Stroker Motor Question!

[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!

[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.