| [GMCnet] Final Drive Ratio Final Decision Rational [message #237634] |
Mon, 27 January 2014 12:19  |
glwgmc
Messages: 1014
Registered: June 2004
Karma:
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Hi Sean and Stephanie,
I think most will answer it all comes down to where you drive and how fast you like to go. With the stock gearing the stock engine is running at an RPM lower than peak torque so any grade you encounter will require opening the throttle further to climb it which will mean running at lower engine vacuum resulting in burning more fuel. If you live in flat land the stock gearing is just fine. If you climb mountains very often it is not. The stock final drive is a ring and pinion design which transmits torque to whichever front wheel has the LEAST traction. The rarer 3.21 is a different physical design which uses a planetary gear set up which will do a better job of transferring torque to both wheels, but is certainly not a "limited slip" design. Some feel it is more robust. All the higher ratio after market final drives that I am aware of are ring and pinion designs.
Peak torque on a stock 455 Olds engine occurs around 2600 RPM (depends on how much cam timing chain stretch has occurred over time). At 60mph with the stock 3.07 gearing your engine will be turning about 2100 to 2200 RPM (depends on tire size and wear). To get to peak torque you need to change the gearing by 18% (2600 - 2200 = 400 / 2200 = ~18%). From the stock 3.07 you would need to go to something around 3.6 to 3.7 (3.07 x 1.18 = ~3.63). Changing the gear and chain at the back of the transmission with the stock 3.07 will give you about an effective ratio of 3.5. The chain and gear plus the 3.21 will give you an effective 3.67. So, the most popular ratio changes, whether accomplished via changing the final drive itself, or changing the gearing at the back of the transmission seem to be in the range of 3.5 to 3.7. Those who regularly climb steep grades or who tow some weight or who drive a slower speeds may like even higher ratios. The Olds 455 does really well at
relatively lower RPMs while the 403s seem to like higher RPMs a bit better.
Net, net, establish the speed you want to go, what grades you are likely to encounter, how much if anything you intend to tow, whether or not your tires are close to the stock rolling diameter and make your own choice. There are no "bad" choices, only good, better or best depending on these factors. We have a 3.21 with the transmission gear in both our 78 Royale and our 77 Clasco, both with 455s. It seems ideal gearing for where we live and our mileage did go up some, and "spunkiness" a lot, when we changed originally to the transmission chain (3.5) with the stock 3.07 in both. Mileage did not change much but "spunkiness" went up even further when we changed to the 3.21 in both. Hope this helps.
Jerry
Jerry Work
The Dovetail Joint
Fine furniture designed and hand crafted in the 1907 former Masonic Temple building in historic Kerby, OR
Visitors always welcome!
glwork@mac.com
http://jerrywork.com
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Message: 11
Date: Mon, 27 Jan 2014 10:37:52 -0600
From: Sean Kidd <fiatkidd@yahoo.com>
Subject: [GMCnet] Final Drive Ratio Final Decision Rational
To: gmclist@temp.gmcnet.org
Message-ID: <3a032.52e68b5f@gmc.mybirdfeeder.net>
Content-Type: text/plain; charset="ISO-8859-15"
I have the stock 3.07...and cannot back-up my driveway without a running start...I upgrading Stephanie's '66 Mustang to a 3.20:1 from the paltry 2.83:1 and realized marked seat of the pants improvement in torque and acceleration....Among other deciding factors, what was everyone's rational for choosing their FD ratio? 3.55 or 3.77 or...
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Sean and Stephanie
73 Ex-CanyonLands 26' #317 "Oliver"
Hubler 1-Ton, Quad-Bags, Rear Disc, Reaction Arms,
Fluorescent Mineral Capital of the World, New Jersey
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Jerry & Sharon Work
78 Royale
Kerby, OR
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