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June 17, 2003
Don...
Dave Vizard's excellent book goes into
the "differences" in good detail.
Usually, the Weber sidedraught carbs. are
utilized along with other performance upgrades such as cam, inlet valve
size
increase, flowed heads, etc., though they
can be used on the stock mechanically sound motor, the benefit in the
latter case mostly confined to more "quick response"
rather than a true "power" increase due to
scads more aiflow available by larger throats.
The operative word in the foregoing is
"mechanically sound". The engine in
question has got to be developing good
vacuum for the Weber is a "fixed venturi"
carb., unlike the "variable venturi" SU,
which by it's "air valve" will "adjust"
it's flow to whatever the engine
is capable of calling for "vacuum-wise".
It is this variable venturi that gives slower
"response" time as compared to the "fixed
venturi" carb in the mechanically sound engine.
On the other hand, it is this effect which
normally gives higher airflow velocity for better low and mid-range
torque once the acceleration process gets underway,
while invariably, the fixed venturi carb. carb provides better flow in
the high rpm range (unless the "variable
venturi" type has been modified for high
rpm flow, in which case it's excellent
low-end flow characteristics have probably been compromised to some
extent).
So, for a worn engine, the SU is far more
user friendly as the action of the air valve
will only allow "in" only what the engine is capable of using in terms
of airflow.
As for the larger throated sidedraft,
larger throats by themselves won't do
much without "flowing" the entire intake
tract into the combustion chamber as
far as "power" is concerned.
This having been said, elimination of the
stock SU log manifold in favor of the
Weber sidedraft's manifold which provides
for a "straighter shot" into the ports does
exactly this!! So, there is some benefit in
this respect in going from an SU to the
sidedraft Weber even on a stock motor providing that motor is
mechanically sound (good compression, minimal
"leakdown").
You can get good low and mid-range
torque while accelerating initially off
the line (with the Weber) by resisting the
impulse to "floor it" until rpm builds to
at least 2,500rpm with the stock cam
profiles, and a little higher depending
upon the power range of more wild
camshafts. In other words, a "disciplined
foot" can to some extent become the
SU's "air valve" in terms of increasing
flow "velocity" by manually "limiting"
the size of the intake "hole" until the engine can "usefully" take WOT.
Matter of fact, when you think about it,
this is the way we drive on the road every
day....we certainly don't go driving around
at WOT with our foot to the floor, do we?
If you have a problem "visualizing" the
immediately foregoing, think "sprint"
cam profile in terms of duration and
lift and a useable rpm range of maybe
5 to 8,000 rpm. Full throttle below 4k
in this instance won't get you anything
but "rough running" and maybe a complete "cut" or "bog".
Cap'n. Bob
'60 :{)
PilotRob@... 22725
Posted on June 17, 2003 05:46 AM
Posted to category(s) Body | Carburetor | Cylinder Head | Engine (external) | Engine (internal) | Exhaust System | Intake System | Restoration | Specifications | Technical Info | Tuning
