Why not aluminum headers?
Thread Starter
Team Owner
Joined: May 2002
Posts: 24,337
Likes: 25
From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
Originally Posted by Alvin
Convection isn't going to save your butt. check out the Cp of air vs. water
The reason why your heads/block/whatever stays cool is because it has a coolant pulling heat from them. If you depended on air alone it woudl take 4 times as much air as water in MASS. Considering the density of water is around 1000 times that of air your talking about moving quite a volume of air to remove the same heat.
thermal conductivity isn't the worst problem.. The problem is the parts would loose their toughness (resistance to plastic deformation) starting at temps much lower than you'll see here.
So in short.. to use a aluminum alloy for a exhaust manifold or header you would have to use some sort of coolant, build them to where the COT isn't going to break anything when the parts get hot, and build them thick enough to where the parts aren't going to deform when the modulus of elasticity goes threw the floor.
The reason why your heads/block/whatever stays cool is because it has a coolant pulling heat from them. If you depended on air alone it woudl take 4 times as much air as water in MASS. Considering the density of water is around 1000 times that of air your talking about moving quite a volume of air to remove the same heat.
thermal conductivity isn't the worst problem.. The problem is the parts would loose their toughness (resistance to plastic deformation) starting at temps much lower than you'll see here.
So in short.. to use a aluminum alloy for a exhaust manifold or header you would have to use some sort of coolant, build them to where the COT isn't going to break anything when the parts get hot, and build them thick enough to where the parts aren't going to deform when the modulus of elasticity goes threw the floor.
Everything you said here is true. You misunderstood me. Heat loss through a stainless header vs aluminum one isn't a big difference because 99% of the thermal resistance is in the boundary layer. Obviously the ID layer will be less than the OD layer, but my point is the pipe ID and OD will be close in temp, but somewhere in between the exhaust temp and outside temp. It'll be closer to the gas temp than the outside temp because of the exhaust velocity, but definitely cooler than the gas temp. Obviously not cool enough though as Pontiac discovered.
The math is already done, I have an old work project using THERM and excel to run 1D and 2D temperature profiles through window frames to check for condensation and heat loss, so I just plugged in some different materials.
Drifting
Joined: Mar 2005
Posts: 1,923
Likes: 0
From: GA
Originally Posted by Alvin
.. They claim "scientifically" that jet fuel isn't hot enough to melt steel.
To their "theory". The tempratures in a jet engine (burning jet fuel) can and will melt titanium.Another reason to vote against alum. headers is metal fatigue. The constant heating and cooling along with the vibration of the engine will soon crack any alum. alloy that can withstand melting from the exhaust.
Race Director
Joined: Jan 2005
Posts: 10,641
Likes: 13
From: Slidell Louisiana
Originally Posted by loiq
Two stroke jet skis use aluminum pipes in race applications. I've been to place that makes them - they're expensive, $1200 for a set for a three cylinder engine. 
Do jetskis have a dry exhuast?
Le Mans Master
Joined: Jan 2002
Posts: 9,036
Likes: 13
From: Edmonton AB
Originally Posted by aboatguy
Do jetskis have a dry exhuast?
to see on Olds 455's in recreational jet boats were wet - cooled by the
coolant circulating in the engine.
.
Racer
Joined: Jan 2004
Posts: 291
Likes: 0
From: Norton ma
Originally Posted by Alvin
Convection isn't going to save your butt. check out the Cp of air vs. water
The reason why your heads/block/whatever stays cool is because it has a coolant pulling heat from them. If you depended on air alone it woudl take 4 times as much air as water in MASS. Considering the density of water is around 1000 times that of air your talking about moving quite a volume of air to remove the same heat.
thermal conductivity isn't the worst problem.. The problem is the parts would loose their toughness (resistance to plastic deformation) starting at temps much lower than you'll see here.
So in short.. to use a aluminum alloy for a exhaust manifold or header you would have to use some sort of coolant, build them to where the COT isn't going to break anything when the parts get hot, and build them thick enough to where the parts aren't going to deform when the modulus of elasticity goes threw the floor.
The reason why your heads/block/whatever stays cool is because it has a coolant pulling heat from them. If you depended on air alone it woudl take 4 times as much air as water in MASS. Considering the density of water is around 1000 times that of air your talking about moving quite a volume of air to remove the same heat.
thermal conductivity isn't the worst problem.. The problem is the parts would loose their toughness (resistance to plastic deformation) starting at temps much lower than you'll see here.
So in short.. to use a aluminum alloy for a exhaust manifold or header you would have to use some sort of coolant, build them to where the COT isn't going to break anything when the parts get hot, and build them thick enough to where the parts aren't going to deform when the modulus of elasticity goes threw the floor.
The external convection resistance is the main resistance in the overall heat transfer from the exhaust to ambient.
You may reduce the conduction resistance by 50%, however if is the conduction resistance is only 1% of the overall thermal resistance from the exhaust to the ambient air the actual change in heat flux is not going to be very noticable.
Headers are very thin, the conduction resistance is pretty small compared to a cast iron manifold
For a quick check assume 200w/m*k for aluminum and a heat transfer coefficient of 5-10 ( roughly natural convection) on the external surface of the pipe.
Le Mans Master
Joined: Apr 2005
Posts: 6,413
Likes: 9
From: The Old Dominion
Originally Posted by CentralCoaster
I don't see why exhaust heat would be a problem, it couldn't be too much hotter than aluminum heads run in the chamber. Even then, they could be ceramic coated inside and out. It's not as if the aluminum is gonna melt.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
Thread Starter
Team Owner
Joined: May 2002
Posts: 24,337
Likes: 25
From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
Originally Posted by 350ws6
For a quick check assume 200w/m*k for aluminum and a heat transfer coefficient of 5-10 ( roughly natural convection) on the external surface of the pipe.
And don't forget about the resistance of forced convection on the ID. It's less, but still a significant factor.
Race Director
Joined: Dec 2002
Posts: 13,348
Likes: 233
From: SCMR Rat Pack'r Charter Member..Great Bend KS
Originally Posted by loiq
Two stroke jet skis use aluminum pipes in race applications. I've been to place that makes them - they're expensive, $1200 for a set for a three cylinder engine.
The exhaust from two-stroke engines is MUCH cooler than the exhaust from a four-stroke. It has to do with the exhaust gases being contaminated by the cool intake charge.
In addition, race applications place performance/weight reduction over durability....I'm betting those pipes require frequent replacement.
Larry
code5coupe
