Gino is seeking advice on optimizing his 850 turbo Ski-Doo's performance with a 20hp tune. Gino's using a bm tune and iBackshift clutching. Im going to see if I can provide guidance on how to address this modification effectively.
Gino] Hey Joe I’m wondering what weights I should run in my 21 fr turbo 154, 20hp bm tuner. I have the dj kit clutch kit in the sled right now
Thanks
Joe] Hi Gino. The bm tune [in particular] loses engine torque from 3500 to 6500 rpms.
I have to get you to remove the red 100/350 primary clutch spring and install a new primary clutch spring, 120/250. You won’t have to change the pivot bolt weight much. It will raise the engagement speed a little to allow the engine to speed up in a pleasant manner and not bog.
Gino] Sounds good. This is a different spring from the one you sell normally in the kit?
Joe] Yes, different spring for the tuner power than the stock horsepower.
Gino] The bm tune, it’s already installed. I install it after at the end of the season, after running stock. Just having hard time clutching. I think it’s bouncing off the limiter at roughly 83-84 rpm or it’s just running like s**t at high rpm.
Impact at power peak rpms 7900
Joe] The impact of a 20 horsepower increase at 7900 rpms; IF the clutch settings are set for 165 horsepower and now raise to 180 horsepower, THEN the engine will flare into overrevving using the 165 settings.
Horsepower to clutch weight rule of thumb
8500 – 7900 = 600 rpms.
200 rpms = estimated 5hp; then you have 5, 10, 15hp [estimated] increase over what you previously had. Its hard to get close to exact guess on the hp because you achieve a maximum rpm rev limit stutter and can’t hold full throttle.
Aaen rule of thumb] the springs [and flyweight] mechanism work against each other in opposite directions.
The side force of the sheave against the drive belt is the result of subtracting the spring force from the flyweight force as they work against each other.
Aaen rule of thumb] The higher the load, the more RPM of heavier weights are needed to overcome it.
Spring forces can be used to influence the shift curve and obtain a desirable "straight shift". in your case, we want 8000, not 8500 rpms.
Instead of starting with changing the flyweight grams, I would rather start the setting change of the spring forces.
What we do is change the primary clutch spring to a bit softer force. The lesser spring “final-force” will have less resistance against the new horsepower.
There is a benefit of changing the primary clutch spring, instead of only-adding pivot bolt weight.
If you simply start to add more pivot bolt weight [in this particular case] then with the lesser engine torque at lower rpms, it can enhance the lag when you quickly press full throttle.
In this case, can change the spring force to optimize the way the clutch pushes, matching the new power curve.
Aaen law; BELT FORCE[rpms at full throttle] = FLYWEIGHT FORCE - SPRING FORCE.
[pg 22 1986, pg 24 2007; Aaen tuning handbook]
You have….
The next spring we have is the 100/250, or the 120/250.
Impact of hp at midrange rpms [3500~6500]
From experience we know that bm tune loses engine torque from 3500 to 6500 rpms.
At 10hp increase there is a certain lag.
At 20hp increase, we know the lag is deeper.
At 30hp increase, I tell owners to go back to 20hp, or, go somewhere else for clutch tuning.
Not my fault the lower end to lower midrange engine torque reduces with a horsepower increase at power peak rpms. giggle, ive got the stories on which tunes brand is worst to best [my opinion], haha.
How do we overcome a lag? You can raise the primary spring start force. You can use a less aggressive start pdrive ramp, like going from a 965[most aggressive/slowest to rev up] to a 984, 992, 903[least aggressive/quickest to rev up]
In this case, we'll use the primary spring start force.
Go from the 100/XXX to a 120/XXX start force, thus raising the engagement speed to help the engine push through that LAG. We'll go from the Dalton Red 100/360 to the Ski-doo Yellow/Yellow 120/250.
Summary for a 20hp increase.
Gino] Hey Joe I’m wondering what weights I should run in my 21 fr turbo 154, 20hp bm tuner. I have the dj kit clutch kit in the sled right now
Thanks
Joe] Hi Gino. The bm tune [in particular] loses engine torque from 3500 to 6500 rpms.
