I'm going to be building an aluminum exhaust for my car pretty soon. I was wondering if you guys had any ideas about making hangers. I know if I try to bend aluminum round stock, it'll break. Do you think I can try annealing the round stock down to T0 and then bend it? I'm leaning towards just welding on flat brackets to go parallel to the rubber hangers, then just nut and bolt it. Any ideas? Thanks.

Tyler

 

Tyler annealing it to i believe the "O" condition will make it alot softer and more malleable.

I would not recommend welding directly to the exhaust as it will more then likely fatigue and eventually break. What you can do is take a piece of expeanded tubing (i like to leave a gap as to put some high temp tape or other material like felt on the inside)and slip that over the tube and weld to that this way you can use stainless or mild steel build your hangers accordingly and not stress the exhaust

 

I second that. You do not want more stress risers than you alredy have if you want the exhaust to last. In my not-quite-expert opinion, the exhaust should be made of a single length of annealed tubing, mandrel bent, then heat-treated to T6. The hangers should wrap the pipe and should be steel, and the connections should either be slip fit with clamps, or perhaps a bi-metal v-band assembly. Even then, I still believe the muffler and downpipe (with flex joint) should still be made of steel.

 

Ok- I gotta know; why the hell do you want to make your exhaust out of aluminum?

 

I would think weight savings.

Very very light!

 

Alot of T4 run aluminum exhuast.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Brandon Clifton &raquo;</TD></TR><TR><TD CLASS="quote">Ok- I gotta know; why the hell do you want to make your exhaust out of aluminum?</TD></TR></TABLE>

Because steel rusts and looks ugly. Thanks for the help guys.

Tyler

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Brandon Clifton &raquo;</TD></TR><TR><TD CLASS="quote">Ok- I gotta know; why the hell do you want to make your exhaust out of aluminum?
</TD></TR></TABLE>

Granted there are other more exotic ways to do it such as thing gauge .035 and thinner 321 stainless inconel and all the other strong alloys that are availble in tube form. Aluminum seems to be the most cost effective way to reduce the weight

 

I could outline what a pointless idea this is based on the density and velocity factors of the Bernoulli equation and the themodynamic properties of Al versus steel, but I suspect there's no changing your mind so I won't waste my time.

 

I want to hear it.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Brandon Clifton &raquo;</TD></TR><TR><TD CLASS="quote">I could outline what a pointless idea this is based on the density and velocity factors of the Bernoulli equation and the themodynamic properties of Al versus steel, but I suspect there's no changing your mind so I won't waste my time.
</TD></TR></TABLE>

Explain how the thermodynamic properties of aluminum would be bad for an exhaust system on a turbo car, where cooler is better. Don't know if you've picked up much 4" tubing lately, but with .065 aluminum at almost 1/3 the weight of .065 stainless (which is about as thin as you're going to find bends), you can save quite a bit of weight by using aluminum. I'd like to hear exactly why it's a bad idea?

 

ahh yes i am sure he is thinking of heat velocity and such, but the thing is that since aluminum absorb heat quickest it would in theory be better for the 1/4 mile drives that we do. just a little something i pulled out of my behind

 

Well, on a turbo car, we're looking for pressure and temperature differential from the inlet of the turbine to the outlet of the turbine. That being said, a 4" aluminum downpipe would work great if it wouldn't fall apart. lol

 

from what i gathered on heat velocity the exhaust would flow the best when the material is hottest?

 

A turbines performance isn't dependant on the temperature drop external to it. The turbine creates the temperature drop as a function of isentropic gas expansion and fluid losses. That temperature drop is a function of turbine efficiency and energy use of the turbocharger. What you drop the temperature to external to the turbine won't make a difference at all to it's performance.

I'm thinking keeping the exhaust hot (pre and post turbo) is still a good plan on a turbo motor, no matter how you look at it. However, keeping the pressure differential across the turbo as high as possible is more beneficial and the easiest way to keep pressure down is using a big *** tube with minimal length and bends along with smooth transitions at piping diameter changes. Even though the 4" exhaust might lose more heat and theoretically that drop in temperature increases airflow restriction, the airflow will still see less overall restriction compared to a smaller exhaust simple because of the increase in flow area.

A smaller exhaust MIGHT see a little higher torque peak, assuming the smaller exhaust can keep up at the torque peak, but with the size of turbos these street cars are using any more, "the bigger the better" is what I would tend towards.

Internally ceramic coating the exhaust might help the whole heat loss issue on the aluminum tubing. Aren't there a couple types of stainless steel materials that won't react with aluminum? 321 maybe?

 

How does air move? From high pressure to low pressure, and from high heat to low heat. Correct?

 

High energy state to low energy state.

Not sure what you are getting at though. I said keeping the air hot will help the airflow. I just said the dominant factor is piping diameter.

 

forget piping diamter for a second, that's a given, big. Assuming all things are equal with the piping. Assuming that pressure was equal on both sides, with one side being hot and the other being colder, would the air not move in the direction from hot to cold? I see what you're getting at though, that cooler air won't flow as good as hot air? I guess at that point it comes down to making the downpipe big enough that flow doesn't even matter much, and allowing the exhaust to cool more after the turbo to promote better airflow through the turbine.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Brandon Clifton &raquo;</TD></TR><TR><TD CLASS="quote">I could outline what a pointless idea this is based on the density and velocity factors of the Bernoulli equation and the themodynamic properties of Al versus steel, but I suspect there's no changing your mind so I won't waste my time.
</TD></TR></TABLE>

lol. WTF. please do.

