"Bullitt Ride" (yesterdayknight)
08/02/2013 at 15:32 • Filed to: None
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"Bullitt Ride" (yesterdayknight)
08/02/2013 at 15:32 • Filed to: None | 1
| 6 |
I've been admiring the workmanship on this header for about 10 minutes now. I found the step up in tube size to be an interesting feature. It was definitely employed as method for tuning the exhaust pulses to either prevent reversion or aid in scavenging somehow. Discuss....
Joe_Limon
> Bullitt Ride
08/02/2013 at 15:45 |
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It definitely was intentional. I agree it probably has something to do with resonance. The first couple inches, is that a off round profile? Dat MERGE
BoxerFanatic, troublesome iconoclast.
> Bullitt Ride
08/02/2013 at 15:45 |
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Increasing the diameter of a flow tube increases it's flow rate, and reduces pressure.
A transition from a given pressurized flow area (pipe cross section) to a higher flow, less restrictive area, acts to slightly accelerate flow, if there isn't a further restriction down-stream.
That is the idea behind a megaphone exhaust, especially, with a gradual transition from a given exhaust diameter to open atmosphere. But that tends to simultaneously increase exhaust sound volume, like a horn speaker.
If there is a flow restriction down stream, an increasing-diameter exhaust system does alleviate some pressure effects, and allows for a less abrupt deceleration of flow and pressure spike (a pressure spike that is reflected and traveling backwards/upstream is reversion), as the larger tube diameter works like a flow reservoir, rather than reversion all the way back to the exhaust valve, and reduce resistance to the exhaust stroke of the piston.
That one I6 header is gorgeously crafted, and looks like titanium. It is probably insanely expensive.
dropthatclutch
> BoxerFanatic, troublesome iconoclast.
08/02/2013 at 16:04 |
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I thought increasing the diameter actually slowed the exhaust gases.
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Joe_Limon
> BoxerFanatic, troublesome iconoclast.
08/02/2013 at 16:17 |
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I thought the flow velocity decreases with increased cross sectional area, thus resulting in lower frictional losses. Also, I throught the volumetric flow would be roughly the same, varying only with the change in flow due to reduced back pressure.
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BoxerFanatic, troublesome iconoclast.
> dropthatclutch
08/02/2013 at 16:23 |
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I suppose it does decelerate on an absolute term, if you are considering just the exhaust tubing, and not it's dynamic effect on the engine. So, I suppose an increasing diameter, unrestricted exhaust technically decelerates exhaust gasses to a lesser degree than a constant-diameter system.
But if there is no obstruction or back pressure, it provides less resistance to the exhaust pressure coming out of the engine, and the engine can be a more efficient pump by not having as much resistance to it's output (assuming that the exhaust flow is the system's bottleneck, and there isn't an internal, or intake-side restriction). Theoretically, it would allow the engine to rev faster, and the engine would be what increases exhaust velocity at a faster rate than a more restrictive exhaust, with less exhaust stroke resistance.
Any bends or obstacles in the exhaust system increase resistance to the exhaust flow, and very few exhausts are straight-line, even if they are relatively un-obstructed.
But any time you put a chambered muffler or resonator, or a catalytic converter in that flow, it acts as an obstacle, and some of the energy of the exhaust is reflected back up-stream, as back-pressure.
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Bullitt Ride
> BoxerFanatic, troublesome iconoclast.
08/02/2013 at 17:07 |
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Yes, if you are just comparing pipe A to pipe B, increasing the diameter will net you a higher flow rate at a lower pressure. However when you look at pipe flow, when you have an expansion where the transition angle is less than 90 degrees (a.k.a a diffuser) you actually get a decrease in velocity and an increase in pressure.
Exhaust tuning is pretty tricky stuff. When the exhaust valve opens it sends a compression wave downstream. When that compression wave hits the expansion it causes a reflected wave of negative pressure to be sent back towards the valve, and that negative pressure wave helps with the exhaust scavenging.
I’m definitely no expert though.