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Old 03-29-2020, 07:37 PM
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Default Do Torque Converters Limit Max RPM?

Just doing some messing around with the top speed calculator and I have a question. Do torque converters limit the amount of RPM you can get? I know there's the inherent belt slip which can rob power, and that torque converters have built-in overdrive, but do they put a "hard limit" on the RPM you can turn? Let's assume an engine with no load can turn out 8000 RPM. Now obviously under load, it can't rev quite as high, so let's say 6500 (rough numbers of course). But I've seen torque converters being sold with descriptions that say "5000 RPM MAX" (specifically the 780 torque converter on Vegas Karts). Does that mean you won't be able to turn more than 5000 RPM (essentially capping your top speed) or does that mean that it's just fully shifted out at 5000 RPM and the engine can still rev higher?
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Old 03-29-2020, 07:40 PM
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Wow that's a really good question now I wonder about that lol
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Old 03-29-2020, 07:46 PM
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When it says 5000 rpm max, that is the limit they've set beyond which it might fly apart from the internal stress.

You can spin it faster than that. How much faster is anyone's guess.

And the speed at which they're fully "shifted out" depends on loading. A stock Tav2 might hit top ratio at 2500 rpm with the kart up on jack stands and just spinning the axle, but actually driving it might be right at 3600. It's called "torque sensing" and it results from the spring preload and the "ramps" on the driven mechanism.
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Old 03-29-2020, 07:58 PM
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Not if you get the JUGGERNAUT!!

Every freakin' youtube channel sponsored by GPS is peddling those now.
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Old 03-29-2020, 08:01 PM
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I had a comet magnum 44c and I absolutely could not get it above 6250 rpm. And trust me i tried !

Then I melted the driver together and had to throw it away. It was on my mini me dragbike from a few years ago in the build off.
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Old 03-29-2020, 08:03 PM
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Quote:
Originally Posted by PullHorse View Post
Not if you get the JUGGERNAUT!!

Every freakin' youtube channel sponsored by GPS is peddling those now.
You must be sponsored by GPS too... That's like the second or third time you've mentioned it in an hour.

---------- Post added at 10:03 PM ---------- Previous post was at 10:02 PM ----------

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Originally Posted by Joe-405 View Post
I had a comet magnum 44c and I absolutely could not get it above 5800 rpm. And trust me i tried !
Even with the kart on jack stands? If you're talking about on the road, that might just be all the power it has.
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Old 03-29-2020, 08:18 PM
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eh what?
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Old 03-29-2020, 08:54 PM
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I'm planning to run a 780 torque converter on a built 420/440. Built 420s/440s (billet rod/flywheel) can rev up to 8000 RPM under no load according to Red Beard's Garage. I'm aiming for 60mph w/ 22" tires and a 6.6:1 gear ratio. That's 59.5 MPH in the calculator, and that's not including overdrive from the torque converter. I want to gear as low as possible for more torque, but still want at least a 60 MPH top speed. End goal is to have something comparable to a Honda Odyssey (The dune buggies not the minivans!). I just need to know if the torque converter is capable of it so I figured I'd ask the experts.

EDIT: Just did a bit more research on this topic, guess I should share for anyone else wondering. Let me just say that YouTube is a wonderful thing.

Vasily Builds runs a 780 on a built 670cc that's turning 7000 RPM (w/o load). Cars and Cameras ran a 780 on their 60 horsepower 670cc drag rail at 6000+ RPM (w/ load) by their estimate. They also ran a 40 series (w/ load) on their 50 horsepower 670cc mower that did 65 MPH on 18" tires. Granted it had about 50 horsepower while I'm probably going to be closer to 25-30 horsepower. No idea what gearing they used; they only said "short gearing", so I'm guessing they geared it for just about 65-70 MPH. They were running that 40 series at almost *3 TIMES* its rated horsepower for quite a while before it crapped out on them. Red Beard's Garage runs 40 series on his built 420s/440s that put out about 30 horsepower and turn 8000 RPM (w/o load) with no issues, and he also ran a 30 series on a Stage 2 Predator 212 which are capable of about 8000 RPM (w/o load).

All of this considered I imagine I'll be fine running a 780 on a 25-30 horsepower engine at probably around 6000-7000 RPM. It seems that these torque converters can be run not only above their power ratings (to a point) but also at high RPMs with little issue. At the end of the day I can always compensate for any loss of RPM with gearing to get to my desired balance of speed and torque. But at least now I'm not as worried about the durability of these torque converters.

