Stroker Motor Question!

[LazeR]

When you stroke a motor what exactly happens ? Does is gain displacement ? thanx ahead of time!

[K_Fulk]

Yeah it gains displacement , the piston moves further up and further back down.

[LazeR]

Where does it give you power at ? I heard Bottom end is that true ?

[K_Fulk]

Yeah.

[InsAyneROostin]

So a stroker wouldn't be ideal for sand? a bore-kit would give you more mid-top end? What's the whole deal behind strokers, bores and stuff?

[hondarider2006]

power:D

[LazeR]

Stroking gains more bottom end, then when you bore it, you gain more top end right ? What does a high comp piston give you ?

[K_Fulk]

Compression gives you bottom end.

[SnellCRP]

Stroker cranks have a longer throw for increased displacement. The gains from the displacement bump are typically across the board, with more power usually found on the bottom end of the powerband. A longer stroke is a longer lever acting on the crankshaft, thus providing more torque. Longer stroke creates faster piston speeds, which means that the piston can create a more accurate pressure drop to fill the cylinder more thoroughly at low rpm for torque. It is important to match the compression ratio with the duration of the cam. While additional cam duration can produce more useable power, to much duration can hurt the performance because too much duration results in lower cylinder compression pressures at low RPM which reduce the low RPM torque and power. Higher compression ratios produce more power, up to a point. The more you compress the air/fuel mixture, however, the more likely it is to create detonation or preignition. The high compression will make power all through the RPM's. :)

[wilkin250r]

^^Hey, somebody's moving in on my territory. I thought I had the monopoly on long technical posts. :eek:

Seriously, good explanation.

Picture the crank, connecting rod, and piston. By moving the connecting rod further out on the crank, you increase the distance that the piston travels up and down, thus increasing the stroke. This is how they make a stroker crank, they move the rod out on the crank.

This will increase torque, especially at low RPM's. However, as SnellCRP pointed out, you are increasing piston speeds. This extra speed will actually rob you of power in the upper RPM range, but it would take twice as long to explain why. With big bores, you don't increase piston speed, so you don't lose as much power in the upper RPMs. This is why they say stroker motors are "low-end" motors, because you get more power in the lower RPMs. For this reason, stroker motors may not work well with really aggressive "race-profile" cams that build power in the higher RPMs. You don't get the best of both worlds, you actually get the worst of both worlds, lousy low-end performance because of the camshaft, and lousy top-end performance because of the stroker.

[SnellCRP]

Thanks wilkin250r. Also, depending on the amount you increase the stroke you may actually have to get a longer rod to keep the piston from being pulled out of the sleeve at BDC. When you increase the bore only you are increasing the volume of air/fuel that can be pulled in to be burned in the combustion process; therefore it would not negatively effect the performance characteristics of the engine at the top or bottom end. If you have your heart set on a stroker try to get a cam without really long duration. You will have to get a short duration, low end cam to keep your low RPM cylinder pressures up to complement your stroker. Otherwise as wilkin250r said you will be getting the worst of both worlds. :)

[wilkin250r]

Originally posted by SnellCRP
When you increase the bore only you are increasing the volume of air/fuel that can be pulled in to be burned in the combustion process; therefore it would not negatively effect the performance characteristics of the engine at the top or bottom end.

True, to a point.

As you piston gets larger (heavier), it takes more energy to keep it moving up and down, and it gets worse at high rpms. This will rob your available power. At low rpms, it's not a big deal, but at high RPMs it becomes a major factor. This is why a 416 will generally rev higher than a 440. The problem isn't as bad as with a stroker crank, but it still exists. You gain great low-end power and torque, but it doesn't rev as high, and the problem gets worse the larger/heavier the piston gets.

However, most aftermarket big-bore pistons are also high-compression pistons. With high compression, there is no downside (except detonation). You gain more power down low, and more power up high. It is truly universal, you gain power everywhere, not just in one spot. So most big-bore pistons will out-rev the stock piston, even though the big-bore is heavier, because of the increased compression.

[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?

[ghak99]

[QUOTE]Originally posted by toby400ex
[B]Here is my explanation in simple terms. Stroking is just a longer rod, which means the piston goes up and down farther in the cylinder. Which makes it use more cc's of the cylinder, in turn making it a higher cc number. Here is my chitty pic i made also


UHHH, Not all strokers have longer rods, in fact many have shorter rods in order to use the stock piston!

[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.

[SnellCRP]

Ideally your engine would be "square". Your bore diameter would be equal to your stroke. If your bore diameter is 89mm so should be your stroke. An engine with a stroke larger than the bore is called "under square," and an engine with a bore bigger than the stroke is called "over square." Technically an over square engine should make more power and rev higher than an under square configuration. What it boils down to is if the the mechanical can take the power, its the breathing and the integrity of the valve train that will limit your performance. Over square engines have an advantage here, in theory. In a big bore engine, the edges of the valve are less obstructed by the cylinder wall. The downsides of a big bore are flame travel and burn time, and a reduction in strength. Generally longer stroke motors produce more torque, have a lower rev limit, and also revs slower. Short stroke motors are the exact opposite. This is becoming VERY interesting!!! I like it!:D

[wilkin250r]

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?

hmm, good question. This is getting a little more in depth than my level of knowledge. I'm not sure of the advantages of an over-square engine (bore larger than the stroke) vs under-square. There are advantages to each when designing and engine and the valve layout, but your engine is already designed.

