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The Gentleman's Express: Tech-Tips from the JIOC

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Differential Ring and Pinion Gearing

Author: Joe Sicignano

This should be titled ‘Ring and Pinion Change: Usable and affordable performance’. For some time, I have been interested in ways to increase the performance of my Interceptor. I have a 1974 which suffered the horsepower deflation of the anti-pollution movement. While the 440 engine responds beautifully in the 60 to 90 mph range, I am unwilling to become any more popular with the Highway Patrol. Driving on the street, I have not relished defeat at the hands of lesser machines.

Several members have solved the problem with major engine modifications, including new cams, manifolds and lifters. Some have convinced their mechanics to break the law and remove pollution equipment. The results can be as spectacular as the cost, one hundred extra horsepower but a bill in excess of $1500. If, by some unfortunate turn of events, an engine needs rebuilding so that good parts are not being discarded, such revisions make excellent sense.

Several club members have expressed annoyance at the hesitation in response experienced off the line, as if the car has to pause and catch its breath before launch. T Wiley of Edelbrock suggested carburetor modifications or gearing as possible alternative suggestions to engine modifications. Since I know two members who have experimented with the Holley carburation only to return it to the stock Carter, I decided to pursue the gearing alternative.

First, I began to look into B & M high stall torque converters which have been used mostly in drag racing applications. The torque converter links the engine output to the transmission and act as a low-low gear from standstill. Currently, the stock torque converter multiplies the final drive ratio by up to 2.5, which means that instead of a first gear of 7.50 (with the 3.07 rear end) the initial gear is actually 18.75. The hitch is that the torque converter locks up and ceases its multiplying function after 1800 rpm in the TorqueFlight transmission, utilizing little of the engine’s horsepower output which peaks at 4800 rpm.

Modified torque converters allow for the multiplication to last up to 2400 or more, though it would cut out at freeway speeds. The major drawback is that the work load on the transmission is increased and thereby the heat, especially in traffic. Having invested much in oil and transmission coolers to solve Interceptor heating problems I felt the last thing I needed was more heat. However, in a cooler climate this modification remains the cheapest route, parts and labor running under $300 plus the cost-of a transmission cooler which B & M strongly recommends in conjunction.

The alternative I chose to have installed was a new ring and pinion, the blood and guts of the rear end which converts the transmission output through the drive shaft to the rear axle. The original ring gear of the Interceptor was 2.88, but as the horsepower dropped, this was later upped to 3.07 in late 1973 production. Gear ratios with low numbers, under3, do wondersforfuel economy and are popular in late model U S cars for that purpose, but contribute to their sluggish response. The 2.88 rear end also enabled the early Interceptors to reach 140 mph, with the 3.07 showing some drop off at the top end. Higher numbered rear ends such as 3.55 and 3.73 give a faster acceleration with some sacrifice in top attainable speed, although the top is reached more quickly. The higher numbered ring and pinions make the engine run at a higher rpm at any given speed, with some noise, a more jolting acceleration, less fuel economy, and a change in driving characteristics. Streetable drag cars use 4.11’s and 4.56’s. Applied to the Interceptor, more of the engine’s horsepower is available at legal speeds and more control of the car is obtained via increased engine braking.

Comparison of Gear Ratios:
First (Low) 7.508.869.10
Third (Final)3.073.553.73

Con-Ferr Jeep of Burbank did the job in three days, but now that they know the sub-type of DANA 60 they should be able to do it in two. They were able to use an adapter to avoid changing the carrier and the live axle (limited slip differential) was maintained. The total price was $412. The speedometer had to be recalibrated ($39) since it reads speed from engine rpm and was 15 mph too high at freeway speeds.

The results are impressive. The hesitancy in starting from standstill is cured. It also turned out that due to improper alignment with my old ring and pinion I was generating rear end vibration which is now gone. The acceleration is as smooth as a turbine. The noise. difference, though noticeable, is minor and acts as effective biofeedback to the temptation to speed. The car enters freeway ramps with unchallenged authority. There is only a hint of wheelspin in maximum acceleration up an incline as the transmission upshifts from first to second at 4850 rpm (versus 4200 prior to change). The rpm at 55 mph is 2800 versus 2400. The downshift at highway speeds will still operate from 3rd to 2nd up to a shade under 60 mph but will disengage almost instantly due to running up to the rpm equivalent of what was 76 mph. As the transmission includes the Trans Go shift kit, it is possible to manually downshift from say 65 mph (3200 rpm) and reach 4400 for a rather exorbitant display of acceleration. The loss in fuel economy is one half to one mile per gallon depending on how much of a show-off you are!

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Contents copyright 2008, 2009 - Jody F. Kerr

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