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
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
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
Same result EVERYTIME... 827 in front of the "drop in" by about 4
lengths stocker quit after 2nd pass because the snow
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
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?
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
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
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
MPH almost every time...about
42mph + or - a few mph..
Drop in kit struggles with the 441's also.
AFTER changing the ramp profile to accommodate the 860, the 860 is
running at 45-65MPH track speed and rpm are SOLID with no
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
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
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
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
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.