Rpm
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maxwell
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its interesting forsure. if you look at 3rd party dyno information the other interesting thing is where these engines make their torque and what RPM the Hp and torque start to fall off.........its why a polaris 850 and a skidoo 850 feel very different.
im not an engine builder so i have no clue
im not an engine builder so i have no clue
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ABMax24
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Skidoo 82mm bore 80.4mm stroke
Polaris 85mm bore 75mm stroke
I think Polaris' shorter stroke moves the power band up the rpm range. But it also has just as much to do with porting, pipe and even intake configuration.
Polaris 85mm bore 75mm stroke
I think Polaris' shorter stroke moves the power band up the rpm range. But it also has just as much to do with porting, pipe and even intake configuration.
Lund
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General rule is longer the stroke the more torque an engine will generate at a lower RPM. Though HP will also be less at peak. Yes porting effect the entire equation but ignoring that and just looking at displacement in stroke vs bore.
So just generalizing by the bore and stroke given the Rotax should generate better torque at a lower RPM but with less overall HP.
The Polaris on the other hand, again generalizing by looking at specs will generate more HP but less overall torque.
Honestly HP mean's poop, torque is what pulls a big track in deep snow up hills, it also keeps you shifted and not back shifted.
But HP is what makes sales not so much torque.
So just generalizing by the bore and stroke given the Rotax should generate better torque at a lower RPM but with less overall HP.
The Polaris on the other hand, again generalizing by looking at specs will generate more HP but less overall torque.
Honestly HP mean's poop, torque is what pulls a big track in deep snow up hills, it also keeps you shifted and not back shifted.
But HP is what makes sales not so much torque.
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maxwell
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Yep, that’s why the doo850 still outperforms the Polaris 850. Torque
Caper11
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The 850 doo should have more torque. The turbo doo makes its power at a lower 7900rpm, the new Polaris turbo makes its power at 8500 according to burant.
SUMMIT TREE
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The way I think of HP and torque is this, torque is the Ability to rotate a shaft, Hp is how fast you can rotate it. You could use a very small engine to haul a big load if you use a big enough gear reduction, it will take a long time to do it, the more Hp you put in the less you have to gear down and the faster it goes......
What they ^^^^ said (with caveats). Power is set by torque at a given RPM. You need to look at the dyno curves. These sleds typically hold their peak torque for a few hundred RPM and there is a small fluctuation in the numbers at that RPM, so it simply could have been the difference of half of a ft/lb at 8100 to 8500 rpm and they took 8500 instead of 8100.
Longer stroke will mean different port timing above and below the transfers and exhaust, which limits power output on the combustion stroke depending on where the exhaust valve opens and stops. Consider that these motors also have 2 different fuel delivery systems, one injects the fuel at the last minute under high pressure and the other is a case-induction motor which affects crank case pressure and case volume. Piston design, speed and rod angle are big items that will determine torque output since the combustion chamber design and bore diameter determine how fast the piston attains full speed on the combustion stroke. Bigger bore, longer combustion explosion (with all other variables the same). off the top of my head I don't remember the specific compression ratio of either motor or the piston and full rotating assembly weights. So many variables between them to measure and discuss. If you remember there was a piston reliability issue in the Liberty motor years back due to the rod angle putting too much pressure on the piston skirt. Spark plug angle and projection will also affect it. Consider that Doo has indexed the plugs. This allows for more even flame propagation across the cylinder and maybe allows them to start timing slightly earlier before TDC. Subtle differences like these can make the difference of the 3-4 HP or 2-3 ft/bs of torque that we are talking about between the 2 motors.
In 2019, Polaris had a different piston design with a thicker top half and also dropped a bunch of weight off the crank. Less rotational vibration with less weight. Thicker piston dome was likely to allow more casting for more dimensional stability and increased combustion heat dissipation to the cylinder, since they also re-vamped the cooling system at that time as well. The crank weight reduction was likely done to offset the increased piston weight. Thicker piston = Less heat in the piston (by transfer to the cylinder and then to cooling system) and less vibration on the crank means more reliability. All this is leading up to.........This motor has a 4 year warranty now.
Keep in mind the patent and warranty issues (see note about 4 yr warranty above). Simply building the same motor configuration as your competition will land you in court, even if it goes nowhere, the legal costs will still mount up and that bad press affects share price. Warranty claims and company write downs affect profit which affect share price.
Another thing is the amount of emissions that each motor can output. These companies are restricted on emissions outputs from a fleet level, so they need to manage cumulative emissions, which may mean a more restrictive exhaust on 1 brand vs another. Don't read into it too much. Usually the sales brochure spec is a little different than production. Many parameters change between the pre-production sleds and the mass produced, little changes like tweaks to fuel
Hope you're not more confused now than when we started.... LOL
o zone guy
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confusious saY when confused sTaY con fused ... HeY who lit the fuse ... i can help .. back in a bit /...i like the waY u THiNK ouT LouD ... beeP !! beep !!
slednk
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Horsepower isn’t an actual force it’s a product of hp=Rpm x torque/5252 that’s it that’s all .. not go being how fast You hit the tree and torque being how far you moved it
ABMax24
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If this statement was correct sleds would be running low speed boosted four strokes, or even diesels, because they both have way more torque than modern 2-strokes.
HP is the only true measure of an engines output. Torque only comes about in engine literature because that's how a dyno measures it, it puts a load on the engine (in the form of torque on the crank shaft) and uses the rpm to calculate power.
Sure different power curves may be easier to clutch for, but at the end of the day more hp wins.
MP Kid
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Lund’s statement is correct....
What makes motorsports units exciting to ride, is their ability to make that power quickly. The current motors (regardless of 2S or 4S) is their power output compared to their weight. The power plant’s output per weight is important for the size of the chassis...
