Oh my, here we go again. I don't know how I can respond without seeming a
curmudgeon, but let's give it a try.
In a thread about overdrive fitment, Bob advocated a change to a 3.9 diff in
an MGA.
I responded: What you might want to do is go the _other_ way! Fit a 4.55 diff
and have enhanced acceleration, still with a more relaxed top end than you
had with
the old MGA diff.
Bob said: "I strongly disagree. With that combination the first gear is
almost unusable...Additionally, it becomes extremely easy to break loose the
rear tires."
To which I retorted: "With your recommendation, (the 3.9 in the MGA) you have
a car with less performance than a stock MGA. With all due respect, that is
NOT a direction in which I am interested in heading!"
At this point, Bob responded either disingenuously, or in deliberate attempt
to save a losing argument with misdirection:
"Bill, That is NOT TRUE. The modifications I have done to my car make
it a great deal faster than a stock MGA. "
Which of course begs the question - it doesn't matter if you have 50 or 250
horsepower in your car, it will accelerate more quickly with a higher
numerical diff ratio.
I thought perhaps that Bob had experienced traction difficulties with the
stock diff ratio, and offered:
"If, as you imply, you have traction problems with a 4.3 diff, the answer
lies not in fitting a 'longer' ratio than the MGA, or indeed than an MGB,
came with (given the 15" wheel size of the 'A'), but in giving attention to
methods of improving the traction."
Bob surprisingly replied:
> Actually, given the difference in tires mounted, the diameter of
>the tires is virtually identicle.
Which is where we are now.
Anyone (except perhaps Bob) knows that the rolling radius of the tires on MGA
and MGB is considerably different.
Maybe he meant that the effective ratio was about the same. If so, he would
be equally wrong.
An MGA with 15" tires and 4.3 diff gets around 17 mph per 1000 rpm in 4th
gear. The MGA Mk2 with 4.1 diff gets about 17.7.
The MGB which was fitted with 155 x 14" tires and a 3.9 diff, returns 17.9,
which is slightly longer legged than the Mk2 MGA.
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, but you
will decrease the acceleration through the gears, which is not a result most
people would seek, hence my suggestion that you get the best of both world's
- fit an overdrive for higher top end, and also a LOWER diff (such as a 4.55)
for even better acceleration than stock.
Bob ended his last response with:
>I Agree to disagree. And I also agree to end this arguement so that
>list members may get on to more important issues
Bob, you aren't engaged in an argument - that implies an exchange of reasoned
points of view. I have given you the facts and numbers, you have simply said
you don't agree. You are flat out wrong when you say that a 3.9 in an MGA
enhances anything but top speed.
I do agree that further discussion isn't warranted - I'm sure anyone that
hasn't stopped reading this from boredom has long ago figured out the issues.
If you haven't, I'd recommend that your time, at least, would be more
profitably spent with a basic mechanical text than in further discussion
about this (that was the curmudgeon part).
Bill
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