The percentage means how much of the engine's output power is being sent to the tires via the converter. Meaning, an 89% converter will send 89% of the engine's torque to the transmission/tires, and allow for some stall (slippage) just like all automatics. A 91% converter will send 2% more. The 91% converters are primarily used for drag and pulling applications where big, really big hp and tq numbers are being made, but not really recommended for street driving. Ask the guys at PDR about that.
Of note - factory stock converters are rated around 60%. So, when your on a hill, and the engine is revving its heart out, and the truck just slows down, it's because the stock converter in the truck is literally bleeding out. There's nothing wrong with the transmission or the converter, just simply they were designed that way from the factory. The reason for that is because the early A-518/46RH transmission's can't handle much more than 250 hp without eventual breakage, so the factory detuned the engines to 130 hp and that made it so they could get the trucks on the market without big problems with the transmission's - they hadn't got the lock-up converters fine tuned yet, so, they got out what they could. Dodge was slated to introduce the Cummins in 1985, but they couldn't get a frame or transmission to handle the torque yet, so it had to wait until 1989. And the overdrives weren't ready until 1991, which is why the 1989-1991 rigs had the 3. 07 gears.
I say 'eventual' breakage (read: failure) because not all of them will break or even wear out, but, big power will probably make a stock transmission fail. With BIG power (aftermarket turbo, like an HX-35 or better) and a small collar (like a 16 or 14cm), where you have quickquick spool up on the turbo and big fuel (POD's or big injectors), the input shafts on a 518 can snap on a hard launch especially with a big load - something has gotta give, and the overdrives have smallish oil passages and can literally cook internally under long-term heavy towing. . under big power.
DTT used to advertise their converters in percentages, but they're now building each converter custom ordered to customer specs. I'm installing my new DTT converter tomorrow. I ordered it from Laser Bob at Laser Track and Alignment in Greeley, CO. These are the specs as I ordered it:
- Up to 300 hp
- 20,000 gross towing weight
- Not a drag race application
- 2wd
- No other transmission mods (including valve body), except for a MHT deep pan.
So, there are other less expensive converters. In my first truck I put in a TCI unit from Summit Racing - it cost $350. Not bad, and a big improvement over stock. Once I get this DTT unit in tomorrow I'll post back, and I bet I post back that it feels like a manual transmission.
It's all about heat. The more the converter 'stalls' the more heat it'll make, and the sooner the transmission seals will fry, and the sooner the transmission will fail. I spent the $750 on the DTT unit, and I'm 100% sure the transmission will not have a heat issue for as long as I drive the rig.
Now, when I build my Megacab rig, it will have a full-boogie DTT-built 47RH transmission with lock up converter - that'll be the only way to handle the 400 hp and 1000 tq the engine will be making at that point... but that's another story

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Hope that helps. Any other questions, ask away.
- M2