So In searching the exhaust section these last few days, Iv noticed a few things, and Im curious on some other peoples input on the topic of 'Back pressure' and 'Velocity' when it comes to exhaust (I know technically I should be saying speed, but lets not be picky here, the direction is in general 'out'). Im reading how more and more people are concerned with having enough 'back pressure' but from my understanding, and Ill explain more, back pressure is absolutely bad, isn't it? Few years ago in my SR year of high-school, I recall getting our Physics teacher (Who was a gear head) and our shop teacher together and asked about exhaust, this is what Iv learned, and basically always believed, so Ill explain, and would love to get some input.
Back-pressure is always bad, an ideal exhaust system has 0 back pressure, and the most velocity possible, so the delicate balance is the fastest velocity, while maintaining the lowest back pressure. Think of Exhaust as a brick on a table. Pushing it uses (X) amount of force, this is the basic amount of force needed to push the exhaust out of the motor, so X=(F)orce needed. If you add piping, and create back pressure, your essentially having someone hold the brick while you try to push it (Y Is the amount of pressure being applied to hold the brick, or Y=Back-pressure), thus the equation is now X+Y=F as the motor is basically having to work harder to not only move the vehicle, but also working to push the exhaust out of the motor, and now through the pipe
Making sense so far? So then in all Back-pressure is bad, why not add huge pipes. Okay, I added big pipes, but now I actually LOST power, why? Velocity (Speed, shut up)!
Velocity is the rate at which exhaust "flows" through the piping. Think of a 2 water hoses, one is a 1/2 inch hose, the other is a 1/4 inch hose. Both hoses are hooked up to a 10 gal. per minuet water source, so both hoses are pumping out the exact same amount of water, (Now lets compare this quickly to your motor, at 2500 RPM your motor will put out the same amount of exhaust, no matter the size piping) However the water has to flow faster in the smaller pipe, to match the output of the larger pipe, correct? so the smaller pipe has a higher Velocity.
Back-pressure is the enemy to Velocity, as back-pressure is basically slowing the velocity down (The more bends you have, the higher back-pressure you have=slower velocity vs a straight flowing faster system)
So then why is velocity good? If the gasses are getting out, and there's no back-pressure, what role does velocity play? When the exhaust gasses get flowing quick enough, they actually help SUCK the exhaust out of the motor by creating a negative pressure behind it (Think of the air bubble you get in the front of the bed, while traveling down the highway. Now throw an empty soda can towards it,it will get sucked in, and most likly the can will move around, but it stays in that bubble, why? Because the bubble is acting like a vacuum, and has a lower pressure then the air around it.) In the exhaust system, that bubble is right outside the exhaust valve. Exhaust gets sucked out of the cylinder, and into that bubble. when the valve closes the bubble goes away and the exhaust gasses that were caught in it now become fast moving fumes like those around it. Going back to that brick, velocity is as if someone tied a string around that brick, and is actually pulling it the same direction your pushing it. The amount there pulling it can be (P) for example, so now our equation is X-P= F
Lets just throw numbers in here so some more can understand. Lets say it takes 10 units of power to simply push the exhaust gasses out of the motor (This is similar to running open headers), so:
And lets say there is 1 unit of back pressure, so:
And to make it easy, lets say there is also 1 unit of velocity (Or 1 unit of suction, helping pull the exhaust out of the motor) so:
With backpressure the equation is:
So 11 units of power to push the exhaust out, whereas the equation for velocity is:
(10)-(1)=9; so it only takes 9 units of power to push the exhaust out.
Obviously you would rather only waste 9 units of power vs 11 when pushing exhaust out.
So, to recap; Back-pressure bad, velocity good. Now the problem is, your exhaust cant be changed while your running down the drag strip (Well it can, and some fancy cars actually do modify the exhaust on the fly, but we wont get into it) So it can only be "setup" for one ideal RPM. usually, this is aimed right in the middle, so that's why when you take a stock exhaust and rip out the muffler to create a nice free flowing setup, you'll possibly see a loss in low end, and a gain in the high end, because low end you'll have less velocity, and high end you'll have more, while decreasing the back-pressure that used to be there. Also this is why some say "This muffler didn't hurt my performance like that one did" Chances are the "better" muffler just lined up your Back-pressure vs Velocity better then the "Worse" one.
Ideally you'd have smaller pipes, or less possible flow at lower RPM's, and bigger pipes or more possible flow in the higher RPM. My suggestion is on performance cars, find a way to set a trigger depending on RPM. My old MSD box had one (and I sure others sell them), some racers use them to trigger NOS at a specified RPM range, but if you hook that trigger up to a small exhaust cut out, for example, you could in fact alter your Back pressure and Velocity, automatically, on the fly.
And don't say that's too much to read, lol. I'm by no stretch of the imagination claiming to be an expert on the topic, but I do feel I am a higher educated individual, and fairly smart, also good when it comes to critical thinking, so I'm hoping there's other like minded individuals on here (Or possibly an expert).
Also to the Nazi's, sorry for any and all missed grammar mistakes, its 2:30 am here, and I've been up for about a day in a half, so suck it.