Super Jet Stroker Cranks

[tshank123]

Here's my question. If a stroker makes more power because of the increased CCs, is it more power than an equivalent amount of increased bore? (does that make sense?). I always hear that the Kawi 1100 tripple is stroked compared to a Yami tripple, therefore its a better option for a conversion. The way I look at it, its still 1100 CCs, so compared to a 1200 CC yami, why is being stroked a benefit?

I know that there are other reasons for going Kawi 1100, just wondering why people always mention the stroke.

 

[Matt_E]

Longer stroke means longer piston travel, which is equated with more torque. Power output of a motor = torque at a given RPM * RPM

A bigger bore only results in more force on the piston with the same stroke.

You can also look at it as work = force * distance. The work performed by each piston equals the force acting upon times the distance traveled. A 'stroker' increases that distance.
A bigger bore increases the force, but it may not be as linear of a relationship as between stroke length ('distance') and work.

 

[623pwr]

Type bore to stroke on Google, go in to Wikipedia. It kind of explains it, this is way over my head and can get confusing if you think too much, like me. I'd put the link but on my phone and its PITA. I know my stock stroke sxr Dasa revs quick and the 78mm Dasa pulls harder but doesn't rev as quick, both stock flywheel set ups. Just my opinion, I'm sure the guru's will have better feedback.

 

[scottie mac]

Think of a baseball pitcher throwing a ball. Now, add a foot longer arm on the pitcher. With that much longer arm, the force of the ball leaving would be higher/faster.

The kawi has the advantage of stroke AND flat top pistons and higher port timing which also create better power when set up right. But, it comes down to set up. Stock to stock, the 1200 motors has more power. Once you set up the kawi to take advantage of the timing, etc, it has the advantage. That's why it takes a 100cc advantage for a Yamaha to be competitive.

SM

Here's my question. If a stroker makes more power because of the increased CCs, is it more power than an equivalent amount of increased bore? (does that make sense?). I always hear that the Kawi 1100 tripple is stroked compared to a Yami tripple, therefore its a better option for a conversion. The way I look at it, its still 1100 CCs, so compared to a 1200 CC yami, why is being stroked a benefit?

I know that there are other reasons for going Kawi 1100, just wondering why people always mention the stroke.

 

[waxhead]

Have a look at any performance two stroke. They have as close to a square bore ratio as possible. You willl find that a big bore will never make the same power as a stroker of the same cc. Its just not going to happen I am afraid. And that not adding in the fact that the big bore cuts into the transfers as well which are already an issue in the 62t platform. The reason the yamaha crank last longer than the kawi is because the bearings on a kawi are alot smaller and there for the loading is higher on them.

Groupk does do a big article on stroker cranks much of which is wrong. I have not seen many people wining races on groupk built engines his are always very lightly and conservative ported engines. while he does have some good and interesting articles this is not really one of them bear in mind it was also written about 15 years ago.

The stroker crank does have higher piston speed, the longer rod tends to reduce the piston acceleration and thus helps the piston live with the higher piston speeds, The stock stroke engines with a big bore run a huge piston of a crank that's not designed for the load that your putting on it and this is why we see alot of little ends blowing up on the big bore engines. You can of course use full complement bearings like the seadoo to try and over come this. Personally I would take a stroker over a big bore any day of the week.

 

[623pwr]

Thanks for clarifying the Yami/ Kawi bearing differences. I wasn't trying to compare the two, I just thought maybe based on some of the other posts made by some really knowledgeable people that the stroke may of been a factor. I also thought the shorter stroke had a higher piston speed, my bad.

 

[waxhead]

Think about it this way. The piston on a stroker crank has to move further each 180 degrees as the stroke is longer so at each given rpm the piston is moving faster to actually end up in the same place as the smaller stroke. It also advances your port timings as the piston moves more each degree of crankshaft rotation this means that your ports get uncovered sooner

 

[tshank123]

Think of a baseball pitcher throwing a ball. Now, add a foot longer arm on the pitcher. With that much longer arm, the force of the ball leaving would be higher/faster.

The kawi has the advantage of stroke AND flat top pistons and higher port timing which also create better power when set up right. But, it comes down to set up. Stock to stock, the 1200 motors has more power. Once you set up the kawi to take advantage of the timing, etc, it has the advantage. That's why it takes a 100cc advantage for a Yamaha to be competitive.

SM

When are you getting to finals?

 

[scottie mac]

Thursday night, but I won't be there til late. I am going straight to Jimmy's house to give his ski a once over, then to bed, as it will be LATE my time. lol

I'll be at the track early friday morning.

SM

 

[masterblaster]

Speaking as someone who has had stock stroke, 5mm, 6mm and 8mm strokers (all lameys) built by Tim Judge, thery is very little noticeable difference (in fact, I'd doubt anyone would be able to tell he difference) in vibration between a stock stroke to the 8mm. Especially if they are both trued and welded by someone who knows what they are doing.

