Motorcycle exhaust back pressure

Motorcycle exhaust baffles serve two purposes: Reduce the noise from wide-open exhaust pipes and create back-pressure in the exhaust system. Limiting noise emission is not only considerate, but is required by law in many localities. Those with knowledge of motorcycle engines also understand a certain amount of back-pressure is needed to avoid damaging a stock engine.

Baffles are not complex pieces of equipment. Typical baffles consist of steel tubing with a series of offsets that combine to reduce noise and create that necessary back-pressure. You can make motorcycle exhaust baffles in your workshop with shop tools. Measure the inside diameter of the motorcycle tailpipe.

The overall length of the steel tubing depends on the number of baffles you intend to make. Typical baffles are 8 inches long. Secure the steel tubing in a bench vise and cut an 8inch baffle using a hacksaw. Mark the baffle for three offsets using a felt-tip marker. The first offset is 3 inches from one end. The third offset is 3 inches from the first mark. Secure one end of the baffle vertically in the bench vise with the offset marks above the vise. Make horizontal crosscuts halfway through the baffle at each of your marks sillones de suiza a hacksaw.

Reposition the baffle in the vise if necessary as you go. Remove the baffle from the vise. Place the baffle on the workbench with the hacksaw cuts facing up. Secure the baffle in place using one or more c-clamps.

motorcycle exhaust back pressure

Position the tip of a cold chisel at the first hacksaw cut. Strike the cold chisel with a mallet to force the outer edge of the cut into the center of the baffle. Repeat this at the remaining hacksaw cuts to create the baffle offsets.

Secure the baffle in the vice with the 3-inch end of the baffle extending from one end of the vise.Exhaust systems are responsible for the disposal of exhaust gas created after engine combustion. Backpressure does not benefit engine performance as it returns exhaust into the engine cylinders instead of releasing it through the tailpipe. Backpressure can lessen fuel efficiency, reduce power and stall the engine.

Backpressure constricts exhaust flow. Backpressure is ever present in consumer vehicles but harmful when excessive. Exhaust system piping diameter varies by vehicle. Proper piping diameter provides an optimal level of exhaust flow and velocity with minimal backpressure allowing an engine to reach full potential and efficiency. Overly small piping will reduce flow but excessively large piping reduces the velocity exhaust is released.

Plugs, obstructions or collapsed materials within exhaust components, including the catalytic converter or muffler, will decrease exhaust flow and velocity, thus returning exhaust gasses into the engine. Excessive backpressure reduces available power, overheats the engine and forces the engine to work harder, which reducing fuel economy.

Exhaust backpressure test kits are available at auto retailers. This article was written by the It Still Works team, copy edited and fact checked through a multi-point auditing system, in efforts to ensure our readers only receive the best information.

To submit your questions or ideas, or to simply learn more about It Still Works, contact us. Motorcycle exhaust image by Crisps85 from Fotolia. Backpressure Causes Plugs, obstructions or collapsed materials within exhaust components, including the catalytic converter or muffler, will decrease exhaust flow and velocity, thus returning exhaust gasses into the engine.

How To Increase Back Pressure On Motorcycle Exhaust

About the Author This article was written by the It Still Works team, copy edited and fact checked through a multi-point auditing system, in efforts to ensure our readers only receive the best information. Photo Credits Motorcycle exhaust image by Crisps85 from Fotolia.There are all kinds of things going on inside there: the internal shape of the pipe combined with the initial header length, points of expansion and internal routing sometimes there are pipes in your pipes all work together to keep the waves of exhaust gases produced working for your engine and not against it.

The exhaust gases that exit your engine do not do so in a smooth flow. That pulsation can result in back pressure. The shape of your exhaust system works very precisely with these exhaust pulses to maintain rearward flow: all of the gases flow toward the exit instead of back up through the exhaust port and into the engine.

A well-designed exhaust will use the precisely-timed pulses to create a similarly precisely-timed vacuum. This helps engine performance, and helps keep your power band nice and wide and usable.

All that precise timing goes away. Flat spot much? The bike may run great at some RPMs and terrible at others: this is often a sign of a poorly-tuned exhaust with too much back pressure. If you've ever experienced a leak in your exhaust system due to something like a failed header gasket or a rust hole, you know first hand the absolute chaos that introduces. Your bike suddenly runs like crap!

