Exhaust Systems, Back Pressure and Torque/HP
 

[EDIT]

There seems to be three ways to think about this.
1 - Headers are where all the magic is. They must be part of the engine system and design for desired results. After the collector, the less restrictive the exhaust the better.

2 - The entire exhaust system from headers/manifolds all the way to exhuast tips must be considered for maximum performance/desired results. Simply increasing exhaust size is not necessarily going to give you any benefit and may even cost you some performance.

3 - All things being equal, the less restrictive the exhaust with larger headers and exhaust pipes, etc., is an inherently good thing and should basically be maximized. Of course there are diminishing returns, but you would basically never loose power by going to a larger and less restrictive exhaust system, from the headers all the way back.

At first I tended to think along the lines of 2. But now that I think about it more, 1 seems more likely. Links to papers and dyno results, anecdotal stories etc. welcome.

From my readings in the past, articles written my much smarter people than I, have said that all exhaust tuning is done with the primary tubes in the header and the collector length. Any restriction past the collector is costing horsepower and torque. This tells me that only the headers are responsible for any scavenging effect the exhaust system contributes to power gains.

Maybe I should make three options phrased a little differently.

1 - Headers/Manifolds must be matched to the engine for intended purpose, but after the collector the less restriction the better, always.
2 - The whole exhaust line from headers/manifolds to exhaust tip matters
3 - The less restriction from the exhaust ports on the better. In other words straight pipes out of the exhaust would be best.

I believe this to be true. However, on a forced induction engine, Zoomies baby.:sm_laughing:

This may be the case because when I did this to my Chevelle I changed the header size (larger primary tube as well as collector size and length) and exhaust size at the same time. I know a lot of drag racers test with different length extentions after their collector and I've read that around 18 inches is usually best but I personally have never tested it.

1 is correct. 2 starts out ok, but finishes poorly. A less restrictive exhaust will always increase hp (assuming there is increase to be had) and providing that the engine is properly tuned/ capable of those gains. But not regardless of the situation. A low end motor with little to no time spent past 4K rpm, would not do well with a 280 cam, big runners and zoomies.

"scavenging effect" is not back pressure. :p

I read an article is Muscle Mustangs and Fast Fords a couple of months back and it dicsussed this very topic. They said that the headers have to be thought about as part of the whole engine build-up. The length and size of everything designed with the characteristics of what would work best with the engine. Can not remember if the article was more about headers than the whole exhaust though. I will try and dig up the article.

This Article is aimed at Honda/Acura but seems to have some good basic theory-
http://www.team-integra.net/sections...sp?ArticleID=2

Ben,
Richard Waitas, the head engineer at Magnaflow, still corresponds about car days. I'll set you guys up when I hear about the next "open show." That guy knows more about exhaust then anyone I know.