Exhaust and Intake Tuning
1. What is Exhaust Tuning?
When I was fooling around with racing 4-stroke motorcycles a long time back, we used exhaust tuning to increase horsepower by cutting each pipe to a specific length. In order to gain the maximum effect the pipes have to be totally separate from each other, with no common manifold between any of the cylinders. That is not always practical, but it's the ideal. You will notice that on drag racing engines there are always individual pipes sticking out the sides, and those pipes are set to a specific length, which is critical.
We used a formula to determine the length of the exhaust pipe. Because the pressure waves in an exhaust system travel at the speed of sound, we had to know the approximate temperature of the exhaust gases because sound waves move faster in hot air than in cold air. Meaning, it travels further in the same amount of time in a hot gas environment versus a cold gas environment.
Then we calculated how long it would take a sound wave to move from the exhaust valve seat to the end of the pipe and back again. A round trip. Next we needed to know the approximate RPM at which we wanted to run at top end, and then determine how long the exhaust valve would be open from beginning of travel to end of travel.
2. Exhaust Tuning in 4-Stroke Engines; Valve Overlap
When we got it right we would have an exhaust pipe that would carry a positive pressure wave of exhaust pulse down the pipe to the open end. There it would collapse and create a negative pressure wave that would return back up the pipe. If the negative wave arrives back at the exhaust valve just before it closes, it will suck more of the exhaust gases out of the cylinder. This lowers the pressure inside the cylinder and makes the next intake stroke more efficient.
On a 4-stroke, the intake valve begins to open while the exhaust valve is still off it's seat. This is valve overlap. This allows the negative exhaust pulse (the reflection of the positive pulse) to actually pull more fresh mixture past the intake valve and into the cylinder. Here's how it works, and it has nothing to do with exhaust tuning as such.
When the combustion cycle begins, the piston is forced downward; this is the power stroke. Near the bottom of the power stroke the energy is mostly spent and the exhaust valve starts to open. It will actually start to open slightly before bottom dead center. The exhaust charge then begins to rush out the exhaust pipe.
The exhaust gases rushing out are further assisted by the piston pushing up on the exhaust stroke. This forms a stream of hot gas in very rapid motion away from the cylinder. This stream of hot gas has inertia and it will tend to continue moving in the same direction out the exhaust pipe even after the piston stops pushing it. This creates a region of reduced pressure in the vicinity of the exhaust valve.
By opening the intake valve just prior to top dead center, while the exhaust valve is still open (overlap), the gases going out the exhaust pipe will begin pulling the new intake mixture in behind them. Or, the intake stream will try to flow into the region of reduced pressure behind the exhaust stream, if you want to look at it that way. So overlap merely takes advantage of the inertia of the exhaust gases and the low-pressure region that it produces near the exhaust valve at the end of the exhaust stroke.
That part of the overlap design is common to all 4-stroke engines in order to gain additional charging of the cylinder with fuel mix at high RPM. The higher the RPM we design for, the greater the intake and exhaust overlap we build into the cam lobes. Most engines are fitted with exhaust manifolds that collect all the gases from a bank of cylinders. They also usually have a long pipe and muffler. So, while the physics of gases in motion will apply there, tuning for the exhaust pulse will not.
3. Intake Tuning on a 4-Stroke Engine
On the 4-stroke, intake tuning is also a consideration. When the intake valve opens, it creates a negative pressure wave which will travel to the end of the intake pipe. It is reflected as a positive wave which then travels back down the pipe. It will create a sharp, supercharging effect if it can be timed to arrive just before the intake valve closes. That requires individual carb throats to each cylinder though, and no common intake manifold.
The old Dodge Ramchargers used that method in the days when carbs were the standard way to get fuel mixture into an engine, and you could see the eight intake stacks sticking up from the hood. Those intake stacks were cut to a specific length for tuning. The intake tuning was used to "ram" a little more charge into the cylinder just before the intake valve closed; hence the name of that racing team, the Ramchargers.
So you can see the principle involved here and why both intake and exhaust systems are engineered for specific lengths if we want to make use of the pulses within the pipes. Two-strokes use the same principles, but the intake side is not tuned. We can see one undesirable side effect of the intake system on our 2-strokes, however, which is spitting back of some of the fuel mixture at lower rpms.
4. Exhaust Tuning in 2-Stroke Engines
A 2-stroke is different of course since there is only a finite amount of fresh mixture available in the crankcase to be transferred up into the cylinder. In a 2-stroke, we want to maximize the retention of that finite fuel charge. We can't afford to "waste" any of it by having it escape out the exhaust port.. This is not a concern in the 4-stroke because the intake flow of fresh mixture is unlimited, or at least limited only by the duration of the intake valve timing. The closing of the exhaust valve prevents significant escape of the incoming charge.
