I have my theories I guess, but let me lay out the particular situation I'm interested in before the variables get to out of hand. And just to get it out of the way, here is a general definition for insulation. Yes, air itself can be an insulator, but read on and you'll know what I'm getting at. I am replacing the firewall in my roadster with a sheet of flat 1/8" aluminum for clearance issues. The 1/8" thick mounting flange is already welded in place at the cowl. I know on long trips that aluminum by itelf probably won't do much to keep engine heat out so I am considering adding a second sheet of thinner material behind the flange. This would leave a dead space the thickness of the flange (roughly 1/8") between the sheets. It could easily be more - probably up to 1/4 or 5/16" if I added some spacers. Now... do you think it would do more to keep heat out with some sort of material between the sheets or would it be more efficient as just a mostly dead space of air? AND... here comes the overengineering. I'm actually thinking my best option would be leaving the space, adding two or three forward facing louvers accross the cowl, and ducting them right into the space. Forcing fresh air between the panels and out the bottom of the firewall on either side of the engine. Am I a total kook for spending this much time engineering my firewall? You might not think so if you've ever been in my car for 2 hours plus. I'll bet it's not that inspired - I believe some race car at some point has likely used a similar system.
If you want it to actually insulate, then make the inner panel out of an insulator! not metal. and add about 1/2" of good insulation material between them. Air is usually captured in insulation material, the trick is that the insulation material keeps the air from moving around, so there is less heat transfer. If you just have air between two metal sheets, you'll get a lot of heat transfer, and the louver thing will only make it worse, unless you duct in air from outside the engine compartment...and you don't drive when it's over 90 degrees out.
I think Styrofoam would help stop the heat transfer better than dead air. I agree with Squirrel about making the inner panel from something other than metal, but some guys want an all metal rod What are your door/interior panels going to be made of, if you're planning on using them? Flatman
But he's talking about air moving through and out, not just sitting there. I see this as a problem when stuck in traffic if louvers are the source of air flow...that's when you'd fry sooner without insulation in there...so go with active motivation for the air. Most inappropriate way for a modified, though a good one mechanically, would be a heater fan stuck through rear wall. Unfortunately, any more electricity than that needed to fire the plugs and satisfy the law is an aesthetic violation here. Fords with hot air heat used a stack with a scoop, looking for all the world like the forward facing type Stromberg topper, to channel fan air into the heat system...but I suspect the area around the front of your engine will be pretty full. So...I say tour the junkyard looking at all the remote air cleaners on 1980's cars, and install one up inside your car's cowl...duct box on carbs back to front of firewall. Look at the air box on the restored Highland Plating Special, and turnitbackwards. Weld in a Brookville deuce cowl vent for an outside feed for hot weather... Another possible source of moving air: Remove the little inspection plate on the bell housing and see if the clutch is tharshing the air with any directional control...Yunick said the rotating clutch moved enough air that he was interested in using the bellhousing as a source of pressurized air for the intake...
Kevin - Squirrel is on the right track... Heat transfer can occur by three methods: Convection, Conduction, and radiance. In the case of the firewall - you can have a ventilated air space and still receive significant heat transfer on account of the engine (and especially the headers) radiate a significant amount of heat. This heat will penetrate the interior compartment - dead air space or no. You're better off using an engineered heat shield material with a reflective surface between your panels, with the reflective surface facing the engine. While you're at it, you may want to install the same material on the toeboard - especially if your exhaust system is tucked in between the frame rails. If you look at the engineering on 'modern' exhaust systems, extensive ventilated heat shielding is used to manage the surplus heat generated by catalytic convertors. Use the same thought process to reflect radiant heat - and the interior compartment will be much cooler!