Now, that's enough scowering the internet for one night...it's 2:40 AM...I should go to bed.
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Old 03-30-2020, 07:00 AM
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a hard limit.. no, certainly not..
a soft limit .. sure does!

We've had that conversation a while ago because of MrMonks TavKit last year,
that had me think about it ...

anyways
here's how I think it goes:

first some facts
A belt transmission always slips;
and that slip is what's causing belt wear and also powerloss
(power turned to heat and belt dust instead of movement)
it's the same principle that wear down your tyres eventually.
that slip happens at the transition from semi circular path round the driver
to the straight path (free air) to the driven and back.
those four transition points cause about 20-25% powerloss on an ideal CVT.

The driver and driven forcefully try to change reduction ratios,
by clamping onto the belt;
that clamping force in case of our torque regulating CVTs
(series 20,30,40 style comets)
is what's regulating the CVT gear ratio,
ideally forcing the belt into a larger radius path;
the driven by torquing against the spring loaded cam on one side (moveable sheave)
and the wheel and eventually the road on the other (fixed sheave),
the driver by torquing against the flyweights and their centripetal force
(yes.. petal, not fugal... nevermind) on one (moveable sheave)
and eventually the engine's crank on the other (fixed sheave).

the more torque applied to the rear wheel, the bigger the active diameter of the driven,
the more torque applied to the crank, the bigger the active diameter of the driver.
And ideally those two level out since the total belt length is a given

The main difference between the two:
the driven can never apply more clamping force than that of the cam spring.
so that is a hard limit on clamping force until fully compressed;
the driver however can increase it's clamping force by increasing the engines rpm,
since the flyweights are rotationally locked to the crank of course.

I think so far we all are on the same page, right?

Long text already.. no limit explanation so far... bear with me

Now, in our "extreme" situation what I think is happenening is this:
first the driven is fully expanded, the belt is running at the lowest active diameter;
but there's still rpms left in the engine to be released.

you increase the throttle and the engine tries to rev up,
the belt can no longer move up the driver and down the driven;
and what you are hoping for is increased road speed;
since that's the only way those additional revolutions could go, right?

Yeah but what if the load on the rear wheel prevents that?
you cannot move faster, since you simply do not have the power at the rear wheel to do so?
you hit a speed wall for whatever wheel-torque you now have!

that power is then fed back into the CVT.. the belt tries to wedge down into the core hub of the driven, but can't go anywhere..
the force then travels back to the driver and
does as it's supposed to, trying collapse the flyweights back
and reduce the active diameter of the driver.
but the flyweights are spinning fast enough to prevent that..
so the belt wedges itself into the driver pulley even harder
increasing the clamping force
making it harder and harder to feed the belt into and pull it out of the driver at both transition points.
(if you ever tried to wedge a 3/4" belt into a 5/8" gap you know how much force is required to do so )
So you loose more and more power ...
and the fewer power is reaching the driven pulley, the more it tries to force the driver into lower gear.

Now two things can happen:
a) the belt looses getting too hot, glaze over, and slips, the driver then just revs freely
or what is more likely, because of the clamping force
b) the belt just transmits all the forces back into the crank overcoming the raw engine torque at that rpm and thus slowing it down.

and voilá, we've hit the soft rpm limit!

Why soft limit?
Well that rpm I doubt is a fixed number for all engines..
it's highly dependend on the specific engine itself and the gear ratio used.

I'd say you can have a unmodded engine but gov removed with a series 30 hit 5k
and the same series 30 hit a wall with a stage 3 or 4 at just 4k rpms

mainly because of the idiotic rpms over torque modifications made.
rpms are mostly worthless if not backed up by torque.

So, why does the juggernaut work then you may ask.
I think the juggernaut driver prevents exceeding clamping forces,
and since no additional power is lost to those raising clamping forces,
it can then use that 'additional' power to actualy make the wheel turn a smidge quicker.

but tbh... that is cosmetic and totally pointless in all cases I've heard so far.

unless you are competetive racing (prices to be won, not friends to be beaten)
there is absolutely no point in a juggernaut..

I'll explain my thinking behind why and how later

'sid
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Old 03-30-2020, 10:06 AM
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Quote:
Originally Posted by itsid View Post
mainly because of the idiotic rpms over torque modifications made.
rpms are mostly worthless if not backed up by torque.
Yes!

