My 2 cents:
Bob wrote:
<<<snip>>>
but the top speed will be slower.
<<<snip>>>
I don't think so. You can over gear just as easily.
I found the following tests of the MGC which is listed as having 145 BHP
and 170 lb/ft of torque, 3.70 final drive ratio and 15 inch wheels.
The Autocar, November 1967
Top speed OD: 120 MPH at 4450 RPM; without OD: 120 mph at 5450.
Road and Track May 1969
Top speed OD: 118 MPH at 4780 RPM; without OD: 115 at 5600 RPM
What does that tell you? The 3.9 with OD may produce about the same
overall gearing as a 3.7 DIRECT. The MGC needs 145 HP to pull that
gear. Do you have 145 HP from 1850 cc versus the MGC's 2912 cc?
<<<snip>>>If one is going to travel long distances at high rates of
speed it may be that one would want lower engine speeds to:
<<<snip>>>
The highest legal limit I know is 75 mph in South Dakota. Except for
Montana just where do you plan on driving at these "high rates of
speed?"
<<<snip>>>
1. place less wear on the engine;
<<<snip>>>
Yes, but how many miles do you plan on putting on this car every year.
<<<snip>>>
2. have a greater reserve of torque for passing, etc.;
<<<snip>>>
Big wrongo if you think the 3.9 gives more torque. You may have more
engine torque from a modified engine but at what RPM? It is almost
always at a higher RPM than a standard engine.
Gearing is a multiplier or reducer of torque. The 3.9 ratio increases
the need for more torque from the engine for the same acceleration
rate. The OD when engaged makes the need for more torque even
greater. If the engine is modified to produce more HP those
modifications almost always
raises the RPM at which the peak torque occurs. That's in the opposite
direction your going with the reduced gearing. What's the torque curve
like now with the modified engine? Two bits says it's now around
3,500. Can the car cruise legally at 3500 RPM DIRECT? Does the engine
turn 3500 RPM IN DIRECT at 60 MPH which is the most likely speed you'll
be going if you want to pass? My TD with a 4.875 runs 3800 RPM at 60
MPH. The 4.5 will produce 3,500 RPM at 60 in a TD!
The optimal engine RPM of the stock MGB engine is close to 3,750 RPM.
That's where the torque curve and the HP curve cross. If the RPM is
higher the HP goes up but the torque goes down and vice versa. Assuming
there is a 500 RPM drop between gears at 4,000 RPM, the engine is at
it's best for both torque and HP between 3,500 and 4,000.
<<<snip>>>
3. gain greater fuel economy is travelling while maintaining good
performance
<<<snip>>>
You're worried about fuel economy in a sports car?
The lowest fuel consumption on the stock MGB occurs at 3,500 RPM.
That's given in a graph published by the "Motor" in September 1962. I
believe Max Heim has put this graph on his web site. If the engine is
turning faster or slower than that, the fuel consumption rate
increases. 4,500 RPM produces about the same fuel consumption rate as
3000. 2,500 RPM consumes more fuel than 5000 RPM.
<<<snip>>>
Bill wrote:
>As everyone but Bob no doubt appreciates, if you fit a 3.9 diff to an MGA
>which has 15" tires, you will indeed raise the theoretical top speed,
Bob wrote:
True
<<<snip>>>
Bill said theory Bob, and that's usually not what happens. When the
horsepower being produced is less than the drag of the car, it won't go
any faster. This can occur at ANY RPM.
Road & Track's test of the twin cam MGA shows it to have 108 BHP at
6,700 RPM and 104 lb/ft of torque at 4,500. Road & Track's test of the
5 main MGB shows it to have 92 BHP at 5,400 RPM and 110 lb-ft @ 3000.
Is the MGB engine you're discussing producing at least
110 HP when the car is going 113 MPH?
You're going too far or the wrong way with the gearing when using OD if
you want the best acceleration, top speed, and possibly fuel economy.
I had a 1960 Rambler with about a 180 CID six and 115 HP and an OD
transmission. It's rear axle ratio was higher numerically than that
for the same car without the OD. My Uncle's 1950 flat head V8 was the
same way. The Fords with OD had a numerically slightly higher rear axle
ratio than non OD cars. I had a 1965 287 CID 155 HP V8 Ambassador with
OD and its rear axle ratio was the same as those without OD. What does
this show? That you needed close to 155 HP to keep the same rear axle
ratio and still have good performance when using OD.
You're going too far or the wrong way with the gearing when using OD if
you want the best acceleration, top speed, and possibly fuel economy.
Following this line of attack, the ONLY increase in performance, if you
can call it that, is greatly reduced RPM's at cruising which makes for a
less buzzy engine. But, you've now made acceleration more difficult
and you need to learn to use the box more.
Blake
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