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Old 04-18-2024, 11:17 PM   #31 (permalink)
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Quote:
Originally Posted by aerohead View Post
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 View Post
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.
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 View Post
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.
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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Old 04-19-2024, 03:32 AM   #32 (permalink)
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Ten years ago (...on a cold dark night?) I posted this (lower left):



It has a 4-2-1 header that terminates in a reflective shell that expands the tailpipe to the entire wake area (ignore the Beetle fenders ) Looked at one way, it's a Coanda nozzle that energizes the wake; the other, the wake is sucking on the tail pipe.

Put that 1200CFM of hot, humid exhaust gases to work.
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Old 04-26-2024, 10:42 AM   #33 (permalink)
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Quote:
Originally Posted by freebeard View Post
Ten years ago (...on a cold dark night?) I posted this (lower left):



It has a 4-2-1 header that terminates in a reflective shell that expands the tailpipe to the entire wake area (ignore the Beetle fenders ) Looked at one way, it's a Coanda nozzle that energizes the wake; the other, the wake is sucking on the tail pipe.

Put that 1200CFM of hot, humid exhaust gases to work.
We're heading off in a new direction here Freebeard but one I am also interested in.

Yes I believe that exhausting in the area of lowest pressure, behind a car at speed, is a no-brainer. (except for laws around this IIRC)
(The 1st Lotus Europas did this, more or less, but sealing the hot vibrating exhaust to a relatively still fiberglass body was impracticable)

Optimizing the shape to better use that vacuum as you have; I like the idea.

Same goes for the engines air intake:
That should be a properly designed bell mouth at the highest pressure area of the car's nose.
Ingested water can be made to settle in the bottom of airbox where flow speed drops dramatically due to the vastly increased 'frontal area' there and then bled off with a small float valve.



Another idea:
For the life of me I cant find the article but basically it was research on a wing where air is sucked into the wing at the trailing edge of the low pressure side and air is blown out at around the point it normally breaks away from the same top surface. (at around the thickest point.

NB that the sucking and blowing was not perpendicular to the surface but more of a slot/step, so the sucking and blowing was parallel to the surface.

The study on this wing showed hard to believe results at insane angles of attack.

IF it works as stated and if one did the same thing in a car by using the engine's exhaust and intake, you would end up with adhered flow on a much shorter tail.

There would also be a fair amount of EGR but unlike std EGR systems; you just cant beat the efficiency of this type of 'heat exchanger..!'

(Perhaps the link is on my other computer. I will look and post again if so)
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Old 04-26-2024, 12:44 PM   #34 (permalink)
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en.wikipedia.org/wiki/Blackburn_Buccaneer#Boundary_layer_control
Quote:
Boundary layer control

A depiction of the Buccaneer, the blowing slots visible on the leading edges, flaps, and horizontal stabiliser are highlighted; these aerodynamic features contribute to the Coandă airflow over the wing

In order to dramatically improve aerodynamic performance at slow speeds, such as during takeoff and landing, Blackburn adopted a new aerodynamic control technology, known as boundary layer control (BLC). BLC bled high-pressure air directly from the engines, which was "blown" against various parts of the aircraft's wing surfaces and horizontal stabiliser. A full-span slit along the part of the wing's trailing edge was found to give almost 50% more lift than any contemporary scheme.[46]

In order to counteract the severe pitch movements that would otherwise be generated by use of BLC, a self-trimming system was interconnected with the BLC system, and additional blowing of the wing's leading edge was also introduced. The use of BLC allowed the use of slats to be entirely discarded in the design.[46]
Also highlighted is the split-tail air brake.
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Old 04-28-2024, 10:54 AM   #35 (permalink)
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Quote:
Originally Posted by freebeard View Post
en.wikipedia.org/wiki/Blackburn_Buccaneer#Boundary_layer_control

Also highlighted is the split-tail air brake.
Ye; like that but ALSO with sucking at the trailing edge of the upper wing surface..

ie: That blown air is directed/pointed towards a facing suction nozzle/slot of some sort, located very close and parallel to the trailing top edge of the wing.

I really must find that write-up.


Last edited by Logic; 04-28-2024 at 11:00 AM..
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