Re: MPH

Posted by Joe on March 03, 1998 at 10:49:37

In Reply to: MPH
posted by Jerome Bokelmann on March 03, 1998 at 08:31:42

Is this possible that I have a different gear ratio than most cars?What should the ratio be?How difficult is it to modify the ratio to get the car more normal.
Yes it is possible.No it is not (that) difficult.
The Model A cars mostly originally came with a 3.78 (or thereabouts) rear end. For the small Model A trucks, (CC pickup, roadster pickup) a 4.11 rear end was available also.The Model AA trucks varied but had ratios even higher than these for the "heavy hauling" service they were built for.
You may have one of the 4.11 rear ends under your car.A lot of mix and matching used to (and still does) go on between available parts.I have been told that you can tell a 4.11 rear end in that the torque tube is a gradual taper from differential housing to transmission connection whereas the 3.78 rear ends were pretty constant diameter for the rear 2/3rds of the tube and then a more abrupt transition to the transmission connection.
I have also heard that the torque tube on the late 31's is the gradual taper type also as was the subsequent Model B's so this method of determination is not absolute.
Obviously the higher the ratio, the less is the maximum speed possible.There are a couple of charts available in the literature showing the engine rpm versus speed that is attainable.
As you have noted, the downside to high speed in the Model A is premature wear on the engine.Specifically, the engine has a tendency to "wipe out" the centermost of the three main crankshaft bearings.
Today there is available a 3.54 ratio set of gears which is probably a good investment considering the highway driving that predominates today.Even with these, a sustained 55 is tough but do-able on the engine.The downside to this though is that "pick up" of the car is somewhat less.Modern cars will do pirouettes around you at stoplights.
In the "old days" the speed boys often bought aftermarket gears at 3.23 ratio.I'm not sure I would want these with a stock engine.
To change the gears is not a venture to be entered into lightly, but it is possible (people like myself even do it for pleasure.)You will first have to deal with the rear spring problem and the compressed energy stored within.There are safe ways of dealing with this, of course.Later, you will want to be sure to put the rear end together exactly as it came apart otherwise you are liable to get three speeds in reverse and one speed forward.Read up on the available literature and you won't go wrong.I recommend the "How to Restore Your Model A" series of 5 books by the MAFCA.
The various aftermarket overdrives you will see advertised are another way to increase the top speed of the Model A Car without beating the engine up in the process.Those who have overdrives generally are pleased with the results.The downside here is one of cost.
I have often thought the best arrangement overall would be to have a 4.11 rear end and an aftermarket overdrive.Thus you won't sacrifice that much at the stoplight and still have a reasonably good highway runner.Any comments from others on this?
Best regards.Let us know how you make out.
Joe

E-Mail Comment to Webmaster