NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/rec/autos/tech/18416

Internet Message Format | 1993-01-28 | 4.5 KB

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From: [e-mail] (Dave Dunbrack)
Newsgroups: rec.autos.tech
Subject: Re: General Carburetor Questions
Message-ID: <[e-mail]>
Date: 27 Jan 93 07:40:09 GMT
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>[e-mail] (Scott N. Dickson) wrote:
>
>Why would a carmaker use, say, 3-2V ("sixpack") instead of a 4V
>if the same flow rate could be had?  Why not a huge 1V?

  That was part of the problem, the same flow rate could not be had
in a 4V carb. The introduction of the Quadrajet is what killed Pontiac's
tripower (the Corvette had it a bit longer). Until the q-jet, your choices
were the Carter WCFB and AFB or the Rochester 4GC, none of which have the
flow potential of 3 twos. However, the Q-jet was far cheaper to manufacture
than the trips and was quite capable in the flow department (the early ones
were about 750 cfm, right?).

>From: [e-mail] (Randy Brown)
>Oh, baby, if you gotta ask!  Status!  Everybody has a 4V, but who has the
>screaming setup of a 6-pack?  Not many.  My GTO had a 6-pack.  Many of
>the GTO's performed better with a 4-barrel.  Another advantage is you can
>have a larger primary with the six-pack (the middle two barrels) to
>provide more of a power range without opening the secondaries.  But when
>you do, stand back!  You are now opening 4 more barrels.  You also have
>a shorter path to the cylinders when opening the secondaries with a 6-pack
>vs. a 4 barrel.  Usually, the 6-pack would have a higher cfm that the
>4 barrels offered on that make of car.  I think my 6-pack was rated around
>900 cfm, which a 389 would have an extremely hard time using fully.

  While status probably accounts for the reason Chevy kept the trips longer
than the rest of GM (olds had trips, as did caddy, and I think buick did
as well but I'd have to check that one), The fact is that factory 4v carbs
were flow-deficient until the advent of the Q-jet. This is not to say that
the status didn't help sell Goats, but there was a legitimate need for the
extra flow -- notice how quickly the trips were dropped from the Pontiac
line in 1967 -- Q-jet in, trips out (damn shame). BTW, the main reason that
most 4V GTO's were faster than thier 3x2 bretheren is due to the fantastic
job the Pontiac engineers did on the factory 4V intake manifold. The
factory 4V manifold is tough to beat. However, a very carefully tuned 3x2
setup is generally at least as fast as the 4V, but they're harder to keep
in tune... (by 4V faster than 3x2, I mean swapping induction systems, not
different engines).

>>From: [e-mail] (Thi Van Ausdal)
>>  A four-barrel, on the
>>other hand, generally has bigger primary throats than the twos, so
>>you'll burn gas less efficiently cruising.
>
>From: [e-mail] (James W. Swonger)
> From what I have seen of 2BBL and 4BBL carbs, the opposite is true
>(for a given engine). A 390 or 500CFM 2BBL is going to have larger throats
>than a 750CFM 4BBL even in the case of a square-bore configuration;
>spread-bore designs like the Quadrajet have itty-bitty primaries, much
>smaller than a typical 2BBL used on a V-8, and humongo secondaries.
>
> Now, having 3 390CFM 2BBLs and using one of them will probably give
>good low-end metering, with a velocity not too far different from a
>4BBL and greater top end flow. I believe the 2BBL carbs used on 3x2 setups
>were smaller than those used singly.

  In the case of the Rochester 2V vs. the Q-jet, the 2V has larger
bores. On the Pontiacs, a smaller center carb (but still bigger than
the Q-jet) was used up until 1965 (the "Chevy-size" single 2V), but
it was replaced with the "normal" 2V for 1966. The latter carb has
pretty large bores, bigger than lots of 4V primaries (I have both a
`64 and `66 tri-power, and a friend has a `65). The carbs were not
necessarily smaller than "single" 2V's, it depends on the application
of the single (I don't remember cfm ratings off the top of my head).
  Also keep in mind that 2V and 4V carbs are often flow-rated at different
vacuum levels - I think it was 1.5" H2O for 4V's and 3"H2O for 2V's;
anyone remember the numbers for sure? Anyway, it isn't always correct
to directly compare 2V cfm with 4V cfm.

    Dave