Beefing Up 904
Math is hard for some people; but geometry is impossible. If you take the time to measure a 904 and a 727 clutch disc do the math, subtract the difference, You'll find that the 904 plate is a little over HALF the friction area of a 727; exactly only 5/8 the area. A six plate direct clutch 904 is 75 square inches of friction material. A six plate 727 is at 118 square inches. A four plate 727 is 79 square inches; a little bit more than a six plate 904. To get to six plates in a 904 it requires a lock up trans as a foundation for parts. And then you need a dummy lockup converter..more expense. A lot of trouble to get to a point that is still weaker than a stock 727. And for what? 1/8 of a second in a quarter mile? The friction material in a 904 has an outer radius that is smaller than the inner radius of a 727. You can easily drop a 904 plate right thru a 727 plate. Consider the difference in the leverage a 727 plate has since the radius is a lot larger. There's more to strength than square inches; it depends on how close to center those inches are. Based on square inches plus leverage, a 727 plate is over twice as strong as a 904 plate.
The point of my posting that link was to show that there were key differences that Chrysler made to the 999 version to make it strong and durable enough for small block v8 use. The 727 is overkill for a small block. Line pressure is helpful with less clutch surface. 727 were beefy enough for use in diesels. They are overkill for a small cube v8 gas engine. The low gear set would allow a higher(lower numerically rear gear for economy while still having reasonable stoplight performance.
These trans with the lockup are plentiful.
Besides the clutch surface are the steel 4 pinion planetaries for heavy duty use. The distance from centre of the 999 low gear set is very similar to the 2:45 727 gearset with a better ratio. Most 904s use an aluminum front planetary which can fail in the splines. The best feature of the lockup 999 that will really increase your street or racing thrill is the 2.74 low gear that came standard in these trans. You will pay a hefty premium to get that ratio in a 727. The 904 comes standard with a riveted roller clutch, the 727 would require an aftermarket $$$ bolt in clutch.
In a performance application of any kind, the 727 user should be using an aftermarket steel or aluminum front drum, if you value your legs. The 904 does not have the problem of the drum exploding in a roller clutch failure. Looking at the 999 from that perspective, the 999 is strong enough and much cheaper to build, and will give better performance.
Yes you would have to do a few things to convert a lockup to non lockup if desired, but that really is just selecting the right parts from a donor trans. There are also performance lockup converters starting to appear on the market. This is why most feel that going from a 904 to a 727 is not really an upgrade.
Earlier I mentioned that the 904 is approximately .15 faster in the quarter mile, built equal to equal. Equal as in same first gear ratios. Changing from a 727 to a 999 with a 2:74 low gear, along with a cam change and a better matched converter dropped my et over half a second. Ymmv