That's because all that engineering that keeps your bike running and your exhaust gases flowing in a beautiful symbiotic dance has gone directly out the window. No amount of tweaking your carburetors or remapping your EFI can help; exhaust leaks mean the gases are not exiting cleanly and your bike can breathe in but it can't breathe out. Back pressure is literally choking your bike.

Home Articles Design. By : Kate Murphy.

How to Measure Exhaust Backpressure for Smarter Exhaust Upgrades

Short answer: Yes, with an "and! Commenting Guidelines. Sign In or Sign Up.Jump to navigation. Engine Performance Testing with a Vacuum Gauge. A vacuum gauge shows the difference between outside atmospheric pressure and the amount of vacuum present in the intake manifold. The pistons in the engine serve as suction pumps and the amount of vacuum they create is affected by the related actions of:.

Each of these has a characteristic effect on vacuum and you have to judge their performance as compared to what is considered "normal". To do this, it's important to judge engine performance by the general location and action of the vacuum gauge needle, rather than just by a vacuum reading. What follows is a list of the kinds of gauge readings you may find.

At idling speed, an engine at sea level should show a steady vacuum reading between 14 in. A quick opening and closing of the throttle should cause the vacuum to drop below 5 in.

With the engine at idle, the continued fluctuation of 1 to 2 inches may indicate an ignition problem. You should check things like spark-plug gap, primary ignition circuit, high-tension cables, distributor cap or ignition coil.

Fluctuations of 3 to 4 inches may point to sticking valves. A vacuum reading at idle that is much lower than normal might indicate leakage through the intake manifold gaskets, manifold to carburetor gaskets, vacuum brake booster or the vacuum modulator. Low readings could also be caused by very late valve timing or worn piston rings. Starting with the engine at idle, slowly increase engine speed to 3, rpm. Engine vacuum should be equal to or higher than vacuum at curb idle.

If vacuum decreases at higher rpm, an excessive amount of back pressure is probably present due to a restriction in the exhaust system. With the engine at idle, the vacuum gauge pointer will drop sharply every time the leak occurs. The drop will be from the steady reading shown by the pointer to a reading of 10 in. If the leak is between two cylinders, the drop will be much greater.

You can determine the location of the leak by doing a compression test. Remember, engine problems can affect transmission performance. If you suspect an engine problem, connect a vacuum gauge to the intake manifold. Note the location and action of the vacuum gauge needle, and use that information to determine the engine problem. Correct the engine problem before doing extensive calibration work on the transmission.Simply put, it's because the two-stroke exhaust system, commonly referred to as an 'expansion chamber' uses pressure waves emanating from the combustion chamber to effectively supercharge your cylinder.

Kaaden understood that there was power in the sound waves coming from the exhaust system, and opened up a whole new field in two-stroke theory and tuning. An engine's exhaust port can be thought of as a sound generator.

Each time the piston uncovers the exhaust port which is cut into the side of the cylinder in two-strokesthe pulse of exhaust gases rushing out the port creates a positive pressure wave which radiates from the exhaust port. The sound will be be the same frequency as the engine is turning, that is, an engine turning at rpms generates an exhaust sound at rpms or cycles a second--hence, an expansion chamber's total length is decided by the rpm the engine will reach, not displacement.

Indeed, the only advantage to this crude pipe system was that it was easy to tune: You simply started with a long pipe and started cutting it off until the motor ran best at the engine speed you wanted.

Of course those waves don't radiate in all directions since there's a pipe attached to the port. Early two strokes had straight pipes, a simple length of tube attached to the exhaust port. This created a single "negative" wave that helped suck spent exhaust gases out of the cylinder.

Ask RideApart: Is Exhaust Back Pressure A Thing?

And since sound waves that start at the end of the pipe travel to the other end at the speed of sound, there was only a small rpm range where the negative wave's return would reach the exhaust port at a useful time: At too low of an rpm, the wave would return too soon, bouncing back out the port.

And at too high of an rpm, the piston would have traveled up the cylinder far enough to close the exhaust port, again doing no good. So after analyzing this cut-off straight-pipe exhaust system, tuners realized two things: First, that pressure waves could be created to help pull spent gasses out of the cylinder, and second, that the speed of these waves is more or less constant, though it's affected slightly by the temperature of the air. Higher temperatures mean that the air molecules have more energy and move faster, so sound waves move faster when the air is warmer.

