Carb suggestions: 66 Caprice - Impala Tech
Engine General Engine Discussion.

 
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post #1 of 5 (permalink) Old 03-26-2015, 03:40 PM Thread Starter
 
 
Join Date: Apr 2014
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Carb suggestions: 66 Caprice

Is a 750-770cfm carb too big for a 396? Is 600cfm too small? Any suggestion? The motor is stock. The car is a cruiser. I'm not a racer HOWEVER I want to have the horsepower just in case someone pulls up beside me and challenges me. (lol)
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post #2 of 5 (permalink) Old 03-26-2015, 04:39 PM
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Your 396 left the factory with a 780 cfm Rochester QuadraJet if it was rated at 325 horsepower, a Holley 650 cfm 4160 vacuum secondary if it was rated at 350 horsepower and a 780 cfm 4150 series Holey vacuum secondary if the 396 was the special high performance 375 horsepower 396. Because only the big full size cars and the Corvette where allowed to have an engine bigger than the magic 400 cube limit imposed by GM's executive board most people who ordered a big block in their Biscayne or Impala ordered the 427 back as soon as it became available as a RPO in 1967. (Regular Production Order).

In 1966 the only way to get a 427 was to build one yourself out of the special production HD service part catalog by ordering the bigger bore block, crank shaft (which differs from the 396 crankshaft as it has heavier counter weights to balance the bigger pistons), and a set of those TRW 12.5 :1 pop up pistons. The open chambered head didn't become available as a service part until 1969 so you were stuck with a bathtub 101 cc combustion chamber.

That being said from 1967 and up few ordered the 396 in the big b-body with the 427 being offered for about the same price. Those who did order the 396 chose the 360 horse version with the Holley 650 vacuum secondary 4160 series carb (no rear main metering block) and a slightly hotter cam than the 325 horse version that wasn't offered in the Impala (only in the Camaro and the Chevelle).

They purchased this engine over the base 275 horse 327 for it's added size and rugged dependability (it was designed to be a high mileage truck engine) and the fact that you cold get a three speed automatic with the big block instead of the aging PowerSlide. I do not know if the 375 horse was offered as an RPO in the Impala as solid lifters conflicted with the Impala's luxury image (no A/C available with a solid lifter car) and that same three speed transmission wouldn't respond well to a the cam in the 375 horse as loose converters hadn't been invented yet. So as with the Z/28 it would require a manual transmission if you did order the 375 horse motor.

So the trend here is that a 750 is a good choice size wise for a 396, and it should definitely have a vacuum secondary. It is the size I chose to run on top of my 406 SBC in my 1989 Caprice. I was running a 3.73 rear gear with a 700R4 that I had built to the max after it's third time in the repair shop behind a big block ZZ502 motor. By the time I had rebuilt it for the third time it had all of the goodies that the aftermarket offered to make it stronger better than before. The supper low first gear, the 3.73 and the fact that a mid-eighties B-body had been down sized and lightened to be about the same size and weight as a late sixties Chevelle, it really could scoot with the 512 horse 406 I had built for it.

Big Dave
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post #3 of 5 (permalink) Old 03-26-2015, 07:01 PM Thread Starter
 
 
Join Date: Apr 2014
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Thanks Big Dave! The information was very helpful... as always.
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post #4 of 5 (permalink) Old 03-27-2015, 07:41 PM Thread Starter
 
 
Join Date: Apr 2014
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I went with the Holley 750 vacuum secondary. It bolted right up to the intake. However when I was driving and took my foot off the gas pedal it appeared that it was still getting gas. I had to rev it for it to idle down. I may need to adjust it a little. or could it be the spring on the gas pedal a little stiff?
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post #5 of 5 (permalink) Old 03-28-2015, 10:19 AM
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If the throttle doesn't close that could be a very dangerous condition and the reason the NHRA requires two throttle return springs (which accelerates throttle shaft wear).

This may be the result of a broken motor mount as well as every Chevy V8 made before 1969 was recalled in 1968 to have a five ton cable hold the motor in place. It bolted to the factory cast iron exhaust manifold and looped around the upper control arm cross shaft. In fact every car from 1958-'68 should have that cable in place at every show if you want to talk about being "original". Chevy went with the cable instead of fixing the problem which is the motor mounts became delaminated (they were held together by vulcanized rubber and a prayer) as most broke when drag raced.

If you have factory replacement motor mounts (the correct size to fit your frame stands) that do not interlock then that could be the reason the throttle doesn't close properly. A broken motor mount allowed the motor to roll to the passenger side pulling the throttle open as the rod was a fixed length. The design was changed in 1969 to a throttle cable that could flex without pulling the throttle open and an interlocking motor mount that used a different frame stand.

If you have a 1958-'68 Chevy then you need the short and wide motor mount. Conversely if you have a 1969-'72 Chevy then you need the tall and narrow motor mount. The two do not interchange because they are dimensionally different.

I have pictures showing the difference if you think that might be the problem. The actual problem is corner discount parts stores don't know that there is a difference not only between base engine V8, and High Performance V8 engines made from 1958-'58 but that after 1969 they changed again so that there are four different possible motor mounts made for the SBC made from 1959 up until they went with only one engine mount for V8's after 1973. They will all give you a 1969-'72 base engine motor mount if you ask for one. You have to go to a reproduction house on line to find a correct fit motor mount if you buy one.

Big Dave
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