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Old 10-03-2022, 04:15 PM   #11 (permalink)
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into the 'open' air

Is that a good thing?
I've never experienced a vehicle which performed without the back-pressure of the rest of the exhaust system.
The entire system was 'tuned' as a unit.
If 'reversion' happened to occur within the system, it would manifest itself as a 'bluing' along some point in the exhaust pipe. Simply cutting the pipe at that point of discoloration constituted 'tuning', and elimination of any 'reversion.'
One might notice that General Motors' highest-performance Corvette engine closes off half of it's exhaust system during cylinder-deactivation in order to 'protect' back-pressure for the half of the cylinders still firing!
Is this 'Unicorn Corral' material?

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Old 10-03-2022, 04:38 PM   #12 (permalink)
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Engines don't need exhaust backpressure. What they actually need is flow velocity, for exhaust scavenging. Closing off part of the exhaust during cylinder deactivation should help maintain flow velocity.
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Old 10-03-2022, 05:19 PM   #13 (permalink)
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Quote:
Originally Posted by Blacktree View Post
Engines don't need exhaust backpressure. What they actually need is flow velocity, for exhaust scavenging. Closing off part of the exhaust during cylinder deactivation should help maintain flow velocity.
* Okay, the interior volume of the exhaust system is a constant.
* The engine rpm varies from say, 600 at idle, to 6,000 at redline.
* A variability of 5,400 rpm.
* 1,000%.
* Are we concerned with acoustic velocity?
* Velocity of blowdown?
* Or the velocity of remnant gases pushed out by the pistons on the exhaust stroke?
* One size fits all?
* 'Perfect' at all rpms?
* Are the other pressure pulses from the other cylinders setting up a 'train' within the system, which aid in the 'scavenging'?
* If we kill these pulses, have we shot ourselves in the foot?
* Under transient loads, 'conventional' exhaust systems appear to have performed okay for over a hundred years now.
* What is Vizard actually bringing to the table, and does it really move the dial?
* Does the 'velocity' of the gases leaving the exhaust port change at any given load and rpm?
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Old 10-03-2022, 05:53 PM   #14 (permalink)
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Quote:
* 'Perfect' at all rpms?
I'm picturing a trombone slide.
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Old 10-04-2022, 06:17 AM   #15 (permalink)
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Quote:
Originally Posted by serialk11r View Post
I don't think exhaust reversion is a huge issue other than at low load + low speed (I read a bunch of papers about exhaust reversion trying to figure out how to optimize my VVT).

At low load, you're pretty screwed because the intake pressure is far far below atmospheric.
Yep. But low load and rpms is what most here are doing.
And doing so without a Branch specifically made for low rpms...
So think the anti reversion doodats will help some.

If you really want to stop ALL reversion of exhaust into the intake tract:
Put Reed Valves (Carbon/Glass Fiber), on the intakes, as close to the intake valve/s as possible:

This allows the use of an extremely 'hot' intake cam, but gives you infinitly variable intake cam timing... Exhaust cam too, to some (overlap) degree...

IIRC
Honda uses/used them in their 500cc single cylinder 'Thumpers'.
Alfa Romeo experimented on car engines:
Lost ~1kw in top end power, for VASTLY improved low rpm drivability.

Last edited by Logic; 10-04-2022 at 06:40 AM..
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Old 10-04-2022, 06:25 AM   #16 (permalink)
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Quote:
Originally Posted by freebeard View Post
I'm picturing a trombone slide.
Been done on 2-Strokes:


Seals are an issue, but mini 'piston rings' not tried.
No reason it couldnt work, but weight and complexity on a multi pistoned engine would be a challenge.
And the weight may outweigh the benifits..?
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Old 10-04-2022, 06:38 AM   #17 (permalink)
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Quote:
Originally Posted by aerohead View Post
Is that a good thing?
I've never experienced a vehicle which performed without the back-pressure of the rest of the exhaust system.
The entire system was 'tuned' as a unit.
If 'reversion' happened to occur within the system, it would manifest itself as a 'bluing' along some point in the exhaust pipe. Simply cutting the pipe at that point of discoloration constituted 'tuning', and elimination of any 'reversion.'
One might notice that General Motors' highest-performance Corvette engine closes off half of it's exhaust system during cylinder-deactivation in order to 'protect' back-pressure for the half of the cylinders still firing!
Is this 'Unicorn Corral' material?
For the PWTB: NO it is not.
Plz feel free to discuss that with David Vizard himself.
(You have heard of him haven't you..?)
He's active on YouTube and various forums IIRC.
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Old 10-04-2022, 07:59 AM   #18 (permalink)
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Quote:
Originally Posted by aerohead View Post
Is that a good thing?
I've never experienced a vehicle which performed without the back-pressure of the rest of the exhaust system.
The entire system was 'tuned' as a unit.
If 'reversion' happened to occur within the system, it would manifest itself as a 'bluing' along some point in the exhaust pipe. Simply cutting the pipe at that point of discoloration constituted 'tuning', and elimination of any 'reversion.'
One might notice that General Motors' highest-performance Corvette engine closes off half of it's exhaust system during cylinder-deactivation in order to 'protect' back-pressure for the half of the cylinders still firing!
Is this 'Unicorn Corral' material?
After further reading I would say that Anti-Reversion is also NOT Unicorn Corral material:
There is just too much about it on the net.
I would suggest some research, assuming your opinion of me (and others) warrents it.

Here is some talk on a step in F1 exhausts, specifically to reduce reversion:
https://www.f1technical.net/forum/viewtopic.php?t=4115

So; if you have a Branch for higher rpms (the std really) then some low cost, relative to making your own long, thin tube Branch, would be AR doodats.

They could be improved with a rounded back and 'sharpened' pipe end to more effectively change reversion flow direction back to 'out' and to 'blow in the face' of any gasses still trying to get up your intake tract.
ie: More the shape of the ideal PWTB, but way smaller, assuming you looked at the link.

Last edited by Logic; 10-05-2022 at 05:27 AM..
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Old 10-04-2022, 12:39 PM   #19 (permalink)
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Quote:
They could be improved with a rounded back and 'sharpened' pipe end to more effectively change reversion flow direction back to 'out' and to 'blow in the face' of any gasses still trying to get up your intake tract.
This approaches the Tesla one-way valve. I was [conceptually] trying to get there with a ring of holes.
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Old 10-05-2022, 05:40 AM   #20 (permalink)
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Quote:
Originally Posted by freebeard View Post
This approaches the Tesla one-way valve. I was [conceptually] trying to get there with a ring of holes.
I would say that a bunch of small PWTBs, with half torus front ends and sharpened from the outside in 'pipes' and at slight angles to each other is the closest to a 3D Tesla Valve one could get Freebeard.

A difficult shape/s to fabricate, but one cant help wondering what effect such would have on exhaust flow and economy at low rpms.

The stock Tesla Valve for others reading this and wondering whatTH we're on about:

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