Here are a few questions so you can learn this a bit better. If you answer all of these correctly, you will
totally understand the difference between:
- Tuned length vs. Backpressure
- Restriction vs. Flow
- Velocity vs. Pressure
Quiz A
Using the diagram below, answer the 4 questions.
The RED line is POSITIVE PRESSURE (Initial Wave) and the
BLUE line is NEGATIVE PRESSURE (Reflected wave). Also note that this is very simple, it will be explained below.
The exhaust port is at the left side of the pipe.
#1. What is the pressure of the red line at the
exhaust port for Graph A?
#2. What is the pressure of the blue line at the
exhaust port for Graph B?
#3. What is the resulting pressure of both waves combined at the exhaust port for Graph A?
#4. What is the resulting pressure of both waves combined at the exhaust port for Graph B?
1. 3 psi
2. 0 psi
3. 5 psi
4. 3 psi
With some simple math above we can see that the
blue line is creating 2 PSI of negative pressure.
- 2 PSI (Graph A)
The Red line in Graph A is producing +3 PSI of exhaust pressure on the inital pulse. That means there is a difference of 5 PSI between both waves. The reflected pulse is leaving a negative zone at the port and literally sucking the exhaust out.
Since the initial wave is +3 PSI, it's going to
come out with higher velocity because of the 5 PSI
differential.
Graph B shows a similar scenario, but the reflected
wave comes back and hits the port at 0 PSI.
The intial exhaust pulse is 3 PSI.
Now there is a difference of 3 PSI.
Since the RED line is still higher pressure than
the reflected wave, the exhaust will move out
of the chamber, but not as quickly (velocity) and
therefore scavenging will be less.
You can now see that there is NO BACKPRESSURE,
but the power output will be less in Graph B.
We can also see that both pipes have no restriction,
and therefore cannot create backpressure on their own.
It is the tuned length that is causing a drop, or
increase in power.
LAST Diagram to PROVE there is no BACKPRESSURE in
a pipe:
In the above picture:
Pipe A is 2.5" diameter x 20 inches long
Pipe B is 2.5" diameter x 10 inches long
Which statement is true
1. Pipe A will flow more exhaust because it is longer.
2. Pipe B will flow more exhaust because it is shorter
3. Both pipes will flow the same exhaust because
their diamter is the same.
The correct answer is 3.
Therefore, you can conclude there is no restriction
in open pipes of varying lengths.
I don't want to hear any more bull about backpressure bringing the world to an end, or it causing cancer. The term you are looking for is "tuned resonance"
when dealing with pressure pulses.
This is why the power band changes when the LENGTH
of pipe is changed, or a cut-out is opened.
"Why is it bad to put a 4" exhaust on my V6?"
The larger exhaust pipe would alter the velocity of the exhaust pulses, which would move them out of tune for a given exhaust length. Theoretically it would be possible to design an efficient 4" exhaust system for a V6, but it would only be efficient at a rediculous RPM due to the smaller amounts of exhaust being expelled and lower exhaust velocity.
Clues:
- The atmosphere has mass and can exert pressure.
- Volume of a pipe increases with diameter and length increase.
So what's the relation between Pipe DIAMETER and Engine Displacement?
I didn't want to get too detailed, but there are 1/8, 1/4, 1/2, 3/4 wavelengths that will enhance the exhaust system.
The trouble is, a secondary, or third harmonic is not as
powerful as the initial reflection, nor is the phase
of a sub-length wave.
Excessive pipe diameters drop low RPM efficiency.
Tests show a loss in lower end power with an increase of pipe diameter. It's not always about the peak numbers, it's more to do with the average of the curve.
3" single is about the absolute minimum pipe
diameter for roughly 350 cubes peaking between
5500-6000 RPM.
4" single will do well with 400 cubes peaking
at 5500-6000 RPM.
2.25"-2.5" dual is about max for a 350 cube motor
peaking around 5500 RPM.
3" Dual is about maximum for a 350-400 cube motor
peaking between 6500-7000 RPM
With a larger pipe, there is more atmospheric pressure to overcome.
The pipe itself with the ENGINE OFF rests at atmospheric pressure. So...the more volume/capacity
the pipe has, the more work the exhaust pulses
have to perform to get out of the system.
Don't forget, the gasses are pulsing and moving
back and forth. Change in direction takes time,
and requires energy (inertia).
When you're dealing with exhaust gas which is less
dense than the air we breath, the exhaust has to
work very hard to get out of the pipe.
Getting the proper length and diameter is a big
deal when it comes to intake and exhaust tuning.
There are other variables which are going to come
into play.
"But what about my catalytic converter...or my
muffler?"
Which is more restrictive:
1. 3 inch inlet/3 inch outlet catalytic that flows 750 CFM @ 28 in./H20
2. 4 inch lnlet/4 inch outlet catalytic that
flows 600 CFM @ 28 in./H20
3. 2.5 inch pipe that flows 750 CFM @ 28 in./H20
The examples are exaggerated to show a point which
I believe is very important:
Restriction of a muffler, or catalytic can be overcome
in a smaller system and be more efficient than
a bigger pipe...as long as the flow is adequate.
Some tuners will go to a larger pipe while using
a somewhat "average" flowing catalytic/muffler.
They see a gain and attribute it to 'larger is better'
only because the surface area of the catalyic honey
comb has increased by 1" diameter allowing a touch
more flow than the 'average flowing' 3" unit.
Again, but investing in a high quality, high flowing
race muffler, or cat., the pipe size could have
remained the same with better results.
"As long as the cat/muffler flows the same cfm as the pipe, it would present no barrier, would it not?"
Not exactly. According to three sources:
David Vizard
John Morrison
National Dragster
A pulse hitting an object will be reflected.
In other words, the honeycomb center acts as the
reflective surface whether it flows well, or not.
This is the same principle as the pulse exiting the
pipe and hitting the atmosphere.
One last little blurb on crossovers.
As for H-pipe tuning and placement, I haven't been
able to find any conclusive write-ups to show
the correct tuning.
Here's a diagram of an H-pipe and two equal waves
per bank.
The pressure waves would normally be offset because
no two cylinder fire at the same time, but it will
help highlight the action of the H-pipe:
Well there's a quick overview of exhaust systems. If anyone has any questions I'll try to help you out the best I can. PM me or email me, or post them here.
I have edited alot of it for clarity, grammar and spelling as well as expanded on a few things that are more thirdgen specific and useful for the people in this forum. 
