This has been a huge oversight in my '81 LS swap. I have the engine in the car, I drilled holes into the water pump, I'm all but ready to button everything up and then I realized the heater box has to go back in the car. Looking at my engine bay, I'm almost certain that this thing isn't going to fit back in the car, and after doing research it appears my options are 1) relocate the coils, 2) convert to vintage air, or 3) just don't run heat at all.

I'm not particularly keen on any of three options - relocating the coils isn't the look I'm going for, vintage air is an expense I'm not ready for at the moment, and I really would like to have heat as I like to drive my cars well into the fall. Are there any other "relatively" inexpensive options to either replace to heater box or get it to fit? Or is my best option to close it shut for now and worry about it in the future?

 

I didn't have AC in 71 C3 swap. No issues with the heater box.
I'm assuming your 81 has AC? Do you need AC?
Can you retro fit an earlier non AC heater box to your 81.
Or maybe retro fit an aftermarket street rod heater/defrost unit if you don't need AC.

 

I didn’t plan on running AC, but I didn’t realize the heater boxes were a different t size depending on if it came with AC or not. Hmmm I’ll have to look into that more.

 

Use a 04-06 GTO heater valve to replace the OEM heater valve. This will allow for proper operation of the LS thermostat, and keep the heater functioning properly.

 

LS fits using the ICT adapter plates placing the motor in OEM location. #8 plug is tight, but it does fit with the factory Heater/Evap box.

 

Sorry, I’m pretty dumb here - so are you saying I should be using the GTO valve AND my ‘81 heater boxes w/ AC? How does this work?

Yeah, the engine is already in with Dirty Dingo sliding mounts, so unfortunately I’m not going to be able to use the ICT adapters 😅

 

Got any pics of the current install?

 

I don’t, and the heater box is out of the car anyway. So I’m using a stock LS1 water pump from a 2004 GTO, and I tapped and blocked off the existing inlet/outlet, and drilled and tapped two new holes on the top and added 45° fittings (that way it wouldn’t contact the control arm).

The heater box that I have is the AC model, which is a tad bigger and has the copper pipes coming out the bottom instead of the side.

I’ll try to take some photos when I get home in a few hours.

 

I used the OEM heater valve bracket with a cushion clamp to mount the GTO heater valve. It’s mounted on the inner fender in front of overflow tank. Top 2 hoses connect to the heater core. Bottom 2 hoses connect to water pump. Stock vac line reaches the actuator without extending. Heater works as factory with the GTO valve installed.

 

No need to remove the OEM heater / Evap box with LS swap.

 

Looks tight, but as long as it fits!

 

So I think this needs to be dumbed down for me…what does this actually do?

 

I think people are talking past each other here. If you don't have a heater core installed, no amount of plumbing is going to get you heat. It's a miserable job, but not impossible, I've done it once with a leaky core. A quick look at my car didn't seem like anything major was in the way.

 

Another way to get heat is a heated seat insert, or even just a heated seat cover.

I'm sure you already have an alternator that works a LOT better than what came with your C3.

FWIW, the Holley mid-mount system has downward-pointing heater ports, if that makes a difference for you.

 

Install this valve and it will allow water flow on the backside of the LS thermostat (valve closed position) to allow the thermostat to open properly. The in the valve open position, water is directed water through heater core when needed.

 

I think you’re right, while I’m getting good information, I think we’ve lost the plot a little. I have the heater CORE installed, but not the heater box/blower. And while plumbing is good to know, I have to make sure I can actually get the heater box in the engine bay first.

If I’m looking at this correctly, I don’t believe your heater box was fitted for AC, am I correct? I think that might be the crux of the issue, as cars that came with AC from the factory have a larger heater box (which is what I have). I’ll try to get it in this weekend, to see if I can get mine to fit, but I’m not confident it’ll work. I used Dirty Dingo sliding mounts which allows the engine to sit further forward and a tad higher, so maybe I’ll get lucky.

All that said, if my heater box DOESN’T fit, I’ll probably just forgo heat and AC for right now, and install Vintage Air or something in the future. If I were to remove the heat all together, what’s the best way to do that? Removing the core and patching up the hole in the firewall is obvious, but what about plumbing? Do I just loop the inlet/outlet ports on the water pump? Would it need to be a long loop? Do I just remove the fittings and plug the holes I made? Do I need to cut holes in the thermostat or something to allow water to flow through the two chambers?

This isn’t necessarily a question for you, ignatz, but more so anyone who might have an answer.

 

I was going to install aftermarket Corbeau seats, so if I can’t get proper heat to work, maybe I’ll spend the extra $200 and get the heating elements put in the seats.

 

Well that'll keep your tush warm but won't defrost your windshield. I couldn't drive anywhere early mornings in winter until my engine warmed up.

I misunderstood the terminology you are using. By heater box, you mean the enclosure? Otherwise you have an exposed core? Yes my car never had AC. I don't know what else is different about AC vs non-AC cars, but might not a non-AC enclosure work? If you can find one? I guess I'm not helping much here, but plumbing was only a mechanical problem in water pump fitment on my car.

I'm going to have to go out to the garage and understand this a bit more to help.

 

Yes that’s correct, I mean the enclosure - sorry. Right now my heater core is exposed, but I need to put the box back in to cover it all up.


This photo actually might help. The one on the left is the box for a non-AC car, and the one on the right is the box for an AC car. I have the one on the right and I’m trying to figure out how in the hell I’m supposed to make that happen 😅

I plucked this screenshot from of a guy swapping out the boxes. Seems like a fairly easy swap out, but I’m sure finding the parts is going to be a pain in the ***.

Man, between this and the power seat making the driver side floor pan deeper…I’m kind of over the luxury options that came in this car!

 

I can't speak to the dirty dingo engine mounts but I have an '81 with AC and no issues with fitment around the box. If they are adjustable, then maybe adjust which holes you are using on the mounts. Regardless, I'd think it would only take some minor modding of the box to fix any issue. Keep in mind if you locate the engine in a non-factory location you may need to look at your driveshaft alignment. Good luck and let us know how you make out.