Exhaust/ Back pressure question
08-17-2009, 10:17 AM
#12
5th Gear Member
Join Date: Jan 2006
Location: Puerto Rico
Posts: 3,926
I was trying to speak more generally, rather than specifically to any particular engine combination. Though the primary tuning component in the exhaust is the header(primaries and collector), when attached to pipes after that, it becomes necessary to evacuate gasses from those pipes quickly enough to prevent them from backing up and having an effect on tuning at the collector. Larger diameter pipes may help or may hurt, it depends on the rest of the system and the engine itself(like you said, it's not divorced from the engine). If the pipes are so large that velocity drops off too radically, it can cause gasses to back up. Yes, increasing velocity comes from increasing restriction(in terms of pipe diameter), but like I said, the trick is to balance the best velocity with the best overall flow. Too small and you strangle the exhaust, too big and you can lose some efficiency in the overall system. There's 2 components at work, reduced restriction from larger pipes, and reduced restriction from higher velocity.
.... the misconception that too big of an exhaust pipe will cause gases to become stagnant within the confines of the system and cause them to be "pushed" creating a scavenging problem. It doesn't work that way, otherwise a backpressure increase reading would show up when checked, and it doesn't.If you look at the dynamics of the enclosed gases, the added cooling effect a big pipe has on the exhaust stream alone, helps scavenging + reduces the possibility of having turbulence generated..... think about what happens to a stream of gas molecules when they are cooled..... inertia, momentum, contraction.
The pulse tuning is primarily affected by header design(primary diameter, length, collector size, length, how they blend etc). Gas velocity won't really effect pulse tuning other than determining the original strength of the pulse when it's generated at a diameter change in the system, mainly when the exhaust gas initially exists the cylinder and creates a pressure wave spike.
But like you said, it all comes down to how the whole system works together. That's why the F1 and Cup guys spend so much time on the dyno just to build an exhaust system...searching for the ideal diameter, ideal length, the most power. For us, unless we have a set of headers custom built, we're stuck with an off the shelf header design, that can only put power in the ballpark of where we want it.
.... but you don't see them focusing on velocity........
And you're also correct, us "regular Joe's" are limited to OTS headers offerings, another reason why past the flange of the header, is where the exhaust tuning opportunities are...... BTSTDT with "stingers" or tuning extensions..... and they always require pipe ID to be increased to ensure proper pulse scavenging.
08-17-2009, 01:11 PM
#14
6th Gear Member
Join Date: Apr 2007
Location: California
Posts: 10,468
http://www.epi-eng.com/piston_engine...technology.htm
Some interesting reading. Based on the current developments in Cup, F1 and Moto-GP racing.
Some interesting reading. Based on the current developments in Cup, F1 and Moto-GP racing.
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