Torque converter/CVT adjustment/tuning

[massmanute]

I am not a Kart owner, but I am interested in learning more about Karts. In particular I want to learn about torque converter/CVT tuning/adjustment. Let me run through a scenario with a question or two at the end.

Suppose in a recreational application I have an engine that is nominally 13 horsepower at 3600 rpm, and the governor is set to limit the engine to 3600 rpm. Let's not get into engine enhancements right now. Initially I am running on a long level stretch, with the throttle set at full, but the governor is limiting the engine to 3600 rpm. Obviously, the gearing is not set for top speed on the straightaway in this case. Let's just accept that this is intentional.

Next, the Kart starts climbing a hill with moderate slope, and the governor starts opening up the engine to maintain 3600 rpm. So far nothing has happened with the torque converter, i.e. it is running in “high gear”. Let's assume high gear corresponds to a 1:1 ratio.

Eventually the hill gets steep enough so that the governor opens the engine up to full power at 3600 rpm. At this point the engine torque is 19 foot pounds, and since the gearing is 1:1 the load on the driven pulley is also 19 foot pounds. Here we are assuming no power loss in the torque converter, i.e. it is 100% efficient. Let's call the condition of the system at this point “Condition S”. Under condition S the engine is delivering maximum power.

I would like the torque converter to start downshifting once the system reaches condition S, i.e. at a 19 foot pound load on the driven pulley with the engine running at 3600 rpm. If the hill gets steeper the torque converter will downshift even more. I would like the torque converter to be adjusted so that the engine continues to run at maximum power (i.e. at 3600 rpm and with the throttle fully open) as the hill gets steeper. This means the engine must continually downshift to maintain the engine at full power. The downshifting of course would ultimately reach a limit once the hill gets too steep, perhaps at a ratio of 2:1 or so.

Question: can the torque converter be adjusted to meet these requirements? If so, how would one go about setting up the adjustment?

As a followup question, suppose the engine is not actually 13 horsepower but due to mufflers and other issues, including torque converter efficiency that is less than 100%, it is actually running at some lower (and unknown) horsepower figure. How does one adjust the torque converter in that case?

Thanks.

 

[Joe-405]

You can adjust the driven spring tension to allow the secondary gear to kick in earlier or later. Other than that it really doesn’t have anymore adjustment other than the driver being able to change the engagement rpms

If it’s constantly changing then you would destroy belts way to fast. And it would be really hard on parts overall.

 

[massmanute]

...If it’s constantly changing then you would destroy belts way to fast. And it would be really hard on parts overall.

Would destroying the belts come from additional wear as the pulleys change diameter?

---------- Post added at 04:35 PM ---------- Previous post was at 04:26 PM ----------

Y...Other than that it really doesn’t have anymore adjustment other than the driver being able to change the engagement rpms...

From what I read there is a way to open up the driver pulley (clutch) and set the engagement rpm. Does that sound right as far as setting the engagement rpm?

 

[J.S.@SMS]

I'm not sure about 20 or 30 series drivers but I believe that you can swapp parts in 40 series and 780s to raise engagement speed.

 

[bob58o]

Read this....
https://web.wpi.edu/Pubs/E-project/...ricted/MQP_Timothy_DeGreenia_Edited_Jan19.pdf

 

[massmanute]

Read this....
https://web.wpi.edu/Pubs/E-project/...ricted/MQP_Timothy_DeGreenia_Edited_Jan19.pdf

Thanks. I will look at it.

 

[Denny]

What you describe is exactly the way a proper functioning CVT is designed to work. See no thinking required.


Denny

 

[bob58o]

Since you are talking big block engine you will probably be looking at a 40 series torque converter. On a 30 series TAV, there are three different settings for adjusting the driven (secondary) pulley and there is also a stiffer replacement spring. Preloading the spring more or using the stiffer spring is useful for off roading and riding in hilly areas. It will delay the upshift.

I don't recall there being different holes for adjustment on my 40 series TC, but there may be. Nor can I recall a stiffer spring for the 40 series. I'm not saying one doesn't exist. I just don't remember if I ever came across one. It would probably be as simple as drilling a few holes to make the 40 series adjustable if it doesn't come like that already

Those three holes in the picture is how to adjust the driven unit on a 30 series. Middle hole is where the spring comes installed when you get it.

