetah cyl and find a bigger bore, cheetah or DM cyl..

[m671054]

Square is same bore and stroke ie 68×68

Stock is under square 54x64

4Mil dm 64x78 is oversquare

Tradional thought is square motor runs better

Also tricked what about 130,132,137 rods

[shanYE west]

my 132 rods didn't look to much thicker then my 115's... I didn't measure.. but.. they look pretty close besides length.

 

[B.b.p builders]

Square is same bore and stroke ie 68×68

Stock is under square 54x64

4Mil dm 64x78 is oversquare

Tradional thought is square motor runs better

Also tricked what about 130,132,137 rods

Yes ok that's exactly what I thought the meaning of "square" was.