4th gear swap for Doug Nash 5-speed

M

Michael

Active Member
I run a Doug Nash 5-speed manual transmission behind a 0.030-over 454. It's gear ratios are 3.27, 2.13, 1.57, 1.24 and 1.0. For my preferences, these ratios are too closely spaced. I'd like a larger spread between 1st and 5th gear, as I'm losing more speed during shifts than I would be in the rpm drop with a wider gear-spread.

There's lore on the internet about replacing the 1.24:1 fourth gear with an overdrive. See for example the last post in this thread: http://www.chevelles.com/forums/archive/index.php/t-65383.html .

Has anyone done this? I'm not able to perform the swap myself, and would need to farm it out to a transmission-shop. Also, I'd like a taller overdrive than what's in the above link; they use 0.84:1; I'd prefer around 0.6:1.

Thoughts?
 
Is spreading through using a different rear ratio an option for you. Typed that before reading both seem viable either change the 1st or rear ratio. A gear calculator or chart is a good way to make a decision on this along with what rpm is your engine happy at.
 
Actually, my ultimate aim is to attain a large ratio between first gear and final gear. In the Doug Nash, this is 3.27:1. In a T56, it is something like 5.32:1. In a close-ratio Muncie, it's 2.2:1.

With a multi-purpose light car and a large engine, the idea is to use 1st gear exclusively for drag-racing launches with slicks, and final gear (whether this is called 4th, 5th or 6th or whatever) for leisurely highway cruising at low rpm. In "normal" driving I'd only be using a couple of intermediate gears. Rowing through all 5 gears in the Doug Nash only slows me down.

A numerically lower or higher rear-end ratio can shift the rpm band up or down for highway cruising, as will tire diameter and so forth. But it will affect all gears in the transmission simultaneously.

With a high-strung, high-performance engine, it's important to remain in the ideal rpm range. But for a large and comparatively lazy engine in a light car, I'm just as fine at 3000 rpm as at 6000 rpm. So my goal is a small number of gears with a large spacing between them... like a 700R4, but in a manual transmission.
 
That transmission-ratio chart is unfortunately misleading, because it's plotted with a linear abscissa. It should really be plotted in logarithmic coordinates. Only then could the various colorful line-segments be directly compared.

Regardless, my immediate aim is to see if anyone here has in fact taken apart their Doug Nash transmission and swapped-out gearsets.
 
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