Friday, January 27, 2012

Loud pipes save.......well....nothing it seems.

editors note: This post now more or less finished.  I had to run out last night at the last minute to avoid being killed and dropped in the soup pot by a very hungry and somewhat insistent Miss Kitty. Read on and be smarter!

Leather Pirate: "I just put drag pipes on my bike, sounds so tough and you should see all the admiration I get now when I ride thru the neighborhood! Everyone tells me I'm number 1!"
Motorcyclist: "Wow, that is really loud."

Leather  Pirate: "Loud pipes save lives!"

Motorcyclist: "How's it run?"

Leather  Pirate: "FREEDOM!!!!!"
(translation: "Not worth a shit, it misses and is really down on power. BUT....I'm coool! Besides I never ride more than 30 miles at a time between coffee shops. And all my buddies have them....."

Motorcyclist: "Yeah....umm, I'm gonna go RIDE now but have fun man."

Leather  Pirate: "Whatever, where's the nearest Starbucks?"

(Yeah yeah, I'm a big meanie making fun of the dim and uneducated....go tell my Mom.)

I can't count the otherwise interesting bikes I've seen ruined by open or drag pipes. Why is it that the myth of open pipes being remotely useful still exists. Who's propagating this bullshit? 

Yes, you can tinker your drag pipes into working.....sorta. 
Yes, you can fuck with your jetting to get'm working.....sorta.
No, you won't make any more power than running a well designed muffler.

The only place open pipes serve any purpose is at the drag strip and possibly a Starbucks parking lot. 
Don't believe me? Listen to this guy then, he's smarter than you....and me.

I've been doing some research, freshening up my exhaust theory as I'm getting ready to design the pipes for the 920, and while the bike in the video is a Harley it's still a very useful and universal bit of info worth passing along. One thing that Bruce, the educated gentleman in the video brings up is reversion and what a great demonstration with the slinky. Welcome to first year physics kids! There are volumes of information available regarding wave front propagation in a tube (ie: those pulse waves in the exhaust pipes), but really he hit it dead-on here for what we need. 
There are things you should include in any well designed exhaust, and while I'm not the biggest expert on the subject I have built a few very good systems that have performed admirably. 

Starting at the exhaust port....

1) Keep the tubing diameter reasonable.
Giant, monsterously large pipes can look pretty neat on the right bike but just like running open, they will kill your power delivery. If you've had OE pipes off before you know that they're likely double piped with the smaller diameter inside portion doing the work and the outer as a pretty skin, especially in chrome pipes. 

2) Don't cheap out on the pipe material you choose.
Good DOM tubing will make a better exhaust than the shitty seam-welded crap you get at the autoparts store. Also, avoid really thin wall stuff, for a number of reasons. 
First, you're building something for the street so it's gotta last. If you're building a race bike then sure, go for the thinnest you can get away with. Thin pipe, aside from simply not being as strong is also less effective at keeping the heat (which supports exh. velocity) inside. Worst.....they ring. Take two pieces of steel tubing of the same length and od, we'll use 1.5"od and 24" long as an example; one with a wall thickness of .045" and one with .080". Suspend them from a piece of string and give a tap with your handy 10mm box wrench. What will you notice? Well, beside the ding you may have just put in the thin wall piece, it'll also ring like a tuning fork where the heavier wall won't nearly as much. That ringing comes from vibrations in the metal itself, just like that tuning fork. In an exhaust this can result in lots of bad stuff from simply extra noise to stress cracks and fractures. Badnews bear kiddies! 

3)Torque cones and other anti-reversion tech
These little guys go in against the port and help fight that reversion by trapping a portion of that energy from smacking back into the port and slowing the charge velocity. I was surprised to find something similar built into some OE XS650 pipes I cut up the other day.

That's where they go!

