Stroker Motor Question!

[K_Fulk]

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

[K_Fulk]

Yeah.

[K_Fulk]

Compression gives you bottom end.

[wilkin250r]

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

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.

[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.

[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.

[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!

[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.