TRANSMISSION gear ratios

[donald w. miller]

Hey Rick, I like the sound of this.

I don't want to give up my 3.07 ratio as it is perfect for flat land
running. Besides, when they get around to having a Western Rally at the
salt flats, I want to go through the traps 100+ in mine.

Few would argue about the benefits of lower first, second and reverse gear
ratios.

With a 3.07 final gear and the transmission modified as you mention:

First gear would be equal having a 3.40 final gear

Second gear equals a 3.26 final gear

High remains at 3.07

Reverse gear equals a 3.58 gear

This would be a big improvement and as you say, would provide the best of
both worlds.

Changing from 3.07 to a 3.42 ratio gives a 12% torque increase. I can
easily find that much additional torque in the engine and for fewer dollars.

It would be nice to have some statistics on final drive failures. Seems I
hear a bit about transmission failures but very little about final gears.

Lower first and second and reverse transmission ratios are interesting.
Hope you are able to find out more about this.

Cheers,

Don Miller
75 Glenbrook
Shenandoah Valley of Virginia

>Date: Thu, 5 Nov 1998 12:13:28 EST
>From: RickStapls
>Subject: GMC: TRANSMISSION gear ratios
>
> With all the recent discussion of final drive and transfer chain ratios,
I
>wonder if anyone has any information/experience with changing the actual
GEARS
>inside the transmission for different ratios?
> I read in a GMCMI newsletter from way back in '85 (newsletter #12,
summer
>'85) that a company in Fountain Valley, California was selling replacement
>gearsets which reduced the 1st, 2nd, and Reverse gears to lower
(numerically
>higher) ratios. Specifically, 1st gear went fro, 2.48:1 to 2.75:1, second
>went from 1.48:1 to 1.57:1, and reverse from 2.11:1 to 2.46:1. 3rd (high)
>gear, involving no gears within the transmission, remains at 1.00:1.
> I also recall seeing another reference to such a project, but I can't
>remember where. ("The first thing to go....") IIRC, they said the
replacement
>gears were straight-cut rather than helical like the originals, making them
a
>bit stronger abeit noisier.
> This sounds like the best of both worlds for those of us trying to start
>off on steep hills over 10,000 feet altitude. The lower 2nd gear would
give
>more power on long pulls, although one might not be able to hold 2nd gear
up
>to 70+ mph as you can now. The stock 3rd gear ratio would allow us to get
>across Kansas with minimum noise and fuel.
> Does anyone out there have any experience/information about such
>modifications? Does anyone know if "Art Car Performance Transmission" is
>still in business, or any other such sources? Any leads appreciated.
>
>TIA,
>Rick Staples,
>'75 Eleganza, Louisville, CO
>

 

[zachary zehnacker]

Don,

Put us on the list for a final drive failure. We had about 100,000 miles
on our GMC (we put on about 65,000 of those towing a trailer with the rest
from the original owner) and we began to hear noises. We didn't know what
it was (our guess at the time was that it sounded like the chain in the
trans dragging on the cover). We drove it for quite a while with the noise
(guess around 5,000 miles) while towing the one-race car trailer. On one
trip about 3 miles from home we began to smell gear oil, we made it home
and even backed up into our driveway with no problems. The bearings in the
final drive had failed. The vibrations from this actually caused most of
the bolts holding the drivers side axle to the final drive to back out.
IIRC there were only 1 or 2 bolts left holding on the axle when we got
home! One of the bearings was totally shot, while the others were not in
good shape (probably because of the metal from the first one). The gears,
however, looked to still be in very good shape. We got the numbers off the
bearings and called around to local bearing places. We were able to get
Timken replacements from a local bearing supplier. We put in the new
bearings, reinstalled the final drive, and put another 20,000-25,000 miles
on it before switching to the 3.42 unit. The final drive was still fine
when we removed it.

We have not really had any total trans failures. We do, however, have our
trans rebuilt every 50,000 miles or so to avoid any problems. The final
drive problem listed above apparently caused some damage to the
transmission, but it was only found on the rebuild and was not noticeable
in operation. We feel the loads we place on the trans by towing the
trailer and the distances from home we travel warrant the more frequent
rebuilds. We also change the fluid and filter occasionally and have
installed transmission coolers in-line with the radiator cooler and an
aluminum trans pan.

We have also considered the different trans gear ratios. We do not
usually have a problem starting out in first. From what I understand, the
425 uses the same gear sets as the 400. This allows one to use the
different and more common 400 gears in the 425. Traction is usually our
limiting factor when starting to go up a hill from a stop. In our case,
the shifts bring the motor down quite a bit unless we really bring the RPMs
up before shifting. The RPM difference would be even greater if the ratio
difference between the gears was increased. We do, however, like the idea
that this method does not affect the high gear ratio.

Zachary Zehnacker
Seemann Motorsports

>Hey Rick, I like the sound of this.
>
>I don't want to give up my 3.07 ratio as it is perfect for flat land
>running. Besides, when they get around to having a Western Rally at the
>salt flats, I want to go through the traps 100+ in mine.
>
>Few would argue about the benefits of lower first, second and reverse gear
>ratios.
>
>With a 3.07 final gear and the transmission modified as you mention:
>
>First gear would be equal having a 3.40 final gear
>
>Second gear equals a 3.26 final gear
>
>High remains at 3.07
>
>Reverse gear equals a 3.58 gear
>
>This would be a big improvement and as you say, would provide the best of
>both worlds.
>
>Changing from 3.07 to a 3.42 ratio gives a 12% torque increase. I can
>easily find that much additional torque in the engine and for fewer dollars.
>
>It would be nice to have some statistics on final drive failures. Seems I
>hear a bit about transmission failures but very little about final gears.
>
>Lower first and second and reverse transmission ratios are interesting.
>Hope you are able to find out more about this.
>
>Cheers,
>
>Don Miller
>75 Glenbrook
>Shenandoah Valley of Virginia
>