3b)Buzz Cones
These work a little different, as well as having a secondary function.
The idea here is that the openings in your perforated cone add up to equal or more area than the pipe itself allowing them to be inserted into it but not adding much obstruction. These act as a damper to that reverberation we're worried about, and used with the torque cones above can make some real improvements over a regular old chunk-o-pipe. As well, they also break up the higher frequencies associated with free flowing pipes, most evidenced by the asshole kids with coffee can pipes on their civics. I add these between the collector/headpipe and the muffler. I've pointed them both ways and haven't found much difference. I choose to point them toward the exit though.

4) The Muffler
There's a lot of good technology available now for getting the most out of your engine without deafening the innocent. There are simple straight thru mufflers with perforated or louvered cores, just like that old cherrybomb glass pack you thought was so cool on your '74 Pinto from highschool.
This is more than likely the core type you're going to find in an aftermarket muffler for a bike. Keep in mind when choosing, while the louvered core will be a bit quieter you give up flow because those louvers protrude into the 'bore' of the pipe. If you pick your inner diameter carefully however you can get good performance from either style.
On the other end of the spectrum are reflection baffle designed like Flowmaster does for cars and trucks. We're beginning to see a lot more of this style of muffler core from high end manufacturers as well as a few high end AFTERMARKET manufacturers as they offer very good performance, no packing to burn away and relatively light weight.
This is a fairly common 'Turbo' performance muffler for a car. You can see the curved internal deflectors that keep the flow going more or less where it needs to. There is some sound damping via those deflectors, as well as the packing. This style exists somewhere between the restriction muffler for Mom's Buick and that reflection setup above. Lots of OE bike mufflers are this style and offer decent performance and not too aggressive sound output.

The bottom end is the restriction style, just like all the OEM mufflers from that old Honda you have parked in your carport to your mom's Buick.
As a general rule, a larger case design will yield better results than a small case. Now, that seems an awfully broad statement to make, but in practice it almost always works for mufflers. A good portion of its overall function is directly tied to how much expansion the exhaust pulses do when entering the case. The more you can allow the pulse to expend energy filling the internal volume of your muffler case, the less restriction required to damp down the most egregious frequencies. 
Ahh, the ubiquitous Emgo shorty. The only thing this is suitable for is your mower powered margarita blender or maybe your kid's totegoat. Seriously....these aren't mufflers, they're thinly chromed weight to hang on the end of your pipe. Don't....just don't. There's so much wrong here: tiny case doesn't offer any expansion volume, the length is also just sill, there's not enough room inside to damp anything. I have a buddy who will remain nameless who has two of these on his CB350. I'm hoping one day to convince him that they aren't helping AT ALL and may actually be making his bike subjectively louder due to the incredibly loud initial CRACK!!!!! with each firing. Bad bad bad.
Here's a good experiment for you to illustrate exactly what I'm getting at. Build yourself a box from some 3/4" particle board, say....18" square, used gorilla glue and deck screws so it's a nice sealed box but make one side of it removable. 
In two opposite sides use a hole saw and cut a 2" in and out. In the output side stick a piece of 2" exhaust pipe and silicone seal around it. In the other end throw a 2" reducer to whatever your pipe happens to be and use a piece of cheap radiator hose and some hose clamps to seal it to your pipe.
Staple some regular old pink fiberglass insulation to inside surfaces of your box and close it up. Connect this soon to be stinky box up to your open header and give'r a crank. What will you get? Likely a fairly mellow tone, not too much 'crack' and generally good flow. From a box. Made of wood.
BUT! It does illustrate the effect of internal volume on noise control.
A last thought for you to consider: 
Physics is physics is physics. Period, get used to it Bunky!
The rules don't change no matter how loud you squawk, no matter how hard you stop your feet. You can however, a little forethought, planning and using just a little mental exercise build a great sounding exhaust for your pride and joy that actually works well, looks good and won't lead the torch and pitchfork wielding mob back to your house after a ride.....


  1. Dynos are pretty well immune to poser bullshit, but then open pipes never were the choice of thinking riders.

    Anyone contemplating "improving" an older, none-too-powerful bike should read this post.