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Author Topic: ISO KX21 rev limiter  (Read 927 times)

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Offline Painlesstom

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Re: ISO KX21 rev limiter
« Reply #15 on: November 08, 2020, 07:08:54 PM »
He sold it to someone and it's probably sitting on a shelf somewhere collecting dust. I recall the purchaser contacted me a while back about getting parts for it like a muffler and linkage etc.. but didn't realize my conversions weren't based on the GX200 style engines or that the Subaru was that much different. He didn't want to ship me the motor to figure something out and never heard from him again. That motor makes it's power way past 5k rpm, it would never be able to reach it's power band in the Mokai with a 1:1 direct drive ratio. Overheating and poor fuel economy and performance would be the result. 
Tom

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Offline happul3

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Re: ISO KX21 rev limiter
« Reply #16 on: November 09, 2020, 06:48:56 AM »
Would it complement your experiments with impeller design? Something like reducing pitch so it can rotate faster and allow kx21 to reach the max power rpms. 

Offline Painlesstom

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Re: ISO KX21 rev limiter
« Reply #17 on: November 09, 2020, 09:04:31 AM »
I tried so many different impeller profiles and came to the conclusion that the stock version is the most efficient at this scale. The KX is quite happy in the Mokai, I wouldn't want to run it at any higher RPM than where it's at. Engine longevity is reduced the higher the sustained RPM's get. The new pump design offers a fraction of the internal friction by using the thrust bearing designed for axial loading, so that parasitic loss is no longer a factor and that power can be converted to more speed. I never would have guessed it was going to make that much difference. More testing will confirm my initial findings, but I'm very excited.  \./8
Tom

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Offline CC-CODER

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Re: ISO KX21 rev limiter
« Reply #18 on: November 09, 2020, 09:32:13 AM »
Are you saying that my Mokai could see an increase in speed?
🤦🏼‍♂️ Chuck 🤷🏻‍♂️

Offline Painlesstom

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Re: ISO KX21 rev limiter
« Reply #19 on: November 09, 2020, 04:48:41 PM »
Are you saying that my Mokai could see an increase in speed?

Increase how? With my new pump design? Possibly, but more testing needs to be done before I can confirm that. Will be converting my 2011 pump next to see how that boat responds. Stay tuned.  \./8
Tom

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Offline happul3

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Re: ISO KX21 rev limiter
« Reply #20 on: November 10, 2020, 07:27:00 AM »
Are you saying that my Mokai could see an increase in speed?

If you are asking about impeller modification with complementary change from ex21 to kx21, then one would certainly expect top speed to increase. As Tom was explaining earlier, swapping for kx21 while still using stock pump does not allow motor to reach rpms at which it produces full power (9.2 hp at 5600 rpms). That's because Mokai folks have designed pump system specifically for ex21. And there is no way around that with direct drive. However, if one can change pump itself, it should be possible to make one better suited for kx21 at its top rpms. Simply reducing pitch of the screw could be enough. That would poorly with stock motor, leaving it underpowered at 4100-4400 rpms, but might show significant improvement with kx21.

However, there are other factors  at play to. That's a lot of work and experimentation to gain just, say, 4(?) mph in top speed (not to mention the cost of kx21 and impeller machining). And higher power/heat production of kx21 resulting in overheating is definitely a concern. But, considering extremely crafty people like Tom are already experimenting with both essential components - motor and impeller - it seemed like a perfect marriage. Personally, I'd be very curious to know just how fast that could work  with all other Tom's upgrades (intake grate, low friction pump, wear ring, etc.)

Offline Painlesstom

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Re: ISO KX21 rev limiter
« Reply #21 on: November 10, 2020, 07:45:47 AM »
Personally, I'd be very curious to know just how fast that could work  with all other Tom's upgrades (intake grate, low friction pump, wear ring, etc.)

