Actual volume of exhaust gas

[theholycow]

Ok, the pulses thing helps, but wouldn't the pulses be a whole lot smaller (from less air/fuel) at lower throttle/RPM and slower (from slower piston speed) at lower RPM?

[dkjones96]

Another thing to point out is a header only has a real benefit if the pipes are of equal length. This evens out exhaust pulses and improves scavenging.

krcperformance is a pretty cool site. I like the 408 stroker kit! 4" bore + 4" stroke = 451hp and 505ftlbs of fun!

http://www.gassavers.org/showthread.php?t=5668 Back pressure being a positive thing or not has been tested here.

**Spoiler Alert**
It isn't positive.

[bobc455]

Originally Posted by theholycow

Ok, the pulses thing helps, but wouldn't the pulses be a whole lot smaller (from less air/fuel) at lower throttle/RPM and slower (from slower piston speed) at lower RPM?

Of course, but they still have enough velocity (for part of the front pipe) that you have to think of them with a pretty high velocity where these little things can still have an effect.

But any part of the exhaust system that makes flow easier will help the engine breathe easier.

-BC

[bobc455]

Originally Posted by dkjones96

Another thing to point out is a header only has a real benefit if the pipes are of equal length. This evens out exhaust pulses and improves scavenging.

Actually since headers typically have broader bend radii and larger volume before the collector, the pulses will have a easier time flowing than through the typical cast exhaust manifold. Header tuning does not do nearly anywhere near as much as people think, it's about as effective as spark plug indexing (a tiny bit of benefit only). There's a lot of untrue hype with the "tuned" headers.

-BC

[thisisntjared]

Originally Posted by bobc455

Actually since headers typically have broader bend radii and larger volume before the collector, the pulses will have a easier time flowing than through the typical cast exhaust manifold. Header tuning does not do nearly anywhere near as much as people think, it's about as effective as spark plug indexing (a tiny bit of benefit only). There's a lot of untrue hype with the "tuned" headers.

-BC

i dont know about that man.... some motors respond very well to aftermarket headers and others do not at all.

most often a header configuration that makes the best power does NOT give the best mpg. case in point: sequential pairing of runners vs opposing pairing. also important is runner (and exhaust for that matter) diameter. larger is NOT better for mpg unless it is down stream from a turbo

[bobc455]

Originally Posted by thisisntjared

i dont know about that man.... some motors respond very well to aftermarket headers and others do not at all.

The highly tuned headers (vis-a-vis nascar etc.) are tuned for a specific RPM range, and if you can keep the RPMs in a fairly narrow range then tuning the length (and equal length) can have more significant gains than on a street car that has a relatively wide range of RPMs (high - to - low). In other words, if you tune your headers for 7800 RPM +/- 10%, the "tuning" will have a lot more benefit than a car tuned for 2000 RPM +/- 50% (and nobody "tunes" headers for 2000 RPM anyhow). In the typical RPM range of a hypermiler, we're just looking to reduce flow restriction so that the engine has an easier time pushing the exhaust gas through a 15-foot long path.

There are some aspects of header design that are the same for hypermiling and high-RPM HP, but some are the opposite.

-BC

[dkjones96]

Some stock manifolds are better than others. The 4.7L found in later Durango/Dakota/Jeep models is said to gain nothing from headers because they engineered the stock exhaust manifold so well it performs the same. Chrysler engineers openly admitted that.

However, the 318/360 like I've got have disgusting log style manifolds that are far from efficient.

[thisisntjared]

Originally Posted by bobc455

The highly tuned headers (vis-a-vis nascar etc.) are tuned for a specific RPM range, and if you can keep the RPMs in a fairly narrow range then tuning the length (and equal length) can have more significant gains than on a street car that has a relatively wide range of RPMs (high - to - low). In other words, if you tune your headers for 7800 RPM +/- 10%, the "tuning" will have a lot more benefit than a car tuned for 2000 RPM +/- 50% (and nobody "tunes" headers for 2000 RPM anyhow). In the typical RPM range of a hypermiler, we're just looking to reduce flow restriction so that the engine has an easier time pushing the exhaust gas through a 15-foot long path.

There are some aspects of header design that are the same for hypermiling and high-RPM HP, but some are the opposite.

-BC

im getting the feeling that you are familiar with only certain engine types with narrowed aftermarket availability. i dont think you can speak in generics when making the suggestions you do because the dont apply to some of the 4 cylinder cars i have owned.

when putting an equal length opposed piston pair header with very mildly larger diameter primaries, secondaries and collector on my old civic i got 2-3mpg highway keeping the stock exhaust(pea shooter). when i put larger diameter exhaust on i lost said fuel economy gains. the same was true with my most recent civic, but i kept the stock exhaust to avoid attention and held onto the little more mpg. but larger is not better for fuel economy.

[bobc455]

Jared-

Sorry if I'm coming across as some kinda knowitall, I'm sorry about that. I work with fluid dynamics every day, but there is always stuff to learn (I think everyone on this board would agree!)...

Of course not only are there exceptions to every generality, there are also consequences of going "too big". There's cam design to consider, as well as a bunch of other stuff. For example if a huge exhaust system cools the exhaust temp so that the O2 sensor doesn't read the same, throwing off the ECM, then it doesn't help afterall. And as we've all heard, have too big of a header can hurt low-end torque in a racecar, and the generality (again, only a generality) is that low-end torque happens in engines with better gas mileage.

-BC

[thisisntjared]

i did not think you came across as a know it all. i just think you are off in what you imply about larger diameter being better. you are VERY knowledgeable in the area that is familiar to you and i respect that.

that said i think we are coming from opposite ends of the spectrum as far as our automotive experiences as a lot of what you have learned to be true comes from larger displacement v8s where i am coming from the 4 cylinder crowd. i think you are also correct in that the cars you know have a much different philosophy in cam design than those i am. i bet its honda that fights generalities as they are the most low-end-torqueless cars i have ever driven but they did maintain good fuel economy.

in short: i believe i didnt consider your automotive background when saying that larger is not not better because there are cars out there with a factory pipe that is way too small and larger would be better.(i cant site examples, but after looking at some cars from the 70s and 80s it becomes clear.