I have to get you to remove the red 100/350 primary clutch spring and install a new primary clutch spring, 120/250. You won’t have to change the pivot bolt weight much. It will raise the engagement speed a little to allow the engine to speed up in a pleasant manner and not bog.
Gino] Sounds good. This is a different spring from the one you sell normally in the kit?
Joe] Yes, different spring for the tuner power than the stock horsepower.
Gino] The bm tune, it’s already installed. I install it after at the end of the season, after running stock. Just having hard time clutching. I think it’s bouncing off the limiter at roughly 83-84 rpm or it’s just running like s**t at high rpm.
Impact at power peak rpms 7900
Joe] The impact of a 20 horsepower increase at 7900 rpms; IF the clutch settings are set for 165 horsepower and now raise to 180 horsepower, THEN the engine will flare into overrevving using the 165 settings.
Horsepower to clutch weight rule of thumb
- 1 gram of weight = estimated 5hp
- 1 clicker position = estimated 5hp
- 1000 feet elevation = estimated 5hp
- 200 rpms = estimated 5hp
- 30 pounds spring force = 5hp = 200 rpms difference = 1 clicker position
8500 – 7900 = 600 rpms.
200 rpms = estimated 5hp; then you have 5, 10, 15hp [estimated] increase over what you previously had. Its hard to get close to exact guess on the hp because you achieve a maximum rpm rev limit stutter and can’t hold full throttle.
Aaen rule of thumb] the springs [and flyweight] mechanism work against each other in opposite directions.
The side force of the sheave against the drive belt is the result of subtracting the spring force from the flyweight force as they work against each other.
Aaen rule of thumb] The higher the load, the more RPM of heavier weights are needed to overcome it.
Spring forces can be used to influence the shift curve and obtain a desirable "straight shift". in your case, we want 8000, not 8500 rpms.
Instead of starting with changing the flyweight grams, I would rather start the setting change of the spring forces.
What we do is change the primary clutch spring to a bit softer force. The lesser spring “final-force” will have less resistance against the new horsepower.
There is a benefit of changing the primary clutch spring, instead of only-adding pivot bolt weight.
If you simply start to add more pivot bolt weight [in this particular case] then with the lesser engine torque at lower rpms, it can enhance the lag when you quickly press full throttle.
In this case, can change the spring force to optimize the way the clutch pushes, matching the new power curve.
Aaen law; BELT FORCE[rpms at full throttle] = FLYWEIGHT FORCE - SPRING FORCE.
[pg 22 1986, pg 24 2007; Aaen tuning handbook]
You have….
- 100/360 pound spring
- Have 8500, need 7900, then 600 rpms.
- Reduce 30 pounds for every 200 rpms needed.
- 360 pounds – 30#s – 30#s – 30#s = 270 pounds
The next spring we have is the 100/250, or the 120/250.
Impact of hp at midrange rpms [3500~6500]
From experience we know that bm tune loses engine torque from 3500 to 6500 rpms.
At 10hp increase there is a certain lag.
At 20hp increase, we know the lag is deeper.
At 30hp increase, I tell owners to go back to 20hp, or, go somewhere else for clutch tuning.
Not my fault the lower end to lower midrange engine torque reduces with a horsepower increase at power peak rpms. giggle, ive got the stories on which tunes brand is worst to best [my opinion], haha.
How do we overcome a lag? You can raise the primary spring start force. You can use a less aggressive start pdrive ramp, like going from a 965[most aggressive/slowest to rev up] to a 984, 992, 903[least aggressive/quickest to rev up]
In this case, we'll use the primary spring start force.
Go from the 100/XXX to a 120/XXX start force, thus raising the engagement speed to help the engine push through that LAG. We'll go from the Dalton Red 100/360 to the Ski-doo Yellow/Yellow 120/250.
Summary for a 20hp increase.
- Remove the 100/360 spring for 165hp. [too much final-force for a 20hp increase]
- Install the 120/250 spring for 185hp. [lower the spring final-force for a 20hp increase]
- Go run the sled with the present pivot bolt weight grams. [Proof of clutch theory/rule of thumb concept]
- Do a full throttle run, check the engine speed, what is it? [validation]
- Adjust the pivot bolt weight to get 8000 rpms again. [success]
