2.5" exhaust

[rfuerst911sc]

I like what you have done except the no muffler ........ that is going to be loud . Report back after you have driven it a while .

[ike84]

Originally Posted by rfuerst911sc

I like what you have done except the no muffler ........ that is going to be loud . Report back after you have driven it a while .

Will do. Ive gotta check alignment and push 996 row tune on it before I can start running her around but I will keep the post updated.

Sent from my POCOPHONE F1 using Tapatalk

[911monty]

While you ultimately wanted to reduce back pressure by using the 2 X 2.5" adapter that actually will increase pressure. Expanding the diameter reduces velocity of the gas, reduce velocity increase pressure. Just my .02.

[ike84]

Originally Posted by 911monty

While you ultimately wanted to reduce back pressure by using the 2 X 2.5" adapter that actually will increase pressure. Expanding the diameter reduces velocity of the gas, reduce velocity increase pressure. Just my .02.

No way dude, I used a level 5 hex to actually cause the gravitational pull of the crankshaft to accelerate flow at the level of the expansion...

Joking aside, I'm not sure I follow you there. Bernoulli's principle certainly dictates pressure inversely proportional to flow velocity over a surface, but that is not the same as pressure within a cylinder. I don't disagree with the fact that this will slow gas velocity at the expansion, and a decrease in flow velocity will certainly change flow dynamics, especially at the boundary layer (between turbulent and laminar flow). But, minimizing heat loss through the system (hence the wrap) should mitigate this and maintain the desired pressure gradient per the ideal gas law (PV= nrt, keep the nrt equal and P and V move in opposite directions) and Pousielle's law (restriction of flow through a tube is inversely proportional to the the 4th power of radius)

If your statement is correct - i.e. change in pressure is directly proportional to the cross sectional area of flow, this would mean that an infinitely large expansion (i.e. from an exhaust tube to the atmosphere) would create an area of infinitely large pressure. If that were the case, then I think we just turned our exhaust pipe into a rocket booster?!?

That is, unless I have completely misunderstood what you were getting at.

[911monty]

Originally Posted by ike84

No way dude, I used a level 5 hex to actually cause the gravitational pull of the crankshaft to accelerate flow at the level of the expansion...

Joking aside, I'm not sure I follow you there. Bernoulli's principle certainly dictates pressure inversely proportional to flow velocity over a surface, but that is not the same as pressure within a cylinder. I don't disagree with the fact that this will slow gas velocity at the expansion, and a decrease in flow velocity will certainly change flow dynamics, especially at the boundary layer (between turbulent and laminar flow). But, minimizing heat loss through the system (hence the wrap) should mitigate this and maintain the desired pressure gradient per the ideal gas law (PV= nrt, keep the nrt equal and P and V move in opposite directions) and Pousielle's law (restriction of flow through a tube is inversely proportional to the the 4th power of radius)

If your statement is correct - i.e. change in pressure is directly proportional to the cross sectional area of flow, this would mean that an infinitely large expansion (i.e. from an exhaust tube to the atmosphere) would create an area of infinitely large pressure. If that were the case, then I think we just turned our exhaust pipe into a rocket booster?!?

That is, unless I have completely misunderstood what you were getting at.

Wow. I like the KISS principle. Real world example, a centrifugal pump or compressor works by taking a fluid and increasing it's velocity (impeller) then routing to a volute (fancy name for wide part of the pipe) which decreases velocity and increases pressure. P&V inverse relationship. As far as temperature is concerned expanding gasses cool. The wrap otherwise is a good idea.

[ike84]

Originally Posted by 911monty

Wow. I like the KISS principle. Real world example, a centrifugal pump or compressor works by taking a fluid and increasing it's velocity (impeller) then routing to a volute (fancy name for wide part of the pipe) which decreases velocity and increases pressure. P&V inverse relationship. As far as temperature is concerned expanding gasses cool. The wrap otherwise is a good idea.

I admit that I had not thought of an exhaust expansion like a volute but I really do not think that they are equal. Lets try to think this through though. The flow dynamics of an exhaust system and centrifugal pump are not the same. Exhaust systems are pulsatile, centrifugal pumps rely on constant flow, and these two phenomenon do not behave the same. Even if exhaust were a constant flow, the geometry between the two are markedly different - small diameter increase over relatively short distance versus marked expansion over relatively longer distances. Look at the role that A/R plays in turbocharger applications - reducing the length of the volute substantially decreases the effectiveness in the design's ability to build top end pressure. Since this is a straight line, the R would be approaching infinity which means that little number divided by really big number is a teeny tiny number. Where there may be a marginal increase in pressure at the expansion joint, the reduced resistance of the pipe to follow it would more than make up for it.

Here is another real world example (I'm a surgeon who sees a fair bit of trauma so bear with me here) - someone comes into the ER bleeding to death. I have a choice - an IV catheter (the part that goes in the vein) that is smaller than the tubing that goes back to the bag, or an IV catheter that is larger than the tubing that goes back to the bag. If you are correct, the larger IV catheter would not flow as well since there is an increase in relative diameter in the system. Trust me when I say that this is not the case. That catheter (called a Cordis) saves peoples lives by replacing blood faster than most injuries can lose it.

Shorter and fatter is the way to go when flow matters. Just ask Ron Jeremy. That is, until you disrupt gas scavenging (complete disruption of laminar flow), and then you've gone too fat.

I know I sound like a smart ass, I am just having fun with this though. Your statements are valid. I wish I had a setup to test it truthfully. I always learn the most when I'm dead ass wrong.

[911monty]

Fun facts.....

