Ok, so my car is fast...

[cody]

He meant if you spray it too, it will last longer. That caught me off guard at first too though.

[sperry]

Originally Posted by MikeK

Wrapping makes it last 5 or 6 years instead of 2 or 3?? Where do I sign!!

I must be the only person that understands sentence structure in english:

Originally Posted by sperry

Even if you go the full 9 yards and use all the different sprays and sealants etc to make the wrapping "waterproof", you've still got a pretty good chance that the wrapping will ruin the exhaust in 5 or 6 years instead of 2 or 3.

"the wrapping" that "will ruin the exhaust in 5 or 6 years instead of 2 or 3" is the wrapping created with "all the different sprays and sealants etc". Granted, the sentence isn't totally obvious, but it doesn't imply that an unwrapped exhaust will only last 2 or 3 years. Context is a sonofabitch apparently.

[MikeK]

Fuckin' context

[sp00ln]

Originally Posted by sperry

Hot gasses do not "move faster", there is a difference between pressure and flow. Gasses that come off the motor are expanding due to their heat. If you let the gas cool, it doesn't expand as fast... i.e. you're losing some of the kenetic and thermal energy that will spin the turbo. Once the gas leaves the turbo, you don't want it to expand, because (just like a balloon) they expand in all directions. Expanded gasses after the turbo push backwards against the turbine blade, and slow them down. Ideally, you'd want hot expanding gas before the turbo and cool contracting gas after the turbo to help scavenge the gasses out of the turbo and reduce back-pressure.
http://store.summitracing.com/defaul...=egnsearch.asp

Thats not what RRE says:

Our DPs have a 2 1/2" stainless steel first bend with a 2 1/2" stainless steel flex section. They then expand to 3" right after the flex section right under the oil pan area. Why start out at 2 1/2"? If you are running on the stock O2 sensor housing, even ported it is 2 1/2" inside diameter. When the exhaust gasses are hot, they flow better, 2 1/2" is plenty for the first 12". As the gasses cool , bigger helps. By going to 3" after the flex section, we allow this expansion and also allow for better clearance at the front where room is tight. From there they are a full 3" back to the Apex 3" flange with 14 gauge aluminized steel piping. We had some Apexi flanges laser cut out of thick 1/2" steel for a good seal and fit.

As far as keeping temps cool under the hood, I think I'll get some of that wally wet or whatever. Oil cooler...? And maybe a new radiator (if i can afford it) I refuise to wrap my nice, chrome IC pipes

[sp00ln]

Oh, and I cant wait to get my car out to the RFR track... let me know next time you guys are going!

[sperry]

That RRE quote sounds like a sales pitch, not anything scientific. Granted I'm just trying to remember whit from a physics class I took like 9 year ago, but I wouldn't want the header/up-pipe to expand in size because that would slow flow and drop pressure in the pipe.

There's a reason no one uses an up-pipe on the WRX significantly larger in diameter than the stock pipe.... it slows down the flow too much. I don't know the details of your motor, but I would expect sticking with 2.5" for the entire up-pipe would help spool time. The only reason to have a pipe that expands to 3" is to provide a smooth transition to a large turbo that uses a 3" exhaust input, and that expansion should be as short as possible while remaining smooth.

[AtomicLabMonkey]

As a pipe's diameter increases, fluid velocity in the pipe decreases, localized fluid pressure goes up, and the friction drag/"backpressure" over the length of the pipe run goes down. Normally this is a good thing, however I don't know how it applies to your funky ass turbo systems.

[JC]

PV=nRT. Volume stays constant so when you increase temperature you increase pressure. Using Bernoulli's equation pressure and velocity and inversely proportional. So when you increase temperature, velocity should go down?

[sybir]

Reread that RRE quote, it's for their DP. Sounds like marketing trying to justify using a 2.5 inch initial section for clearance reasons, then going to the full 3". A full 3" setup on the waste side of the turbo is going to let you clear stuff out more effectively; notice they're not talking about scavenging or anyhtign like that, they're saying "yeah, 2.5 is *ok* for this section, because it's like stock, then we flare out.

Sounds like a copout to me. Not saying a 5" DP is necessary, and there's evidence that the big bellmouths like the TurboXS can hurt torque a bit down low at the same time that it opens up a little bit of HP on top, but I wouldn't base an argument on a sales pitch.

[Dean]

Originally Posted by JC

PV=nRT. Volume stays constant so when you increase temperature you increase pressure. Using Bernoulli's equation pressure and velocity and inversely proportional. So when you increase temperature, velocity should go down?

V= infinity... It is not a closed system, so PV=nRT is not directly aplicable without significant modification. It is a really a fluid/gas dynamics problem.

And exhaust gas starts contracting pretty much the second it stops combusting(Heat source). There is no additional heat in the headers/uppipe, so anything you can do to retain the existing heat will minimize contraction, and maintain velocity to the turbo. There is even a theory that pipe diamater should decrease in the uppipe to increase velocity as the gasses enter the turbo. The gotcha with this is that it produces increased back pressure in the headers which can diminish power.

