One of my winter projects was to replace the blower motor in my 73 with AC. One popular retrofit is to use a C4 motor (with homemade spacer) which makes much better air flow. This thread came up the same time I was working on getting my old blower motor out. https://www.corvetteforum.com/forums...ence-73ac.html I ended up doing the same thing the OP doublell tom did in his car. I am of the same mindset as the OP in that thread and didnt really want to use a spacer if I could avoid it. Full disclosure, I made one months ago but I ended up not using it this time, though I might in the future. I will explain.


I have 4 blower motors.
1) Original C3 from my car
2) replacement C3 motor from Summit (the same one Zip sells for $80, Summit has it for $55)
3) Original C4 motor I purchased off ebay last year
4) New replacement C4 motor (without cage) I bought online made by Four Seasons.

I wanted to compare all 4 of them to see if there is any real benefit to the popular C4 retrofit. One of the first thing I did was hook them up to a 12V car battery I have on the bench and measure the current draw from each motor. This is in free air and not installed so bear that in mind that once installed the current draw will be higher than my reading as the load on the motor will certainly be be higher. Pushing air through the ducts work will increase the motor's load as compared to spinning freely. All were tested with the cage they came with except the new C4 Four Seasons motor which had no cage when I bought it... so swapped the OE C4 cage to get a reading. One thing to note, the C4 motor both OE and the replacement is a more powerful motor however that does not necessarily correlate to the physical size of the motor. For example, the original C3 blower motor is physically larger than the C4 replacement however the C4 replacement is bigger in terms of how much more work it can do.
In any case, when hooked up to the battery and my ammeter this is what I got:

Original C3 motor: 10 amps
Original C4 motor: 12.5 amps

New C3 motor: 7.5 amps
New C4 motor 10 amps

How much air they move is really subjective and I don't have any numbers to put to them but for certain the C4 motors move more air. Which makes sense since the C4 upgrade has been done many times with many testimonies as to the benefits. But its not quite that simple. For the (not so) fun of it, I wanted to test it in the car since I'm already here. So I installed the new Four Seasons C4 motor with the spacer in my 73. I then swapped it out for the replacement C3 I bought from Summit. The difference here is also subjective but I will say the C4 motor did seem to move a little bit more air but not by much. The C4 motor and fan makes a little more noise but also not by much. (Side note, you cant put the C3 squirrel cage on the C4 motor. The "offset" on the cage is much different and so you would still need a spacer in that scenario)

In the interest of not making this a mile long post I will summarize.
The replacement C4 motor with the spacer moved the most air, more than the new C3 replacement. Again its my perception here, I don't have numbers to put to that. But if you need that little extra then the new C4 replacement with the spacer may be the way to go. If you don't, or you just don't want to fuss with a spacer then your not sacrificing much by going with the C3 replacement, and its a little quieter if that matters to you.
For sure one thing is perfectly clear, you don't want to use a 50 + year old C3 blower motor. What's also clear is you really don't want to use a 40+ year old C4 blower motor (if you can avoid it). The newer blower motors are much smoother in operation, run a little quieter, draw less current and move more air. While fussing with the motors on the bench its clear the newer motors have noticeably stronger fixed magnets. Heat can kill a magnet and over the last few decades the OE magnets have been exposed to enough heat from the engine compartment they most certainly have lost strength. Newer design and materials in the newer motors may also play a role. I ended up using the C3 replacement without the spacer for now however I may decide to go to the new C4 and spacer next summer.


If you already have a C4 motor in your C3 and it works then dont waste your time swapping it out. Its probably not worth it at this point. However, if your still running the original C3 blower motor and you live in a climate where you can benefit from the upgrade then its probably worth doing.


A few notes, none of them come with gaskets but it looks like GM used butyl so thats what I used. Got it from local Napa. The C3 replacement is also the same part you would use for ang G Body replacement. Also, that little air hose from the C3 motor will not work on any new motor including the 40 year old C4 motor. I used Microsoft AI copilot to find this little gem. Also from Summit. FOUR SEASONS HVAC BLOWER FAN HOSE | 75769. It fits the new motors and it was really easy to install.


I hope this helps anyone looking to replace their C3 blower motor

 

I run a C4 blower motor with home made spacer. My spacer is plastic. Something I found in a home hardware store in the plumbing department. And modified.
Many years I ran the C3 blower. If you put your hand in front of the upper center vents. You could feel the air blowing. As you pulled your hand away, about a foot and a half back you could no longer feel air flow.
C4 blower motor, now driving down the road I can feel air flow on my face. Huge difference! I need to turn it down sometimes now.
A tip for those considering this, the 84 and 85 C4 blower motor is wired the same. I spliced the c3 and c4 cooling tubes together in the center, as my C4 blower motor came with a cooling tube.

 

No question about it. The 50 year old C3 blower motor is about useless.

 

I have a 1978 that blows OK, but if I replace it I would want to go to the C-4 motor, but I am not familiar with the spacer needed. Any photos available ?
Ed

 

There are so many old threads on this mod. Most with photos. Basically you just extend the sealing flange. You can just use your motor for a patern. 7/8 inch thick will do. Many use plywood. I went plastic. Really easy.

 

If thats the route you choose, dont use an original C4 motor pulled from a 40+ year old car. Get a new one. They're not that expensive. The new replacement motors are a lot more efficient and draw less current.

 

I installed a new C3 blower motor from Napa along with cleaning out some debris and it really made a big difference over the original motor

 

I did the C4 blower swap 20 years ago and glad I did. To me the second highest fan speed now feels like high did with the C3 blower so it has given me an additional speed. For the spacer I bought a super cheap plastic kitchen cutting board and cut it with a jig-saw. Some black paint and it looks factory.