footwell insulation

[Blas]

The Stainless Steel plates that stand-off the fiberglass footwells are quite good by themselves. The air gap between the SS plate and the fiberglass is quite a good insulator since air is flowing thru this area when the car is moving. The plates should be Stainless Steel and not aluminum and polishing them increases their effectiveness.

[G-Pete]

Originally Posted by Blas

The plates should be Stainless Steel and not aluminum and polishing them increases their effectiveness.

Not really- stainless steel is heavier, harder to work with and cost more. Polish them makes no difference because you handle heat radiation and not light radiation.

The aluminum cool down easier and they don't need to be very thick. All what a heat shield does is keep the heat directly from the surface.

With an 0.5 inch gap and a 18ga alu shield we talking about 250f (1 inch from the header) before and ambient temp after 0.5 inches.

What helps the most is to wrap the headers (150f one inch from header), in combination with the heat shields. That helps to lower the overall engine department temp.

[Blas]

Conductive Heat Transfer through Aluminum with thickness 2 mm and temperature difference 200 C (382 F)
q / A = 250 (W/mK) 200 (C) / 2 10-3 (m) = 25000 (kW/m2)

Conductive Heat Transfer through Stainless Steel with thickness 2 mm and temperature difference 200 C (382 F)
q / A = 16 (W/mK) 200 (C) / 2 10-3 (m) = 1600 (kW/m2)

The emissivity coefficient - ε - indicates the radiation of heat from a 'grey body' according the Stefan-Boltzmann Law,
compared with the radiation of heat from a ideal 'black body' with the emissivity coefficient ε = 1.

Stainless Steel, weathered emissivity coefficient = 0.85
Stainless Steel, polished emissivity coefficient = 0.075

Way to technical for me, and while I have not personally confirmed the above technical information , I did stay at a Holiday Inn last night........

[G-Pete]

It is just amazing how the internet creates all kinds of engineers.
One click and you have the right answers and formulas...

WRONG!


I like to visit my last statement:

"The aluminum cool down easier and they don't need to be very thick. All what a heat shield does is keep the heat directly from the surface."

Ergo, alu heats up faster and the heat conductivity is higher. With other words, if there is any little airflow striking the alu the radiation will be lower, because of the rapid change of molecules (principle of heat sink).

Both shields will heat up to the same temperature inside the engine compartment eventually (no airflow) - the alu just a little faster, but is cooled down and radiates less on the other side when exposed to airflow.

As soon the fan kicks, in you have airflow.

As I said before, there is no gain (SS) with higher cost, more weight and harder handling.

Google tip: https://www.google.com/search?q=automotive+heat+shields&hl=en&client=fire fox-a&hs=sbb&rls=org.mozilla:en-USfficial&prmd=imvns&source=lnms&tbm=isch&sa=X&e i=0P5hUM2EIoje9ASNkIDACA&ved=0CAoQ_AUoAQ&biw=1785& bih=913

[Blas]

I think the issue here is how you and I perceive the function of the heat shield as used in this case.
You stated: …’ alu heats up faster and the heat conductivity is higher…” And I totally agree with that statement, but then wouldn’t all that additional heat that is transferred so much more efficiently thru the aluminum plate once again be interacting with the footwalls’ fiberglass and ultimately with your feet? Especially during periods of low air flow behind the heat shield plate. I don’t think the idea here is to create a heatsink here with aluminum, the goal is to reflect or slow down the efficient transfer of heat from the headers to the footwell fiberglass And using a material that does not conduct heat as easily thru itself, like stainless steel, prevents the efficient transfer of heat back to the fiberglass on the footwell. We are not trying to extract heat from the headers or area around them and actually attempting to “cool” it.
And you were also correct in saying the stainless steel is a higher cost, has more weight and harder handling then aluminum, but these facts are not relevant to the question of heat transfer are they.

And regarding your opening statement… --------------------------!

[Ron61]

They may not even make this now but I used Thermotech on my Cobra. Easy to cut, bends as easily as a blanket and it will withstand up to 1500 degrees. I even had my transmission tunnel lined with it and worked great. It came in a roll and you just unroll it and either glue it in or use screws. I think it was about 1/4 inch thick with the shinny side out. I even replaced some of my aluminum heat reflectors with it as it lowered the temperature behind the shield more than the aluminum did.

Ron

[ACBill]

I expect there is always some airflow within the air space, when using a foot box deflector plate, (regardless of material used), via convection, caused by the heat itself..Hot air rises, drawing in the cooler air from the bottom. This takes place regardless if your moving, or if your cooling fan is running. Granted, it might not be as rapid an air movement, but the hot air is not trapped.