Friday, December 14, 2007

Making a Non-AC Heater Box for 3rd Gen F-Bodies


You guys have my attention.  

There are at least 3 threads out there now with “where did your non AC heater box pics go” since the server that was hosting them crashed and burned… honestly I never expected anyone to miss them.

Anyway, lets discuss this for a second, put up some info on how I made it and some pics.  I’ll have to apologize ahead of time: the whole thing might be a bit disjointed, especially the pictures, since I’m taking it from 3 different places. I’m also sadly missing the pictures of the actual fiber glassing since between the resin that sticks to everything and the very abrasive dust generated cutting and sanding ‘glass I probably would have destroyed a camera.

For those of you that aren't coming from the assorted forums looking for this- some quick background. Basically, for a whole slew of reasons the 3rd gen f-body guys have been getting rid of their air conditioning style heater boxes, “my AC doesn't work and I don’t feel like converting to a new refrigerant,” just to clean up the engine bay to get rid of some clutter, make room for modifications or because it’s more of a race car and what does a race car need AC for? For those of you asking why does a race car need heat either I’ll say for a consistent drag car you need consistent engine temps, with a heater you have a ready built, small radiator and fan assembly to use for just that; that and to keep your hands warm on those mineshaft, incredible conditions "this thing will run the best times ever," nights- ask my brother, he removed his and now drives to the track with a little electric heater and his hands between his knees looking like he's doing something he shouldn't at stop lights.

In my case I originally did it because:
  1. It didn’t work
  2. It was extra weight in a car that I owned for no better reason than to go faster
  3. It took up tons of space in a place that I could use for a turbo down pipe.
To give you some idea, here are a few pics of my “new” project car with it’s complete AC box.  Not only is it impressively large (it’s really about 2x what you see in the pictures, you can’t see the bottom half that extends almost all the way down to the frame rail, hell, you can’t see the frame rail, the firewall, and you can’t get to a large part of the engine on that side), but all the stuff around it is actually connected to it.

In contrast, here is my older car which has now become a parts car that has my box in it. There is actually more wiring harness around it because this car has been converted to use a different ECM, programming and 2 barr MAP sensor (mounted on the firewall above the air box with the green plug), and you can see what an enormous difference it makes, you can see all of that side of the engine, the firewall, frame rail…

Now there is another solution. A few f-bodies came from the factory without AC, but this has it’s problems also. The non AC boxes were borrowed from the S10’s and were steel, unlike the FRP (Fiber Reinforced Plastic) AC ones, and the steel ones just didn’t look right, they stuck out like a sore thumb. They also had a smaller fan housing, which used a smaller fan and motor which were setup for 3 speeds as opposed to the 4 that the rest of the setups have, meaning different wiring and dash pieces that you have to dig up somewhere. Lastly, this is the big one, they’re not that rare, but people have gone insane with their prices. You can find them pretty readily on ebay or assorted message board sale boards, but I’ve seen dented and rusted out ones going for $150-300.

So there’s a perfect reason to waste a bunch of my non-existent free time stinking up the house and killing a few of my dangerously lacking brain cells with fiberglass resin. Lets get dirty!

Actually, first a quick note about the design (sorry for the false start). This thing would have been much easier to make with some pleasant curves and nice lines and usually somewhere in that universe you hit an elegant state of “form follows function,” but that was not one of the goals here. I wanted something hard and angular, with the occasional awkward curve, like the original box it was replacing. The idea was that it should fit in. I’ve had guys very familiar with these cars closely inspect around under the hood trying to figure out how I made so much space on that side never even noticing the “different” heater box. This is a case of “form follows GM.”

So here is what I started with:

There’s 2 significant things here that you might not immediately realize. First is that the AC fan box is the right size and fits the 4 speed fan that we want and has the shape that we want, so lets use part of it… I found a convenient place to cut it from the rest of the box. Second, I took a piece of cardboard and made a pattern for the base plate. One that covered all the necessary holes in the firewall…. Once I had a shape I liked the fan box got taped to the top of it (you might be able to see it better from this angle:
At this point I had to work out the rest of the shape, and cardboard was the perfect medium. I took a piece and scored it to make the hard, square corners, glued/taped it down to the cardboard base plate and added whatever shapes I needed to make the thing up. At this point I took and blended/sanded/ground the fan box edges so it’s shape flowed smoothly into the rest of it.