Quote:
Originally Posted by itsid View Post
there is absolutely no point in a juggernaut..
At least they corrected their advertising somewhat, and are no longer claiming "huge benefits" to stock motors, I can respect that.

I would assume the main allure is the reliability under high load/ speed.

Anyways I ordered one to try out and test on a couple different builds, ill let yall know the results (against a regular 30 series driver with same engagement speed).
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Old 03-30-2020, 02:47 PM
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Excellent response itsid, very helpful. That perfectly addresses the main question of this thread.

As for my end goal with this project, I'm confident that I will be able to reach my desired top speed of 60 mph with my planned setup. I believe the original FL250 used a 780 driver (they were made by a company called Salsbury at the time but they look identical to a modern 780), and I know it had a gear reduction of 9:1 thanks to the guys over at the Odyssey forums (I'm guessing to compensate for the 2-stroke engine's lack of torque). Max RPM on the FL250 engine was around 8000 RPM unloaded, and they put out around 16 horsepower. Top speed was around 50 MPH. The engine I'll be running will undoubtedly have more horsepower and way more torque, so running around a 6.6:1 gear ratio while also achieving 60 MPH seems possible, even with the "soft limit" that itsid mentioned. Most guys would run a 40 series above its rated power here while I'll be running a 780 well within its rated power, so I don't think I'll have issues with burning belts. More expensive but I'm willing to pay the price for reliability, and if I'm interpreting itsid's comment correctly, it may help prevent power loss as well as improve belt life.
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Old 03-30-2020, 09:43 PM
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The Fl250 used a Comet Duster 94c with I believe a 30mm tapered shaft. The 94c would be a better option than the 780. I kind of brought up the topic several months back when asking about 2 stroke and 4 stroke power ratings and if they mattered. They do by the way.

http://www.diygokarts.com/vb/showthread.php?t=42430
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Old 03-31-2020, 12:11 AM
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30 series limited my 5 hp briggs to 5200. A race clutch let it rev out to 7200 rpm (valve float)

20 series unknown

40 series till valve float (6500 rpm)
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Old 03-31-2020, 12:39 PM
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Quote:
The Fl250 used a Comet Duster 94c with I believe a 30mm tapered shaft.
Thanks for the correction, I wasn't able to find any definite info on what clutch they used aside from pictures and my best guess lol

Quote:
40 series till valve float (6500 rpm)
Good to hear, sounds like it might rev a bit higher still if the engine was capable of it. I know 780s can also spin to 6000+ RPM thanks to a little YouTubing, so I think I'll be fine regardless of whether I go 40 series or 780.
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Old 03-31-2020, 01:33 PM
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Quote:
Originally Posted by SquidBonez View Post
Just doing some messing around with the top speed calculator and I have a question. Do torque converters limit the amount of RPM you can get? I know there's the inherent belt slip which can rob power, and that torque converters have built-in overdrive, but do they put a "hard limit" on the RPM you can turn? Let's assume an engine with no load can turn out 8000 RPM. Now obviously under load, it can't rev quite as high, so let's say 6500 (rough numbers of course). But I've seen torque converters being sold with descriptions that say "5000 RPM MAX" (specifically the 780 torque converter on Vegas Karts). Does that mean you won't be able to turn more than 5000 RPM (essentially capping your top speed) or does that mean that it's just fully shifted out at 5000 RPM and the engine can still rev higher?

Great thread.
I wanted to make a comment about overdrive. On the small engine overdrives there seems to be two different definitions.
1. CVT pulley ratios. When the pulleys' flip over the 1:1 ratio, as in changing from the drive pulley spinning faster to the driven pulley spinning faster.
2. When the CVT hits full shiftout and the machine continues to speed up due to increased engine rpm's.

My experience with small engine CVT's says its the pulley ratio.

What's everyone think?
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Old 03-31-2020, 02:06 PM
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there is a high (final?) drive ratio and low drive ratio(starting). depending on the series of torque converter, as well as the driven unit diameter, you will get varying high/low ratios.

There is a balancing act here with gearing, weights, and the actual load that the torque converter has.

In an ideal world, the starting ratio will be maintained until the target rpm (wether that be peak torque or peak hp or whatever you choose) is met, then at that point the ratio will change as the kart/bike speeds up, maintaining that target rpm until it reaches the final ratio, then the rpms will be allowed to climb.

Shifting your gearing, weight/load, drive unit springs/weights as well as driven unit spring tension all work together to try and accomplish this "ideal secenario" listed above.

