Good Idea???????

[travsstuff]

I don't have my car yet, but I am trying to decide what I am going to do with it whne I get it. I am thinking about just hacking the exaust system off and running a 6in. pipe out the back. Also putting high performance plugs and a homemade tam air system. Just poor mans mods. Are there any other poor mans mods that I could do?

[-Jayson-]

a 6inch pipe? thats gotta be one hell of an engine. . .

[travsstuff]

no I just want it to sound big=)

[-Jayson-]

well u will lose a ton of back pressure and your car will be slow as shit

[psychobadboy]

yea, not to mention it's also illegal

[Reed]

you could gut the interior by removing the rear seat and any fabric and whatever you find behind the fabric. you could also gut the doors. the stuff in doors weighs a lot. my friend just made a turbo setup for his civic for about $400 by using left over stuff from some of my other friends, you could think about that.

[Auto_newb]

Originally Posted by -Jayson-

well u will lose a ton of back pressure and your car will be slow as shit

Backpressure doesn't make an engine run... It's the combustion inside the cylinders that eventually put power to the wheels. Backpressure is something that you don't want.

[-Jayson-]

ok i know how an engine runs dont insult me, and doesnt back pressure produce scavaging?

[Sluttypatton]

Your right, you don't want backpressure*, but it does have something to do with exhaust velocity, which is something that you do want. You can't just bolt on a huge pipe so you don't have any backpressure, as that would kill exhaust velocity and exhaust gasses would hang around the exhaust ports a lot longer. Low velocity exhaust doesn't scavenge very well (or at all) and leaves you open to reversion, which will dilute your intake charge and can very easily ignite it prematurely as the exhaust is very hot. This diluted intake charge, in combination with the lost scavenging effect, is what makes a motor loose power in the absence of any backpressure.

*Technically you don't want any backpressure, but I am using the term loosely (read incorrectly) to describe both actual backpressure and delta pressure. However I didn't feel like flinging around the term delta pressure to describe it more accurately, so just remember, when you bolt on huge exhaust you loose velocity.

Also, dude, 6 inch diameter piping is huge.

[Auto_newb]

Are you saying that without any backpressure, the exhaust will go back into the engine? Don't the valves stop that?

[Sluttypatton]

Valves would stop it if the motor had zero valve overlap, however all modern 4-stroke motors have some degree of valve overlap. Valve overlap means that the intake valve will be open for a fraction of a second during the end of the exhaust stroke in order to further facilitate scavenging. Normally what would happen is that the high velocity gas creates an area of negative pressure behind it as it leaves the cylinder, then the intake valve opens allowing fresh fuel and air to be drawn in by the negative pressure and help flush out the remaining exhaust gasses. Then the exhaust valve closes and the intake stroke begins. However, if the exhaust gas has little velocity it will hang around the exhaust ports and will not create the negative pressure area behind it in the cylinder that high velocity exhaust does. This means that it doesn't scavenge, and during the intake stroke there is no negative pressure in the combustion chamber to suck in fresh fuel and air through the intake valve.

[Reed]

as an extremely general rule of thumb for naturally aspirated engines, a bigger exhaust will actually make you loose a little torque at low rpm but gain it back at high rpm. some backpressure actually helps to make low end torque by doing the things already posted. this is why if you put a turbo on a car and then put it on the dyno you sometimes see increased torque at low rpm before the turbo even starts to come up from vacume, because of the restriction of the turbine blades.

I'm sure someone will say I'm wrong but this is what I have seen in my limited experience.

[59elcooldsuv]

Phhffft! Ha LOL

Sluttypatton wins the prize for the best attempt to answer a complicated question.

Reed wins the prize for Not Getting It.

1) backpressure is NOT the issue - VELOCITY is what you get from smaller pipes. VELOCITY contributes to scavenging. Backpressure impedes it.

2) a 6" dia straight pipe will drag the pavement. Even professionally built headers will often drag the pavement.

[Sluttypatton]

I did my best to answer it as simply as I could, but I should not have used backpressure in place of delta p. I thought it would make it easier to understand, but they are two completely different things. For the sake of clairity, delta p is the pressure drop between two points, backpressure is the resistance to flow in the exhaust tract. So you can see that delta p isn't pressure at all, it is the difference of pressure between two points.

[59elcooldsuv]

I have been working in Engineering, or more precisely 'doing the dirty work' for Mechanical & Piping Eng's (draftsman & designer), for over 25 years and I STILL have trouble conceptualizing & visualizing a 'chunk' of hot, high pressure gas flowing down an exhaust tube and dragging a low pressure wave behind it.

Watching how traffic 'flows' (I use the term loosely) on the freeway (another term used loosely) helps me to grasp it. No kidding, a perfect model of what a fluid system would look like if you could actually "see" pressure.

It gets even more complicated when you try to visualize how that high pressure wave bursting into a collector creates a low pressure wave that 'sucks' on the other tubes of the header set or manifold.

Whoa! Boggled mind! And to think, I'm actually considering welding up my own set of headers for an engine & body combination for which there are none commercially available.

Thermodynamics & fluid flow is a fascinating subject, but not easy to grasp without spending a great deal of time carefully studying all the related subjects.

I can't hold it against anyone for not "getting it" - only suggest that they make some effort to try to think about it before jumping to a contrary conclusion.