So I put my Emgo reverse cone muffler on my  otherwise stock s40, and for those of you who don't know what that is, its basically a straight pipe, and man is it ever loud. Now I'm at 500 feet above sea level if thats relevant, but my real question is should I change my air filter at all, and if so either with a cone or with oldfeller's nufoam filter mod? I haven't re-jetted or changed the mixture screw but I'm running pretty rich.

When I put the Emgo on I saw a 15% bump in hp and 5-10% bump in torque. Would a cone or more free flowing filter really be beneficial without increasing wear on the engine?

Any comments are appreciated.

lg_fry, that doesn't make any sense, if your bike was stock in the first place. If you uncork the exhaust you should be leaner, not richer. (more air + same fuel=leaner).
Also running a straight pipe usually means less low end torque, not more. It might feel (and sound) like you have more power, but 15% with just a muffler change is highly unlikely.
That being said, if you put a K&N slab filter in the stock airbox, rejet to maybe a stage three, you will get that kind of power increase, and maybe more.
Dynojet claims a 32% increase  to 33 hp at the wheels on an S40 with a Raask pipe and their stage three jet kit, and a foam filter and have a dyno graph to prove it.

fry, A horsepower increase on almost any engine is a careful ramp up of intake, exhaust, timing, carb, etc. I'm with Serowbot on this one. I suspect that he and I have about the same amount of Snake Oil stored neatly in the back of the shop as a reminder you can't get something for nothing.  

I wouldn't recommend a free flow air filter without a jet change...

If your jetting is stock,... the open pipe alone may make you lean enough to cause damage...

Unless you ran it on a dyno before and after, you can't really be making any real claims about changes in horsepower or torque. Plus your admission that you're not that mechanically savvy throws more doubt into the matter.

First off, overly lean bikes can backfire too, so backfiring isn't necessarily a sign of being too rich.

Secondly, despite the claims of aftermarket companies, just changing the muffler isn't going to give much more than a 5% increase in horsepower, and then only at the top end of the power curve. Numbers like the ones you gave come from things like larger bores and more radical cams, not from mufflers alone.

Thirdly, stock bikes need to meet federal emissions and sound regulations. That means the manufacturers balance the capabilities of the carb and exhaust. There's no point specifying a carb that puts out power the muffler is just going to choke off. So just opening up the exhaust isn't going to give big performance gains. You need to increase the carb's capacity as well. That means changing the jetting and filter. Otherwise you're just making more noise, not more power.

Humid air is denser -- more air molecules for a given volume. Without any other changes, dense air leans out the mixture. So the change in weather made you bike leaner.

Exhaust length, diameter and flow capacity effect torque. It's not really about "back pressure." It's about getting the exhaust pulses to work together properly.  The chances of hitting that magic combination spot on just by putting on a universal-application muffler are exceptionally slim.

Serowbot wrote on 07/06/11 at 12:55:14:


You're right. I was thinking cool air.

>To see why humid air is less dense than dry air, we need to turn to one of the laws of nature the Italian physicist Amadeo Avogadro discovered in the early 1800s. In simple terms, he found that a fixed volume of gas, say one cubic meter, at the same temperature and pressure, would always have the same number of molecules no matter what gas is in the container. Most beginning chemistry books explain how this works.

>Imagine a cubic foot of perfectly dry air. It contains about 78% nitrogen molecules, which each have a molecular weight of 28 (2 atoms with atomic weight 14) . Another 21% of the air is oxygen, with each molecule having a molecular weight of 32 (2 stoms with atomic weight 16). The final one percent is a mixture of other gases, which we won't worry about.
Molecules are free to move in and out of our cubic foot of air. What Avogadro discovered leads us to conclude that if we added water vapor molecules to our cubic foot of air, some of the nitrogen and oxygen molecules would leave — remember, the total number of molecules in our cubic foot of air stays the same.

>The water molecules, which replace nitrogen or oxygen, have a molecular weight of 18. (One oxygen atom with atomic weight of 16, and two hudrogen atoms each with atomic weight of 1). This is lighter than both nitrogen and oxygen. In other words, replacing nitrogen and oxygen with water vapor decreases the weight of the air in the cubic foot; that is, it's density decreases.

>Wait a minute, you might say, "I know water's heavier than air." True, liquid water is heavier, or more dense, than air. But, the water that makes the air humid isn't liquid. It's water vapor, which is a gas that is lighter than nitrogen or oxygen. (Related: Understanding water in the atmosphere (http://www.usatoday.com/weather/wwater0.htm)).

>Compared to the differences made by temperature and air pressure, humidity has a small effect on the air's density. But, humid air is lighter than dry air at the same temperature and pressure.

Add heat to that humidity and the air gets even more "lean," meaning the jetting gets richer. But I still doubt it got sufficiently richer to counteract the mods to the exhaust.