Most guys who own a 47RE or 48RE can tell you it's a four speed automatic. What they can't tell you is what is actually happening inside that case every time they push the throttle. This episode changes that.
Todd walks through the entire mechanical layout from the ground up, starting with how the 47 and 48RE trace their roots back to the three speed 727 and what Chrysler did to turn it into the four speed that ended up behind millions of Cummins trucks. From there he covers all three internal shafts, the differences between output shaft configurations, and the Simpson planetary arrangement these transmissions share with the Turbo 400.
Each gear is shown physically on the bench so you can see the rotation and ratio yourself. First gear, second, third, reverse, and overdrive all get broken down into exactly which clutch packs and bands are applying and why. The overdrive section is particularly worth your attention, including how an 800 pound spring keeps the overdrive planetary locked out during the lower gears and how the overdrive piston releases it to spin your output shaft faster than the intermediate at a 0.69 ratio.
If you want to actually understand what is happening inside your transmission before you build one, tune one, or just talk intelligently with the guy rebuilding it, this is the episode to watch. Subscribe on YouTube and follow on Spotify or Apple Podcasts so you do not miss what is coming next.
Everything covered in this episode comes straight from the Power Driven Diesel shop floor. Head to PowerDriven.com to see everything they carry for your diesel build.
Shop Power Driven Diesel: https://www.powerdriven.com
0:00 Intro: 47 and 48RE fundamentals overview
0:30 Four-speed automatic with lockup explained
1:01 The three shafts: input, intermediate, and output
1:07 727 origin story and how overdrive was added
1:46 Output shaft variations: 4WD, 2WD, and RH hydraulic governor
2:47 Simpson planetary arrangement
3:08 How planetary gears work: sun, ring, and planet gears
3:52 Sun shell and the direct drum connection
4:15 Forward clutch assembly
5:22 Direct drum and direct clutch
6:10 Second gear band and its relationship to the direct drum
6:29 Low reverse drum
6:51 Assembling the planetary and visualizing power flow
8:13 Neutral and how the transmission idles without output
8:22 First gear: forward clutch and low reverse band applied
9:23 Second gear: band swap and 1.45:1 ratio
9:51 Third gear: planetary locked out, 1:1 direct drive
10:29 Reverse: direct clutch plus low reverse band
11:15 Overdrive planetary assembly overview
12:36 The 800lb spring and overdrive direct clutch pack
13:36 Overdrive piston and how the overdrive brake applies
14:54 Overdrive ratio: 0.69 and what overdrive actually means
16:31 Second gear band and double-sided servo deep dive
17:54 How third gear pressure releases the second gear band mechanically
19:17 Low reverse band: double wrap design and servo setup
20:46 Why manual first matters for drag racing and heavy loads
21:13 Outro and what is coming next
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