Kevin - Squirrel is on the right track... Heat transfer can occur by three methods: Convection, Conduction, and radiance. In the case of the firewall - you can have a ventilated air space and still receive significant heat transfer on account of the engine (and especially the headers) radiate a significant amount of heat. This heat will penetrate the interior compartment - dead air space or no. You're better off using an engineered heat shield material with a reflective surface between your panels, with the reflective surface facing the engine. While you're at it, you may want to install the same material on the toeboard - especially if your exhaust system is tucked in between the frame rails. If you look at the engineering on 'modern' exhaust systems, extensive ventilated heat shielding is used to manage the surplus heat generated by catalytic convertors. Use the same thought process to reflect radiant heat - and the interior compartment will be much cooler!
the aluminum sheet spaced away from the firewall may work more like a heat shield than insulation heat would be absorbed by the firewall and transfered to the air between the panels the aluminum would reflect the heat back to the air the heated air could be vented to the outside. if it's not vented the aluminum would act as a heat sink and radiate heat I'm with the squirrel on this one if you want to insulate, use insulation edit; or what chuckspeed sed
I know it's not high tech enough but I'd use some of that 1/8 bubble pack insulaion on the back of the aluminum and plywood on the inside. The plywood cuts down on noise and heat transfer. Sealing all the penetrations is just as important if not more so. There is a reason that all the old houses have their empty stud cavities filled with insulation to improve performace. Mostly it's air infiltration.
First choice,for maximum insulation,would be the high tech NASA developed insulation the NASCAR guys use on their floors. You can buy it in different sizes from BSR. Not cheap. A foil back insulation,with the shiny side towards the heat source,will reflect some of the heat.
This is exactly what I want to do. So do the insulation product proponents agree that the firewall will be cooler with insulation than with a constant supply of fresh air? I'm not so sure I get it. I'm thinking even if it's well into the 90's air from outside and blowing between the panels would work better. At some point the air trapped inside the insulation has to reach the engine compmartment temperature. Otherwise ice kept in a cooler would never melt? I remember sleeping on the insulated and carpeted floor of my Dad's van on the way to a BMX race and waking up because the floor was HOT. This all overthought for sure, but maybe the vented air wouldn't be as cool initially - but would maintain a more consistent temp. over a long trip?
grimlok- my first "fun" car was a '65 triumph spitfire. you couldn't drive the thing for more than 45 minutes without cooking your feet. it was terrible. my buddy and i would trade off driving so the passenger could hang thier feet over the side of the car for a few minutes to cool them off. i tried a couple of things. the first was to snag some carpeting and padding from a carpet store dumpster. not much help there. next was some hot water heater wrap from home depot. again, not much help. so you know what made an immediate and noticeable difference? some cheap header wrap. i'm not sure if that would violate your aesthetic vision... but it made a huge difference for me. hope this helps, ed ps- i don't think your air space idea would work out too well. the aluminum is just going to radiate the heat to the air, which will then transfer it to your second firewall. maybe consider using this product between the two firewalls (with the aluminum side out toward the engine)?? pps- how well sealed is your firewall?
If you are traditional enough to still put your coffee in a Thermos Vacuum Bottle thermos, then you are insulating FROM heat loss with a vacuum trapped in a double layer glass bottle, (or plastic or stainless if it's a newer one.) Your idea of having replacing air is next best but a vacuum is better. Now we all know that vacuum is a relative term and just means a difference in air pressure at best. That's how air conditioning works in a closed system. The compressor compresses the gas, traditionally Freon which boils at -17º, into the "condenser" raising it's heat becasue of the pressure change, and is stuck out in the wind so the wind can remove some of that heat, then it is allowed to pass thru a nozzle expanding into the "evaporator" in the car's interior, and the expansion allows the Freon temperature to drop, rapidly, cooling the air that passes thru the evaporator. You can use the same principle on your firewall, by running the two panels you are suggesting, and sealing them well, except for a vacuum pump that will draw the air out of the area causing a cooling vacuum. You already have that pump in the car. It's the engine itself. Just duct the carburetor's air cleaner to that firewall chamber (but do it prettier than NASCAR) and do it in such a way that it draws the air thru the whole firewall area and you have it.
