MyE28.com

[QUOTE="Shifty"]The problems that arise from reducing the backpressure in the exhaust system only existed in carbureted cars. Nowadays, the only negativity that you get from reducing backpressure is increased airflow which, in turn, leans out the mixture. This can, of course, be solved by enriching the mixture which, of course, increases power (more air/fuel flow at the right mixture = more power).[/QUOTE]

[QUOTE="dotrp"]So is that the current concensus? I've been passively involved in some spirited debates about backpressure, and in the end I tend to agree with Shifty's viewpoint: less is better. But how, if at all, is that related to the issue of pipe size? It's tempting to think 'less backpressure is better' equates to 'bigger piping is better.' But doesn't exhaust size affect the shape of the power band? Is there a body of wisdom associated with exhaust sizing for the m30 and/or m20?[/QUOTE]

Shifty is not entirely correct, in that neither "leaning" nor "enriching" the mixture detracts from or improves power until you know whether or not the mixture is correct in the first place. Mixtures that are too rich will decrease power just as lean mixtures will, albeit being safer for the engine in terms of combustion temperatures.

As for exhaust size, for a normally-aspirated engine, the exhaust should be sized for best scavenging for the RPM range you're interested in. This is a combination of exhaust pulse timing and momentum along with backpressure. For a turbo engine, zero backpressure is the goal, as there's really no scavenging per se in a turbo engine (turbo guys, I know you'll correct me on this if I'm incorrect). There's lots of exhaust theory out there -- have a look on the internet.

I did some figurin' on E28 exhaust sizes and have a spreadsheet for it:
"Shawn D.'s Exhaust Calculator" Note that this is very simplistic -- just a general examination to get you in the ballpark.

[Edit by Shawn D. on [TIME]1109260507[/TIME]]

[QUOTE="Shawn D."]Shifty is not entirely correct, in that neither "leaning" nor "enriching" the mixture detracts from or improves power until you know whether or not the mixture is correct in the first place. Mixtures that are too rich will decrease power just as lean mixtures will, albeit being safer for the engine in terms of combustion temperatures.[/QUOTE]

Shawn, my comment on leaning and enriching the mixture was assuming the engine is set up properly in the first place. Of course if you're running stoichiometric, you're already too lean for max power, or if you're running the right mixture for max power and you lean out a little you'll be closer to stoich but too lean for max power.

[QUOTE="Shawn D."]Shifty is not entirely correct, in that neither "leaning" nor "enriching" the mixture detracts from or improves power until you know whether or not the mixture is correct in the first place. Mixtures that are too rich will decrease power just as lean mixtures will, albeit being safer for the engine in terms of combustion temperatures.[/QUOTE]

[QUOTE="Shifty"]Shawn, my comment on leaning and enriching the mixture was assuming the engine is set up properly in the first place. Of course if you're running stoichiometric, you're already too lean for max power, or if you're running the right mixture for max power and you lean out a little you'll be closer to stoich but too lean for max power.[/QUOTE]

Very good, my friend -- you is on your way to becomin' a true guru!

[QUOTE="Shifty"]The problems that arise from reducing the backpressure in the exhaust system only existed in carbureted cars. Nowadays, the only negativity that you get from reducing backpressure is increased airflow which, in turn, leans out the mixture. This can, of course, be solved by enriching the mixture which, of course, increases power (more air/fuel flow at the right mixture = more power).[/QUOTE]

[QUOTE="dotrp"]So is that the current concensus? I've been passively involved in some spirited debates about backpressure, and in the end I tend to agree with Shifty's viewpoint: less is better. But how, if at all, is that related to the issue of pipe size? It's tempting to think 'less backpressure is better' equates to 'bigger piping is better.' But doesn't exhaust size affect the shape of the power band? Is there a body of wisdom associated with exhaust sizing for the m30 and/or m20?[/QUOTE]

Try How to Build Horsepower by David Vizard. This guy has written about exhausts for years and is among the most knowledgeable on the subject.

If you want to spend too much, Metric Mechanic makes an exhaust they call system 300. The PO of my car dropped the $1,000 for this setup. :~ That's SS system money.

http://www.metricmechanic.com/pg104.htm

I think the muffler is actually off the 524td. It flows well, and is pretty quiet at all RPM's. For the money I would have just gone to my favorite local exhaust shop and had a custom system built. Probably would have been about $600 with an equivalent cat and a nice Magnflow.

Just came across this recently:

Hot Rod exhaust system test

In their tests, on a "mild" 454, a dual 3" system picked up around 13 hp and 15 ft/lb tq across the entire range over a dual 2.5" system.

Also, people have recorded gains of 5-7 whp, with no low-end losses, going from 2.5" to 3" exhaust on a 240SX (2.4L 16v 4-banger) with intake and header.