As for the cr500 rod cranks, both my 6 and 8mms had cr500 rods. My race blaster motors revved in the 7800-7900 rpm range and had as much bottom end hit as anything south of 1000ccs in the free ride community. Just because you are a free rider, doesn't mean you want your motor set up to rev lower. Rpm should be controlled by your index finger, not your ignition or pump set up. Two skis with identical set ups, one revs 7500. The other revs 7850. The one revving 7850 will hit harder because it revs faster with a dry pipe.

Reliability on any crank from stock to 16mm is 50% how well he crank was built and trued and 50% owner maintenance. I've got more time on strokers than probably 99% on here and most of that time was at 7800rpm plus I've never had a crank failure. Wonder why?

SM

That small stroker you had in the b1 was a beast,it surprised me for sure riding it. most are refering to lower comp pump fuel freeride/rec motors on wet pipes as alot are here that are looking for power/reliability balance. Even built and balanced perfect,a 220lb high comp race fuel stroker is not going to outlast a 180lb pump fuel stroker motor. With out the race fuel setup,these low comp setups really do need to be setup with lower duration and hit early when we need it,other wise they just feel soft and late all over the low rpm range.
Ive rode many a _ _ _ _ stroker setup that felt like a wet dog on pump fuel,then came to life on race fuel setups. this is the challenge in building a good powerful long lasting freeride motor. I agree on the maintenance,though I know yall take them apart and check more than many of us,so how many hrs did you get out of them on average?

 

[scottie mac]

My motors went the entire season of racing, 7800 plus, being loaded in turns and run ragged by me. I'd be willing to bet I put more, harder time on a motor in a year when I was racing than a freerider does in 4 years. It's a heck of a lot harder on a motor to load in the turns than it is to jump waves or blip to do a trick. That's why detonation is such an issue with race skis. That's why you guys can run 195lbs on pump gas and not destroy piston crowns. Even still, the only maintenance we did to our motors was one top end in March, one top end before Nationals and a crank rebuild every other season.

I have ridden pump gas set ups that would blow away most on this forum. Again, it comes to set up and the entire package.

The trick is to set up the ski to get the desired result. It's funny how the billet DASA motors have very similar port timing to the race motors from 10 years ago. Those same motors that some people say have no bottom end. Perfect example is our spec motors. 99% of the people on this forum will tell you a 61x cylinder is better for bottom end than a 62t because the 62t has higher port timing. This is not true. If you prop the 62t to get to the right window of rpm, it will always hit harder and pull better. Proof? Look at all the fastest sport spec blasters. They are all 62t set ups.

Set up has more to do with any package than what stroke, bore, color, flux capacitor, etc. any ski has. And, if you are talking about freeriding and surf, the MOST important thing is nuts. Zimmy goes bigger, higher and crazier than just about everyone out there and until recently, was rocking a 650.

IMO, the best set up for the budget minded rider is a TPE stock stroke, ninya pipe, some 46s and some timing. I've ridden that set up and it is more than most would ever need in a freeride ski.

If money is no object, and there are a lot of guys on this forum who act that way, then by all means, knock yourself out, go 10 mil, 16mm, whatever.

SM

 

[kevbo]

Is this why OEM Yami cranks tend to outlast Kawi OEM cranks, due to stroke? I've always heard 68mm cranks are very reliable. Or is it just a bearing issue with the kawi OEMS?

Kawasakis tend to break rods at high sustained rpm, it happens consistently enough to be a design flaw. Every engine has limits based on design and quality of mfg. Quality is usually the weak point of mass produced products. As a consumer the only questions should be about life expectancy and failure mode so you can stay in front of problems. As for performance, I have found that longer stroke smaller bore designs are better with tuned pipes. Open exhaust engines have better power/weight with short strokes and larger bores since peak power ends up nearly the same. This is the reason there is no long stroke chainsaws or outboards. We should be much more interested in porting, pipe, and pump specs. As a group, size is all we know.

 

[McDog]

My 5mil crank has had four seasons on it at 195 compression and total loss and one season around 220 without ever feeling rough or loose on a yearly inspection running 86mm pistons. I know I'm past due on a rebuild but that still says something about longevity. I ride 100% freestyle/rec flatwater.

 

[CARBONX2]

Kawasakis tend to break rods at high sustained rpm, it happens consistently enough to be a design flaw. Every engine has limits based on design and quality of mfg. Quality is usually the weak point of mass produced products. As a consumer the only questions should be about life expectancy and failure mode so you can stay in front of problems. As for performance, I have found that longer stroke smaller bore designs are better with tuned pipes. Open exhaust engines have better power/weight with short strokes and larger bores since peak power ends up nearly the same. This is the reason there is no long stroke chainsaws or outboards. We should be much more interested in porting, pipe, and pump specs. As a group, size is all we know.

