Retarder or Exhaust brake for MT647


New Member
I've been trying to source parts to convert my MT647 to an MTB (or find a used one) - The only place I'm finding with parts is Defeo Mfg ( Do they have a decent reputation? I'm starting to consider an exh brake instead.

There's a few of us Foretravel owners out there with the older 643s and 647s. We're usually told fitting an inline exhaust brake isn't worth it. My Allison parts book shows my 647 will lock up in 2nd, 3rd and 4th, so shouldn't an exhaust brake work reasonably well with a 647 on a Cummins C8.3 mechanical?

I ran across a tech note in my parts book (SPI-1-TR-95, dated 1 Jan 95) for modifying the 600 series trans for extended lockup with an exhaust brake (via the external lockup port). Is this a worthwhile modification? My A/N is 23042335 and I believe its standard modulation meaning I would likely have to also buy a new seperator plate.