Well, it is good to see someone trying to make something useful out of a
Tri**ph ;-) Actually, being an old hot rodder, I have been following your
efforts. In some perfect would that exists only in my mind unless I win
the lottery, I would have two of every interesting car that I can think of.
One would be bone stock and the other would be "improved" as Dan is doing
to his pickup, excuse me, TR-6.
Choosing the proper gear ratio is very complicated but a good simple system
that I have heard recommended is to choose your gear ratios such that a
shift at maximum horsepower rpm would drop you to maximum torque rpm at the
lowest. Since you are using a crate motor (correct?) these numbers should
be readily available. Assuming 3,000 and 5,000 rpm, this would mean that
the .72 or .73 top would Drop you to around 3600. Even the .59 would still
keep you at around 2950 after a shift at 5000. This is the beauty of
V-8's- the ability to pull wide ratios.
On the other end, the same calculation with the 2.95 ratio would drop you
to 3300 while the 3.35 would go to 2895 which would still be OK since you
won't be power limited at the low ratios- traction limited is more like it.
Your choice for rear end and top will give you about 2300 at 60mph and,
given the 2.95 low gear, around 6 mph at idle. The car will be buzzy by
modern standards but probably OK for it's stated purpose. It will also
crawl around car shows just fine with that first gear. The 3.5 rear with
the same tranny would give you about 2100 at cruise and about 7 mph at idle
so it would probably be acceptable too with slightly more relaxed cruising.
A better choice for relaxed driving may be a 3.35 1st, .68 5th, and 3.5
rear end would give you a 6 mph idle and 1950 rpm at 60.
The 3.89 give you a usable speed range (peak torque to peak power) of
29-49 in second and 42-70 in third where the 3.50 gives you 32-54 and 47-78
in the same gears.
So, getting down to brass tacks, I think your initial choice is acceptable
and will give a good useable speed range in 3rd. The wider ratios and a
3.5 rear would make the car more pleasant day to day with no real
performance penalty but the 3rd gear ratio would be slightly less optimal
ranges for your stated use.
Regards,
Bill Eastman
61 MGA with a big hole between 2nd and 3rd.
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