After the turbo, it is all about minimizing back pressure, velocity is not terribly important. There is actually something to be said for not having the larger piping start until after the cat, so the tendency will be for the again expanding gasses to exit via the lower pressure higher volume piping, but on a turbo, the increased back pressure at the turbo probably negates this.

[cody]

The reason DP's are the widest part of the exhaust system at their leading opening (idealy no more than 10% larger than the exhaust port of the turbo) is to destroy the vortex the turbo creates. The gases would take longer to travel the length of the DP if they are swirling.

[AtomicLabMonkey]

Originally Posted by Dean

VThere is even a theory that pipe diamater should decrease in the uppipe to increase velocity as the gasses enter the turbo. The gotcha with this is that it produces increased back pressure in the headers which can diminish power.

Hmm... I would be wary of following this theory because turbines are energy driven devices, and energy is not confined to velocity alone. Deltas in heat, pressure, & velocity should all combine to spin the turbine. I wouldn't want to sacrifice one for only a modest increase in another...

[AtomicLabMonkey]

Originally Posted by cody

The reason DP's are the widest part of the exhaust system at their leading opening (idealy no more than 10% larger than the exhaust port of the turbo) is to destroy the vortex the turbo creates. The gases would take longer to travel the length of the DP if they are swirling.

As I recall from NACA data fluid is pretty turbulent for at least 10-15 diameters down a pipe run after a fan/compressor no matter what you do to the pipe inlet.

[cody]

Originally Posted by AtomicLabMonkey

As I recall from NACA data fluid is pretty turbulent for at least 10-15 diameters down a pipe run after a fan/compressor no matter what you do to the pipe inlet.

Interesting.

Corky Bell is who I got the no more than 10% number from.

Also check out Turbulent Flow Is Helpful from this scan of "Turbochargers" by High MacInnes.

[sperry]

Originally Posted by Dean

And exhaust gas starts contracting pretty much the second it stops combusting(Heat source). There is no additional heat in the headers/uppipe, so anything you can do to retain the existing heat will minimize contraction, and maintain velocity to the turbo.

I remember reading somwhere that the gasses are nowhere near done expanding due to the heat of combustion until they're into the headers and on their way up the up-pipe. The expansion of the exhaust gasses isn't instantaneous. However I would agree that the rate of expansion is decreasing from the instant the combustion event occures.

Also, if you have an expanding gas volume between the piston and the turbo's turbine blades, doesn't the math start looking a lot like PV=nRT? Granted it's not totally closed, but it's also not just an open pipe.

Thirdly, what about the exhaust pulses? There is energy in the form of a compression wave traveling down the pipe. I know lots of N/A cars take advantage of this and tune the pipe length/diameter such that the resonance frequency of the pipe matches the opening of the exhaust valves on another cylinder, allowing the low-pressure part of the waveform to assist in extracting exhaust from the cylinder. I think the turbine in the exhaust might limit the usefulness of that effect on a turbo car, unless you went with one of those funky rear mounted turbos.

[Dean]

Originally Posted by sperry

I remember reading somwhere that the gasses are nowhere near done expanding due to the heat of combustion until they're into the headers and on their way up the up-pipe. The expansion of the exhaust gasses isn't instantaneous. However I would agree that the rate of expansion is decreasing from the instant the combustion event occures.

Also, if you have an expanding gas volume between the piston and the turbo's turbine blades, doesn't the math start looking a lot like PV=nRT? Granted it's not totally closed, but it's also not just an open pipe.

Thirdly, what about the exhaust pulses? There is energy in the form of a compression wave traveling down the pipe. I know lots of N/A cars take advantage of this and tune the pipe length/diameter such that the resonance frequency of the pipe matches the opening of the exhaust valves on another cylinder, allowing the low-pressure part of the waveform to assist in extracting exhaust from the cylinder. I think the turbine in the exhaust might limit the usefulness of that effect on a turbo car, unless you went with one of those funky rear mounted turbos.

Muddy waters. Pressure in the cylinder > pressure in the headers, so yes, the gases "expand" as they exit the cylinder, but that is pressure equalization, not really heat based expansion. Once your heat source is gone, all you have left is volume, and pressure.

And as the exhaust valve opens, the volume of space available to that specific exhaust charge mass increases. As it increases, the mass cools, and pressure drops. Being a gas, it continues along the headers seeking the ultimately lower pressure of the end of the exhaust. And yes, i am using PV=nRT sort of, but only in principal, because it doesn't perfectly hold because it is an open system.

Yes, everything pulses as each cylinder fires in order, but I think on a turbocharged car, you are probably just trying to keep pressure even as opposed to trying to have a scavenging effect as you might on a non-turbo car.