Re: Custom Exhaust
There are several concepts involved in this issue.
Energy- something with energy has the ability to do work
Force- equal to the mass of an object times it's acceleration.
Work- equal to Force multiplied by distance traveled.
Power- work divied by time taken. (same job twice as fast means twice the power required)
Force, work and power are "vectors"- they have magnitude and direction. The importance of this will become apparent shortly.
Pressure is equal to force divided by area on which that force is being applied.
When the piston travels upwards, it compresses the air, thus producing a higher pressure. The fuel is ignited, causing a massive pressure spike. This forces the piston downwards. Now on the way up, the net force is on the top of the piston, but it is moving against the force (here's where direction is important). This results in a negative work- ie work is REQUIRED to perform this operation- ie negative direction force (downwards) x positive direction movement (upwards) = negative work.
On the way back down again, the force is negative and the direction of movement is negative, so the result is positive work (negative force x negative movement direction)
The negative work for the compression comes from the flywheel. It stores energy in the form of velocity. (Energy cannot be created or destroyed, only transformed from one form to another, eg chemical (petrol) to heat to kinetic (velocity))
Now when you introduce "back pressure" into the equation this means that you are deliberately increasing the resistance to flow of gas from the cylinder, which increases cylinder pressure during exhaust stroke. If we let 1MPa be the normal pressure and 2MPa be the high pressure, use 0.1m as the distance and 0.008m^2 as the piston area, then it works out as follows:
(-1x10^6) x 0.008 x 0.1 = -80Nm (-80 Joules) (LOW BP)
(-2x10^6) x 0.008 x 0.1 = -160Nm (-160 Joules) (HIGH BP)
Remembering negatives because pressure is acting downwards onto piston.
Now if that all happens in a period of 1/10th of a second then
-80/0.1 = -800Watts (-0.8Kw) (LOW BP) (or -1.07hp approx)
-160/0.1 = -1600Watts (-1.6Kw) (HIGH BP) (or -2.15hp approx)
...and so in that the back pressure story has been totally disproved. That analysis applies to all piston engines regardless of revs, fuel, whatever else you care to name.
The idea behind scavenging is this- clearing the cylinder as much as possible on the exhaust stroke will allow more intake air to be admitted, thus increasing power.
This is accomplished through the momentum of the exhaust gases. Momentum = mass x velocity
This momentum means that when the piston reaches TDC, the molecules are travelling forwards. This causes a lower pressure "behind" the air charge being expelled. That low pressure means that there's less mass of air remaining in the cylinder at the close of the exhaust valve- more air in on intake stroke. So we want to increase the momentum of the air charge.
As can be seen in the equation there's to ways to do this- increase mass which isn't possible because that is determined by cylinder volume etc. or increase velocity. A velocity increase can be accomplished by reducing the diameter of the exhaust valves or the ports or the HEADER TUBES.
Now according to Bernoulli's equation, as velocity increases, pressure must drop. This is how a carby works- narrowing in the inlet passage (ventury) causes a low pressure area. A small pipe is attached at this point. This pipe runs to the carby bowl and due to pressure difference the fuel is "sucked" up.
So the whole point of this bit above is that gas velocity is your friend and it can be improved by using a smaller rather than larger diameter header tube.
This bit runs into problems into higher revs though. When the gas velocity reaches the speed of sound in the gas (dependant on gas composition, temperature and a constant) then the mass flow rate has achieved it's peak value. The only possible way to increase the flow rate is to increase the diameter of the pipe. While supersonic flows are possible, they won't occur in an exhaust system and the mass flow rate doesn't increse anyway. I haven't gone into detail because it would require half a textbook worth of writing to effectively demonstrate the concepts.
Now this problem generally doesn't occur at low rpm's, but it does cause a problem higher up. If you have a totally inadequate exhaust system, then perhaps it may occur lower in the rpm range, but this is very rare. After this point, torque output would begin to drop away. Gently at first but progressively getting steeper. At the point where the torque curve is dropping at the same rate angular velocity (RPM in different units) is increaseing, then maximum power is reached. This is because power is directly related to torque and rpm. After this point, power drops away.
Obviously it is desireable to have this point at a high rpm so that there is a high power output. If you have a restriction anywhere in your exhaust, then this will lower the rpm at which it begins and hence lower power output.
The problem (and the confusion) lies in this point- An exhaust with large primaries and large bore will give great peak power but will destroy bottom end power.
Small primaries and small exhaust will give better low end power but destroy top end performace/peak power.
At this point it's time to realise that the percieved restrictionof smaller diameter tubes is NOT the factor that contributes to good low rpm performace. Ultimately, the restriction of the tubes plays little part until gas velocity reaches it's maximum. The primary effect is gas velocity and the result is more effective scavenging.
Any musical note you hear is due to resonance. Doens't matter what instrument- resonance is what it is. Resonance is what happens when an object (including air) is made to vibrate at a frequency which boosts the wave height rather than interfere with it/destroy it. In a tube filled with air (like an exhaust....) this mainly has to do with the pipe length and gas temp. The aim is to produce a resonance so that the pressure at the exhaust valve remains as low as possible every time it opens. The same applies to the inlet, but with a high pressure to allow more air in.
That's fine if you have an engine that only runs at one speed. With a range of speeds, a range of different lengths would be required. That's the reasoning behind the 6 cyl. Ford Falcons (Australian) "snail shell" inlet manifold- two length runners allows for a wider spread of rpms where this concept works. So basically, you want an exhaust runner length that gives you a power boost at a suitable rpm range. I say range because most headers are an interference design and such have a wider range of rpms where they are most effective. Tuned length headers also work over a range, but there will be a noticeable peak in torque on the rev range at the resonance point/s.
4. Bringing it all together.
So we want minimal back pressure system, headers with small diameter for scavenging and lengths to suitably boost power output in a desired location.
We also need large diameter primaries/exhaust for high rpm performance, which conflicts with the other needs.
Generally what is done is the maximum gas flow of the engine is considered and the header tube sizes are selected from that, which is larger than ideal for low end power. Then the tubes are made extra long to give the low end a boost. The tubes are either all the same length (tuned design) or close to it (interference design) so as to prevent gas from two cylinders arriving at the collector at the same time, which causes backpressure.
If top end output is the sole consideration, then shorter headers can be used to move the resonant frequency further up the rev range at the sacrifice of bottom end power.
So what to look for when considering headers-
*Are they equal length or close to it? If not then they're not an efficient design. I've noticed that some shorty headers are a LONG way from being equal length. While undoubtedly better than most crappy factory maifolds, there are even better options for minimal if any price increase.
*Are they large or small diameter primaries? The largest on the market will likely hurt low end performance whilst the smallest are likely to give minimal gains up high in the rev range.
*How long are the tubes? Longer = better low rpm power.
* What sort of mufflers will be in the exhaust? Straight through mufflers are exceptionally quiet but give very good performance as they are very free flowing.
*What diameter is the exhaust? remember- you can go too big.
...and the most important one of all:
*What is the main purpose of your car?
1987 Jaguar Sovereign - Metallic Green
3.6L DOHC 24 Valve