Very true. Everybody wants to buy simple off-the-shelf bolt-ons.
If the porting and pipe are matched, significant gains are made in HP. The problem is to try get Temp, Tuned Length, recommended Exhaust Duration, etc. from an exhaust manufacturer

 

[waxhead]

Very true. Everybody wants to buy simple off-the-shelf bolt-ons.
If the porting and pipe are matched, significant gains are made in HP. The problem is to try get Temp, Tuned Length, recommended Exhaust Duration, etc. from an exhaust manufacturer

Yeah its surprising they dont give that information out

 

[mthomps]

Stroker cranks are probably the most misunderstood upgrade parts for our skis. There is more to it then what others have posted, ie if 4 is good 10 is great right? Well that's not really the case. The rod length itself plays a large role in the way the power is delivered. The rod to stroke length ratio is the term used to describe this. If the rod to stroke length is small the piston will experience extreme acceleration velocities and high side loading which leads to early piston failure and rod fatigue. Honda saw it prudent to put a 144mm long rod on 79mm stroke cr 500 to keep piston speeds in check on an engine that peaks rpms in the high 6000's . Contrast that to a similar 78mm stroke 10mm stroker Yamaha crank with kawi based 133mm rods on an engine that commonly has an msd rev limiter set at 8k = piston speeds that are very excesive. Stroker cranks also make port timing more radical. If we have an engine that was ported optimally for a stock 68mm stroke crank and decided to throw a 4+ into it the port timings would get whacked out enough that everything we gained from the stroke would probably be out weighed by the poor porting...especially in the transfers. This is why we use a different cylinder between our ss and our 4mil even though it would fit.

Another element to consider is vibration. Vibration is hp. The violent shaking that those rubber engine mounts are absorbing is energy - all which never makes to the prop. The less vibration that you can make the more power that gets to where you want it and less of it that goes into shaking rubber and beating up bearings.

Bottom line a common 10mm+ stroker 133mm rod crank will have a shorter life span than the smaller strokers will with higher piston side loads and more vibration. For those looking for every little bit of power they can get at any concession the 10mm may be the best choice. For those looking for some longevity there are better choices.

Thanks..

Do you remember a little while back the thread on the 6m6 motors? You mentioned an option for 650 guys on small budgets was turning it in to a 4mm stroker (maybe it was 6). There were a few guys interested in what you were saying but the thread died. When you get a chance could you tell us more about that? And compare the power vs reliabilty with a stock 650 and a limited 701 set up?

Dont mean to derail guys, just interested in the idea of a smaller bored pump gas stroker.

 

[TheBuzzard]

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?

without reading thru all this, i dont know if this most basic of questions was answered. throwing a stroker crank in a stock motor is not going to give you big gains. some bad ass cylinder porting is going to make a lot more power than a stroker crank. a stroker crank thrown on top of porting, big carbs, ignition , high comp, will take it to the next level. a stroker crank thrown at a motor that hasnt been set up correctly, will have less power than a SS set up right. i've seen this happen this year.

 

[BruceSki]

My 12mm wiener makes 19inchpower with massive stroke. If I smallen up the combustion chamber a bit I can get the rod to throw frequency back into the timing spec. That's when I run into problems with hitting the firewall though.

Still haven't figured it all out but I've been running WTFO and replacing pistons as soon as I'm done pumping away. Also random sentence generator.








I think we should start a competition. Who can do the best on a bone stock 650 in surf. Any modified oem hull you want, since there are too many aftermarket. My guess is you would be surprised at the riding more than the dinky little power plants we all oogle over.

 

[kevbo]

Thanks..

Do you remember a little while back the thread on the 6m6 motors? You mentioned an option for 650 guys on small budgets was turning it in to a 4mm stroker (maybe it was 6). There were a few guys interested in what you were saying but the thread died. When you get a chance could you tell us more about that? And compare the power vs reliabilty with a stock 650 and a limited 701 set up?

Dont mean to derail guys, just interested in the idea of a smaller bored pump gas stroker.

A +4 crank and 6m6 cyl will work with its low transfers but suffer from lack of port area compared to a 701 cyl. A 701 cyl on a stock 6m6 low end is the cheapest way out of a 650.

 

[chasingdaylight]

My 5mil crank has had four seasons on it at 195 compression and total loss and one season around 220 without ever feeling rough or loose on a yearly inspection running 86mm pistons. I know I'm past due on a rebuild but that still says something about longevity. I ride 100% freestyle/rec flatwater.

Yep me too... 4 seasons on my x-scream 8mil stroker. right around 200lb on compression. just stock ignition with a msd. sank it in hurricane bill surf tore it apart and cleaned it all up and still running strong. Anyone that knows me knows how well i maintain it... Not... Yep i'm past due too but think i'll wait till 2014 daytona freeride is over..