Excuse my ignorance, but I have an exhaust question. What is the rule about exhaust back-pressure on a naturally aspirated car? In forced induction it's pretty simple... the more you have coming in... the more you need to let out. I've been told that it can be detrimental to have to much exhaust flow on a non turbo/supercharged engine. Is this true? Any links to detailed information would be appreciated...

 

Under optimum conditions, you should have at or very near .0 Back Pressure at idle, and certainly no more than 1.5 at 2K RPM. If you are concerned, then try testing your BP to make sure. If you are in the 3.5-4.0 range, then you may have a partial blocked up exhaust system. Here's an article on how to use the tester. You want to get as much exhaust pressure out of the system as you can. That is why we use "X" pipes. http://www.troublecodes.net/articles/jds/

 

Soaring,
Good reading and education...

 

I know that I need to get bigger pipes on my car.

 

I have the 2 1/2" pipes with the 'X" pipe, and mine seems to breath easy. The factory used the 2 1/4 and that was good if you didn't add any mods. However, if you add any mods where more air is coming in than factory specs , then the larger pipes might be necessary. The best addition though is the "X" pipe. If nothing else, the "H" pipe. But it is not as efficient . The 289, 302 and 351's come factory set for BP tolerances, but if you add mods such as changing the 2v to a 4v or a blower or whatever mods you put on it to create more cold air coming in, then the exhaust has to be modified to expell that exhaust in order to keep the BP at it's lowest. Just adding mods to the top of the engine will not give you maximum HP. You have to pay attention to the exhaust ratio.
On edit: You might want to read this article to acquint yourself about how the "X" pipe works. Enjoy.......Article Here

 

I don't know if back pressure is a good way to look at it. I think what you need, and it may result in back pressure, is some velocity in the exhaust pipes so it will have a scavaging effect on the cylinder with the exhaust valve currently open. Does that sound right?

 

You can call it what you like, but if more exhaust volume is produced by the engine than the exhaust system can handle, you will get a loss of HP and torque. That is why we used to run open headers on the track. Maximum flow.......

 

I've read all the posts and here is what I've understood. Too much back pressure is obviously bad (which I knew). But from what I'm hearing is that there really isn't too little flow. If this is the case (and like Soaring said) why not just run open headers (which wont work on the street) or just run a straight pipe? wouldn't a straight pipe flow better as long as the dimensions did not create back pressure. I would think back pressure would be determined by the size of the pipe and/or the route the pipe takes... am i wrong in thinking this?