Question: I've been a loyal reader of your magazine since I was 14 years old when I became bitten with the 'wheeling bug. Probably the part of your magazine I have enjoyed the most over the years is "Techline." I have always found the advice given in your column to be quite informative.
In the April '06 issue someone with a Suzuki/Ford 302 had a question as to which transmission/transfer case combination would suit him best. I'm sorry to say that your response had a few errors in it. First, the low range of the NP207 transfer case is not 2.72:1, it is only 2.61:1. The 2.72:1 ratio was used in the NP231 and 241 transfer cases. Second, you stated that you would not use the C4 transmission but rather swap in an AOD because it offered a lower First gear and an overdrive. While the AOD does offer an overdrive, the First gear ratio is actu-ally slightly higher than a C4. The AOD uses a 2.40:1 first and a 1.47:1 second while the C4 uses a 2.46:1 first and 1.46:1 second gear. The only Ford automatic that uses substantially lower gearing than this was the 4R70W (for all purposes, an electronically controlled AOD that utilized a wide-ratio gearset), which used a 2.84:1 first and 1.55:1 second. The 4R70W's overdrive isn't nearly as steep either at 0.70:1 compared to the AOD's 0.67:1.
In my opinion, he should use the C4.While only being a three-speed auto, it is very light and saps very little torque from the engine. In fact, it is in the same class as the Powerglide in the amount of power it uses to operate. Durability shouldn't be a serious issue either, because the C4 can be seriously beefed up with better turbine (input) shafts, larger servo piston(s), and replacement steel six-pinion input planetaries to replace the failure-prone stock aluminum three-pinion planetary. The C4 doesn't require a computer to operate it, either (neither does an AOD, I guess).
Even if the AOD was geared lower, I wouldn't use it due to the power flow in Third and Fourth gears, non-selectable lockup feature, and the fact that its shift pattern uses a crappy three-quadrant setup which only offers Overdrive, Drive, and First, with no selection available for Second. While you can get a Second gear by manually shifting from First to Drive and then back into First before the transmission can make the shift into Third, this is not a good idea. It will quickly fry the OD band and direct clutch, as well as eliminate engine braking in second.
You also can't get away from the lockup feature due to its entirely mechanical nature. Plus the 60/40 (60 percent mechanical, 40 percent hydraulic) torque flow in Third gear isn't the best either, especially in high-load, low-speed situations when a vehicle would need all of the torque multiplication of the torque converter in these situations (or at least more multiplication than this setup will offer). There is an aftermarket turbine (input) shaft offered that will entirely eliminate the OD lockup feature and split torque flow in Third, but this can add a lot of heat to the transmission, especially during extended cruises in OD. All of these problems are addressed by certain aftermarket companies, but the cost of all of these modifications usually raises the cost of a rebuilt AOD to anywhere from $1,200 to $2,000. Ouch! You could use a later-model version of an AOD (an AODE is better, but the 4R70W is best), but it would necessitate the use of an aftermarket stand-alone computer or swapping in a new engine/transmission combo from a donor vehicle.
I think he should stick with the C4. It's lighter, just as strong, sucks less power, is simpler in design, doesn't need a computer, and costs much less to rebuild/modify. And if you want a lockup torque converter, get a C5. The C5 is nothing more then a C4 with a lockup torque converter.
Answer: Thanks for catching me on this. You seem to be very knowledgeable about Ford automatics. You're right, I did make two mistakes, though I should have gone into more detail in my answer. I say most AODs because there were actually some AODs that had the 2.84:1 ratio right from the factory. Even if they are hard to locate, the parts are not. I don't have a current SVO Ford Racing Catalog, but my old one lists an AOD wide-ratio upgrade kit that basically uses the 4R70W transmission gearset. Art Carr has a similar kit available.
Yes, you did point out some problems with the AOD transmission and how expensive it would be to rebuild to solve these problems, but you also made note of the modifications needed to beef up the C4. I don't think that $1,200 to $2,000 is out of the ballpark when it comes to a performance automatic transmission. The performance transmission guys tell me that the AOD can be made plenty stout and handle any amount of power that the 302 could put out.
Oh, I will agree with you as to the C4 being the better transmission to use for his conversion.