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Just a little about
Exhaust Design
The cycles of a four stroke engine are (1) intake, (2) compression, (3) expansion, and (4) exhaust. Although the exhaust cycle is the last process, if done properly, it may have the greatest impact. That's not to say the other cycles, intake compression and expansion, aren't important. Each cycle is important and if any one cycle doesn't work properly, it will negatively impact the others. That said, I think the exhaust has the greatest individual impact.
Think of the combustion chamber as a room. The fuel is mixed, ignited, burns, and creates power in that same room. The exhaust cycle cleans the room so the next batch of fuel can start in a clean environment. If you understand this, you understand volumetric efficiency.
Without a clean environment,
- Not as much fuel can enter, as the room (combustion chamber) is already partially full.
- The incoming fuel will be contaminated with exhaust residue.
- The contaminated fuel won't burn as completely, and as a result, won't produce as much power.
That is why volumetric efficiency is important. Basically, if the engine cannot exhaust itself adequately, additional modifications will result in very little improvement.
Pumping loss
Of the four cycles, only expansion directly creates power. The other cycles, including exhaust, reduce power and are known as pumping loss. One goal of a good engine design is to reduce pumping loss.
Some pumping loss is reduced by the camshaft profile. That's done by opening the exhaust valves before the piston reaches bottom dead center (BDC, when the piston reaches the bottom of it's stroke). If that sounds odd, you may have expected the exhaust valves to open when the piston is on it's way up. That would require the upward motion of the piston to force all the exhaust out of the cylinder. Pushing the exhaust out requires energy, which make it 'pumping loss'. If the valves are opened before BDC, the combustion pressure will be used to start forcing exhaust from the cylinder. This is referred to as blowdown. The blowdown period is responsible for approximately half of the exhaust flow. On the downside, opening the exhaust valves before BDC releases pressure that would otherwise be used to continue to force the piston down and power the crank. However, this reduction in thermal efficiency is far outweighed by the reduction in pumping losses. This is mainly because the majority of the useful work occurs during the first third of the piston travel, from top dead center (TDC) downward.
Velocity
On the Internet, it's common to read things like engines need backpressure. This is untrue. High performance engines need exhaust velocity. In this case, velocity is referring the speed the exhaust gases are traveling. Based on cam design (if you need some background on cam design, read this), velocity can cause engines to have a little reversion, but I think, the more velocity the exhaust has, the better. Velocity is important for one reason - inertia. Newton's law of motion basically states an object in motion tends to stay in motion. When applied to exhaust gases, once the exhaust is moving, it will continue to move until something forces it not to.
Consider this: After the piston passes TDC, the exhaust valves are still open. At that point, the piston is on it's way back down, a vacuum is beginning to form, the intake valves have opened (some overlap), and yet that exhaust valve is still open. Why? Inertia allows the exhaust gases to continue to pass through the exhaust port until the exhaust valve shuts. This is called scavenging. For this example, the speed of the exhaust isn't important. The fact that the combustion chamber is being emptied of exhaust (scavenging, which increases volumetric efficiency) is the key. I know of two ways to increase exhaust velocity; (1) decrease the cross section of the size of the tubing (which can have a detriment), and (2) increasing the volume of exhaust that's flowing though it.
Decrease the cross section (bigger is not better)
If decreasing the inner diameter of tubing sounds odd, hopefully I can explain it's benefit. At some point in engine design, it was determined that multiple exhaust valves were better than a single exhaust valve. One reason is because multiple valves have multiple ports, and (although short) multiple exhaust tracks. Those multiple valves/ports/tracks may have less volume (smaller) than a single large valve, but they flow better. What makes them better is the exhaust flows faster. This faster flow creates more inertia. The flow is so much better, that it has enticed some manufacturers to go with five valve heads. Ever hear of someone who had their heads ported and the engine produced less power? They made the ports too big, which killed velocity. Without velocity, the power dropped off. The smaller valves, with their smaller ports and tracts cause the exhaust to move faster. Velocity was increased, as was power.
