40 series torque converter and axle gear size

Abtheman28

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Hi guys,

I've recently upgraded to a brand new GX390 clone which is currently running in stock form. I'm running a rear axle gear size of 48 teeth on 8 inch tires and currently have a 14 tooth centrifugal clutch. I'm loving the mid to top end power, however bottom end sux!

I've been told I require a 40 series torque converter to achieve better low end grunt.... but I'm unsure what size (how many teeth) I need on the rear axle gear to achieve great low, mid and ideally top end speed if possible.. bit definitely low to mid range I want, or will my current axle gear do the trick with the addition of a torque converter? All new to this and need advice before I unload money as I'm based in New Zealand so have one shot at getting it right due to import delays....dam covid! Cheers
 

ONE-EYE

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What size is the bore for your engine? What size engine? A torque converter will definitely help with adding torque. As far as sprockets go, a 60T will give much more torque but if you're wanting more mid, you could always opt for the 54T. It really depends on where you'll be riding most, how much weight you're pulling, and the engine itself. Most 212cc and lower have a 3/4" bore and use a 30 series (which is much cheaper). The larger 301, 420, etc. have a 1" bore and will use the 40 series. Switching the upper sprocket to a 10T will help as well. These normally come with the torque converter. You will probably need a new chain as well as the setup you have now probably uses #35 chain. The setup I just explained will use a #40,#41, or #420 chain (normally these 3 are interchangeable and will fit fine but the #40 is slightly thicker).
 

Abtheman28

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Hi mate the engine size is GX390 and is the bigger sized 1inch bore or 25mm crank. The chain itself is 420 pitch and also has been from stock form. I was always led to believe a smaller axle gear would create more torque, or a bigger axle gear would create more top end speed?
 

ONE-EYE

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Hi mate the engine size is GX390 and is the bigger sized 1inch bore or 25mm crank. The chain itself is 420 pitch and also has been from stock form. I was always led to believe a smaller axle gear would create more torque, or a bigger axle gear would create more top end speed?
Yeah you're in good shape for a 40 series. And yes and no. For torque you want your upper sprocket with less teeth and your lower sprocket with more teeth. It would be the exact opposite of you were wanting top speed. Since you like the mid tho, you could try getting the torque converter and trying out the 10T sprocket that comes with it and then later decide to switch the axle sprocket if you want. May save you some time/money if you like it. A 40series itself will add quite a bit. If you're still wanting more torque than that though I'd go for the 60T axle sprocket.
 

ONE-EYE

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To find the gear ratio, divide the lower tooth # by the upper tooth #. So 48/14=3.43. That means your gear ratio is 3.43:1....by switching to a 10T and 60T sprocket setup (60/10=6), the gear ratio would be 6:1
 

Abtheman28

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Yeah you're in good shape for a 40 series. And yes and no. For torque you want your upper sprocket with less teeth and your lower sprocket with more teeth. It would be the exact opposite of you were wanting top speed. Since you like the mid tho, you could try getting the torque converter and trying out the 10T sprocket that comes with it and then later decide to switch the axle sprocket if you want. May save you some time/money if you like it. A 40series itself will add quite a bit. If you're still wanting more torque than that though I'd go for the 60T axle sprocket.
Awesome thanks for your help appreciate it! So in the mean time if I'm wanting more torque with my centrifugal clutch I'm guessing I would be better off getting a bigger rear axle gear? Keeping in mind the clutch has 14 teeth.... because right now bottom end power is crap
 

Abtheman28

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To find the gear ratio, divide the lower tooth # by the upper tooth #. So 48/14=3.43. That means your gear ratio is 3.43:1....by switching to a 10T and 60T sprocket setup (60/10=6), the gear ratio would be 6:1
Thanks mate. From what I've seen online I can't seem to find any 10 tooth clutches with a 1 inch bore Size with chain pitch of 420? I'm unsure whether the gear on the clutches are interchangeable or not....
 

ONE-EYE

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Awesome thanks for your help appreciate it! So in the mean time if I'm wanting more torque with my centrifugal clutch I'm guessing I would be better off getting a bigger rear axle gear? Keeping in mind the clutch has 14 teeth.... because right now bottom end power is crap
Yes unfortunately. Unless your sprocket on your clutch can be switched with a 10T. Normally with the bigger engines, bigger tires, and bigger karts you're going to want to use a torque converter instead of a clutch anyway. I bet that thing is pretty quick though lol...oh and don't forget, you may have to search items for #40. Your #420 chain should do just fine though. That's what I have on mine.
 

Abtheman28

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Yes unfortunately. Unless your sprocket on your clutch can be switched with a 10T. Normally with the bigger engines, bigger tires, and bigger karts you're going to want to use a torque converter instead of a clutch anyway. I bet that thing is pretty quick though lol
It's insane lol took it for its very 1st spin on a long gravel driveway yesterday 🤣 ... must say I was pleasantly surprised. My wife said the smile she saw as I flew past was from ear to ear. Considering it's speed governed at the moment and stock everything.....I can only imagine the power at full potential which is on the cards in the future. For now it's great fun
 

Abtheman28

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It's insane lol took it for its very 1st spin on a long gravel driveway yesterday 🤣 ... must say I was pleasantly surprised. My wife said the smile she saw as I flew past was from ear to ear. Considering it's speed governed at the moment and stock everything.....I can only imagine the power at full potential which is on the cards in the future. For now
 

Abtheman28

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Any idea of best brand of torque converter? Want one thatl last... I see we have the tav2 brand here in nz but I'd have no idea if it's good or bad 🤔
 

Denny

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Forget what these guys have said. Go to the GoKart Supply site and do some reasearch. They can explain all about centrifugal clutches, CVTs and they even have a gear ratio calculator. There you can get all your questions answered properly.
 

madprofessor

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Here's the info Denny's talking about. Read the Comet torque converter belts page (first link below), all the text at the beginning, but especially scroll down to the bottom and read all of that. The other link is for the gear ratio calculator. Read up, learn up, build up.................
Comet Drive Belts | Comet Torque Converter Belts | Go Kart Belts (gokartsupply.com)
Go Kart Sprockets | Go Kart Sprocket Hubs (gokartsupply.com)
NOTE: Whatever type or size of torque converter you use now or on other karts, always pay the extra for a Comet belt. It's cheaper in the long run, and just saving the hassle of frequently having to replace cheap belts and/or getting stranded far from home even once makes it worth it.
 

ONE-EYE

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Forget what these guys have said. Go to the GoKart Supply site and do some reasearch. They can explain all about centrifugal clutches, CVTs and they even have a gear ratio calculator. There you can get all your questions answered properly.
What do you mean by "forget what these guys said"? I'm the only one that was posting on his feed and I literally gave him the info from a quick Google search. He said he was running out of time so I kept it simple. He said he was looking for more torque vs top speed and what I told him will definitely accomplish that. Oh and the link for the calculator at gokartsupply doesn't work properly so I used the one at BMI KARTS. They spec theirs for a "safe junior kart" so people aren't killing themselves. As far as the gear ratio though, it's saying the same thing I said.
 

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madprofessor

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I think Denny was trying to help out a new member by directing him to a lot more info than anyone can (or would want to try typing) put down here. Also there comes with that the confidence of the established written word being wise, where one can only hope for wisdom from various voices out of the ether.
I know that has certainly been my longtime intention with many links hunted down and pasted here.
 
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