I've seen posts regarding an inexpensive upgrade to the C3 a/c system that involves putting a C4 cage on the C3 blower motor.

Can anyone explain how this improves the system? I get that the cage is longer, but that means a spacer is necessary to maintain clearance with the firewall. Since the spacer shrouds the extra length of the C4 cage, I'm not sure how it improves the system. It seems that the size of the cage hasn't changed relative to the air chamber. Can someone help me understand this? Thanks.

MajD

 

I havent done this mod but the pitch of the cage and the performance of the electric motor could be why it improves the air flow.

 

The C-4 wheel has a different pitch and the new motor helps and even though the spacer covers part of the new wheel, you end up with better air flow. I've done this to several C3's, each time I replaced the motor and wheel and made up a spacer that was 3/4" thick. The air flow was greatly improved on all of them. There are several other things that will improve the air flow as well, such as cleaning all the leaves and debris from the evaporator box.

 

Thanks '75, that's just the info I looking for. Can you tell me how much difference the motor makes? I'm assuming it spins faster. I was planning to use my stock motor since it works just fine. What do you make the spacer from?

 

The new motor is stronger than the 30 some year old stock one even though the original still works it's likely weaker. I made the spacer out of 3/4 plywood and painted it black. The other thing that helps is to increase the size of the ground wire to the blower motor. The stock ground wire is much smaller than the power wire. Ive always purchased the motor and wheel for an 86 locally, no need to buy online.

 

Awesome info, thanks. Do you weatherproof the plywood?

 

No, just some black spray paint to help the color blend in.

 

Thanks for the info. Is that all? Motor, fan, spacer and ground wire. All other components can remain as is?

This is definitely what my system needs.

 

Here's the difference between the two wheels. The stock wheel has a lot of end clearance in the housing that is 3-1/8" wide (if I remember correctly). And then the fins of the wheel don't start until about 1/2" from the face of the motor. By installing the longer wheel with less end clearance and all fins in the 3-1/8" deep housing about 5/8" to 3/4" more fins are doing the job. It requires a 3/8" spacer (I used plywood) and I did spray my spacer with urethane just for the heck of it.

In the end you'll get a LOT more air out of your vents so its a modification that is worth the effort and minimal expense (the 4-Seasons wheel cost me $16).

 

Wow. I'm toying with the idea of tackling my under-dash wiring and my HVAC system as a winter project.

I had no idea this upgrade existed.

Very simple fix and timely thread!

I love this forum.

 

I did the same thing basically, I just sealed the plywood with a fiberglass resin for moisture resistance and painted it black. I had to use 3/4" plywood for clearance and ended up running 10 gauge wiring for the high speed blower. I'd say overall it gave me a 25-30% increase in available airflow. Also, take the time to find a rubber hose that will provide ducted air back to the windings. Not only keeps water and dirt out of the windings and armature, but the winding need that air to keep them cool.

 

Thanks too broke, it sounds like you used your stock a/c motor. Is that true?

 

Thanks. Did you get a specific high speed motor, or just order the stock C4 motor from a specific year?

 

Yes, I did use my OEM motor but right away I found I had to use my hydraulic press to push the new blower wheel's hub back a whole 3/4". If you use a later motor like 1978 or later you don't have to alter the blower wheel at all. So if you're planning to do it use a later motor because the position of the motor's flange is about 3/4" different from the early motor's.

The housing is about 3-1/8" wide and the stock blower wheel's fins only takes up about 2-1/4" of that 3-1/8" width. With the new wheel and motor spaced out 3/8" you end up with about 5/8" more fins in that 3-1/8" of space and that's why the longer wheel blows so much more air. I estimated it'll blow about 1-1/2 times as much air; so much you can actually feel the air hitting your face as you're driving.

 

Here's a picture of my 3/8" plywood spacer. Note how the screw holes aren't concentric with the bore of the spacer because that's how the motor's flange is made. And here's picture of the eight 3/8" holes I drilled thru the face of the hub to allow cool air to cool the motor. Your OEM cooling hose will JUST make it with a 3/8" spacer.

 

I remember I had to use a carbide cutter on my air grinder to enlarge the big hole in the housing because my new wheel was an itsy bit larger in diameter than my OEM wheel but it was no big deal. If I recall correctly the new wheel was rubbing the top of the hole by about 1/16" but the carbide cutter removed the material real fast (the plastic housing has a steel sheet metal backing).

After I installed my new blower wheel a friend of mine with an '80 did the same thing but as he has the later motor he didn't have to push the hub back like I did. So I suggest using the later motor to save yourself some work.

You'll really like the outcome...............

 

FWIW, the C4 motor turns the same speed as a new C3 motor. If you're still running the original motor - it's led a good life and going to a new C3 or C4 motor will get you some extra RPM. But if you already have a new C3 motor, you won't realize any advantages with the C4 motor.

Now, upgrading the wiring is definitely worthwhile - the grounds in particular are weak. The nice thing about the C4 motor is that it has a dedicated ground wire, while the C3 motor uses a case ground via one of the fan screws.

Another thing to look for when considering this mod is any loose ducts or bad seals - to really get air flowing through these vents, you need to make sure you aren't losing any in the process.

 

I ordered the stock C4. The motor has a lot more torque, and due to the pitch of the squirrel cage, it moves a lot more air.

 

i would check all the seals and the case in the engine compartment.

 

Sorry for reviving this thread so late but I was out of town. I have a '79 and I measured the cage that came with the motor. It is 2.75 inches deep and the housing is about 3. When I put the motor into the housing, and lay a level across the housing, there is about 1/2 inch of clearance between the level and the edge of the blower cage. From what I can tell, the cage is recessed about 1/4 inch into the hole where the motor mounts, so that would account for the extra space.

So my first question is how much clearance do you set between the edge of the housing and the blower cage? The hole in the firewall has a lip that could hit against the cage if it's too close or slightly misaligned.

Second, is the opinion that this mod is still worth doing over just putting the old system back together? I may only get an additional 3/8 inch of fan length. It seems that the additional torque from a new motor plus the pitch of the blower fan make this a good mod regardless of the additional length. Is that the consensus / experience of anyone with a late model C3 that's done this? Thanks.