If you add HP, do your clutches need modifying? Does your clutch REALLY tell you anything about the power your engine is producing? I think the answer MAY surprise you!

 
 

 

There is the perception that anytime you add HP, that the engine's RPM will increase and that if your RPM do not increase, then you did not add any HP.

Well.. I completely disagree with this and will ATTEMPT to explain why this line of reasoning is FALSE!

 

OK.. let's get in to this a tad further,

NOTE: I will reference the Ski Doo clutches and components in my explanations.  ALL CVT style clutches function the same.

The same theories apply to the Arctic Cat and the Polaris Clutches

 

OK, Here we go...

If the assumption is made that the flyweight is ALWAYS overcoming the engine acceleration, then if acceleration is slower (possible less HP) then the flyweight will over-power the acceleration and rpm should drop.. BUT.. IMO, the engine acceleration is the one overcoming the "resistance" from the flyweight!

NOT the other way around..

This changes everything!! and I , honestly, feel that this is the case MUCH of the time.. Not always.. it depends on what type of clutching set up you are running..

One example.. more to come...

Back in 2003 (
ZX chassis) I had Stever Decker design and build me some custom twin pipes for the 800 engine. When I received these, I was more than anxious to test them out but did not want to test them in the back-country and have issues. So, I opt'd to test them on a groomed road at 8000ft on a nice cool and sunny day.

We took up 3 sleds, 1 Stock 800HO (
ZX chassis), 1 2001 Drop in kit, and 1 2001 827 kit. 3 people and the time to do some testing. All sleds had the same tracks.
So, we lined up on the wide road.. drop the hammer and ran them about 1/4 mile back and forth 3 times in a row. Switched riders and everything..

Same result
EVERYTIME... 827 in front of the "drop in" by about 4 lengths stocker quit after 2nd pass because the snow injestion and defeat was too much to bear.

So, now we we're down to 2 sleds only. Again,, results were always the same.. BOTH sleds were on their game spinning 8400-8500 rpm.

So, we skip on back to the trailer.. decide to install the twins on the drop in equipped sled.. OK, we went back to the road and lined them up again.

Now, "drop in" kit sled's skis were at the tunnel of the 827
everytime!

Cool we are making power!! Pipes are working!! BUT.. Guess what?? RPM were down 400 rpm?? Hmmm.. rpm down, but sled is faster?? How can this be?? If we are making power the rpm should not be lower.. right? :shrug

So, we went back to the trailer and changed the clutching to get the rpm back up to 8400-8500. Went back to the
road..lined them up again.. "drop in" kit out in front by 1-2 lengths EVERY-TIME and rpm back to normal.. YET, we had to DROP/LESSEN pin weight to get there.??? :shrug BUT.. sled gained 6 lengths in a drag race!

So, what next?? Well, of course, "we better put the twins on the 827".. So, that is exactly what we did... Now 827 is out so far ahead, it is not worth measuring and , AGAIN, pin weight needed to be lowered to achieve correct rpm.

 

ANOTHER EXAMPLE: Don't worry, it will all be tied in together very soon!

With an STOCK 800R Summit the 441 ramps work very well in the mountains and will hold rpm pretty well in a steep powder hill pull. They will usually be in the 36-43 MPH range depending on hill steepness and snow depth around here.

After installing an 860 in the sled.. the 441 ramps are horrible for holding rpm and the sled suffers from a substantial rpm fluctuations.... In fact the 441 simply cannot be used in the steep and deep with the 860. The rpm "Tank" .

Now, here is the "kicker" the 860 DUMPS the rpm at the
same MPH almost every time...about 42mph + or - a few mph..

Drop in kit struggles with the 441's also.

More info

AFTER changing the ramp profile to accommodate the 860, the 860 is running at 45-65MPH track speed and rpm are SOLID with no fluctuation.

Installing the SAME 860 clutch components in the stocker (with a little less pin weight) and the rpm are solid like the 441 ramps were and track speed is the SAME as before with the 441's.

Installing the SAME 860 clutch components in the "drop in" (with the
same pin weight) and the rpm are solid like the 860 BUT track speed is lower.. between 42-56 mph.

OK, NOW, let's get to the REAL reasons behind all this!

 

The primary ramp/weight profile has everything to due with the rpm the engine will maintain. Same with the helix design.

So, if your "pre-mod" sled (we'll talk mountain set ups here) is not capable of getting over 40MPH on a hill pull, then you will NEVER reach ANY point on the ramp profile that is past the 40 MPH "point" .. 40 MPH is 40 MPH no matter what set up you have. What CAN change is the profile of the ramp at 40MPH . So, say a 441 ramp vs. a 412 ramp. The shift ratio at 40 MPH is exactly the same (belt length does not change) BUT the profile on each ramp at that shift ratio can and will vary.

OK.. So, your pre mod set up is a 40 MPH sled.. Your ramps and helix combo support this 40MPH point good and you have solid rpm and 40 MPH track speed.

NOW. you mod the sled.. Now, your sled is doing 45 MPH.. Guess what?? You are NOW at a
totally different point on the ramp and on the helix.. Hmmm.. So, say the ramp profile gets VERY aggressive (fast shift ) at 42MPH.. and you are doing 45 MPH... What happens??? You over-shift and rpm DROP! This is exactly what happened in both scenarios I used as examples.


So, higher track speeds but lower rpm.. DUE TO DIFFERENT shift ratios being achieved!!

In the example with the 860 and the 441 ramps.. The 441 ramp profile is so aggressive after about 42MPH that you have to slow the shift or it will over-shift..

This is not a problem for the stocker!! WHY? because you
NEVER get to that shift ratio!!

So, if you add power and lose rpm.. You need to check track speed. Chances are you are at a higher track speed at the moment before the rpm tank.

You must alter the ramp profile to adjust for this new shift ratio.

To state that if you lose rpm you lost HP is a huge misconception IF you are at a higher shift ratio.

 

In short, WHEN you are making more HP, the TORQUE curve of the engine usually changes. THIS makes the engine deliver power MUCH quicker. When you deliver the power quicker, you shift out faster, when you shift faster your rpm will usually get LOWER not HIGHER! In other words, if you have an engine that just wants to "SHIFT" there is a very good chance that your rpm will be LOWER BUT GROUND SPEED will be higher. So, once you alter your clutching (sometimes LESS WEIGHT (yes I said that)) and get your rpm back to proper... your will then have the ultimate set up..

When people ask you how much more weight your MOD sled is pulling and you say "LESS weight actually" and they tell you that you lost power.. GO contest them and then see if they still think you lost power!!

Now if you are at a lower shift ratio and lose rpm, there is a good chance you did lose HP.. but, to make a blanket statement that lower rpm means lower HP is simply FALSE!! WHY? See above..

OK, now if you add HP and you rpm do increase, what happened? Well, you have a
CVT set up that is less than ideal and can NOT adapt to the new found HP.. What's the fix?? Adjust the CVT to allow for the higher shift ratios.

Say you add HP and the rpm stay the same (where they need to be) and track speed is up or acceleration is up.. or both.. Well, consider yourself fortunate.. You have a properly working
CVT set up that is able to correctly adapt to the new shift ratio..

So, there is MUCH more to this than meets the eye.. These blanket statements simply
mis-represent.



 

 

 

 

 
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