 

[Denny]

99times out of 100, 30 or 40 series CVTs work properly out of the box. Bob let's not confuse the poor guy until he at least installs one and tries it out. Then we can work on fixing it if he has problems.



Denny

 

[massmanute]

99times out of 100, 30 or 40 series CVTs work properly out of the box. Bob let's not confuse the poor guy until he at least installs one and tries it out. Then we can work on fixing it if he has problems.

Denny

Thanks for the comment. Nevertheless, I would like to learn about what is involved in tuning the device to the load characteristics, in particular how to assure that downshifting starts at a certain torque/rpm point, and also how much downshifting occurs for each increment of load.

 

[massmanute]

A related question. On a series 30 torque converter, what is the difference between using a stiffer spring vs. moving the end of the spring to a different hole?

 

[bob58o]

Just guessing... because I have never used my yellow (stiffer) spring or moved the spring away from the middle hole on any of my toys... the stiffer spring probably has a more significant change in delaying the upshift.

I planned on doing a bunch of testing using the different holes and springs, but like Denny said the CVT's work pretty well right out of the box and I've had too much fun to notice if they weren't perfectly tuned.

I always remove the governors and build my engines up to spin twice the rated RPM's. My minibikes don't bog down on the hills by me. The CVT works as it should. Instead of the engine RPMs dropping, the CVT downshifts and the engine just keeps on keeping on.

So Denny is right, it is hard to predict how a tune a hypothetical project. Build it, test it, and fahgettaboutit. Soon you'll being having too much fun to remember you wanted to tinker with the CVT.

I do however try to tune the DRIVER (primary) unit to engage at where I expect peak torque to occur.

 

[KartFab]

Eventually the hill gets steep enough so that the governor opens the engine up to full power at 3600 rpm. At this point the engine torque is 19 foot pounds, and since the gearing is 1:1 the load on the driven pulley is also 19 foot pounds.

You are assuming peak torque happens at 3600 rpm, when in actuality, peak torque happens at approximately 2200 rpm.

Also, at 3600 rpm with governor functional, optimal hp is not being reached as the governor is varying airflow. If you were to hold the throttle butterfly open at 3600 rpm and load the engine that limits rpm at exactly 3600 rpm, you would get a higher peak hp output vs when the governor was functioning. It is under these conditions that these engines are testing, e.g. hp, when in a real world scenario with a funcitoning governor, you might see 11 hp at 3600 rpm. That is why you dont see any more manufacturers with hp numbers, just when peak tq occurs, what the tq is, and when max power output is displayed in newton meters at what rpm. Manufacturers were inflating the hp numbers, and when tested independently, the numbers were coming up short. Now thanks to that and getting sued left and right, engine manufacturers now display the model number or CC's on the recoil starter cover instead of hp numbers.

Here we are assuming no power loss in the torque converter, i.e. it is 100% efficient. Let's call the condition of the system at this point “Condition S”. Under condition S the engine is delivering maximum power.

I would like the torque converter to start downshifting once the system reaches condition S, i.e. at a 19 foot pound load on the driven pulley with the engine running at 3600 rpm. If the hill gets steeper the torque converter will downshift even more. I would like the torque converter to be adjusted so that the engine continues to run at maximum power (i.e. at 3600 rpm and with the throttle fully open) as the hill gets steeper. This means the engine must continually downshift to maintain the engine at full power. The downshifting of course would ultimately reach a limit once the hill gets too steep, perhaps at a ratio of 2:1 or so.

Question: can the torque converter be adjusted to meet these requirements? If so, how would one go about setting up the adjustment?

As a followup question, suppose the engine is not actually 13 horsepower but due to mufflers and other issues, including torque converter efficiency that is less than 100%, it is actually running at some lower (and unknown) horsepower figure. How does one adjust the torque converter in that case?

Thanks.

• Torque converter AND engine automatically adjust to whatever conditions they are subjected to. THEY ARE NOT INDEPENDENT OF ONE ANOTHER.
  
  An engine can overcome the spring pressure of the driven unit.
  An engine cam overcome the friction force of the belt against the sheaves in overdrive.
  A torque converter can be adjusted to engage at a higher rpm than stock by replacing in any combination, weights, and springs that are heavier and or lighter in the drive unit.
  A torque converter can be adjusted to delay shifting into overdrive by changing the spring position in the driven unit. There are three holes for this. This adjusts the clamping force of the driven unit against the belt.
  