A complicating factor here is that changes in the shape of the tube cause reflections, or changes, in the sound waves: Where the section of the tube grows in diameter, there will be sound waves reflected back towards the start of the tube. These waves will be the opposite of the original waves that they reflected from, so they will also be negative pressure waves.

The next important discovery was made--by gradually increasing the diameter of the tube, a gradual, more useful negative wave could be generated to help scavenge, or pull spent gasses out of, the cylinder.

Putting a divergent cone on the end of a straight pipe lengthens the returning wave, broadening the power band and creating a rudimentary expansion chamber. So, to sum up, when the negative wave reaches the exhaust port at the correct time, it will pull some of the exhaust gases out the cylinder, helping the engine to scavenge its spent exhaust gas. And putting a divergent cone at the end of the straight parallel "head" pipe broadens the returning wave.

The returning negative wave isn't as strong, but it is longer, so it is more likely to find the exhaust port open and be able to pull out the exhaust gases. As with plain, straight pipes, the total length of the pipe with a divergent cone welded on determines the timing of the return pulses and therefore the engine speed at which they are effective. The divergent cone's critical dimensions are where it starts the distance from the exhaust port to the start of the divergent cone is called the "head" pipewhile the length of the megaphone and the rate at which it diverges from the straight pipe determine the intensity and length of the returning wave--A short pipe which diverges at a sharp angle from the head pipe gives a stronger, more straight-pipe-like pulse.

Conversely, a long, gradual divergent cone creates a smaller pulse of longer duration. In addition, the negative wave is also strong enough to help pull fresh mixture up through the transfer ports. And while adding a divergent cone to the head pipe produced great tuning advantages, it had its limitations, too: The broader negative wave from a megaphone can still arrive too early and pull fresh mixture out of the cylinder.

That's exactly the problem that Walter Kaaden had with the factory MZs. He realized that putting another cone, reversed to be convergent, on the end of the first divergent pipe would reflect positive waves back up the pipe. These positive waves would follow the negative waves back to the exhaust port, and if properly timed would stuff the fresh mixture that was pulled into the pipe back into the exhaust port right as the piston closed the port.

In addition to head pipe length, divergent and convergent cone lengths, an expansion chamber has three more crucial dimensions. The length of the straight 'belly' between the divergent and the convergent cones, the length of the tailpiece 'stinger', or muffler, and the diameter of the belly section. The stinger acts as a pressure bleed, allowing pressure to escape from the pipe.

Back pressure in the pipe, caused by a smaller-diameter or longer stinger section, helps the wave action of the pipe, and can increase the engine's performance.

motorcycle exhaust back pressure

This, presumably, happens since the greater pressure creates a more dense, uniform medium for the waves to act on--waves travel better through dense, consistent mediums. For instance, you can hear a train from a long way away by putting you ear to the steel railroad track, which is much denser and more uniform than air.

But it also causes the engine to run hotter, usually a very bad characteristic in two-strokes. Kaaden immediately realized a large power gain, and the expansion chamber was born. The length of the belly section determines the relative timing between the negative and positive waves.

The timing of the waves is determined by the length of the straight pipe. If the belly section is too short, positive waves have a shorter distance to travel, and return to the exhaust port sooner. This is good if the engine is running at a higher speed, bad if you want to ride on the street.If you insist on using drag pipes on your bike, there is something you can do to improve the low and mid range power produced by the engine.

Even with the improvement listed here, the streetable engine power is not going to match power output of a good or exhaust system. Motorcycle Performance Guide does not recommend drag pipes or porker 2" pipes for serious street engines, but the performance fix listed here will improve the power of your drag pipes. Results have been confirmed by dyno results. If you are serious about making horsepower on the street with your Harley-Davidson, drag pipes will not fit into your engine program plans.

Serious street power requires a serious exhaust system. The familiar sound of a drag pipe may be music to a "bikers" ears, but the performance rider hears the labored acceleration as the motorcycle moves by. Drag pipes do have their place. It is on the drag strip where the engine runs in a very narrow RPM band. On the street, stick with proven winners. The first item to get modified or changed on most new Harley-Davidson motorcycles is the exhaust system.

Getting the proper Harley sound always seems to require increasing the decibel level out the exhaust, with many riders installing drag pipes as the exhaust system for the proper sound. The rider often believes that by reducing back pressure in the exhaust system the engine will also increased power.