Now to use the whole thing as a plug mold, as you can see in the pictures it needed a shiny surface, it got wrapped in duct tape. Without a shiny surface the fiberglass would have never released from it. The whole assembly then got stapled to a big sheet of plywood which was wrapped in plastic wrap (again, the resin won’t stick to shiny smooth surfaces). If I really intended this thing to be reusable for multiple uses I would have given it a heavy coat of shiny paint or gel coat to make it shinier and less sticky to the resin.
Next the whole deal got slathered with a few heavy coats of car wax to act as a mold release. You can use proper mold release, but anything that gets a good layer of wax where you don’t want things to stick will work.

Now this is where the fun part started, basically I put on layers of resin and ‘glass cloth, carefully pressing them down to squeeze all the excess resin out and to make sure that the cloth was tightly following the lines. Since this thing had all these different shapes and cloth doesn't stretch like mat does I used it in strips, kind of like paper mache.

The 2 layers of glass gave me a light, thin, and adequately strong part to work with. The more layers I’d put on top of the plug mold the more it would cover/obscure the shape of the part, so at this point I popped it off the mold… well, more accurately, since the mold was not totally smooth I pulled up on it and the mold came up with it, pulling the staples out of the wood. The mold stuck where the edges of the tape was, but I wasn't really stressed about it, and broke it up and pulled it out (again, if I meant it to be reusable I would have given it a heavy coat of shiny paint).

At this point I went to turning it into a useful part. 

The base plate put a ridge around the lower inside of the part which was easy to follow to cut out and sand to shape. From there I gave the part it’s structure by applying multiple additional layers off ‘glass mat (it’s easier to form into intricate shapes) mixed with cloth (stronger, lighter, really not necessary for this, I ended up with a part that was super light but strong enough for me to stand on) to the INSIDE of the part.
I didn't care at all about the finish on the inside of the part, but the outside I did. I gave the outside a single, thin layer of mat/resin since mat has a random pattern to it and doesn't have the same tendency to “print through,” as cloth. Then I did what I consider “cheating” the final finish, saving me a good deal of the sanding and other finishing that I would need otherwise by taking some bondo body filler, mixing it with some ‘glass resin (both are polyester based products) and spreading that thinned out goop across the top surface of the part, letting it smooth itself out.

What was left was sanding, sanding and more sanding, some paint and finally cutting out and drilling the holes for the motor/fan assembly and the coil assembly (plate that screws to the top of the duct to cool the coils used for fan speed control).  Slap it together and you’re ready to go.

This is what it ended up looking like (before cutting out the hole for the fan speed coils):

Here is a view from the inside assembled:

And finally, a view from the top assembled, with the motor cooling hose installed, all screwed up, ahem, together:


Anonymous said...

Thanx for the info,brings me back to my fiberglass days!did some.. yes sand sand sand your truely a fabricator,and beyond. great work!

willmoschella said...

Do you have one of these heater boxes for sale? I am not any good with Thank You

Mark said...


You don’t know how many times I’ve been asked that. Unfortunately, it’s basically a one off job, and that being the case it took way too much time for me to make to sell at anything that would resemble a reasonable price.

If I was really interested in going into production the deal now would be to put a glossy gel coat on the part, spray it with some PVA and then make a mold off of it so I could cast copies fairly quickly/easily, but to be blunt, I have a day job that I spend way too much time at and don’t really have the space or incentive to setup a full on ‘glass shop. OTOH, that may change since I’ve been playing with the idea of making a totally custom hood for the car since nothing out there for them really suits my needs/has the right look.


Vess said...

Thanks for the idea. I made mine out of stainless and mig welded it together

Vess said...

Thanks for the idea. I made mine out of stainless and mig welded it together

Mark said...

Vess- I'd like to see a better picture of yours, I wasn't able to make your link work besides showing me a small thumbnail of it.