This ideal scenario actually happens with the jr dragster torque converters that pull the fastest times. They are tuned to such a degree of accuracy that the target rpm is maintained for a big portion of the race, which makes the FASTEST times.

in the real world, in my experience, is that the 30 series units perform well enough with a 5-7 hp engine as long as it isnt modified. Once you start modifying the engine, you end up going into the final ratio, and skipping the transition from start to final. This can also be compounded by improper gearing.

I have had this happen on the 40 series with my 24-26 hp big block build to the point where i can force the belt to slip at ANY rpm because the torque being put out exceeds the clamping ability of the driven unit.

My main issue here that I have thought about a lot is that the drive unit tends to get more clamping force (hence it DRIVES the driven unit), often to exceed the clamping force of the driven unit in high hp applications. i wish there were stronger springs that would help clamp down on the belt and prevent "fast shift" into the final drive ratio on the 40 series.

On the 30 series side of things i have seen spacers, stiff drive unit springs, light drive unit weights, and extra heavy duty driven unit springs. One of the winners of the drag race i filmed a year or two ago had his 30 series set up with a spacer to prevent it from going into overdrive, had like a 4k engagement rpm, and the beefiest springs on the driven unit, all to prevent this scenario from happening. he had good success with it as well.

I was at gopowersports last year and one of the guys there (taylor) was showing me the difference between the $60 torque converters, the gopowersports torque converters, and the genuine comet torque converters. Just the driven unit he was showing me.... the spring tension was just complete crap on the driven unit of the cheap china ones, and the gps ones actually had mroe spring tension than the genuine comet ones, which i thought was pretty cool. They were talking about the (then unnamed) juggernaut and how it would be a game changer with their driven unit. fast forward a year or so and here we are.

Now that the juggernaught is out, it fixes almost all of those crappy problems with the 30 series units......there is even some evidence that the juggernaut cannot go fully into the overdrive ratio, or that the belt constantly slips if it actually makes it to full overdrive at high rpm.... but this is with an engine that makes 18.5 hp on my dyno, probably 22-24 hp on someone elses... Either way the inexpensive cost of the juggernaut as well as the ability of it to allow the engine to rev out beyond that 5k rpm wall make it a real winner in my book for small engines. The belt wear on the juggernaut units is just amazing, it has very little noticeable wear in high hp applications. At that endurance race, people with the 30 series units were shredding belts left and right. Some of the time finishers and the top 1st place winner all had the juggernaut torque converters.

I always have had issues with the drive units (20 and 30 series units) due to the crappy design they have. The weights and posts in the weight pack end up rubbing on each-other and wallowing out, and the center hub gets worn out and catches. The 40 series, and 780 series dont have that issue. It is a real advantage to have the rollers on a torque converter like the juggernaut, 40+780 series have. They just seem to work well compared to the hub/weight pack.

long winded somewhat of a rant. Those are my thoughts.

my ideal setup for a smaller 3/4" shaft engine would actually be the following
1) juggernaut drive unit
2)driven unit with heavy springs to increase clamping force on the belt even at high rpm.
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Old 03-31-2020, 03:03 PM
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Quote:
I have had this happen on the 40 series with my 24-26 hp big block build to the point where i can force the belt to slip at ANY rpm because the torque being put out exceeds the clamping ability of the driven unit.
Would it have been better to run a 780 then? They're rated for 30+ horsepower. That's why I plan on running one.
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Old 03-31-2020, 03:14 PM
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I saw a difference of 4 mph when swapping my torque converter with a centrifugal clutch. The both of them were a 12 tooth setup. Same kart. Same engine.
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Old 03-31-2020, 03:14 PM
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That 30+ rating is on a 2 stroke. 4 stroke rating is 16hp.
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Old 03-31-2020, 06:10 PM
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Quote:
Originally Posted by Solomonster212 View Post
I saw a difference of 4 mph when swapping my torque converter with a centrifugal clutch. The both of them were a 12 tooth setup. Same kart. Same engine.
Did you gain 4 mph with the clutch or lose 4 mph with the clutch?

Quote:
That 30+ rating is on a 2 stroke. 4 stroke rating is 16hp.
GoPowerSports markets them as "Ideal for 670cc Predator Engine" (which is already 22 horsepower stock) and "18-30 horsepower", no mention of 2 or 4 stroke. Plus they're used a lot on built 670s pushing 50-60 horsepower with very little issue. Why would it matter for 2 or 4 stroke? Differences in torque or something?
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