This is wrong. By properly re-jetting the carburetor and adding a free flowing air cleaner to an engine with drag pipes, the maximum horsepower produced will improve over the stock engine. But there is a difference between usable power and maximum horsepower. The maximum horsepower of two engines may be similar, but the horsepower torque curves may be different. The area under the horsepower and torque curves defines the "power" the engine produces. The more area that is under the curve, the better the power.

A typical drag pipe produces a horsepower curve that initially rises very slow. As the RPMs start to rise above mid-range power, the curve begins to rise at increasing rate until maximum horsepower is achieved. Once RPMs have passed maximum horsepower, the curve drops of rapidly.So I've talked before about how you measure intake restriction and how you can actually find the source of restriction by using a differential pressure meter.

However, while I've alluded to the fact that you can also do something similar for exhaust back pressure, I haven't done anything about it until now.

How NOT to Modify Motorcycle Exhaust for Better Flow & Horsepower

Today I want to share with you how you can analyze an existing or aftermarket exhaust system using a very inexpensive gauge, some tubing, a spark plug defouler, some jb weld and a fitting from your local home hardware store. It's like tuner MacGyver, but trust me, it works great. Most enthusiasts know that reducing backpressure in the exhaust system will typically yield a performance gain.

However, there is much speculation about what exactly makes a big difference and what does not. Sure, a bigger pipe will almost always reduce backpressure as will a higher flow muffler with either a straight through or significantly less baffled design. But what about the catalytic converter, is it really a restriction in your application?

On some cars it may be a huge restriction, while on others it doesn't seem to make much difference. What about the resonator? Is it really causing any back pressure at all? Is the back pressure mostly coming from the cat back? Knowing where in the system the restriction is can be useful if you're on a budget, or, if you're trying to get the most power out of your system and want to identify areas of additional improvement without excessive cost.

Like the intake restriction measurement technique I've described elsewhere on the site, the methods that I want to tell you about today allow you to pinpoint EXACTLY where the restrictions are. Today I'm just going to talk about how you test an existing system and perhaps the modifications you make after that.

In other words, how to figure out where restrictions are in the stock system, and how to measure changes in backpressure caused by modification you make. In a later article, I'll explain how you can actually test mufflers, resonators and cats OFF the car so that you can locate a very nice flowing cat, muffler, or resonator at the junk yard or exhaust shop in case you might want to use junk yard parts to create a great inexpensive sleeper performance exhaust system.

Bad news is, it's very difficult to get pinpoint accurate back pressure measurements because exhaust pressures are pulses, not constants. In other words, because exhaust gasses go back and forth up and down the extractors and exhaust system VERY fast, you will get a very bouncy needle on the vacuum gauge we use to measure back pressure.

Good news is, it really doesn't matter much because you're going to be mostly interested in the "peak" number and you're also going to be looking for BIG changes, not tiny ones, so the measurement still works out quite well. Just know you'll probably have to have a friend help you when you go out to take measurements.

Measuring back pressure is not rocket science. As a matter of fact, the hardest part is either finding a good place to tap into the exhaust system and how to keep tubing from melting. Aside from that, it's no big deal. Fortunately, the difficult parts I've already solved for you. The first thing you need is a pressure gauge that can read up to about 20psi.

motorcycle exhaust back pressure

They sell these at auto parts stores and many can read vacuum as well. For exhaust back pressure, I prefer mechanical gauges because they're generally cheaper to replace and less delicate. Spark plug defouler purchased from autoparts store with fitting from hardware store to attach transmission fluid hose. Next thing you need is a spark plug defouler which conveniently has the same threading as an O2 sensor.

Go to the hardware store after you get one of these and find a fitting that will screw into it. I found one at the hardware store that had a similar thread but still fit and used JB Weld to secure the threads and make sure everything was nice and tight. Then you need transmission line hose.

motorcycle exhaust back pressure

You need this or something rated for even higher temps because the tip of the fitting we made above will get relatively hot, relatively quickly. You also need hose clamps that will allow you to really clamp the hose down onto the fitting or the exhaust backpressure will blow the hose off. Once you've got these items, run the hose to your fitting, screw the fitting into an O2 bung and the other end of the hose will need to go into the vacuum gauge.

No worries if you have to use a few different kinds of adapters to make the connection work, the most important thing is that the line isn't kinked and that the connection is sealed.

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