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Curious about 425 vs. 401


jframe
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Does the dual carb 425 have a serious seat of the pants difference in performance over a single carb 401? I realize the ratings are different, but does anybody think it's a true horsepower difference, or does the 3.42 gearing in a GS make most of the difference? I'm interested in driving impressions from you folks who have experienced both. I've only driven my 401 powered 65 that has I THINK a 3.08 gear; can't really see the numbers on the bottom of the axle well enough to determine.

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If properly tuned, the dual four barrels, more cubic inches, curved distributor,  modified valve body in the transmission, lager exhaust, and the 3.42 gears will make an impressionable difference.

 

A couple of years ago, I drove a guys '65 Gran Sport that was in really poor shape,  The trans wouldn't kick down, the second carb wouldn't open, etc. etc.  My single four barrel 425 in my '64 with the 3.07 rear end was much more drivable and from what I though, much more responsive. On the other hand, another friend from out of town has gone to much effort to make sure that his Gran Sport runs perfectly.  It will definitely sit you back in your seat.  This is all seat of the pants reactions, nothing like an accelerometer or timing slips to substantiate what my butt felt. 

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Engine torque and gearing have more impact on initial acceleration than horsepower. The '65 Gran Sport with a has two things going for it that help increase its acceleration rate when compared to a regular Riviera equipped with a 401. Firstly, the GS has a 3.42 rear end which allows the engine to reach its torque peak sooner. Also, its larger displacement produces more torque - the 425 is rated at 465 ft lb vs. 445 ft lb for the 401 mill.

 

Just as important is the transmission. In 1964 Buick introduced the legendary Super Turbine 400 (precursor to the almost identical Turbo Hydra-Matic 400 used for years in all GM heavy-duty applications). In ’64 this transmission had a fixed-pitch torque converter and unique valve body which although the transmission had 3 speeds, only allowed the driver to select Low and Drive ranges.

 

For 1965 this transmission was upgraded to incorporate a new feature called "Variable Pitch" (also known as “switch pitch”) that allowed the torque converter stator blades to assume two angles, changeable between performance (high stall) and economy (low stall) angles. When in high stall mode it gave 2.5 times torque multiplication, a 25% increase. The low stall mode gave a bit better mileage. In a heavier vehicle such as the Riviera, the variable pitch feature provides a reduction of 0-60 time of up to 10%, and also gave smoother shifts.

 

The switch pitch feature makes a noticeable difference in that 'seat of the pants' feeling when accelerating from a standing start. Part throttle ‘tip-in’ acceleration is also improved.  It’s almost as if the car had an extra low gear. If I disconnect the switch pitch solenoid on my Gran Sport (located on the throttle linkage) the car feels noticeably ‘soggier’ on take-off. 

 

The switch pitch feature was installed in many big GM cars from 65 through 67 only. In the high stall mode, it let the engine idle with less "creep" at a stop light, and the low stall mode provided less slippage at highway speeds.  This feature was dropped in 1968 to save $ as the increasing displacement and higher torque of the big car line engines was deemed sufficient to make it unnecessary.

 

 

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  • 2 weeks later...

That rear center section was a Buick only item.  It has a 9-3/8" ring gear (hot rod guys think a Ford 9" is big, LOL).  The last production year for this style drop out center section was 1965. After that Buick went to a rear end with an inspection cover,   There is not enough demand for anything aftermarket for this unit.  Used is the only thing available.  The rear end center section is not just a Riviera part, it fits all full sized Buick.  It ended in 65 but I don't know when it began.  Find a shop that has a Hollander Interchange manual to see what other years and models used the same part.  

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The basic ring and pinion was the same, but the carrier, axle size and spline count changed I believe from '62-'63. There are also a fair amount of 3.23 gears around as it was OE more with the '63 Dynaflow -where 3.07 was found more with the '64-5 400 trans.

Dan   Mpls. Mn.

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