Saturday, December 10, 2011


Well, the trusty-- and crusty--12-bolt I'd purchased to replace the bent 12-bolt in my 29 Model A turned out to be a lousy benefactor for the Model T. It was no less than six inches too wide, and being well used not worth spending the time and effort of narrowing. So I've bitten the bullet and I'm buying a new 9" Ford unit from John's Industries. Mercury Charlie will be ordering it next week and the only unanswered question is what the rear end ratio should be. Seeing as the 425 Nailhead is a very torquey motor and the car is going to be well under 2500 lbs -- probably two thousand lbs. less than the car it came in -- I won't be needing a steep gear. The turbo 400 gives me three gears and no overdrive. So I need to get the rear gearing to where the RPM's are comfortable for cruising speeds, figure 65-70mph. I'd like to be taching between 2100-2300 at freeway speeds. I narrowed it down to 3.0, 3.25, or 3.50 as my options.  A quick post on the HAMB revealed the following feedback based on a 30.20" diameter rear tire (below):

2016 RPM @ 60 MPH with 3.00 gears
2184 RPM @ 60MPH with 3.25 gears
2352 RPM @ 60MPH with 3.50 gears

Considering the fact that the 425 Nailhead reaches maximumum torque at 2,800 RPM, we'll go with the 3.50 gear set which will provide the following RPM's at the speeds noted using a 30" tire:

2352 RPM @ 60MPH
2548 RPM @ 65MPH
2547 RPM @ 70MPH
2940 RPM @ 75MPH
3136 RPM @ 80MPH
3332 RPM @ 85MPH
3528 RPM @ 90MPH
3724 RPM @ 95MPH
3920 RPM @100MPH

While I'm on the subject of the rear, I've decided to go with a non-posi unit, as I'm not planning on doing any racing in it and feel that the single track nature of the rear end will be a more comfortable choice for the way I'll be using the car, which is primarily for casual cruising. It also saves on the overall cost, as a posi TruTrac would add about $900.

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