67 Air Conditioner Pulley Size vs Pressures
Got a friend that's considering purchasing a new compressor for his 67 SB coupe. One he looked at has a 5" clutch/pulley while the other has a 6". I know mine has a 5.5" pulley. Would there be any advantage or disadvantage to using a 5 or 6" one over the original 5.5". I'm sure there will be a difference in the speeds the compressor turns, but will the 1/2" diameter make a difference in pressures within the system, so as to be damaging to components? TIA Al
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Re: 67 Air Conditioner Pulley Size vs Pressures
As long as the belt doesn't slip, the power supplied to the compressor from the engine should not change with pulley diameter. However, the compressor speed may change, which can affect compressor capacity. The larger diameter compressor pulley was used by the SHP solid lifter cars to reduce compressor RPM at high engine RPM..........and perhaps sacrifice some capacity for compressor life.
A smaller pulley will speed up the compressor and provide more capacity.........and may or may not have any effect on compressor life depending on how the car is driven.
If I recall correctly, there were also a couple of different cu. in. displacements available with the A6. This may also factor into the compressor pulley size used.
You can change out any pulley size for another if you want.
The pressure within the system is more dependent on the refrigerant, design of the system, and air temperature (both ambient and cabin).
If use change compressor pulley size, you will probably need a different length belt as well.
Larry -
Re: 67 Air Conditioner Pulley Size vs Pressures
Larry,
You certainly know your stuff, and from what I read and seen they went to a dished piston in late 64 and onward with the A6. The bore still remained at 1.5" and they went to the 5 3/4 pulley as you know and the pulley was a bit smaller than 5 3/4'' as Al said his was. the 365 HP 327 went to the 6 3/8'' because of RPM.
I tried my luck a E BAY with a restored 68 which was dated for a build date and was quickly shut down by the others for sale that said (fits Corvette).
Sure they fit, and as you pointed out with the 5" passenger car pulley it would need a different belt.
Al when you get down to it, it depends if they want the correct case,pulley, date code, and so forth. yours is correct in all aspects.
I'm missing the 64 manual but the 65 manual described the change and why I believe.
DomComment
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Re: 67 Air Conditioner Pulley Size vs Pressures
Don't forget the rear gear ratio may also impact compressor speed especially at highway speeds. Lower ratios (numerically higher) gears would call for a larger pulley size as to not over speed the compressor. I can tell you my original 1967 L79, M21 with 3.70:1 posi rear is fitted with an A/C pulley size of about 5.660" outside diameter. I hope that helps.ROCCO SCOTELLARO
1967 Lynndale Blue/Black Coupe L79, M21, G81 (3.70:1), A31, A82, C60, K66, N11, U69Comment
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Re: 67 Air Conditioner Pulley Size vs Pressures
Hi Dom, I suggested sending his to you, but he doesn't want to ship across the country or put that much into a non-original motor unrestored vette. Tried to convince him, you were the man when it came to AC. Hope things are going well for you! AlComment
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Re: 67 Air Conditioner Pulley Size vs Pressures
If you think about it, the A/C compressor has to operate over an rpm range from idle to redline. The compressor has to provide adequate performance at idle, while not throwing belts at redline. I'm pretty sure there is an internal system to regulate the maximum pressure at redline.
A C2 327/300 has an idle rpm of about 600 rpm and a practical redline of about 5000 rpm. A C2 327/375 has an idle of about 900 rpm and a practical redline of about 7000 rpm. So, the "sweet spot" for A/C compressor pulley diameter will be different for the two engines.
I think that a larger A/C pulley diameter helps reduce the probability of tossing the belt at redline, at the expense of possibly providing less cooling capacity at idle. On the other hand, most of the high performance engines have higher idle speeds than the base engines.
So, the "best" A/C compressor pulley diameter will typically be larger for a higher performance engine. I think this is what you will find if you look at the A/C pulley diameters for various C2 engines. Note that the actual rpm of the A/C compressor depends not only on the A/C compressor pulley diameter, but also on the crankshaft pulley diameter.Comment
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Re: 67 Air Conditioner Pulley Size vs Pressures
FWIW: The A6 is rated at around 27,000 BTU (2-1/4 tons refrigeration) at 2000 RPM (compressor RPM) The pulley diameters used on the compressor and the crank will relate this back to engine RPM. I THINK/BELIEVE the crank pulley diameter is approx 6-1/2 to 7 inch on the AC cars........
LarryComment
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Re: 67 Air Conditioner Pulley Size vs Pressures
Joe, that makes a lot of sense! I passed on the info to him last night, so it's his decision now to make on what size to get. I will call him back with your info this morning during his break (oops, missed that) maybe at lunchtime, now.Comment
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Re: 67 Air Conditioner Pulley Size vs Pressures
I thought I would ad this practice that could keep things lasting longer. I know there are compressors that cycle on & off which has to be hard on the clutch.
The A6 clutch is metal to metal dampened by the front clutch rubber. So I try not to turn on my compressor at high RPM's. Sounds stupid but I push in the clutch or shift the automatic in neutral to start the A/C. I started doing this when I saw a clutch throw sparks one night when it was turned on at high RPM in a shop.
DomComment
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Re: 67 Air Conditioner Pulley Size vs Pressures
Yes, this is an interesting topic. The original GM system was designed to run the compressor continuously whenever the A/C was on. If the compressor was generating more "cooling capacity" than needed, the excess was simply wasted.
I'm sure that GM designed the clutch so that it could withstand being engaged at cruising speed (say, 70 mph), since this would not be an unusual thing for customers to do. However, once engaged, the clutch would stay engaged as long as the A/C was on. So, I suspect that GM expected some limited number of clutch engagements at cruising rpm, and designed the clutch accordingly.
Modern A/C systems cycle the compressor on and off to try and keep the average evaporator temperature just above the point where ice would form. This is a much simpler control system, but the clutch is continuously engaging and disengaging every minute or two whenever the A/C is on. Those clutches are explicitly designed for that type of duty.
There are aftermarket kits that will replace the vintage "always on" controls with a cycling control, which will cycle the vintage clutch far more often than the original system did. I have seen some discussions where people worry out loud about the vintage clutch no being designed for such operation, but then others have responded that they installed a cycling control system on their vintage always-on system, and the clutch has held up just fine. Maybe GM put enough margin into the original clutch design.
However, I prefer to try to keep the original always-on control system in good working order. The only advantages of the cycling system are that the controls are simpler and there is a theoretical improvement in gas mileage compared to the always-on system. There is no improvement in the actual cooling capability of the system.Comment
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