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Thread: GM big blocks

  1. #1
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    GM big blocks

    Was there a corporate limit on displacement in GM outside of Cadillac? How did Olds, Pontiac, Buick, and Chevrolet all wind up at 455 or 454 cubes?

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    I think when ci increase from the 400-425-455 , they tired to keep things in a certain under - over square range? Cadi just needed more big cube torque from the 425-472-500 I'm guessing to keep things civil and smooth with low rpm grunt.

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    i think there was a 10 lbs per cubic inch rule in the 60's. all the big cars probably came in around the same weight - 4500 lbs.
    Josh Peterson

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    Nova's excluded. They had Corp rule about cubes and vehicles, intermediates likes the Cutlass stayed at 400ci until the special order Hurst received the 455. From then going forward into 70 the 400 was dropped. The big cars, Delta etc had the 455 first. Could be ordered with the 390HP/500torque. The Toro was available with a 400HP rated 455. The Cadi being the flagship sounded better with a 500 ci. Although some of the big wagons could have used more ci.

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    It was the big engine cube race. The BBC started as a 396. Big bore was 427. Throw a longer stroke crank 454. Evolution.

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    Don't know but I imagine racing sanctioning bodies rules played a part.

    Not to mention with bigger cubic inch engines you get higher piston to cyl. friction with lower rod ratios. So to overcome larger bore/longer stroke friction you have to run longer connecting rods and that means taller blocks. You get into the law of diminishing returns.
    You get to a point on a production vehicle where it doesn't make sense to keep making larger and larger passenger vehicle engines.

    https://www.enginebuildermag.com/201...ng-rod-ratios/

    "Rod ratio is the mathematical relationship between the overall length of the connecting rods and the stroke of the crankshaft. Divide rod length by the crank stroke and you get the rod ratio.
    The lower the rod ratio, the greater the side forces exerted by the pistons against the cylinder walls. This increases wear on the piston skirts and cylinder walls, and creates a higher level of vibration inside the engine. The increase in friction can also elevate coolant and oil temperatures."
    Last edited by CDave; 06-16-2019 at 11:13 PM.
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    5" bore spacing, the goal was for even more displacement than 500
    https://www.hemmings.com/blog/articl...c-472-500-v-8/

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    My mom drove a 69 Rivera. 455ci and close to 400hp/ 500tq that car would burn both tires as far as you wanted it to. The old big blocks were torque monsters.

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    NASCAR limit then was 427. No NHRA reason for 455 cubes. Bore stroke ratios were different. How did 4 divisions arrive at 455 - yes I know Chev was 454

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    A bit to do with marketing, the advertised cubic inch figure was simply rounded to the best sounding number for marketing. Just one example, the hot 375 HP 396 was actually a 402 but 396 sounded better. All the company’s did this Ford had 3 different 351 engines were they all actually the same cubic inch? Very doubtful. I’m to lazy to do the math but most engines are rounded to the most marketable number.

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    Besides all the above, most cars between division shared hood lines, frame rails, engine bay spacing. Not that a lack of space was an issue. How long or tall did they need to build a block to maintain the under/over square range. Let's take a look at the bunch, each 455 , V°, bore and stroke, deck height.

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    The 402 didn't happen until 71or 72

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    '70.

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    Buick 455 4.31X3.9 with 4.75 spacing
    Pontiac 455 4.15X4.21 with 4.62 bore spacing
    Olds 455 4.125X4.25 with 4.625 bore spacing
    Chev 454 4.25X4 with 4.84 bore spacing

    All different engines.
    My guess is an internal GM policy lead to the same displacement. Too big of a coincidence otherwise.

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    There were policies for engine size at the time. 400 ci for mid-size cars, and probably a similar policy for large cars.

    https://books.google.se/books?id=mE8...%20455&f=false
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