Quote:
Originally Posted by tpepmeie
Right on, FR. In fact, a 2.5" diameter choke would not be unexpected for my 700 hp engine. Recommended, in fact. There is a relationship between primary pipe size, and length to the choke size, too.
todd
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The discussion is missing the point, methinks... We're not talking about a 2.5" path vs. a 3" path and choke points or tapers reached over a significant transition... No! Dom's issue is a transition length near zero, which is quite different from a "megaphone" type transition (like Ryan pictured).
I don't know how laminar flow is affected by pulsing, but that aside for a minute, maintaining laminar flow thru a transition from one diameter tube to another depends on the RATE of transition between pipe diameters.
Gas has mass, and therefore a given volume of flow from say a large to a smaller tube requires the gas to
accelerate. And, we all know the (equal but opposite) force the mass has against the force attempting to propel it increases with the rate of acceleration, right?
So, as this applies to the volume of a gas pulse traveling down a pipe, if the pipe diameter is decreased, then the gas will have to accelerate. But, it is that
rate of acceleration that determines how much force will be required to accelerate the mass. If the transition is relatively gentle (like the choke transition Ryan showed us), so will be the acceleration rate and the resistance will less. But, on the other hand, if the transition is instantaneous, the transition
time factor goes to zero.
Power = work/
time and so it would take an
infinite amount of power to make that happen...and therefore it won't. So, what happens is the resistance to the sudden change is in the form of the pulse energy being reflected back; colliding with other gas pulses and the result is laminar flow is replaced by turbulence. Like rocks in a stream of fast moving water, you get disruption of the flow - rapids!
So, I believe the same is true for gasses in (Dom's) exhaust meeting an abrupt obstacle. As Todd pointed out,
Quote:
Originally Posted by tpepmeie
There is a relationship between primary pipe size, and length to the choke size"
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We're not talking about laminar flow through a tapered transition, but "square lips" (as Ryan once referred to such transitions). And, what would reflected energy mean to flow should the reflection arrive at the exhaust valve at the moment it opens??
This is the place where I start getting a headache - starting to revive the math to determine null and nodes and at what frequencies... So, for my 2 cents, my intuition has me totally thinking Dom was right in removing that ridiculously abrupt constriction, especially as close to the exhaust valves as it is. But, w/o doing the numbers, I wouldn't want to say for sure just what practical effect that (abrupt) choke point would cause. Yep...Could be a lot about nuttin, I suppose. (But I eliminated that same POS choke point in my system for the same reason Dom saw fit to do so...Great minds n all that chit!...For what it's worth.
P.