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Hi guys,

I was reading another topic around here somewhere and they were saying a lot of sleds lose power with an aftermarket silencer/can. (F7's were losing 5 horsepower). Many cans claim to have 2-3 horsepower increases or stay the same. I don't really understand this seeing on how an aftermarket silencer would be less restrictive. I always thougth more air in/more air out always equalled more power. Just curious of what you guys think? Here anything about F6's losing power?

Thanks again,
Joe, WI
 

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Hi guys,

I was reading another topic around here somewhere and they were saying a lot of sleds lose power with an aftermarket silencer/can. (F7's were losing 5 horsepower). Many cans claim to have 2-3 horsepower increases or stay the same. I don't really understand this seeing on how an aftermarket silencer would be less restrictive. I always thougth more air in/more air out always equalled more power. Just curious of what you guys think? Here anything about F6's losing power?

Thanks again,
Joe, WI[/b]
I can tell you this JasonF had a Jt can I think on his 600sdi the sled
would only top out at 78mph ok? When he took it off, the Sled ran
close to a buck! To this day we don't know why!!! :confused:
 

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Does a aftermarket can flow more - yes most likely,
On a two stroke (and even a 4 stroke to a lesser degree) does removing back pressure make more power - absolutely not.

A two stroke engine is designed to work with a certain amount of back pressure. Most of this is engineered into the pipe. The expansion chamber causes pressure waves that push air and fuel back into the cylinder and allow for good cylinder loading. This keeps the sled from wasting fuel and loosing efficiency by reducing the amount of raw fuel that exit the exhaust (this is why you never see a performance 2 stroke running straight pipes).

Most sleds today are engineered to achieve maximum efficiency, performance, and power with out sacrificing reliability. If you change the can you may very well be changing the total exhaust push / pull/ pressure effect enough to sacrifice power and speed - all to gain a little noise that almost always ends up pi$$ing someone off.

Cans suck, they slow most sleds down and only aggravate the non-snowmobiling public. Spend your money where it will make a real difference - clutching.

Later,
 

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I can say that I took my can off mine (brand name) and went back to stock and the stock set up is definetly better. My can wasnt a loud one,,, changed to tone to a growl and I really liked it but the performance was absolutely horrible. As for the 5 lb of weight savings,,, I can loose 5 lbs and take care of that!!!
 

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The answer is in the clutching. A person won't gain a thing usually by just slapping a pipe or can on. On the bigger hp sleds, the clutching has to be adjusted also. There are many good aftermarket cans out there, but there are probably just as many that get a bad rap because people don't have the knowledge or take the time to retune the sled.
 

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What happens is that on a dyno, it appears to gain more power. The addition of a aftermarket silencer will tend to flow more air (they have less baffling and a larger outlet diamater to get them to make more "noise").

What happens on the dyno when they "proove" to you how well they work is that they can usually net a couple more peak HP by moving the RPM band up a couple of hundred RPM. This is basically trickery. If you had 100hp at 7700 and 103hp at 8100rpm, the 100hp at 7700 is actually making more power (HP is calculated off of torque multiplied by RPM. the higher you raise the RPM, the more HP it will make on the dyno, even though you have LOST TORQUE)

The manufacturers have spent many many more hours engineering the exhaust systems on sleds nowadays. There is so much there to be made or lost, they have too.

Now 2 stroke is VERY backpressure sensative. If you have ever been to a grass drag or ice drag event, look at the full blown mod sleds or improve stock sleds and their open ended pipes. You will see the end diamaters of these pipe are VERY SMALL. Some down to 1/2" inner diamater. The same sleds stock had exit diamaters of 1 1/4". The smaller the hole, more backpressure, the more backpressure, the more power to be made. The 2 stroke exhaust system is what is called an "expansion chamber" where the sound waves bouncing back off the far end of the exhaust will actually generate greater levels of compression.

The 2 stroke exhaust sytem works similar to that of a turbo, it squeezes more air/fuel into the combustion chamber. The only diffence with the expansion chamber is all of this air is pulled thru the engine allready, but a large part of the un-burnt stuff is sent back into the chamber via this sound wave. The more the back pressure, the more pressure that can be made.

Most of these silencers (and I have seen the operation of one of the larger silencer builders) they just build ONE SILENCER to fit all of the sleds out there. All that changes are the way they bend the pipes to get it to fit under the hood. The silencer itself is the same.

One of the best conversations I have ever had about this is with Jim at DynoTech. DynoTech is the main dyno guru on the east coast, and does all the dyno pulls for the American Snowmobilier shootouts. When ever the big east coast shops (Bender, D&D, Crankshop, DynoPort to name a few, and all of them make aftermarket silencers) bring a sled for the shootout of for a third party verification on the horsepower they are making NONE of them ever have an aftermarket silencer on it. They have spent thousands on these machines to make every HP they can, but they all have the stock cans on them. If the aftermarket ones worked, they would have one on.
 

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Wow! Nice description of the operation of a 2 stroke exhaust system.
I have taught a few 2 and 4 stroke performance classes and I don't think I could have given a better description if I tried.

Well said!
 

