I'm finally asking it...

[Waternut]

I've been afraid to ask this in the past but I want to know... NOTE: Please don't turn this into a kawi vs yamaha thread.

Anyway, I've got a 750 (sx not sxi) and a 701 (61x/61x) that are virtually mod for mod all the way down. They both have TL, factory pipes, race logic porting, wiseco pistons, high compression heads, stock carbs, and stock reeds. Both have bored nozzles, wet wolf cones, and the props are very similar as well. Both ski's produce very similar power.

My question is this... The kawi has bigger reed cages, a 6mm longer stroke, smaller couplers to reduce rotating mass, and 40cc's more displacement. If you told an engine builder that who didn't know anything else about the engines, he'd probably tell you that the kawi engine would be the better without even second guessing. So why is the power roughly the same? Or more importantly, where is the major flaw is the kawi engine that isn't allowing it to produce full potential? Is the 140mm kawi pump just not as efficient as the 144mm SJ pump?

Please help me understand...

 

[suva]

The sx is probably heavier, but I'm sure there's more than that to it.

 

[SuperJETT]

Yamaha - bigger pump, lighter ski, flatter hull = outperforms the Kawi except in top speed.

 

[Proformance1]

Supposedly the bottom end issue is because the volume in the case is larger on the Kawi also, smaller voulme means faster brap, for less cash!

 

[BruceSki]

port height is a factor as well... the kawi port height is generally geared towards high rpm power where the yamaha is geared towards more low end power.

 

[SUPERTUNE]

port height is a factor as well... the kawi port height is generally geared towards high rpm power where the yamaha is geared towards more low end power.

Also Kawasaki transfer ports are huge and flat angled, this makes for slower air speed, less hit right off the bottom.
The Yamaha ports are smaller and angled up and make power lower in the power range.

 

[Waternut]

I think we're starting to get somewhere now. I understood the kawi ports were a little higher but also kind of considered that due to the increase stroke. I never realized that the case volume was that different and also didn't realize that the transfer ports were so half @ssed.

 

[WFO Speedracer]

I am not sure that is all there is to it either,if you bolt an SXR cylinder on that bottom end I believe you will find your lost brap,better port timing than a 750 cylinder for sure,that is the combo I am going to start with instead of dicking around with a 750 cylinder.It also makes a difference if you are talking small pin or big pin cylinder,the big pin port timing is defiantly geared more for top end power.

 

[Carbon Blaster]

Even puting engine performance aside, the 140mm pump vs's the 144 mm yamaha pump is a big difference.
Pumps , props, nozzles, intake grates, pump cones, etc can make as much difference in performance feel as modifying your motor.
So if you had a yam 701/760 motor in both your Kawi & SJ hull, the difference in driveline setup can give you the feeling that one motor has noticably more performance than the other.

 

[SuperJETT]

Darren Hedlund did a guest chat with us on pwctoday back when he was with Kawasaki and someone asked the exact same question. He said you can build a Kawi to be just as strong as a Yamaha, which makes sense if you completely report one and are going for race purposes.

Then again, most of us don't have .01% of the knowledge/ability Darren Hedlund does.

 

[Waternut]

Darren Hedlund did a guest chat with us on pwctoday back when he was with Kawasaki and someone asked the exact same question. He said you can build a Kawi to be just as strong as a Yamaha, which makes sense if you completely report one and are going for race purposes.

Then again, most of us don't have .01% of the knowledge/ability Darren Hedlund does.

I totally agree a kawi can be built just as strong as a yamaha. I pretty much have proof of it. I just think there is so much more potential from the kawi engine that has been lost and quite possibly can't be recovered. Not that I even remotely have the capability but maybe a little bit of case modding and an aftermarket cylinder with proper exhaust port timing and better transfer ports could be a budget alternative to some of the sweet engines we all drool over but couldn't even remotely dream of or justisfy buying.

I mean if we could get a 750 crank and case for $200, mod it to fit whatever hull we wanted, and then bought a nice aftermarket cylinder for $1000 or so... That could be an incredibly strong budget engine and I'd bet a lot of people would jump on it.

 

[madscientist]

great discussion considering the pistons most commonly used for big power in a yamaha motor are actually kawie 750 small pin pistons and the smaller yamaha strokers use kawie rods, go figure?

so thinking on what chuck said if we would fill our transfers and then re cut and shape them along w/ some exhaust porting it should by all discussion out perform the yamaha. then a small pump mod it would once again be the king.

a budget perormance kawie top end is in the works.

 

[WFO Speedracer]

Fine then since the stroke on the Yami is 68mm and the Kawi is 74mm just make a 6mm stroker plate to bolt the Yami cylinder to the Kawi case = po mans stroker.

 

[madscientist]

no it becomes a kawie w/ better porting

 

[Waternut]

Fine then since the stroke on the Yami is 68mm and the Kawi is 74mm just make a 6mm stroker plate to bolt the Yami cylinder to the Kawi case = po mans stroker.

I was actually thinking of this very thing a few months back but abandoned the idea because I was too lazy to measure the distances between the crank rods.

 

[WFO Speedracer]

I was actually thinking of this very thing a few months back but abandoned the idea because I was too lazy to measure the distances between the crank rods.

If that dude can put a 951 Rotax top end on a Kawi bottom end the Yami setup should be a cakewalk,I know the center to center rod spacing on the 951 is huge compared to the Kawi engine. :arms:

 

[madscientist]

so will it be a stoke stroke kawie w/ custom cylinder or a yamaha stroker????

 

[tor*p*do]

Kawa-hama dude!

 

[Waternut]

What about a Yakawami?

 

[djkorn1]

Porting and Weight from what I have seen. The SXR cyllinder hits wayyyy harder than the 750.

My SXR with the lightweight hull only has a wetpipe and flywheel (and only 155 lbs compression) and I guarantee it will chew up a SJ with a b-pipe and flywheel. The weight made all the difference in the world.