As I explained in my earlier reply, the KX is happy in the Mokai at the rpm it operates at and I wouldn't want to run it any faster. Not good to run high rpm in a block that was originally designed to run sustained 3600 rpm. KX uses the same block as EX, the difference is in the head, carb, flywheel, only. Less than 4400 rpm is ideal and will ensure longevity. If you check out my posts about the KX in my ES-kape, as well as the posts in my New Pump Design thread, you will see how fast the ES-kape can go with my wear ring, new pump design, and aluminum wear ring etc..

A customer did put a Kawasaki 440 jet ski motor in a one piece hull some years ago, I rebuilt the pump and made a coupler for him. At 6000 rpm he saw 32 mph. For the scale of the pump, the impeller is well matched from what I've seen.
Tom

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Offline happul3

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Re: ISO KX21 rev limiter
« Reply #22 on: November 10, 2020, 08:26:45 AM »
Not to drag this matter further, but why wouldn't you want run kx21 at its rated rpms? I'd think that good people of Subaru do not make up inflated specs. Or is the 9.2hp@5600 rpm number is meant only for short bursts?

Offline Painlesstom

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Re: ISO KX21 rev limiter
« Reply #23 on: November 10, 2020, 08:38:19 AM »
In a gocart with the engine out in the open, short bursts to 5600 in a race motor are what get you to the finish line. In the Mokai it's in a box, cooling is not 100% efficient regardless of what is done to improve it, and reliability for the long haul is absolutely necessary. The EX21 was never intended to run past 4000 rpm, but it's a well designed engine and has proven it can handle much more, but if you ran it over 5000 rpm in the Mokai you would just be making more heat to deal with and accelerating the wear without much of anything to show for it. Could a different impeller profile that allowed the KX to get more into it's powerband make it faster? Maybe, but I doubt it would be anywhere worth the expense or cost to reliability.
Tom

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Offline happul3

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Re: ISO KX21 rev limiter
« Reply #24 on: November 10, 2020, 09:29:49 AM »
In a gocart with the engine out in the open, short bursts to 5600 in a race motor are what get you to the finish line. In the Mokai it's in a box, cooling is not 100% efficient regardless of what is done to improve it, and reliability for the long haul is absolutely necessary. The EX21 was never intended to run past 4000 rpm, but it's a well designed engine and has proven it can handle much more, but if you ran it over 5000 rpm in the Mokai you would just be making more heat to deal with and accelerating the wear without much of anything to show for it. Could a different impeller profile that allowed the KX to get more into it's powerband make it faster? Maybe, but I doubt it would be anywhere worth the expense or cost to reliability.

I understand your reasoning now. Thank you!

Offline CC-CODER

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Re: ISO KX21 rev limiter
« Reply #25 on: November 10, 2020, 10:01:31 AM »
Increase how? With my new pump design? Possibly, but more testing needs to be done before I can confirm that. Will be converting my 2011 pump next to see how that boat responds. Stay tuned.  \./8
Sorry for the confusion, think I meant to post this question on your new pump design thread, at least you seemed to understand. The sun glare at camp made me do it. Hee hee
If your new design gets me 1 mph faster, that's 15 less lbs I'll have to lose this winter.  :)
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Offline Painlesstom

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Re: ISO KX21 rev limiter
« Reply #26 on: November 10, 2020, 10:29:53 AM »
Sorry for the confusion, think I meant to post this question on your new pump design thread, at least you seemed to understand. The sun glare at camp made me do it. Hee hee
If your new design gets me 1 mph faster, that's 15 less lbs I'll have to lose this winter.  :)

I figured as much. I'm hoping for that result too, would be such a welcomed bonus after all these years!  \./8
Tom

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Offline Painlesstom

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Re: ISO KX21 rev limiter
« Reply #27 on: November 10, 2020, 10:30:46 AM »
I understand your reasoning now. Thank you!

You're welcome, glad I could explain it better. Not my strong suite but I try.  :D haha
Tom

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