Stroker Motor Question!

[SnellCRP]

wilkin250r that is some very good info. I don't have time to reply right now, but I'll be back on later to reply.

[SnellCRP]

Originally posted by AndrewRRR
Actually the rod length isn't what makes it a stroker, you move the mounting place out further on the crank. A lot of people go to a longer rod when they do a stroker because it is a convenient time to do it and a longer rod puts less side load on the connecting rod reducing chance of failure.
I don't know about a 12.7mm stroker. That is HUGE. Most stroker cranks are 4mm or so. A safe way to go is make the motor "square", with bore and stroke equal. My R is a big bore 76mm, so I got a 4mm stroker crank, making it 76mmX76mm (bore and stroke)
I have a question for wilkin and snell: To what extent does high compression affect high rpm power? I have heard really high compression motors can't have high rpm power because the piston/crank/etc have to work extra hard to put that much squeeze at TDC which hampers it at higher RPM's. But I do see a lot of alcohol banshees that don't seem to mind revving high even though they are at 200+ psi.

I am not 100% sure, but I don't think high compression will have a negative effect on high RPM power. Mainly the increase in compression will give you a broader power curve, and better throttle response. The only thing I can think of may be the added weight of material in the piston to create a dome for the increased compression may cause it to rev slower, but I don't think you could ever feel the difference.

[SnellCRP]

Originally posted by ghak99
Very good post!

I am looking into a big bore kit to go with the stroker crank I just bought, and was wondering how big I should go to "balance out the motor" (for a lack of better words). The crank will be a 12.7mm stroker which will be a total of 70mm in a 250x.

Any ideas?

I'm not sure of how far you can stroke a 250x crank, but if your crank is a 12.7mm stroker that means the pin has been moved 6.35mm. Who did the machine work on the crank? If yours has been moved that far won't you have to have a longer cam chain, space the jug further away from the cases,and grind out the sides of the cases for rod clearance? Maybe even a custom piston with the pin moved as close to the oil ring as possible.

[SnellCRP]

I guess to make it really simple you can make the engine be able to mechanically handle a large volume of air/fuel, but if the cam and head are not able to feed the beast you will not be getting the max reliable power out of the engine.

[LazeR]

This is actually what i wanted this post to turn into! I wish i would of named it something differen't but eh! Very good information people!


I am new to this "Motor Work" stuff so bear with me :P


Say you bore to 426, would it even be worth it to stroke it as well ? and what would you stroke it to ?

[wilkin250r]

Originally posted by SnellCRP
I guess to make it really simple you can make the engine be able to mechanically handle a large volume of air/fuel, but if the cam and head are not able to feed the beast you will not be getting the max reliable power out of the engine.

True, for such a large displacement you may not be making max power. You've spent all this money on the stroker crank, and now you are looking at a big bore and piston, probably a cam, you should probably spend a little money on headwork. I think oversized valves would be a great addition to the total package. What good are all these modifications if your engine has asthma? Open up the intake and let that baby breathe!

[ghak99]

Originally posted by SnellCRP
I'm not sure of how far you can stroke a 250x crank, but if your crank is a 12.7mm stroker that means the pin has been moved 6.35mm. Who did the machine work on the crank? If yours has been moved that far won't you have to have a longer cam chain, space the jug further away from the cases,and grind out the sides of the cases for rod clearance? Maybe even a custom piston with the pin moved as close to the oil ring as possible.

Since, this is a 250x they have a very short stock stroke, this kit is basically like putting a 300ex crank in with a 4mm stroke. The 300ex and 250x are the same except for the crank there should be no rod clearence issues. The rod on this stroker is not stock length, it has been changed so that there will be enough room in the top of the cylinder to avoid the valve train and head when using a stock type piston.


For what it is worth, Powroll sells a 12.7 mm stroker crank and rod for the 250x.

[ghak99]

Originally posted by wilkin250r
True, for such a large displacement you may not be making max power. You've spent all this money on the stroker crank, and now you are looking at a big bore and piston, probably a cam, you should probably spend a little money on headwork. I think oversized valves would be a great addition to the total package. What good are all these modifications if your engine has asthma? Open up the intake and let that baby breathe!

You are 100% correct! These parts are going to be set on the shelf until I get my spare head totally worked over and a good pipe. I have a ported & polished head now, but it has stock valves in it and I don't think they will be big enough to handle this volume of air.

[wilkin250r]

Originally posted by LazeR
This is actually what i wanted this post to turn into! I wish i would of named it something differen't but eh! Very good information people!


I am new to this "Motor Work" stuff so bear with me :P


Say you bore to 426, would it even be worth it to stroke it as well ? and what would you stroke it to ?

Stroking is very expensive. I would say you are probably better off going with a 440 big bore than a 426 and a stroker crank (which will end up at about 450cc) The 426 and stroker will make a little more power, and be MUCH more expensive.

[LazeR]

So say you went with the 440, then stroked. What would it end up being ? Or how do you figure it out ? and about what would it cost ?