Super Jet Stroker Cranks

[227]

Will one of you gear heads please explain to me what a stroker crank does? I know the rods are a different length than OEM but what advantage to they provide? More bottom end power? If they're so much better then why didn't Yamaha go with them from the get go? Are they not reliable? Do they only work in conjunction with a ton of other Engine tweaks or can you drop one in OEM cases and Cylinder and expect big gains?

 

[zzylstra]

I would try to explain this to you but it wouldn't come out the way I would want and I'd get put on blast lol.

 

[Mentzel]

Move the crank con rod pin off center additional length. IE 2mm pin move = +4mm stroker crank.

Increases displacement of engine since stroke length increases. Piston travels higher 2mm and lower 2mm.

No you can't just drop in a stroker crank without a cylinder base spacer and port alteration.

 

[Tyrant1919]

I'll take a shot.

Strokers increase the size of the combustion chamber by increasing the piston vertical travel. Yay more CCs!
Torque is increased since the distance from the center of the crankshaft is longer. (Think a lever and a fulcrum, further the end of the lever is from the fulcrum, the more leverage you have)

Rods are lengthened to limit the angle the rod most extend to reach the far extremes of the crankshaft. Short rods have to rock back and forth on the wrist pin further (Larger angle) than a longer rod.

Cases have to be moded to clear a stroker crank. All of the spacing has to be correct. Modifying domes, adding spacer plates to meet clearances. May have to around a bit more depending on how your ports will line up after all is said and done. Wear on your pistons and sleeves are increased. Sideways forces are greater on the connecting rods if they aren't lengthened appropriately. I'm sure I'm missing some .

Yamaha probably determined their engine specs by figuring out what is cheapest for what they are looking for in reliability and power. Besides, even if their cranks came stock 78mm instead of 68mm, we'd all want more still.

 

[kevbo]

Will one of you gear heads please explain to me what a stroker crank does? I know the rods are a different length than OEM but what advantage to they provide? More bottom end power? If they're so much better then why didn't Yamaha go with them from the get go? Are they not reliable? Do they only work in conjunction with a ton of other Engine tweaks or can you drop one in OEM cases and Cylinder and expect big gains?

A stroker crank makes the engine bigger, so does larger pistons. People are spending a lot of money looking for more power, that is why all these parts are being made. There are many larger OEM engines that could, and in hindsight, should have been adapted from the beginning. Racing kept people from doing this so we ended up with huge aftermarket support for our little Superjet engines. The market needs to be able to sell parts in a piecemeal fashion order to survive since very few can afford an entire custom ski.

 

[Half flip95]

more powa!

 

[NVJAY775]

Are you looking at doing a mild stroker for some extra hit, or just curious about the hype and talk? I'm looking at going that way, but I'm high elevation and 99% flatwater.

 

[Bobert540]

After riding a few different skis this summer, stroker is the way to go for flat water. 4-5mm would probably be a little less maintenance than say a 10mm+ motor.

 

[Matt_E]

4-5mm would probably be a little less maintenance than say a 10mm+ motor.

Why?

 

[KTM434]

4mm stroker crank uses stock rods and 10mm uses longer rods. I'm assuming he's referring to the increased side loading and rod angle with the increased stroke. The longer rods actually counteract that and lessens the rod angle which decreases side loading on the pistons and cylinder walls so it's not as much different as you may assume.

 

[chadman154]

Some cranks are also lightened as well to reduce rotating mass. I know PHP offers a lightweight crank +4mm, i cant speak for the other companies

 

[Bobert540]

I guess "maintenance" was not the right word but when it comes down to getting in the stroker game 4-5mm stroker motors are more feasible for most vs stepping up to a 10mm+ motor. TPE, Jetworks, and PHP have really stepped into that market with great motor options that offer a better "hit" than say a SS motor. This is just my 02. as the market is trending towards the lightweight hull that is backflip capable. I realize this is all hearsay as Lenzi, Rick Roy and Malone were flipping shortened SJ and octane hulls not too long ago but I bet those guys had stroker motors in there skis to achieve those goals. It all comes down to your "budget"

 

[Bobert540]

4mm stroker crank uses stock rods and 10mm uses longer rods. I'm assuming he's referring to the increased side loading and rod angle with the increased stroke. The longer rods actually counteract that and lessens the rod angle which decreases side loading on the pistons and cylinder walls so it's not as much different as you may assume.

Thank you for sharing that, I was always under the assumption the side lower was higher, the higher you went in stroke mm.

 

[mthomps]

Why isn't there a good write up on why the yamaha driveline hasn't changed over 20 years??? I wish there was.

No, I'm not trying to say the 701 is the sweetest thing since candy...or anything like that.

But it works well so why was it never developed further? When I was growing up in the 90's it seemed 'real' jet skiing was popular enough to warrant it. Maybe yamaha was just lazy?

For a longer stroke to work everything basically needs to be bigger?

Many are happy with a stock 701 with b-pipe. I've ridden many boats oem and aftermarket with those and it's so much power! (I ride a stock 6m6, but love it.). I think if Yamaha had better clues towards the way jet ski riding went (surf/freestyle) they would have took the time to implement engineering changes to the driveline.

 

[MADMAT]

Clearly Yamaha wants a motor that will run 10 seasons and be like new! And they achieved that IMO. All I know is my 8mm stroker needed checked after 1 season and rebuilt after 2....never again for me. I don't know why but if you read in here enough that is every strokers story....as far as I have seen over these years anyway...

 

[mthomps]

That's the problem then right? The up keep on the engine. I've seen many threads about making the highly modded strokers run. Is it possible to make more power via stroke length and keep that same oem yamaha reliability we all love? Or is what we have now the best physics allows?

 

[Tommygunz]

As far as longevity of a stroker crank goes I ran my PHP 4 mil crank in my Powermouse motor for half a season and then a year and a half in my 898 before it needed new bearings...and we figured it to be well over 300 hours of ride time so as far as worrying about crank life, that wouldn't be my concern for our 4 mil crank. The new lightweight is goin in mine for next year, prolly another two year run at least

 

[MADMAT]

Yea I have heard/read all these years that the 4mm is not so bad.....but anything over that gets worse. Even at 4mm....a 2 year crank life is not the 6 to 10 years of a beautiful stock yami crankshaft....and my 8mm actually wore the sides of the pistons down first.....then when you go to rebuild....you check your crank....she ain't perfect...and bam.....your already expensive new top end just became a top end and crank rebuild. Just silly to me.

 

[Tommygunz]

Yea I have heard/read all these years that the 4mm is not so bad.....but anything over that gets worse. Even at 4mm....a 2 year crank life is not the 6 to 10 years of a beautiful stock yami crankshaft....and my 8mm actually wore the sides of the pistons down first.....then when you go to rebuild....you check your crank....she ain't perfect...and bam.....your already expensive new top end just became a top end and crank rebuild. Just silly to me.

The first year of my cranks life was spent in a Gen 2 Bob that had tons of water pushed through the motor as I was learning flatwater flips. And my two years of riding equates to a normal riders 5-8 years. I blew out stock cranks faster than my 4 mil

 

[MADMAT]

The first year of my cranks life was spent in a Gen 2 Bob that had tons of water pushed through the motor as I was learning flatwater flips. And my two years of riding equates to a normal riders 5-8 years. I blew out stock cranks faster than my 4 mil

What makes you say, "And my two years of riding equates to a normal riders 5-8 years"? That is a guess AND a 2 year range. You blew out brand new OEM Yamaha cranks (plural) faster than you blew 4mil cranks (plural)?