You don't run an aluminum exhaust for it's "thermodynamic properties." You run it because it is the most cost efficient way of saving weight @ the exhaust system. Period. It's not all that complicated.

 

WTF does the Bernoulli equation have to do with it? The Bernoulli equation has absolutely nothing at all to do with heat.

P2 = P1 + rho*v²/2 + rho*g*height

I think someone just pulled a phrase out of his butt to try to sound smart?

 

Tony, I see what you are saying, but heating one side of a tube while cooling the other won't force air to flow a whole lot. It will cause the molecules to move a round a bunch, but I don't THINK it will cause a large amount of directional airflow. Now if that tube was vertical so the less dense hot air could rise due to fluid buoyancy, you would be on to something.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 99_GS-T &raquo;</TD></TR><TR><TD CLASS="quote">Tony, I see what you are saying, but heating one side of a tube while cooling the other won't force air to flow a whole lot. It will cause the molecules to move a round a bunch, but I don't THINK it will cause a large amount of directional airflow. Now if that tube was vertical so the less dense hot air could rise due to fluid buoyancy, you would be on to something.</TD></TR></TABLE>

The greater the pressure and temperature differentiation between the turbine housing and downpipe = faster spool and more power. Maintaining exhaust velocity flow through proper sized exhaust tubing comes second and isn't as important as maintaining the greatest amount of pressure/temperature differentiation.

An aluminum downpipe would create more power/faster spool because it would be able to absorb more heat from the exhaust after the turbine housing. The only problem with AL is the low melting point and will cause the AL DP to fall apart after several heat cycles from deformation and fatigue.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by dasher &raquo;</TD></TR><TR><TD CLASS="quote">The greater the pressure and temperature differentiation between the turbine housing and downpipe = faster spool and more power. Maintaining exhaust velocity flow through proper sized exhaust tubing comes second and isn't as important as maintaining the greatest amount of pressure/temperature differentiation.</TD></TR></TABLE>

I don't know where you guys are getting that gas flows from high temperature to low temperature. There is absolutely no physical law that dictates this. The flow in an exhaust is purely pressure driven.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">
An aluminum downpipe would create more power/faster spool because it would be able to absorb more heat from the exhaust after the turbine housing. The only problem with AL is the low melting point and will cause the AL DP to fall apart after several heat cycles from deformation and fatigue. </TD></TR></TABLE>

No, an Al DP would *NOT* affect spool or create more power. Absorbing more heat post turbine has absolutely no bearing on anything(except post turbine exh. temperatures to some very small degree). In fact, Al will absorb less heat energy than the same mass of steel, but it can conduct it with less thermal resistance - so you're wrong on that point too.

You're right that the Al DP will fall apart in quick order.


The thermal properties of your exhaust components matter VERY little as long as they stay together. Cross-sectional area is the big factor in post turbine exhaust flow - PERIOD.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Def &raquo;</TD></TR><TR><TD CLASS="quote">I don't know where you guys are getting that gas flows from high temperature to low temperature. There is absolutely no physical law that dictates this. The flow in an exhaust is purely pressure driven.
</TD></TR></TABLE>

Pressure and Temperature have a unique relationship.

Ideal gas law
PV=nRT

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Def &raquo;</TD></TR><TR><TD CLASS="quote">
In fact, Al will absorb less heat energy than the same mass of steel, but it can conduct it with less thermal resistance - so you're wrong on that point too.
</TD></TR></TABLE>

Are you trying to insinuate that SS has a greater specific heat than AL?

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Def &raquo;</TD></TR><TR><TD CLASS="quote">
Cross-sectional area is the big factor in post turbine exhaust flow - PERIOD.</TD></TR></TABLE>

You seem to hold cross sectional area in the exhaust tubing with so much importance in turbocharged applications. In your opinion what is the correct size downpipe and exhaust for various turbochargers for best spool and power characteristics?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by DIRep972 &raquo;</TD></TR><TR><TD CLASS="quote">lol. WTF. please do.

You don't run an aluminum exhaust for it's "thermodynamic properties." You run it because it is the most cost efficient way of saving weight @ the exhaust system. Period. It's not all that complicated. </TD></TR></TABLE>

This whole thread has gotten to such a high level of stupid that I am, in fact, going to work through the equations to either:
1) Prove you dumbasses in the "an aluminum exhaust is better because it absorbs more heat" camp wrong, or
2) Disprove the opinions of myself, as well as the rest of the automotive and hot-rod community at large, wrong.
I include the rest of the automotive and hot-rod community is based solely on the use of header wrap and heat shields since the late 1940s.

I do agree that ideally there should be no pipe after the turbine so it can vent directly to atmosphere. However, since that isn't really an option the next best thing is to have a exhaust that is so large it simulates dumping to atmosphere. I'm not out to disprove that, I'm just out to set the record straight that absorbing heat is more of a detriment than a benefit.

Follow this thread for the update.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by beepy &raquo;</TD></TR><TR><TD CLASS="quote">
WTF does the Bernoulli equation have to do with it? The Bernoulli equation has absolutely nothing at all to do with heat.

P2 = P1 + rho*v²/2 + rho*g*height

I think someone just pulled a phrase out of his butt to try to sound smart? </TD></TR></TABLE>

beepy- the Bernoulli equation has density in two different places (rho). Density has a direct correlation to temperature as well as pressure. See below:

http://www.engineeringtoolbox.....html

Brandon