EXHAUST--Buyers Research Guide - Motorcycle-Journal Forums

**intimid8er** · 12-20-2005, 01:54 PM

The following guide is intended as a general reference for those looking for answers to the perennial debates that rage about exhaust systems. It is intended as a guide, and is a result of a ton of research on my part. The guide is mainly geared towards exhausts for V-Twin engines, but is applicable across the board. I just felt the need to pull it all together in one place. For those that know. It’ll be a refresher. For the noobs, it’ll help immensely in starting the long process of selecting an exhaust system, other than “I saw them on the same bike at my local bike night, and the guy that has them swears by them.”

When first selecting exhaust. One has to ask oneself some important questions about the true reason they are looking for a new exhaust system. The reasons each person has are varied about why they purchase an aftermarket system, but they can be broken down into two categories. 1. Looks. 2. Sound. Those are the most basic. However, in my own personal opinion, performance should be first and foremost, then let vanity creep it’s way in.

An internal combustion engine at it’s most basic is an air pump. Plain and simple. It sucks air in, and expels air (not really air, but you get the point) out. This is accomplished by the burning of fuel in a combustion chamber. Air is mixed with fuel in the combustion chamber, ignited with a spark, the expanding gasses force a piston down (work), and the gasses exit through the exhaust port. Simple? Yes and no. As everyone knows, air is everywhere around us. Standard pressure at sea level is one atmosphere or a nominal 15 psi. (14.7 psi per Pocket Ref, Second Edition, Thomas J. Glover) I’ll use 15. At the intake you have 15 psi, and the exhaust you have 15 psi. We all know, that without and exhaust pipe, the engine will run. Albeit terribly, let alone the catastrophic consequences of running it in such a state. Thus, we need an exhaust system to help in this process.

When the intake is opened during its stroke, a pressure differential is created (negative) and air flows in to fill this void. The exhaust valve opens, and another pressure differential is created (positive). This rise in pressure is followed by a vacuum signal or a low pressure behind it in the pipe. Simply put there are two waves that exit the exhaust. One is an energy wave that travels at a nominal 1600 feet per second, and two is a spent gas wave that travels at roughly 300 feet per second. The energy wave has no mass, and the spent combustion wave does. When the energy wave hits a wider part of the exhaust system or the end of the pipe where the atmospheric pressure is greater, it rebounds off the greater pressure and heads back up the pipe with no loss of speed. On the way back up the pipe, it encounters turbulence from the exiting combustion and energy waves, but does not experience any significant decay. If it hits the exhaust valve at the exact time it is closed it simply heads back down the pipe. Timing the exhaust event to happen after the reversion bounces back again is critical to creating a low pressure event that will help your engine scavenge more efficiently, more or less “suck charging” the engine. This is where diameter of the pipe, length, thermal efficiency, and a ton of other variables come into play.

Keeping the above info in mind, now we can reflect on Bernoulli’s Principal:

Quote:

As a fluid passes through a pipe that narrows or widens, the velocity and pressure of the fluid vary. As the pipe narrows, the fluid flows more quickly. Surprisingly, Bernoulli's Principle tells us that as the fluid flows more quickly through the narrow sections, the pressure actually decreases rather than increases!

Essentially, this means the smaller the diameter of the pipe. The faster the gas can flow. In a V-Twin, the faster the reversion waves can create low pressure events in front of the exhaust port, the better the engine can breathe. Which we all know makes us all types of power. The Suzuki 50 cubic inch engines come from the factory with 1.25” diameter primary pipes. Larger diameter pipes (i.e. 2's and higher) are more suited to bikes of displacement upwards of 1700 cc and beyond. Putting a larger diameter pipe on an engine such as the 50, is like putting a plug in the exhaust. You are slowing down the speed of the energy pulses, which again, is slowing down the timing of negative pressure in the exhaust pipe near the valve area, which retards the efficiency scavenging.

Now it’s easier to start looking at lengths and diameters of systems. As I have shown, the wave arriving at the right time at the exhaust port affects the efficiency of the engine. This is one reason exhaust manufacturers spend countless hours on the dyno tuning pipes. RPM or wave tuning depending on the length of the pipe affects the timing of the reversion event, and this in turn affects power output. If you cruise the Internet and Google “exhaust pipe shootouts”, read them, and pay attention to dyno numbers if they provide them. Look at the types of pipes compared, and notice something. Longer length pipes somewhat preserve torque with somewhat of a loss of horsepower up top. Shorter length pipes give up torque on the bottom for a gain of horsepower on top. There is no Holy Grail for horsepower and torque, but it is generally agreed a 2:1 system will preserve both and be the most efficient. Also, a “crossover”,“equalizer” or “balance” chamber will equalize and flatten the torque peak or widen the power band by using the volume of both chambers like a 2:1 system. One of the reasons a 2:1 system is so efficient is this. When an energy wave from one primary hits the collector tube in a 2:1 system, the negative pressure behind the energy wave helps pull the waves from the other primary along faster. This reflects the term I used before of “suck charging”. If you’ve heard the Jardine 2:1 pipe with your own ears, it would most likely explain the “faster” idling sound of the pipe.

The next issue to address is backpressure. Backpressure is not necessary for an engine to run. Backpressure does not make horsepower either. Backpressure helps with torque by interrupting the energy waves. Backpressure can be manipulated by mufflers, baffles, and other devices. These things do nothing more than fool the pipe into behaving like something other than it was designed to be. A properly designed pipe with the correct diameter and length will still be more efficient that an improper diameter and length pipe fitted with baffles or devices to interrupt the reversion waves.

There is no right or wrong in exhaust design or selection. The main thing is to decide what you want out of the system you are about to pay money for. The confusing array of exhaust systems is daunting to look at. But when you look at the basic physics of how they work. You can at least have a basic understanding of what you are looking at.

Some conclusions I’ve drawn:

Long pipes will increase power earlier in the powerband.
Short pipes will increase power later in the powerband.

Large diameter pipes will cause you to lose low end torque and horsepower.
Small diameter pipes will preserve low end torque but lose upper end horsepower.

Crossovers will fatten and flatten (no peaks or dips on a dyno chart) up the torque curve and give a more usable powerband.
2:1 systems will do the same.
(Due to using the combined volume of two pipes to scavenge both cylinders versus one pipe: one cylinder.

(http://www.motorcyclecruiser.com/acc...e03/index.html)

(http://www.motorcyclecruiser.com/acc...ear/Pipes2000/)

This does not necessarily mean that Drag pipes or 2:2 systems are a bad thing. It is still possible to maintain a healthy torque peak (notice I said peak) and horsepower peak with the proper pipe for your application. Equal length (same size) pipes with the proper diameter will still do what you ask of them. Best results from the conclusions above would also suggest a rather lengthy pipe. That is only if you are finicky about where and how you want your power made. Most V-Twin riders ride in the 2300-6000 rpm range with the majority of it spent between 2300-4500 rpm. Power in that range seems to be the most usable for cruiser pilots. Then ask yourself, do you care if you twist the throttle, the bike jumps forward, then falls flat (torque peak falling off on a drag pipe), and then gradually builds up steam to the top speed? Or do you want to twist the throttle, and have instantaneous acceleration (2:1)? Again it’s up to you.

If you just want to look and sound “cool” then just select what you want. However, if maximizing, or getting the most out of the system you paid for and having the best of all worlds (style, sound, and performance) means more to you. Then I hope this little guide helps out.