Do You Have The Need For Speed?

Do you want to keep up with the moderns?  If you are looking for a little more speed, you have several options. 

Why not try the high-speed rear end?   The stock Model A rear end ratio is 3.78:1.  In other words, the drive shaft turns 3.78 times for each tire rotation.  The high-speed rear end increases the ratio to 3.54:1.  At 45 MPH, the stock Model A engine is turning 1926 RPM’s.  Maintaining the same RPM’s, with the high-speed rear end, you can push your car to 48 MPH.  You gain 3 MPH.  Excited?

OK, so you are looking to go just a little bit faster, think overdrive.  There are several options on the market like Ryan, Mitchell, and Borg Warner.  Borg Warner units are popular with Penn-Ohioans.  Borg Warner units have the tallest overdrive reduction at 30%.  You can set it up with a manual or electric shift.  They also have some setbacks.  With a Borg-Warner overdrive as engineered and built by Bob Greene or built, installed and serviced by Lloyd Young, they do not free-wheel unless and until the overdrive is manually (or electrically) disengaged by the operator.  The holdback effect is not as great as in third gear, just as third is less than second gear.  You need to be sure to disengage out of overdrive before shifting into reverse.  Ask the man that owns one.  Another popular choice is the Mitchell with a 26% reduction.  It is fully synchronized, does not free-wheel downhill, and can be shifted into reverse without damaging the unit.  You do have to be handy with a wrench because you have to install it.

What is the advantage of the overdrive?  There are two advantages.  First, you could continue to drive at your normal speed while in overdrive.  If you were driving 45 MPH, instead of turning 1926 RPM’s, the engine is loafing along at 1414 RPM’s.  That is the same as a stock Model A running 33 MPH.  In this case, the overdrive could be saving you from an expensive engine rebuild.  Still have the need for speed?  If running in overdrive at 1926 RPM’s (45 MPH for the stock A), your actual speed is 61.3 MPH.  55 MPH comes in at 1699 RPM’s (40.3 MPH for a stock Model A).  The overdrive allows you to choose between reducing the stress on the engine or to speed up with traffic.  The only other questions are how fast can you stop?  And, how fast do you want to go on four skinny tires?

There are two other choices, the overdrive transmission or the modern engine conversion.  The overdrive transmission will give you similar results as a Mitchell or Ryan overdrive, however it may be easier to install.  But there are some units on the market that require you to cut the frame to make them fit.  Swapping for a modern engine may create even more problems.

By the way, do you know how fast a stock Model A should go?  Peak horsepower is achieved at 48 MPH.  The Model A was originally designed to travel at 60 MPH.  The top end is 70 MPH at 3000 RPM’s.  Sorry, no guarantees.  Drive it at your own risk.

This tech tip was provided by Bob Hudec and was printed in the Jan/Feb 2003 “A” Quail Call.

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