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I think it has to made even more clear that the "back pressure" that is used to build power is acoustic back pressure. One of the primary tuning factors is the acoustic shockwave reflected back from the end of the pipe to reduce loss of intake charge escaping before the piston closes the exhaust port. This is designed to be optimized at a specific internal temperature because the speed of sound within the pipe is dependent on the internal temperature. This is why combustion temperature and exhaust gas temperature is critical for optimal power output. You can have the acoustic shockwave traveling upstream without severely restricting the exhaust out flow. You always want to minimize the physical restriction on the pump to maximize the output within the optimal parameters.
 

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I guess I was lucky with my setup. I put D&D twins and muffler on mine. It worked very well, and I had to spend very little time setting it up. I did take it to my dealer and have him make a custom clutch kit for my sled and those pipes. I didn't get all the hp they claimed, but I did get about a 19 hp gain. The biggest surprise for me was the weight savings. That stock can was so friggen heavy. Even going to twin pipes, I still saved a lot of weight. I'd like to get a set for my Tcat to save some weight, but I really like the stock setup, and will probably just leave it alone.
 

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Jim,
The reason the D&Ds worked is that you ran twin pipes. The entire system was changed out for two pipes per cylinder, which is optimum for a multi-cylinder sled.

The miss-conception that by swapping out the silencer on a stock or trail sled is going to gain some magical power. Again, the factory has spent a ton of time and money to develop the exhaust system to make peak power. The effect of killing noise is all AFTER the fact.

10 years ago, yes, the OEM used "generic" exhaust systems on all engines (500s, 600s and 700cc sleds all had the same pipe and silencer) this is far from true. In order to meat EPA regs, the factory has to get very specific with the exhaust design in order to get max effeciency (and great power at the same time) out of these systems.

I have seen .50 cents worth of steel welded into exhausts that will make more power than will ever be seen by a aftermarket silencer. A old school grass drag trick was to tack weld a REDUCER into the end of a stock pipe between the silencer and pipe and it would make nice gains in torque and midrange power. The engines would be SLIGHTLY choked off on the top end, but they just geared the sled up. Peak power would drop 100rpm or so, but the gains in torque would make it worth it in 500ft. Bill Cudney sold a TON of these on the 700 and 800 Series 1 engines when they first came out.

As far as weight goes, 5 years ago the silencers were heavy, but the OEMs have done much better. Yes, you can save 5-10lbs, but at a price of $300 (that is $30-$60 PER POUND) it is a very costly weight savings. Most sleds can loose that much weight or more by mearly going to a set of USI or C&A skis that will net you the same weight savings, and the sled will handle better, be more predictable and more fun to ride!

If 10 lbs is all that is the difference then I must not be good enough of a rider to tell, casue that is the same amount of weight as carrying extra tools or not carrying extra tools.

For the same money go to lightweight driveshaft, change to 10 tooth drivers, invest in clutching, a lexan hood, invest in a lighter track. All of which will net better results all around than swapping out the thing on the end of the exhaust.

Heck, drop the oil injection tank, hoses, and oil and you can save that much weight for free, as long as you don't mind mixing gas, and it would be free!
 

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94ZR - I agree 100%. A properly tuned expansion system does not just send back "pressure" but the front part of the pipe (known as the trumpet) also will "suck" more air/fuel thru the engine. The expansion chamber works the sound wave both ways. Both on the way away from the engine earlier in the pulse and "re-vurbing" the wave back towards the cylinder towards the end of the exhaust cycle (after the piston has closed off the intake ports in the cylinder, but while the exhaust port is still open)
 

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EFI will run with a can, it will just be slower.
The efi system in the Cat's is not like in your vehicle. The auto efi is always adjusting to make it run at peak power.
The Cat snowmobile uses what they call a "blind" efi system with a few sensors that make programed changes.
This means that the ECU is programmed to put a set amount of fuel at certain throttle positions and rpm's.
This is why when you do a mod you need to buy a Boondocker to add fuel for your changes.
All Cat efi's all have a "stutter" or "bog" that goes away after about 10 minutes into your ride. This is when it has warmed the coolant to normal operating tempature and the coolant sensor is switching the ecu from enriching mode to normal mode.

When you take and add a aftermarket can to your efi, It loses all the back pressure for the return pulse charge at wide open throttle that forces the hot gasses back into the engine, Making it run rich, making less power.

It will sound cool,but will be slower on top end with a efi.

On a carb sled, you can adjust the main jet to make it run leaner and gain a few ponies with a can
 

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Given optimum air/fuel for a stock sled and a stock sled with a can, the stock sled will still make more power, and be faster.

It is possible to make more peak power on the dyno, but again the peak power RPM will raise, giving an ILLUSION that it is making more power. What the dyno CAN'T tell you is how much power you are loosing in the meat of the power curve, 5000-7000RPM, where one rides 98% of the time.

Addition of a silencer will kill bottom end and mid range power, period. It is a fact of physics. These rules apply not just to sleds. Atv's, Dirt Bikes, etc.... all suffer.

Heck, FMF is making expansion chambers for 4-stroke bikes now since they make so much torque and power!


It is not that the sled makes less power due to improper jetting (rich condition), it is the fact that the exhaust is not working as effecient as it once had. Louder does not mean more power.
 

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Dave, are we going to get together this year? What trips do you have planned so far?
 
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