Diesels make good power, but their requirement to be physically constructed to handle extreme pressures are just not practical for Motorsport use.
Go ahead and mount an inline 6 Cummins in the chassis of your choice....!!
ABMax24
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You completely misread my post. My point is that torque is not the be-all end-all specification for engine output. HP is all that matters. Torque really doesn't mean squat in a sled that has a CVT to utilize the peak power band. I used a diesel engine as an extreme example of this.
Torque isn't what moves a sled as was claimed, power does. Anyone that understands high-school physics understands this, power is the ability to do work, the more power the more work that can get done, or the faster the same amount of work gets done. Torque is solely the rotational force, at 0 rpm it does 0 work. Now when there is rotational movement with torque there is power, again power is the ability to do work.
slednk
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Well every aspect of how a engine makes torque has a ton of variances depending on the application low torque hi reving engine like formula one to give them a broad torque band. like the 2s engine in our sled the broad band works with cvt transmission but more how to make sled easy to control the output of the engine ... a super torquey engine that would topically have a narrow rev range would be hard clutch efficient but super hard to handle in the trees besides the weight and super hard on belts hahaha
Lund
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I think you need to do more research before posting but if that is what you believe...ok
Here is a clue, HP = is energy available, more HP more energy. Torque = is ability to work, more torque more capable to do harder work.
Combine the two and you have something that can do a lot of work quickly.
Check it out with your friend Google, but hey what do i know, i just build custom sleds on my free time for people.
I'm trying to stay out of the pissing contest but here's some theory for you to ponder. Its easy to understand the Why, not so much the How? Torque is an instantaneous measurement of a force over a distance in a certain direction (rotation). HP is a measurement of that torque over a specified time. Both are relevant to this discussion so I fail to see the need to argue over which is "more better". Measure torque at higher RPM and you get more HP, because its doing more work in less time. 2 strokes need to do that work in less time because the exhaust ports open half way down the cylinder (relatively for this discussion). In theory you could move them lower for a longer power stroke but you simply wouldn't be able to sufficiently scavenge the cylinder in time from the bottom of the port to BDC and back. Similarily, the intake charge will not have sufficient time to come in unless you raise it as well but if you have a lot of overlap then a significant amount of intake charge will go out the exhaust and your motor makes good power but becomes inefficient. you can play with both port heights but you have to choose either power or efficiency in a 2 stroke.
Some of this can be compensated for with RPM, but when you start revving things to the moon you need to introduce complexity to keep all the parts in check. This involves counterbalance shafts etc. all which add complexity and weight. Better power at higher RPM but CVT does not like higher RPM because of the differential between the primary and the secondary......and before you ask - we don't have gearboxes because of rolling resistance and weight. CVT's would be better off on bikes as well but the gyroscopic effect you get from them cause an imbalance on a bike. Sleds have a front end supported on 2 skis so its not as pronounced.
Same Torque output but at a lower RPM is better for all things in an engine, including a CVT. The closer the primary clutch RPM and secondary clutch RPM are, the less slippage and less heat are generated. Problem is, 2 strokes don't make great torque down low. A high torque, low RPM diesel would be perfect for the CVT. Just curious - Has anyone tried hand pull starting a high comp diesel at -20? glow plugs or grid heaters and diesel starters both need a robust battery setup. We're back to the power to weight ratio thing for something that needs to float on snow.
Some of this can be compensated for with RPM, but when you start revving things to the moon you need to introduce complexity to keep all the parts in check. This involves counterbalance shafts etc. all which add complexity and weight. Better power at higher RPM but CVT does not like higher RPM because of the differential between the primary and the secondary......and before you ask - we don't have gearboxes because of rolling resistance and weight. CVT's would be better off on bikes as well but the gyroscopic effect you get from them cause an imbalance on a bike. Sleds have a front end supported on 2 skis so its not as pronounced.
Same Torque output but at a lower RPM is better for all things in an engine, including a CVT. The closer the primary clutch RPM and secondary clutch RPM are, the less slippage and less heat are generated. Problem is, 2 strokes don't make great torque down low. A high torque, low RPM diesel would be perfect for the CVT. Just curious - Has anyone tried hand pull starting a high comp diesel at -20? glow plugs or grid heaters and diesel starters both need a robust battery setup. We're back to the power to weight ratio thing for something that needs to float on snow.
Caper11
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This thread seems to be a deja vu of hp ,and torque discussions on a Diesel engine thread.
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NoBrakes!
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everyone makes the same point but my question is why.
Why do they chose to run more/less rpm and is there gains to be had? you rarely hear about ralphs/linderman porting anymore.
anyone run more or less rpm than suggested and clutch for it?
Why do they chose to run more/less rpm and is there gains to be had? you rarely hear about ralphs/linderman porting anymore.
anyone run more or less rpm than suggested and clutch for it?
Rotax_Kid
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I wish I could find it. A number of years ago Hot Rod magazine built two identical sized big blocks and arrived at the same displacement by using inverse bore and stroke numbers. Ex: One had a 3" bore x 4" stroke, while the other had a 4" bore x 3" stroke. I can't remember what the actual numbers were, and I know they were not completely inverse because surface area of a circle is exponential.
End result - both engines produced nearly identical horsepower and torque....
As modmad and Lund said things really well above. One other thing to keep in mind - both engines approach port timing slightly differently with how, and when their exhaust valves open. In theory,maybe not in practice, direct injection should allow for port timing to be a little more aggressive, pending fuel.
End result - both engines produced nearly identical horsepower and torque....
As modmad and Lund said things really well above. One other thing to keep in mind - both engines approach port timing slightly differently with how, and when their exhaust valves open. In theory,maybe not in practice, direct injection should allow for port timing to be a little more aggressive, pending fuel.
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