My gut instinct says get the largest piston you can. At this point, I can't see any advantage to a smaller piston, other than price.

Keep this in mind, your compression ratio is going to be higher than expected. If you buy an 11:1 piston, it's going to be higher than 11:1 for your application. Compression ratio is calculated by dividing the displacement by the combustion chamber volume. The ratio of an 11:1 piston is calculated using a displacement of 246cc, but your displacement is much bigger with the stroker crank, so your compression will be higher.

[wilkin250r]

Originally posted by AndrewRRR
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.

The higher compression shouldn't have a negative affect at high rpms. If anything, it should perform better at high rpms than lower rpms because the inertia of the flywheel helps the "squeeze".

[wilkin250r]

Originally posted by SnellCRP
Ideally your engine would be "square". Your bore diameter would be equal to your stroke. If your bore diameter is 89mm so should be your stroke. An engine with a stroke larger than the bore is called "under square," and an engine with a bore bigger than the stroke is called "over square." Technically an over square engine should make more power and rev higher than an under square configuration.

You should add the words "for a given displacement". An under-square 600cc will make more power than an over-square 400cc.

All other things being equal, displacement equals power. Burning 500cc of fuel/air will create more power than 400cc of fuel/air. Like I said, get the biggest piston you can to get the most displacement you can.

[ghak99]

Originally posted by LazeR
When you stroke a motor what exactly happens ? Does is gain displacement ? thanx ahead of time!


Just think this post started with a simple question, and has gone straight over most peoples heads.



I guess I am going to have to think about this bore size a little longer, I am thinking 80mm.

[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 ?

[SnellCRP]

I would go with just the bore and increased compression. There are many other things like thin gaskets, increased timing, better carb, headwork, cdi, lighter flywheel that will make power without the major engine modifications required to build a good stroker. You can always add a stroker after all the above mods if your bike is still not powerful enough, but as you probably know the more engine mods the more heat, decreased reliability, increase in maintainance, and higher motor octane rated fuel you will have to run. Just try and decide if you want to trade reliability for power. Don't forget we like to ride the quads more than work on them. Hope I've helped. ;)

[SnellCRP]

To get cc displacement of your stroker use this formula: (bore x bore x 0.7854 x stroke x number of cylinders). For example if you had a 440 piston and a 4mm stroker crank it would calculate like this: 89 x 89 x 0.7854 x 74 x 1 = 460365.3516 so your cc would be 460cc. To test the equation you can replace the #'s with the stock #'s to verify the equation is accurate. 85 x 85 x 0.7854 x 70 x 1 = 397216.05 so you know what the stock cc's of your EX are (397cc's) you have just verified the equation is correct. Good luck with your build and keep us informed with your progress.
:D

[wilkin250r]

Originally posted by SnellCRP
To get cc displacement of your stroker use this formula: (bore x bore x 0.7854 x stroke x number of cylinders). For example if you had a 440 piston and a 4mm stroker crank it would calculate like this: 89 x 89 x 0.7854 x 74 x 1 = 460365.3516 so your cc would be 460cc. To test the equation you can replace the #'s with the stock #'s to verify the equation is accurate. 85 x 85 x 0.7854 x 70 x 1 = 397216.05 so you know what the stock cc's of your EX are (397cc's) you have just verified the equation is correct. Good luck with your build and keep us informed with your progress.
:D

Just to clarify the fine details. Stock bore is 85mm, stock stroke is 70mm. If you convert these two to centimeters, just move the decimal place left one digit, so the bore is 8.5cm and the stroke is 7.0cm.

Plug these numbers into the formula, and you get 8.5 x 8.5 x .7854 x 7.0 = 397.216cc.

If you have a 4mm stroker crank, you increase the stroke from 70mm to 74mm, and you can convert it centimeters it becomes 7.4cm, and you can calculate.

[SnellCRP]

Sorry, I was calculating with mm not cubic centimeters.

[wilkin250r]

Right. I just didn't want somebody calculating, and then figuring they must have done it wrong because they came up with three hundred thousand...

Of course, it may be easier to use mm, but just change the .7854 to .0007854.

bore x bore x .0007854 x stroke

85 x 85 x .0007854 x 70 = 397cc :D

[Cody_300ex]

What would we all do without wilkin250r or SnellCRP? :p :eek:

[LazeR]

Thanx for all the info guys! I actually don't have any plans to do any motor work anytime soon! I just have always been interested on how everything works so i decided to ask :D Never hurts to know ;) !!!!! thanx again guys!