Re: [Amc-list] sending so I don't forget (diesel MJ)

["Swygert, Francis G MSgt 436 CES/CECM" <Francis.Swygert3@xxxxxxxxxxxx>]

Date: Friday, April 6, 2007 08:21 AM
From: andrew hay <adh@xxxxxxxxxxxxxxxxx>

" From: "Swygert, Francis G MSgt 436 CES/CECM"
<Francis.Swygert3@xxxxxxxxxxxx>
"
" The .55 OD gear is probably NOT a good idea!! The engine wouldn't be
" turning enough rpm to build any power, so 6th gear would be near
useless
" unless going down hill. With a diesel it might be better, but even
they
" need to turn a minimum rpm to produce power! Automotive diesels aren't
" like the big stationary types that produce gobs of power at 1500-2000
" rpm -- with a redline of about 2200!

true enough - but what if you [he] are running deep axle gears, like
4.10s or lower?

-------------------------

A good point, but then the 3.52:1 first gear is useless! First gear
reduction would be a really deep 14.43:1. I surveyed AMC cars from
around 65 to 83 and discovered that a good rule of thumb is to have
first and final gear ratios add up to about 6.5 (between 6 and 7) for a
manual transmission. Anything more than 7 is too low and anything less
than 6 is too high. For example, if running a 2.53 rear axle you need
about a 4:1 first gear for good take-off. So find a trans with a 3.5-4.5
first gear. Automatic transmissions can get by with a lot less gear due
to the torque multiplication of the converter -- a good range is 5-6 for
an auto trans. Larger engines generally use gears closer to the lower
number, smaller ones the higher number. Weight of the vehicle has the
same effect -- heavier vehicles to the larger end of the scale. 

I discovered this when trying to figure out what total reduction was
used on the average AMC. The actual gear reduction is found by
multiplying both ratios, not adding. Adding is just a lot easier rule of
thumb when deciding what minimum ratio is needed. It's a lot easier to
add 2.73 and 2.45 in your head than multiply (73 Hornet rear axle/904
first gear ratios). Of course desired performance comes into play, but
this is just to get you in a workable range for adequate take-offs. 


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