Kevin, 30+ plus years of practical ( impracticle?) experience tells me that dead air is a poor insulator. I know its not trad or old school but I got some spaceage stuff here that will work wonders, doesn't weigh much and will be almost not noticable on the bach side of your firewall. ya need to come after your clock anyway we'll lop you off a piece while you're here. But here's an example for you if you must. Friend Joe has a '55 Stude. It was uncomfortable inside, he installed a vintage air unit. his feet were still getting hot even though his face was now getting cooled air. Solution was a shiney stainless sheet between the firewall and the engine, about a 1/8" air gap. little or no change in the comfort level inside the car. Took a heat gun for a ride. Toeboard temps were in the 108-110 degree range(hot day). Pulled the stainless and sandwiched the space stuff in there, took it for a drive, temps dropped about 15 degrees.
That was actually the example I had thought of against the dead air - and what sparked my interest in getting the air to move. Obviously I'm biased in a particular direction here. In all honesty, good insulation and a thin sheet of plywood might work just as well. But the vented air through the cowl should be fun to work on. I can always unbolt the firewall and add insulation later if it proves to be a huge failure. I was going to try to keep the intake kind of low key but Bruce's suggestions have given me some ideas. And to the point of constantly moving air, I'm generally not stuck in traffic around town so I'm not super worried - though the clutch inspection cover idea is cool.
Yes, the insulation does let some heat flow thru it, and the temperature on the interior side of the firewall insulation will eventually rise, but not to the temperature of the engine side. There will be a "temperature gradient" across the insulation, the engine side will be hot, the interior side will be much cooler. The heat energy that does make it through the insulation will dissipate in the interior air of the car. The insulation slows down the heat flow rate, so you will feel cooler, compared to not having insulation.
It's interesting but there comes a point, to me anyway...where it isn't worth the cost and effort? If you seal up the firewall so that the hot air can't enter the cabin and insulate from radiant heat transfer, will you be able to measure any additional benefit from a firewall cooling system? I kinda Fin doubt it. It's an interesting discussion but not too practical IMHO.
yup...it would be better to put the insulation on the firewall as normal, and then blow cool air into the passenger compartment. Kind of like 1930s cars did with the firewall pad and cowl vent. no...wait...that can't be right!
Did you ever answer if your floor/cowl had any holes in it? I know it's not really trad, but I put Fat Mat in my coupe, and it stays unbelievable cool. My '46 was a SAUNA. Bare floor and firewall, no insulation. It sucked.
Haha. Yeah, I was having a discussion not too long ago about how wierd my car is. The things I consider important or not important. Like not finishing the welds on the door tops, but taking literally weeks to figure out nose and headlight placement. Some of the fasteners are whatever I could find at the time that fit the hole, and some are carefully selected, backed by copper washers and perfectly clocked. I'll be spending hours engine turning a firewall as perfect as possible and bolting it into a body that looks horrible up close by most people's standards. Strange I guess. Interested to see how this firewall works out though.
Well right now it's all hole. But the goal is to have it completely sealed.. esp. from the engine compartment. That's why I'm adding the new firewall.
I have used this product line with good results on non-HAMB related cars. I'd just put it inbetween the two metal panels to hide it from view: http://store.summitracing.com/egnsearch.asp?Ntt=heat+shield&N=4294845359+115+400356&Ntk=KeywordSearch blocking 90+% of radiant heat sounds good to me and much simpler than messing with a louver type system.
Space blankets or fire tents are what we were issued in the Forestry service to use in the event we were overrun by flames. They are aluminized and work by reflecting radiated heat away from you as you cower inside on the ground hoping that all available oxygen is not consumed resulting in your asphyxiation. Never had to put it to the test in an emergency but used them in training.
For a cheap, but very efficient material. Rob the floor padding of an old Mercedes Benz at the wrecking yard. They use thin sheats of lead combined with normal insulation material. In any case, the most important is to have an absolout sealed firewall. The tiniest hole will transfer noise and heat. In modern houses all insulation is vented, mostly to avoid condensation and moisture.
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