Increasing volume
Increasing volume is easy to envision. Increasing the engine speed increases the amount of exhaust produced. This is important to consider because everyone wants an exhaust that's good across a rev range. You may have seen where someone brags that they bolted on an exhaust and peak HP increased 15%. If the dyno chart shows the average HP across the rev range has gone down, that new exhaust is worse than the previous one. This can be proved by comparing quarter mile trap speeds, or watching the owner and his new exhaust get pulled away from while exiting a low speed corner. A peaky engine is good for bragging, but when it's off the pipe, the car is slow.
With regard to velocity, these are the key concepts: Exhaust velocity increases proportionally to RPM. Basically doubling the RPM doubles the exhaust velocity. Exhaust velocity is inversely proportional to the inner diameter of the area it's flowing through. If the size of the exhaust tubing is doubled, velocity will be cut in half. If the size of the exhaust tubing is cut in half, velocity will be doubled. The size of the exhaust must provide velocity across a range, without impeding high RPM performance. Get the velocity up, and it will take a fair amount of energy to impede inertia, which is great for scavenging.
Pressure waves
Pressure waves can be used to help inertia. Before getting into how pressure waves can help inertia, you'll have to understand them. Entire books have been written about pressure waves. Hopefully I can explain it in a few paragraphs.
First, if you've ever heard a Harley Davidson, you know that exhaust leaves an engine in pulses. The exhaust pulse is different than the pressure wave. This is important to remember.
When an exhaust valve opens, a strong positive wave will begin traveling toward the end of the exhaust pipe. When it reaches the end of the pipe, the positive wave will change to a negative wave. That negative wave will travel back up the exhaust, toward the exhaust valve. If the pipe is just the right length, and the engine is turning at just the right RPM, the negative pressure wave will arrive at the exhaust port at the same instant the exhaust valve opens, creating a low pressure area. This low pressure zone helps scavenging.
This isn't as simple as it sounds. Unless you have a Harley Davidson or a race bike/car, this is a little more complicated. There is a lot of junk in the exhaust that will impede the wave. That wave is really a sound wave. That means it travels at the speed of sound, which is much faster than the exhaust pulse. There is a fair amount of junk in the exhaust to impede sound waves. Fortunately, the negative wave can created before hitting the catalytic converter, resonator, or anything else that will ruin it. In a best case scenario, the wave leaves the exhaust valve and heads down a header tube. Rather quickly, the wave hits the header collector. The collector is a low pressure zone. This causes the wave to have the same reaction as it would if it reached the tailpipe. A negative pressure wave is reflected in the opposite direction up ALL the header's primary tubes. What started as one positive wave, reverses direction and splits in to four negative waves (four into one header, naturally). Big money header manufacturers have done the math and one of those exhaust ports can take advantage of the low pressure zone.
The 'math' involves
- Determining the correct primary tube diameter. This is based on the cylinder size, and does not change the speed of the pressure wave. It does impact the speed of the exhaust pulse.
- Calculating how long it takes a single wave to travel down the primary header tube (as a positive wave), and then back up the four primary tubes (as a negative wave). Note: four into one header in this example. Tri-Y headers are different.
- Determining how often the exhaust valves open at a given RPM, and how long the primary tubes will have to be. The tubes must be a specific length to get the negative time wave to arrive at the port just as the exhaust valve opens.
That's why good headers cost so much. Someone to think this through and use the correct primary tube length, primary tube diameter, collector length, and collector angle for a specific sized engine with exacting cam specs. If the engine doesn't have headers or the headers are not built to exacting specs, the pressure waves will still be present, but the exhaust system won't take advantage of them. Fortunately, velocity isn't dependant on pressure waves.
That explains a few aspects of exhaust design. The intent of this page is not to make a specific recommendation, but to give you enough knowledge to make informed decisions. There is a lot of hype out there about what works and to what degree. Some well placed parts will have surprisingly beneficial results. Using other parts can have expectedly bad consequences.
© 2009 Marcus Blair Fitzhugh
Signal to Noise