At 2:49 PM -0500 1/13/2002, WSpohn4@aol.com wrote:
>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.
Ditto.
>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.
What you said is faster. I would call that quicker. It is a
direction one might go if one was drag racing. one will reach top
speeds more rapidly, but the top speed will be slower. If one is
going to travel long distances at high rates of speed it may be that
one would want lower engine speeds to:
1. place less wear on the engine;
2. have a greater reserve of torque for passing, etc;
3. gain greater fuel economy is traveling while maintaining good performance
>
>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, if power was to remain the same what you say would be true.
However, the 1800 has more power than the 1500, or either version of
the 1600, 95 vs. 68 or 73 for the 1500 and 85 for the 1600. I believe
the late 1600 may have had 90 horse power, and I think they had a
4:10 final drive ratio, but Im not certain about the 1622.
However, with a .040 over bore on an 1800, a good idea on a used
engine, a header, some mild head work and a mild cam, something in
the range of 270degrees, one may easily get 115-120 horse power out
of the 1800. One will then have a car that is both quick and fast.
>
>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.
Yes, it is quicker, but it is also slower at its terminal speed.
>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."
The question now becomes which compromise does one want to make. I
chose to have more power allowing greater acceleration with a lower
ratio. Hence I have a car that is both fast and quick. It is a
solution which I find to be flexible and which acquits itself well in
everyday driving as well as solo racing, which is what I wanted it to
do.
>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.
Assuming that we are speaking of 165/15/S tires and 175/70/14 tires,
about as near stock as are readily available, according to the Tire
Rack specification sheets, the revolutions per mile range from about
900 to about 950 revolutions per mile. Look it up for yourself to
check the information. This varies by the different models of tires,
and my the manufacturer, and to be sure, some of the tires in both
size will be both above and below that range. However we find all of
them to be of a similar range. If we made a scatter plot of them we
would find a very similar result. I do not believe it would be a
significant difference, but I have not done an input of data to do a
multiple analysis of variance to be certain.
>
>Maybe he meant that the effective ratio was about the same. If so, he would
>be equally wrong.
>
>
>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.
Our '74 B came with the 175/70/14 tires. I know my A came with 155/15
bias ply tires when it was new, but most are now running the 165/15/S
tires.
>
>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,
True
>but you
>will decrease the acceleration through the gears,
Also true if power remained the same. But power does not remain the
same. Power is increased with the addition of the 1800. There is the
added benefit of 5 main bearings (I know, it may not rev so freely,
but it is a stronger engine with real seals on both ends of the
crank) and there is room for additional power at little cost while
refreshing the engine while getting it ready for use.
>, 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.
Or one might, as I chose, build a more powerful engine in order to
maintain acceleration and increase top end. Hence we find different
ratio, often with a higher (lower numerically) final drive in more
powerful cars that have similar or faster rates of acceleration. We
have three considerations: mass, power and final drive ratio.
Differing adjustments will give similar results. We are not changing
mass. This leaves only the final drive and power. But There are
different results in different compromises, but I prefer both better
acceleration, and more top speed. I achieved this with more power and
a higher (lower numerically) final drive. You may disagree, and that
is your right
>
>>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.
Perhaps I was trying to be too brief. If so, I hope that I have
explained myself better now.
>
>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).
OK. And I suggest you review your high school physics... never mind.
I do not do curmudgeon well. I will defer to you on that.
>
>Bill
And with this, not matter how insulting the reply, I forever put this
thread to rest. If any one is interested in building an A with a 5
main engine and a full synchro overdrive, I will be glad to
communicate off list. If you value a car that is as it came from the
factory, you should make no modifications. Many value such a car. It
is your car. Do with it that which helps you enjoy it most.
--
Bob Shaw
Check out Shaw's Garage at http://www.mlcltd.com/shawsgarage/
My British Car is NOT leaky - it's merely marking its territory.
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