The exhaust tuning of a 2-stroke is designed to preserve the positive pulse going down the pipe and reflect that positive pulse back up the pipe. A racing engine exhaust system uses an expansion chamber at the end of the pipe which is basically two cones that reverse themselves prior to the end "stinger" where the spent exhaust is allowed to exit the chamber.
That sort of expansion chamber creates a very strong positive wave reflection back up the exhaust pipe, and this positive pulse will reduce the amount of fresh charge escaping out the exhaust port, and in some cases, force back into the cylinder any charge that may have escaped before the positive pulse arrives.
On 2-stroke racing engines, we will see an immense change in power delivered when the exhaust system is tuned for a particular RPM range and the engine revs into that range. It's called "coming on the pipe" and the burst of sound and power that occurs when that happens is easily witnessed.
5. Practical 2-Stroke Aircraft Engines
The engines we use for flying do not have a true expansion chamber but rather a semi-expansion chamber and muffler combination. You can imagine what happens when the positive pulse arrives at the piston a bit too early or too late. The benefits of a tuned exhaust in enhancing the retention of the fuel charge are not fully realized. When it's just right, however, it will push some or most of the fresh fuel charge that has started to exit the exhaust port right back into the cylinder just before the exhaust port closes.
The exhaust side is semi-tuned since we use a muffler and not a true expansion chamber. The most highly tuned 2-strokes will have an individual exhaust pipe and expansion chamber for each cylinder, and the resulting increase in horsepower output will be dramatic, but only in a narrow band. This is not good for a wide power band like we need on aircraft engines. Also, the noise would be unacceptable for use in local flying.
The exhaust manifold on our 2-strokes almost always joins into a Y on the 2 cylinder models, and this creates a further problem. The pulse will travel down the pipe, but when it returns it will try to go back up both pipes in the Y. At certain RPM ranges it will be causing more harm than good relative to conservation of the fuel charge.
Rotax spent a lot of time and trouble in designing the exhaust system for their engines. While it's not a pure tuned exhaust such as used in racing, it is nevertheless tuned, and the shape and distance from the exhaust port to where the elbow enters the muffler chamber is very important.
The shape of the exhaust pipe is not simply that of a constant diameter pipe, such as on a 4-stroke engine. The pipe on our tuned 2-strokes is gradually expanding in diameter from the engine to the point where it enters the muffler. It is shaped like an elongated cone when viewed from the side. That shape is an important part of the exhaust tuning.
The length of the exhaust pipe, prior to the muffler, is another important factor due to the speed of the pressure waves mentioned earlier. The length must be such that the exhaust pulse can travel to the end of the cone and be reflected back up the pipe to arrive at the exhaust port just before it closes.
Higher RPM will require a shorter pipe and lower RPM a longer pipe to produce the maximum effect at the RPM we want to use for maximum horsepower. It's not practical to vary the pipe length, so we compromise with some specific length that will deliver the best push at near top RPM.
The Max continuous RPM for the Rotax 447 and 503 is 6,500. (The 582 is rated for only 6,000 Max continuous.) I don't remember the exact distance from the exhaust port to the end of the cone on the 447 and 503, but I think it was set by Rotax at about 29 inches. Technically, that is a centerline distance and not just the measurement of the exterior surface of the curves on the pipe.
The shape and distance is critical and can't be ignored or changed without changing the Rotax power band numbers published on their graphs. So when people change these exhaust systems on their own, they are unknowingly changing the design power curves. Unless they know exactly what they are doing and why, they will likely end up with less power at 6,500 RPM, or unpleasant operating characteristics that are no longer suited to what they are trying to use it for.
As you know, common homeowner 2-stroke engines don't use any exhaust tuning. Chain saws, weed whackers, and the like, usually just have a little muffler box bolted to the side of them, which is not exhaust tuning. This works just fine but misses out on the additional "push" we could get from the engine by tuning the exhaust.
The physics of a 2-stroke engine is very complex compared to that of a 4-stroke when we are trying to optimize horsepower, reliability, and reasonable width of the power band. The 500 cc 2-stroke racing motorcycles that used to be popular in the 500GP races would produce peak power output of around 180 HP, but in a very narrow RPM range. In order to get that same amount of horsepower from a 4-stroke racing engine, it has to be about 1,000 cc. However, the 4-stroke does have a much flatter torque and horsepower curve and is better than the 2-stroke at pulling hard from lower RPM's.
As someone said, it all gets pretty complex with a lot of compromises. There are times when a two-stroke engine acts rather strange, and it's all related to both the intake and exhaust tuning with the major influence coming from the semi-tuned exhaust system.
Author: Bud Connolly