Good question Larry,
I also have two 4bbl manifolds. Both different.
Intake casting # 1349520 has primary bore size of 1.35 inches
the secondaries are :
1.55 inches
I will be using the stock Rochester carb 1.310 primary
throttle bore
1.450 secondaries
the other manifold is casting # 581830X Primary 1.450
Secondary 1.700
I believe either will feed a 215 just fine. I've port matched the smaller
one for my
Buick 215 going into a B GT. The other will eventually go on a stroked
215.
The theory is that for street motors, smaller ports maintain mixture
velocity
thereby aiding low and midrange power ( that's where we play mostly )
If the bores are larger than the carb sitting on them, opening them up
more
may hurt not only velocity, but distribution as well ( wet fuel falling
out of suspension
and onto the manifold walls as droplets )
With larger cubes and lower gear ratios in the rear the motor
needs more
mixture, thus the larger manifold would probably work better under these
circumstances.
Why 2 different manifolds ? I don't know. Any Historians out
there with the answer ?
Greg
On Mon, 5 Jul 2004 23:05:23 -0600 "Larry list account"
<list@marketvalue.net> writes:
> I have a couple of GM stock 4bbl intake manifolds that fit the
> 215/3.5
> aluminum V8.
>
> Each has four intake holes on the top; but one of them has smaller
> primary
> holes and smaller secondary holes than the other. When I compare
> the
> intakes with the venturi holes on a Carter 400 and a Edelbrock 500
> carb;
> even the intake manifold with the smaller intake holes has larger
> holes than
> the carb venturi holes. (are you still with me here? Even if I got
> all that
> right you might need to read it twice!)
>
> Are there any advantages of the manifold with the larger holes; or
> perhaps
> the manifold with the smaller holes?
>
> Does anyone know why there is a difference? What they may have come
> off of?
>
> Thanks in advance,
>
> Larry Hoy
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