matthewk

There are two major scientific concepts that make exhaust systems work: inertial scavenging and wave scavenging. When the two concepts are controlled and merged together correctly it means horsepower. Inertial scavenging begins when the exhaust valve opens. Then hot exhaust gases escape past the exhaust valve and exit down the exhaust pipe toward the atmosphere at a few hundred feet per second. With individual tube headers, the exhaust of each cylinder is kept separate from the others until they all meet at the collector and feed into the secondary pipe. It has been found that the best power is produced at an exhaust gas speed of 300 feet per second. At this rate, it takes roughly l/100 second for these gases to travel the length of a 36-inch-long primary tube. When the exhaust valve seats, it closes off the engine side of the primary tube. The exhaust gases continue to move down the tube at 300 feet per second. However, this leaves behind an area of low pressure that expands as the exhaust flows away. If the low pressure spills over into another primary tube for a cylinder where the exhaust valve is just starting to open, the result is beneficial. The combined low pressure will suck the exhaust gases right out of the cylinder during the exhaust stroke, leaving less remaining behind to dilute the incoming charge of fresh fuel and air. The combined low-pressure area will promote the draw-through effect the exhaust has on the next shot of fresh air/fuel mixture. This will tend to pull more of it in and increase how full the cylinder gets before the intake valve closes. The lowered-pressure area due to inertia improves both the exhaust-scavenging effect and the draw-through effect, creating better cylinder fill that results in higher volumetric efficiency. It all works because of inertia, the actual physical movement of the exhaust and intake gases. This effect that headers give a race engine is known as inertial scavenging. Inertial scavenging works across the entire engine power. Tuning the headers to a specific rpm band is done to complement other engine characteristics, such as the camshaft profile and the demands of a specific track or style of racing. Varying the length of the primary tube changes the time that the vacuum pulse reaches the collector. At high rpm, header primary tubes are frequently shortened up because there is less time between each cylinder firing - the lowered pressure area has less time to get down to the collector and mix with the other cylinder gases. One other important aspect of exhaust tuning for gas speed is tube diameter. With all other factors equal, the higher the exhaust gas speed, the better the scavenging effect. The exhaust gas speed can be controlled up to a point by changing the diameter of the primary tube. The smaller the diameter, the faster the exhaust will flow. So the primary tube diameter also has an affect on inertial scavenging. By varying the length vs. diameter, the primary tubes can be tuned to provide the greatest degree of inertial scavenging in the rpm range needed most; say, coming off the corners. If the exhaust tubes are too large in diameter they will become ineffective and cause lazy gas flow, meaning that a larger primary tube diameter creates less velocity and possible loss of horsepower--the engine could fall on its face. Wave scavenging is different from inertial tuning because it does not involve the physical transport of exhaust gases up and down the header tubes. When the exhaust valve first opens, a sound energy wave is created that races out of the exhaust system to the open atmosphere at the speed of sound. At the pressures and temperatures present in exhaust, the speed is about 1700 feet per second. When the energy wave hits the atmosphere, a large part of its energy is reflected back up the exhaust system. This has an affect on what's going on inside the cylinders while each exhaust valve is open. Another method of controlling gas now and velocity is using tube steps. By introducing small variations in tube diameter, called steps, exhaust gas speed and wave speed can be slightly increased and decreased as necessary up and down the tube of the exhaust. Placed at strategic locations in the primary tubes, these steps help tune the arrival time of the waves and diminish the effects of high pressure in the headers. One major component that has been overlooked, and which is a link to horsepower, is the exhaust collector. A X-pipe or H-pipe minimizes the reflected wave, so it has less affect on the inertia pulses. A X-pipe or H-pipe will make the exhaust pressures, the temperatures, and the pulse velocity transitions smoother. X-pipes or H-pipes serve as the control center of the exhaust system, they are designed to help control or harmonize the exhaust pulses from each individual cylinder and the primary exhaust tubes.

The hourglass shape of the X-pipe tends to regulate the high/low pressures and pull or speed up the velocity of the gas flow. The shape also helps equalize the transition of the individual exhaust tube gases. But collecting the exhaust tubes from the header is not the only function of the merge collector; it also helps equalize the transition or merge of the exhaust tubes.

The most critical design of X-pipe or H-pipe are their length and the angularity of the tubes coming into the collector. The internal pyramid, which is made up of individual, angled collector tubes, also plays an important part in controlling the gases so they don't re-enter and travel back into the low-pressure area of one of the other exhaust tubes. Even the neck of the merge collector plays a critical part; the smaller its diameter, the more it will accelerate the velocity of the exhaust gas flow as it is carried into and out the secondary tube.
By varying the angularity and length of the X-pipe tubes, along with the diameter and length of the neck portion, the powerband range can be increased throughout the rpm range. One point must be considered when using exhaust and a X-pipe or H-pipe, the exhaust temperatures might increase because you are pulling more mixture out of the cylinder and burning it in the exhaust tube; leaning out the air/fuel mixture.


Bolt on horsepower, ya gotta love it!

L.S.

what specific changes must I make to make the car perform fastest 1)at the drag strip 2) daily driving ie. under 5000 rpm. Just kidding....now if only someone had an exact formula maximize performance for what Matthew said above.

mark@velosaudi

i believe my system gives the best balance of low end torque and top end hp

John

What are the max torque gains and HP gains with your system - Interested? (Chipped & unchipped)

DK

Great write up. Totally makes sense for naturally aspirated engines.
I was wondering how the turbo affects scavenging?

Thanks
DK

Corey S

Now that was a mouth full. Interesting read.

RAudiS4

And that the science of exhaust systems is more complicated than just getting one with the fattest pipes.

It would behoove one to pay attention to those tuners who combine research on the effects of their respective SW and hardware mods.

Obviously Audi tuned the 2.7TT to it's exhaust. We are modifying those variables significantly when we chip and just throwing on a bigger pipe MAY NOT be the most effective solution. We need to find a tuner(s) that has "done their homework" in this area.

-mike

ErikR

The turbo is a natural anti-reversion design, it does not allow gas to flow in reverse from the exhaust, but it will produce back pressure at the cylinder and some reversion from cyl. to cyl (yet another reason why you don't want valve overlap in a turbo system). This is the heart of the mythology of turbo exhausts.

The turbo produces its own backpressure. It needs no additional backpressure. (This is not true in some extreme racing designs with very large turbos.) The exhaust impeller and wastegate stops any resonance wave coming from the exhaust (that itself is arguably minor at best due to the F/R cat. con. faces and the resonator dampning).

Audi doesn't design its exhausts to maximise flow. One look will tell you that they face packaging issues with underside clearances, resonances, overall exhaust noise, and tubing costs.

RM

This is Normally Aspirated stuff-it does not apply with turbos. Best exhaust is no exhaust.

donp

Thanks for this writeup... I learned something today. : )