3.5 diff gears and a/c

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Avert1247

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Looking at a 71 Mach 1 with a 429CJ and 3.5 traction lok. It's had a/c added later on its life. According to the 429 megasite, you could only get a/c on a 429CJ car with 3.25 gears. So two questions, what was the technical reason for this? and will I experience any drivability issues?
 
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From what I remember the stated reason was to avoid turning the compressor too fast for prolonged periods of time. Others may know something different. Chuck
 

machphil

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If over driving the compressor is the only issue, one could put an underdrive crank pulley on it to slow the accessories down.
 
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Looking at a 71 Mach 1 with a 429CJ and 3.5 traction lok. It's had a/c added later on its life. According to the 429 megasite, you could only get a/c on a 429CJ car with 3.25 gears. So two questions, what was the technical reason for this? and will I experience any drivability issues?
Ford engineers felt that an A/C compressor on powerful engines that were going to be flogged would likely not survive at high RPMs. Rather than simply providing a larger diameter "driven" pulley they simply said you had to have a 3.25:1 or 3.0:1 rear axle ratio. Our 1969 Shelby gT500 has a 3.0:1 rear axle ratio. Luckily its 428CJ has gobs of low end torque so it still performs nicely when I goose the throttle. But, I know there would be a lot more spirit coming from the powertrain if I were to have had a 3.5:1 or lower rear axle gear ratio. No regrets, it performs just fine, and is an excellent cruiser art highway speeds.
 
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As mrgmhale posted, it came down to the limitations of the AC compressors. Ford was utilizing York and Tecumseh AC compressors which were as good as any in the 60s and 70s. The Tecumseh had a cast iron housing and was production installed in many F600-F800 trucks, yet I still have seen them come into the shop with a large "Ventilation" hole in the side of the compressor.
The main problem with the 3.91, 4.11, and 4.30 cars was the load imposed on the compressors when the sudden increase of extreme RPM's subjected them to stress they just were not engineered to handle. I believe the 3.50 AC restriction was a little warranty wiggle room for Ford and the inevitable warranty claims they knew would be coming if they didn't draw a line in the sand. In the 60s and 70s, you could not get AC with any solid lifter engine since Ford knew how they were geared and how they would be driven. This restriction would have included all 271-289s (High perf 289), '66 428 Police Interceptor, 427's, and all Boss engines.
You could do as I did in my Mach 1 and just cut the AC off if you thought you were going to get involved in some sort of red light "Confrontation" or any high RPM cruising. My '85 GT has a micro switch at the carburetor, so when you go to WOT, it opens the circuit to the field coil. Nice to have, but also just as easy to turn the AC off.
I switched Money Pit #1 ('71 M code with AC) from a 3.00 to 3.50 ratio, and the smile factor outweighs any extra cruising RPM or milage drop! 😊
 
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The 3.25 or lower was just to keep the compressor speed down. You will be fine with the 3.50 gears, don't sweat it too much.
With our 73 Mach 1 the prior owner replaced the original 2.79:1 rear axle gear set with a 3.5:1 TractionLock unit. Using P235R60x15 tires, and the original C4 (it was originally a 302 2v engine, now 251W [not C] built for street/strip) the engine RPM was just about 2,900 at 60 MPH. Tolerable, it did not sound like the engine was about to grenade at highway speeds. But, when the C4 began to slip in its WOT 1-2 upshifts I decided to drop in an AOD vs rebuild the C4. Now, in OD/4th gear at 60 MPH the engine RPM is only about 1,900 or so. The Mach 1 is much more pleasant to drive and cruise in at highway speeds. What a difference. But, before the AOD I felt the 3.5:1 ratio offered a nice compromise between drivability and performance. That said, had I known the AOD would be dropped in I might have gone with 4.11:1 TractionLok gears - the AOD makes that much difference. But, with friends who have 4.11:1 gears in their vehicles I would not want to be driving at highway speeds for very long unless there was an OverDrive gear. And, I would be concerned about the A/C compressor spinning at that speed without an oversized/underdrive pulley.

I think you will be fine with 3.5:1 gears re: engine RPM and compressor RPM - especially if you have a modern Sanden unit like we got with our Classic Air A/C kit for our 72 Mach 1 (not an A/C car originally).
 
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