7580 drivetrain compared to 8550

A thread on another forum eluded to the drivetrain on a 7580 not being able to handle the power of a 8.3 Cummins. How different was the drivetrain components of a 8550?   Those were 300 from the factory right? I have a 7580 but no experience with a 8550.

Don't hold me accountable for mis-information, but I'll try. I think the drop boxes in both models are the same, but I beleive that the 8550 has a heavier duty transmission. Not sure about front ends, but I don't think they've ever been problematic. I think rear ends are also the same, straight out of a 7080, I beleive. If I am wrong, please, someone correct me. Darrel

I have run an 8550 for 35 years and in that time have had a rod come out the side of the block and the transmission gears and power director fail. The engine was a problem on those tractors, but mainly because the designers tried to use the 20 speed power director transmission out of the 2 wheel drive tractors to deliver 305 HP to a heavy 4wd where there was no wheel slip. The engineers modified the 2 wd transmission by making some of the gears a little wider (maxing out the width) and using special higher load bearings in the same gear case as the 2 wd used. In the big 4wd they set the torque limiter to 600 ft lbs to protect the transmission that was originally designed for the torque of a 426 engine that ran at 26-2800 rpm. Horsepower is derived from Torque X RPM so by increasing the RPM you decrease the torque. This was the design logic that AC used to not have to design a whole new transmission for the 300 HP tractors. But it mystifies me why they would take a perfectly good engine like the 844 cid that made really good power and TORQUE, decrease the bore and stroke(which took a different crank) in order to make the 731 engine that ran at crazy high rpm (25-2800 rpm)for a big heavy engine. It would have made so much more sense to have beefed up the transmission to handle the 800-900 ft lbs of torque that a 300+ hp engine would produce at a comfortable 1800- 2200 rpm. Fuel economy, longevity, pulling torque all would have been better with the higher torque engine combo. They did correct this partially in the 4W305 which ran about 2300 rpm. It was a pretty good tractor. Another interesting thing was that AC sold that 20 speed Power Director transmission to Steiger to get a pto in their CR and KR series tractors. Up to 280 HP in a 3406 Cat engine the tranny seemed to last very well. I replaced the 731 in my 8550 with a small cam Cummins 855 set at 350 HP. If you run the transmission in 5th gear, low range in the drop box, and low in the power director, you are transmitting power straight through the main shaft of the power director and you will not have the problems of heat and overload on the small gears and bearings that were designed for 160 HP from a 426 engine at high rpm. I have no experience with a 7580 or 4W220 but would say with fair certainty that the transmission in them would be little different than the big brother. I have not been able to find an engine torque spec for any of the 426 engines but from my experience with them they were not very torquey, specially at low end. That seemed to be designed into the fuel pump- maybe to protect the transmission and the engine. The PD trans had a low pressure light that came on below 1800 rpm which indicates it should not be run that low. Just checking on specs on a 8.3 Cummins- 660 ft lbs torque @ 1300 rpm on the 250 HP setting. 1200 ft lbs torque @ 1300 rpm on the 400 HP setting. That is a whole different world of power and torque than what these tractors were designed for. That being said, my 8550 with a 350 Cummins (900 ft lbs torque), pulled at 19-2150 rpm in 5th gear has made close to 4000 hours, and is still a good running, reliable unit, with little trouble and amazing power, torque and great fuel economy(8-9 Imp gal/hr vs 12-14 on the AC 731). In over 6000 hours I have had zero trouble with rear ends, hydraulics or pto. Over the last few years several knowledgeable posters on this forum have made very good posts on this subject. I thank you all for your contributions. Especially 427435- Mark. I miss your voice of knowledge and reason.