351MooseStang wrote:
Thank you for the extra details, TKO. It turns out that a bit more patience paid off and I have a used -246 transmission on the way. It has less than 120,000 miles on it so hopefully it isn't too thrashed. As far as the V6 models, I was thinking that the ratio differences in 2nd and 3rd were the difference in strength (265 lb/ft vs 300 lb/ft). Is that it or is it something else.
Either way, I'm happy that I'll soon be ripping it apart for some fun!
2nd and 3rd has nothing to do with the strength rating of the transmission. Here's my tirade on the absurdity of how those ratings are determined:
Torque ratings are determined by the manufacturers putting the transmission in 1st gear and then shock loading it with progressive amounts of torque until it fails. Once it does, the highest load it withstood is determined to be its torque rating. Boy, is there a whole lot wrong with that. Really what this test measures is the strength of first gear in an application with perfect traction. Yeah, because those conditions exist in the real world. The only transmissions I've ever seen break first gear were drag cars launched on slicks with an aggressive clutch. I'm talking pull the front wheels on the launch kind of traction. In the real world so few cars have enough traction or clutch to load 1st gear enough to break it.
So what determines how a transmission does on this test? Primarily the relative size of the 1st speed gear and its counterpart on the countergear. The transmission will be strongest when those two gears are approximately the same size. The larger one gets relative to the other the more likely they will break. The larger gear applies more leverage to the smaller one, and the contact area on the smaller gear gets smaller, so the applied force goes up considerably with relatively small changes in actual diameter. Typically the lower the 1st gear ratio the "stronger" the transmission. This is why a Z spec T5 is rated at 330 lbs/ft. compared to the 300lbs/ft. rating of the factory Fox T5s. Is simply because it has a 2.95 1st gear instead of a 3.35.
Like most myths there's a shred of truth to it, in that when the manufacturer does something like say changing the alloy of the steel used for the gears, it does indeed benefit overall strength, and the transmission will do better on the "test". This is why the early WC T5s were rated at 265 lbs/ft (same as the NWC ones) and the later ones are rated at 300 lbs/ft. The additional strength observed in the "test" is directly correlated to the additional strength of the steel.
In the real world, I've never seen a T5 with a broken 1st gear. 3rd gear is the common one, because in 3rd, even most street cars have enough traction to shock load it enough to break, especially on a powershift with an aggressive clutch.
So how much will a T5 really take? No one knows, and that's why people get so hung up on the ratings. They want assurance that the car they built won't break a transmission. All you can really do is play the likelihood. A stock T5 in a moderately powerful car is actually at more risk than in a high powered car. Once traction in 3rd is out the window its unlikely it will break. I built a Z-spec T5 in a GM case for a guy years ago that runs behind a 383 making over 500HP. Going on 15 years and its been fine.
Driver skill also makes a big difference, but that's another story...
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