I was just reading up on exhaust cutouts and saw that it works best with cars that require no back pressure. What exactly does back pressure do?:stupid:

 

Backpressure takes away the engines ability to breathe!!
The best exhaust to make the most power possible would be one just like you see on the Top Fuel dragsters,but this is a totally useless exhaust for driving the car at anything less than wide open.To make an engine useable at all speeds from idle to WOT,you have to compromise.To begin,all things that have mass also have inertia,you remember,all things at rest tend to remain at rest,and all things in motion will remain at motion until acted upon by an outside force.
The things we are concerned with are gas molecules.In a header pipe with the exhaust valve closed they are at rest.When the exhaust valve opens they want to stay at rest.The piston will eventually persuade them to move however and then they will want to keep moving.If the engine was to operate at just one speed the exhaust could be tuned to have the preceding slug of exhaust gas just starting out the end of the head pipe as the valve starts to open.This would cause a partial vacuum in the head pipe and would help the piston by drawing the new exhaust charge into the head pipe.
This isn't possible with an engine that runs everywhere from 600 RPM at idle to 6000 RPM at redline.
With our one speed engine if you speed the engine up the exhaust doesn't quite create the vacuum in the exhaust that it did at a lower speed,and at a slower speed the vacuum has already been there for awhile and is now pulling air from the outside back into the exhaust.This is called reversion.It's not necessarily all bad but it will wreak havoc if not controlled.In a properly tuned engine you can use it to pull the fuel air mix out of the cylinder and jam it back in just before the exhaust valve closes.Reversion waves can also be created by obstructions in an exhaust.Anything that can cause part of the exhaust slug to be reflected back toward the valve will cause a reversion wave.A Harley with drag pipes can show this easily.You might have heard a Harley with drag pipes hiccup or back fire when passing throughthe 3000 to 3500 RPM range.(It's really noticeable when you're riding it)This is a reversion wave that upsets the carburation of the mix as it tries to get into the combustion chamber.Harleys have a pretty good valve overlap and when the reversion wave gets back to the cylinder both valves are still open and part of the incoming fuel air mix is actually pushed out into the intake.This is in an engine with two cylinders and two exhaust pipes,think of how hard it would be to design one that has eight pipes all connected together!!!
A well designed exhaust that makes power across the wide power band we have to have to be able to drive on public streets is a pretty tall order.So we get a compromise.To control reversion there must be some damping in the exhaust.You want the exhaust to go as far as it can before it starts bouncing back toward the valve or allowing outside air to start filling the vacuum.All this causes backpressure,there's just no way around it.It's the last thing you want in your car but you just have to live with it.A good set of long tubes go a long way to help even out the pulses and help with tuning out any bad vibrations.
A set of cutouts with a linkage that would leave them closed until you get to about 5000RPM would be a nice addition to a street car.

 

<applause>

 

wow. great knowledge teach!

 

I hope he copied and pasted that from somewhere...thats alot of typing for a little question like that.

 

Bravo :clap: :clap:

 

Ok, can you now explain it as if I knew very little about cars...:surrender

 

well.. my experience with backpressure is from when I installed LT's, O/R Xpipe, and a new cat-back. I lost some backpressure because my exhaust was a lot more free flowing. Basically I lost low end power because of this, it was too free flowing for lower rpms. But it helped open it up in the higher RPMs so it could breathe better and i gained hp cuz of that. Who needs lots of hp in low rpms anyways

 

Alright, something I can understand. Thanks

 

Where do babies come from?!?!?

 

The stork duh?:shifty:

 

for people who speak only english and not ed-joo-muh-cated english, exhaust moves like music, in pulses. Your exhaust pipes are the equivalent of a pipe organ.

You don't randomly pick which piano key to strike cuz the melody will sound like azz. Similarly with ever changing frequency of exhuast pulses it's very difficult to tune the pulse timing with the length of exhaust tubes and still meet noise law requirements, have catalytic converters or direct it any way but straight up and out. If you had a 4 pack of 3' tubes poking out the side of your engine bay you'd make more hp but you'd catch people on fire every time you tag the gas.

Ironically, you tend to lose some tq by running individual straight and short pipes like a dragster does. That's because if you connect each cylinder bank's pipes together somewhere near the head you get a vacuum or scavenging effect as pulses interact which helps to clear the chamber of whatever cylinder has an exhaust valve open while the previous slug of exhaust is moving further down the tube. That scavenging helps to generate as big and clean a blast inside the cylinder as possible since there's less fouled air in the chamber to begin with. It also helps to pull in the intake charge thanks to the slight amount of valve overlap that is ever present in the IC engine design.

Between the reversion pulse and the scavenging effect there are 2 opposite forces at work during the exhaust cycle. They're both representable as sin waves and will occur predictably depending on the engine configuration, exhaust piping from cylinder on, piston speed, RPM, firing order, etc... The challenge for engineers is to create an exhaust that sounds ok, scavenges well, flows well, meets noise requirements, doesn't shoot fire into traffic, and contains some sort of pollution control system.

Hopefully that confuses Othic a little more.

 

It probably would if i read it.

 

And that is why it takes a while for a company to make a good exhaust for a car, trial and error will take a long while, unless you can create a computer program so precise it calculates in real world figures, it is much easier to do trial and error basing the exhaust off of a companies past knowledge of the subject...
some exhausts are random as ****.
the 350Z and F-body cars arent true dual *lol*
although you can see, they may not need it, one pipe would prolly increase the backpressure, which they prolly need...