Originally Posted by ike84

I admit that I had not thought of an exhaust expansion like a volute but I really do not think that they are equal. Again see First Law of Thermodynamics and Boyle's Law Lets try to think this through though. The flow dynamics of an exhaust system and centrifugal pump are not the same. Exhaust systems are pulsatile, centrifugal pumps rely on constant flow, and these two phenomenon do not behave the same. Even if exhaust were a constant flow, the geometry between the two are markedly different - small diameter increase over relatively short distance versus marked expansion over relatively longer distances. Look at the role that A/R plays in turbocharger applications - reducing the length of the volute substantially decreases the effectiveness in the design's ability to build top end pressure. Since this is a straight line, the R would be approaching infinity which means that little number divided by really big number is a teeny tiny number. Where there may be a marginal increase in pressure at the expansion joint, the reduced resistance of the pipe to follow it would more than make up for it.

Here is another real world example (I'm a surgeon who sees a fair bit of trauma so bear with me here) - someone comes into the ER bleeding to death. I have a choice - an IV catheter (the part that goes in the vein) that is smaller than the tubing that goes back to the bag, or an IV catheter that is larger than the tubing that goes back to the bag. If you are correct, the larger IV catheter would not flow as well since there is an increase in relative diameter in the system. Flow is limited by pressure (the height of the bag), viscosity of the fluid, the tube diameter and the Reynolds number of the friction of the tubing. Throwing a restriction will certainly slow flow such as placing your thumb on your garden hose. Flow is then converted to pressure. Trust me when I say that this is not the case. That catheter (called a Cordis) saves peoples lives by replacing blood faster than most injuries can lose it.

Shorter and fatter is the way to go when flow matters. Just ask Ron Jeremy. That is, until you disrupt gas scavenging (complete disruption of laminar flow), and then you've gone too fat.Apt metaphor but you realize this proves the theory. Desired laminar flow is maintained and velocity remains constant without disruptions such as heat loss or pipe diameter changes. To maintain velocity you would want constant pipe diameter.

I know I sound like a smart ass, I am just having fun with this though. Your statements are valid. I wish I had a setup to test it truthfully. I always learn the most when I'm dead ass wrong.

[911monty]

See the First Law of Thermodynamics (Law of Conservation of Energy) and Boyle's Law.

Originally Posted by ike84

No way dude, I used a level 5 hex to actually cause the gravitational pull of the crankshaft to accelerate flow at the level of the expansion...

Joking aside, I'm not sure I follow you there. Bernoulli's principle certainly dictates pressure inversely proportional to flow velocity over a surface, but that is not the same as pressure within a cylinder. Boyle's law states they are the same I don't disagree with the fact that this will slow gas velocity at the expansion, and a decrease in flow velocity will certainly change flow dynamics,(Bravo you are correct! The flow dynamic you reference is a conversion to pressure which is the point. Reference the First Law of Thermodynamics) especially at the boundary layer (between turbulent and laminar flow). But, minimizing heat loss through the system (hence the wrap) should mitigate this and maintain the desired pressure gradient per the ideal gas law (PV= nrt, keep the nrt equal and P and V move in opposite directions) and Pousielle's law (restriction of flow through a tube is inversely proportional to the the 4th power of radius)

If your statement is correct - i.e. change in pressure is directly proportional to the cross sectional area of flow, this would mean that an infinitely large expansion (i.e. from an exhaust tube to the atmosphere) would create an area of infinitely large pressure. If that were the case, then I think we just turned our exhaust pipe into a rocket booster?!?
And this is just being ridiculous according to the First law of Thermodynamics and energy Conversion which states that energy is not created or destroyed it is converted
That is, unless I have completely misunderstood what you were getting at.

[BYprodriver]

Originally Posted by 911monty

While you ultimately wanted to reduce back pressure by using the 2 X 2.5" adapter that actually will increase pressure. Expanding the diameter reduces velocity of the gas, reduce velocity increase pressure. Just my .02.

Also just a fact !

[ike84]

Originally Posted by BYprodriver

[/B]



Also just a fact !

On that note...

Sent from my POCOPHONE F1 using Tapatalk

[911monty]

Originally Posted by ike84

On that note...

Sent from my POCOPHONE F1 using Tapatalk

And this really? According to the first Rule of Thermodynamics it is a fact.

[ike84]

Originally Posted by 911monty

And this really? According to the first Rule of Thermodynamics it is a fact.

Dude, what are you trying to get at? I'm not denying your point that increasing the diameter of the exhaust will slow the flow. You're hurting your own line of reason by citing boyle's law though - as that volume expands the pressure will drop across the surface - it has to, it's conservation of energy. This is not physics 101 though and there will undoubtedly be more factors at play - CFD is stupid complicated. Flow patterns (laminar vs turbulent flow), boundary layers, vortices, standing waves and resonance patterns, all of this stuff will play into what actually happens in the real world. You clearly understand general physical principles so resistance should not be a foreign concept here. Resistance is the "thin line" so to speak of when it comes to flow. Too much and you choke the system, too little and you lose patterned (laminar) flow which ultimately will siginificantly disrupt the system.

If you don't believe this, take a tour of a major engineering complex (like GE for example). They have an entire team of engineers with impressive credentials whose job it is to model flow systems on computers that rival deep blue.

This part of my project started with the fact that the stock exhaust system is undersized for the hp rating of the vehicle at baseline, and the VE of the stock setup is nothing to write home about. This is my attempt at getting out of the motor what stuttgart left behind 20 years ago. You clearly understand physical principles, so don't you think it's a bit shortsighted to look at one aspect of this entire system from the viewpoint of one principle and then call me dumb for it?

Nevermind, please don't even bother answering that question. I am truly over this thread and I apologize to everyone who has read it.


Sent from my POCOPHONE F1 using Tapatalk