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Burnt Valves?

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delta

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Why take the chance that a valve may experience any problem due to temperature fluctuation.
Get some balls that fit, link them together, and use 'em like your life depended on them. Besides, the big guys do it. they look cool, and they'll keep the dust out if nothing else. This is the opinion of one guy that experienced a chipped exhaust valve and has absolutely nothing to do with short stacks or flying...
 

bmcj

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Why take the chance that a valve may experience any problem due to temperature fluctuation.
Get some balls that fit, link them together, and use 'em like your life depended on them.
Yep, you can certainly plug the stack to reduce rapid cooling IF you can get them in place quickly enough, but you may want to include a desiccant on the end of that plug because burnt gas exhaust has a significant water vapor content. You don’t want to trap that in there where it can promote corrosion without taking measures to absorb it with a desiccant.
 

BBerson

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My tractor has a steel cap that flaps shut after the engine stops. Eight of em might look silly.... but it works.
 

TFF

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Yet your people made your first exhaust with it in mind?
Back pressure is a tuning aid. An advanced EFI can essentially force an engine into characteristics that are not natural. Carbureted, some back pressure can help drivability on an engine that is tuned a little more than it should be. A carbed race engine on the street is a bear. If it has to be driven on the street, it becomes a compromise that needs to be taken.
Race teams, manufacturers, suppliers are constantly running scenarios of resonance of intakes and exhausts to find where they can use essentially back pressure to their advantage. It’s all a game with thousands of solutions that are all correct.
 

TXFlyGuy

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Yet your people made your first exhaust with it in mind?
Back pressure is a tuning aid. An advanced EFI can essentially force an engine into characteristics that are not natural. Carbureted, some back pressure can help drivability on an engine that is tuned a little more than it should be. A carbed race engine on the street is a bear. If it has to be driven on the street, it becomes a compromise that needs to be taken.
Race teams, manufacturers, suppliers are constantly running scenarios of resonance of intakes and exhausts to find where they can use essentially back pressure to their advantage. It’s all a game with thousands of solutions that are all correct.
They are not “my people”. My wife and I drive Ford vehicles. That does not make FoMoCo my people. The person who designed the exhaust was simply mistaken in the concept, falling prey to the myth.

Right now, I’m in the camp that believes back pressure in an exhaust system is not good. In fact, I spent big bucks on an extractor exhaust for my 69 VW to eliminate back pressure.

Where is the handbook on designing back pressure in exhaust systems to be found? Certainly there has to be volumes published on the subject.
 

TXFlyGuy

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Interesting reading, thanks for posting.

Quote - "Thus, an ideal exhaust system should produce a highly negative pressure at the exhaust valve at both EVO and again as soon as possible after dissipating the P wave. This negative pressure should be made to persist throughout the overlap stroke so that favorable scavenging can occur."

With Bernoulli's Theory, will there not be an area of low pressure formed at the exit point of each pipe on the short stack exhaust, when the slipstream is flowing by at 100 to 220 knots? And will this not help with the desired scavenging effect?
 

Toobuilder

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A good exhaust completely evacuates the combustion chamber of spent gasses safely overboard and also allows the induction system to refill the combustion chamber with a fresh charge. An "ideal" exhaust works in harmony with the induction system and camshaft to produce higher than ambient pressure in the combustion chamber. This is expressed as Volumetric Efficiency (VE), and some highly tuned examples can exceed their static displacement in certain conditions. NASCAR engines being a prime example, can do better than 125% VE. This takes a highly refined cooperation with the entire engine from inlet to exhaust and is influenced by OAT and DA, among other things.

This level of tuning is far beyond "reducing backpressure", and not a consideration at all for the supercharged Merlin installed in the P-51. In the Merlin's case, combustion chamber charge was assured by the supercharger, so exhaust scavenging dropped way down the priority list and became a simple "dump overboard" plumbing issue.

In the case of the T-51 replica, the short stacks are simply there to emulate the look of the real thing. Unfortunately, the LS engine is NOT supercharged, so the exhaust becomes an important part of the system again. And while those short stacks have very little backpressure, they will suffer from reversion and have zero chance of achieving resonance. In short, they are costing you power compared to a long tube exhaust.
 

TXFlyGuy

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Yes, we are well aware of the power loss with a short stack.
It will never be as good as a long tube race header. But we can design it to be much better.
 

BBerson

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Well, that should avoid "shock cooling".
But I was suggesting a tuned exhaust with fake short stacks that do nothing.
 

TXFlyGuy

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We are looking at 1.75 to 1.85" diameter pipe coming off the head, into a common manifold, with individual chambers sporting 6 live exhaust tubes.
 

BBerson

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That looks like four C-150 mufflers welded end to end.
(the C-150 has two cylinders into each muffler chamber)
 
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