On Tue, 16 Mar 1999, Phil Vanner wrote:
> It's my (limited) understanding that any given intake valve is only drawing
> air from one of the two SU's in a dual set up with the stock manifold.
> Given the path, down a carb throat, around the bend into the balance pipe,
> and around another bend, into the opposite intake port, I can believe this.
> That route must flow incredibly poorly. If I had a single, larger, SU on a
> well-designed manifold, one where all the intake ports had a reasonably
> straight shot to the air cleaner, could I expect to flow more air? Could I
> expect higher intake charge velocity, since there are more cc's of air per
> minute being sucked through this set-up? Would it fill my cylinders better?
>
> Why is a dual carb set-up favored over a single larger carb?
> The experts out there must have opinions on this.
>
I don't know about the experts, but I'm still here <g>.
Given the area calculation *and* the fact that only one intake valve is
open at a time, it would seem that each valve would see 36% more area with
a single HIF6 than with two HIF4s. Yes, I forgot this consideration.
The manifold is indeed very important, and the area calculation doesn't
consider that.
Airflow is a nutty business... air with fuel droplets doesn't like to go
around corners, as you mention above.
If there's no balancing tube/plenum, then yes, the back cylinders draw
from one carb and the front from the other. Given equally well designed
manifolds (hah!) there is more of a straight shot if the carb for
cylinders 3 and 4 is between cylinders 3 and 4, and not between 2 and 3.
Same idea for the front two cylinders.
In addition, you can make each path equal length with 2 carbs, but not
with 1 (without fancy geometry). In the 1 carb setup, cylinders 2 and 3
might have a better flow than 1 and 4. Thus the 2 carb setup might run
smoother than the 1.
Optimally you'd have one carb for each cylinder... and a little mechanic
tuning them as you drove. Oh wait.... that's computerized fuel injection.
The actual geometry, and the existence of a balancing tube/plenum
complicates intake immensely. That's why there are flow benches and
dynamometers...
Sorry to be misleading in my previous message.
--
John M. Trindle | johnt@tsquare.com | Tidewater Sports Car Club
'73 MGB DSP | '69 Spitfire H Stock | '88 RX-7 C Stock
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