Quote:
Originally Posted by aerohead
I'm going to lunch, but I'm about half way through David's video.
* An inconsistency in his logic is that, an open Mini Cooper header 'HAS' backpressure. It's not 'zero.' David simply has not accounted for it. 
* The exhaust gas encounters backpressure at:
1) tulip valves
2) initial mandrel 90-degree bend
3) internal friction within the length of each four primary runners
4) interference where 180-degree out-of-phase primaries siamese
5) where siamese twins combine into collector
6) length of the collector
7) interface with the atmosphere |
I think David is trying to simplify things for less technical people:
If you are looking at the back pressure specifically added by the exhaust; then considering any back pressure resulting from things before it (in the head) is counter productive to a simple explanation.
(He does give very good instructions on gas-flowing heads is other of his literature)
Quote:
Originally Posted by aerohead
I
8) the 'pressure wave termination boxes', while a 'void' are not at atmospheric pressure, as with the open header.
9) both 'pressure wave collection boxes' DO have bell-mouthed entries, which WOULD kill vena-contracta losses when entering the exit pipe to the rest of the exhaust system.
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No they are not, but for simplicity again...
Bell Mouths:
Yep!
Never underestimate the ability of a properly designed Bell Mouth to improve flow by a surprising amount!
My research on the subject points to an Ellipse with a 3 to 1 ratio, with the minor diameter at 20% of pipe diameter as being damn close to optimal while being easy to make.
No doubt you will make me dig up the relevant research..
Quote:
Originally Posted by aerohead
10) In 1996, Sport Truck Magazine commented that straight-through glass-pack mufflers actually hurt performance do to internal turbulence caused by tube perforations and spirals, producing more back pressure than the muffler they replace.
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Here I agree. However I do recall reading somewhere that someon was racing with a PWTB and glass packs.
Other teams (who saw the exhaust) were so pissed off at the fact that such a cheap muffler was not causing the expected flow drop that they got a "OEM exhaust only" law passed.
It's somewhere on the few forums where Davids PWTB is discussed.
In a nutshell:
Tuning the lengths and diameters of the pipes in a 4 (into 2) into 1 system for a specific rev range will save fuel.
The last, into 1, pipe length (and diameter) is also important.
The PWTB will do a good job of fooling the exhaust branch's last pipe into 'thinking' its exhausting into open air.
(I also think/guess that:
Thx to the compressibility of gas, that the large volume of the PWTB turns the pulses of each exhaust stroke/pulse into a smoother flow.
One more akin to the flow one gets from a fan or tank of compressed air.
This decrease in peak flow requirement, or averaging of the flow to a value below the pulses of peak flow = less flow restriction by the silencing part of the exhaust)
Anti Reversion Doodats help when the exhaust system you have is tuned to help extract gasses at a higher rev range than the rpms you prefer to use for economy.
They do so by somewhat disturbing/reducing the reverse flow that would cause reverse flow into the cylinder and intake tract.
ie: Reducing the high pressure downstream of an opening exhaust valve, in a system tuned for a higher rev range.
As proof:
A variation of them is used in/by F1 to give better low rpm tractability to their engines and the general consensus on forums is that they work well.
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