It describes the process of the four cycle engine.
I want to direct your thoughts to a little more advanced thinking. What happens when the exhaust valve opens besides letting the exhaust escape into an exhaust pipe? There are other events that occur when the exhaust valve opens. Not only does the exhaust gases escape, but also sound waves and pressure waves are released. First there will be a sound wave that will travel down the exhaust pipe followed by a pressure wave.
Click to watch this video of an explosion. The explosion occurs near the end of the video, so you may want to fast forward it. The video is taken at a long distance away.
Notice that just after you visually see the explosion, a split second later you can hear the “boom”. That is because the “boom” is traveling at the speed of sound. Notice the pressure waves that followed shortly behind the “Boom”.
These are the two waves that need to be studied and controlled. These waves can be manipulated to help scavenge the cylinder and combustion chamber.
On a typical engine during the 4 cycles of the engine, the exhaust escapes and a new intake charge is allowed to enter the cylinder. Unfortunately not all of the exhaust gasses can escape, and there will not be a full fresh air charge (100%) into the cylinder. Remember the basics, the more or bigger air/fuel charge that you can get into the cylinder, the more power that it will make. The amount of charge that can be used by the engine to make power is called “VE” Volumetric Efficiency.
One way that you can increase the VE of an engine is by tuning the exhaust. The problem with most exhaust systems is that they have to be routed around obstacles such as frames, foot pegs, engine components and have to made to appeal to the eye. There are too many companies that have designed the “Best” exhaust on the market. Most of these exhaust are designed around eye appeal and the tone of the sound that they make. “It’s louder so it must have more power.”, “The exhaust needs more back pressure.”, “This megaphone helps scavenge the exhaust.” “The exhaust has too many bends.” “Adjustable baffles and shims.” …You’ve probably heard it all. Maybe you should look at companies that spend millions of dollars on research designing exhaust but doesn’t sell the exhaust. If anything, they try to keep it a secret. Look at Formula 1 cars and how they build their exhaust. Remember that these engines are making about four horsepower for every cubic inch without turbos or superchargers.
This is a variable exhaust. The vac. pod opens and
closes the exhaust pipe which will cause it to reroute
the exhaust travel. This lengthens or shortens the exhaust
travel depending on which way the ECM wants it to travel.
Where Are The Baffles?
Notice that there are no
Shim Plates in the exhaust.
"OLD SCHOOL THINKING"
These Engines Can't Run Right Because They Have
"No Back Pressure"!
Imagine these waves traveling down the exhaust pipe as if they were a syringe. Visualize the wave being the plunger. There would be high pressure in front of the plunger in the direction that the plunger is being pushed. There would be a low pressure on the back side of the plunger. Visualize the plunger if it could exit the tube. A negative pressure (low pressure or vacuum) would travel back up the tube. This is called a “Reflective” wave.
Now we can take advantage of the reflective (low pressure wave) if that wave travels back up the exhaust and arrives at the exhaust valve during the time that the exhaust valve is open. This reflective wave (low pressure or vacuum) will enter the cylinder causing a scavenging effect to suck out exhaust gasses. Kind of like hooking a vacuum cleaner to the tail pipe. Since there is a vacuum inside the cylinder, what would happen if the intake valve was also opened? The vacuum inside the exhaust pipe combined with the vacuum inside the cylinder will provide a double source of vacuum sucking the fresh air charge from the induction system almost like "supercharger". Both intake and exhaust valves will be open during “Valve Overlap” caused by the camshaft. Now you can see the benefit of timing the reflective wave to reach the exhaust valve during valve overlap.
The gasses traveling down the exhaust at approximately 300 feet per second. You can change the timing of the wave by modifying the length and diameter of the exhaust. The exhaust temperature also affects the speed of the gases. There are many online exhaust length calculators that will make this calculation easier. Remember that the calculators can get you close but not exact. You will need to fine tune the exhaust by adding or subtracting a little length, then dyno the engine to see if there are improvements.
You should also consider using a two into one pipe. The gasses and waves will also create a negative pressure in the opposite pipe when they pass the “Y” section where the two pipes join at the collector. Not only is the primary pipe length and diameter important, but also the angle of the two merging pipes and the length and diameter of the collector.
Most people can’t afford an exhaust mandrel bender to fabricate their own exhaust. You may have to purchase a mass produced exhaust and make modifications to it. As far as mass produced exhaust goes, I am a fan of the Vance and Hines Pro Pipe with a modified tuned to length and diameter collector pipe. I have tested a lot of pipes and have found this to perform the best with the cam timing that I like to use. One of the other important areas of the primary pipe to be considered is the angle at which the primary tube bolts to the exhaust port. Watch what you purchase so you don't get stuck with a bad pipe. Another way to shortcut to your decision is by looking at dyno graphs posted by people and builders. Granted, the numbers will not be the same on other dynos and the numbers may also be inflated. Since the numbers can't be reliable, you can still look at the flow of the HP & TQ lines. Look to see when they start compared to where you want them to start. Look at what RPM that the TQ & HP will max at.
Look at the ramp to see if the powers climbs like you would want it to.
Remember, that the cam timing v/s exhaust and induction will determine the HP/TQ lines of flow. Other modifications will raise the HP/TQ numbers.
Also remember, that the timing of the waves will either help you or they will hinder you by obstructing the other waves if not tuned properly. This is why some motors make more power than others when an exhaust change is the only difference.
Here is an example of how small differences in pipes makes large differences in performance. I was using a White Brothers E- Series pipe on my build. It just didn't have the power that I was expecting to have even after tuning the pipe. One day when work was slow, I had a V&H Pro Pipe off of a Softail Heritage laying in the shop. For curiosity , I decided to see if could make it fit my Ultra. It did with very little problems. That pipe really brought the engine to life. Later I ordered a Pro Pipe that was made to fit the Ultra. After installing it, It lost several horsepower. I tried to tune the pipe but never could get it to perform as well as the Heritage pipe. The Ultra now has the Heritage pipe installed again. The only difference between the two pipes were a couple of bends but, those bends caused a lot of power loss. The pipe designed to fit the Ultra would probably work alright if it was matched up with the correct cam.
Another way to tune the waves of the exhaust with out modifying the exhaust system is by adjusting the cam timing. Adjust the cam timing so that the exhaust valve will open when the negative wave arrives at the valve. In the past there were basically only a +5 degree and -5 degree gears offered. Now we have the option to adjust the timing were ever it needs to be with the Hemrick Performance Adjustable Cam Gear. After installing the gear, make a dyno run to get a base reading. Then move the cam timing +5 or -5 degrees and make a dyno run and compare the run to the previous run. When you determine whether it need to be advanced or retarded. Then fine tune the timing until you get the desired power that you want.
Check Out The Induction Article.
"Induction"