• Real World Scenarios (assuming honda gx390 engine and 40 series torque converter):
  
  • Geared for 50 mph at 3600 rpm, torque converter does not shift into overdrive until 30-40 mph. Belt slips and burns up while climbing hills.
  • Geared for 30 mph at 3600 rpm, torque converter does not slip, and go kart wheelies. Go kart cannot remain in "base ratio" as there is little resistance to go into overdrive.
  • Geared for 40 mph at 3600 rpm kart performs optimally, on hills and on straights like it should.
  
  Stock drive unit springs and weights, stock driven unit spring position (center). Torque converter performs optimally.
  
  Stock drive unit. Change driven unit spring position to "less spring tension" hole. Shift into overdrive happens sooner. Kart may accelerate faster if the engine produces enough toruqe across the RPM band. May perform worse at hill climbing.
  
  Stock drive unit. Change driven unit spring tension to "most spring tension" hole. Shift into overdrive happens later. Engine accelerates to max rpm, and feels like it "hits a wall" with acceleration. Engine shifts into overdrive eventually. Performs best when climbing steep hills, as the belt slips less due to increased clamping force on the belt, and delay in going into overdrive. Still transitions to overdrive, but may not accelerate as fast as center position on driven unit, or "least spring tension" position.
  
  Drive unit has a few options. heavy weight, and medium weight. Yellow springs (light stock) and red springs (heavy red). lighter weights will change engagement rpm to a HIGHER rpm. heavier springs will change engagement rpm to a HIGHER rpm. If you wanted OPTIMAL launch (e.g. drag race). you would set your drive unit rpm engagement to around the peak torque rpm of the engine itself.
  
  Any combination of drive unit weights, drive unit springs, and driven unit spring position can be used to alter engagement rpm, and general perfomance under load.
  
  Beyind this, remember that, gearing, tire size, driver weight, kart weight, and engine output ALL AFFECT the ability of the torque converter to engage and vary the drive ratio.
  
  You can exceed the ability of the torque converter to work by:
  • climbing a hill that is too steep
  • putting a fat guy on a kart that weighs 1000 lbs
  • modifying an engine to produce more than 18 hp
  • having tires that are too large (combined with bad gearing)
  
  So just keep it simple. A regular dude, riding a regular kart with stock springs, weights, and driven unit spring position in the center position will give you the best perfomance geared at about 40 mph at 3600 rpm.
  
  
  
  If you want to step up your engine game. lets say "hypothetically speaking" it is a honda gx390 or predator 430 engine. You could shave the head for a 10.2:1 compression ratio, and put in a marine grind NR racing 280 cam, and heavy duty single rate springs. You would max out at 5500 rpm and would do great with the torque converter. Possibly adjust driven unit to "max spring tension"

 

[massmanute]

...I always remove the governors and build my engines up to spin twice the rated RPM's...

Speaking of governors, how far can one go in adjusting a governor up without taking it off? For example, could I adjust the governor of a Predator 420cc to run at, let's say, 4500rpm? If so would the higher rpm put so much strain on the motor as to cause a lot of reliability problems or greatly reduced lifetime?

 

[bob58o]

Not sure. I don't adjust my governors. I throw them in the trash can where they belong. LOL. On a 212, gx200, or clone... you could adjust the governor to about 5k RPM.
I cannot quantify the lifetime reduction, but I wouldn't be too concerned about reliability.

My 420 has has no governor, 32mm Flatslide carb, Head Milled 0.085", 0.032" Head Gasket, Valve Job, Offset Timing Key, HD valve springs, stock connecting rod, stock flywheel, stock camshaft.... ???

On a 390 / 420 the stock valve springs will probably limit your RPMs below 5k RPM. So no need to adjust the governor - just throw it away. As long as you don't upgrade the valve springs - you aren't going to turn many RPMs anyway. IIRC, a stock 420 with no governor will turn about 4300-4500 RPM. Even if you do upgrade the springs, the stock cam probably isn't going to turn much higher than 5k RPM anyway. The big blocks don't like to turn fast unless you put some series coin into them.