I would lean towards the 2" carbs for the simple reason of maintenance.
The square inches of choke is a bit of a red herring with your engine.
While the 2" carbs will give you more CFM in airflow, you won't need it with
your engine unless your running at very high RPM. You can do some math to
figure this out by calculating your displacement per minute at max rpm and
comparing that to the efficiency CFM of the SU's.
With respect to "straight shot" to the inlet valves, you don't really get a
"ramming" effect unless you have individual runners such as Webbers so, I
would consider this a non-issue.
With SU's you don't have to worry about loss of vacumn with larger chokes
(and related torque) at lower RPM because they are variable choke carbs.
With fixed Choke Carbs (Webers, Holley and most other carbs), larger chokes
become an issue (especially with a modified Camshaft). The reason being is
that when you open the butterflies suddenly, the vacuum drops suddenly which
affects the fuel drawn through the venturi's (this causes hesitation upon
acceleration). This is why fixed choke carbs have to have an accelerator
plunger that gives a shot of fuel when the butterflies are opened....they
can't rely on vacuum to draw the fuel. SU's don't have this issue. It is
this changing of vacuum and trying to compensate for it that poses the
problem that people have with webers. However, when properly tuned Webers
are great carbs and will be more responsive than SU's.
All this to say....go with the 2" Webers.
Sorry for the long explanation...got carried away.
Paul
-----Original Message-----
From: healeys-bounces@autox.team.net [mailto:healeys-bounces@autox.team.net]
On Behalf Of Greg Lemon
Sent: Tuesday, March 16, 2010 4:18 PM
To: I Erbs; healey help
Subject: Re: [Healeys] which carb set up?
Area = pi *r * r (pi times radius squared)
1" x 1" (radius squared) x 3.14 (pi) x 2 (carbs) = 6.28" sq. in. for double
2" carbs
.75" X 75" x 3.14 x 3 = 5.30" sq. in. for triple 1,5" carbs
This is of course the theoritical "maximum pipe" for airflow, actual will
also be affected by the throttle plate and such, but theortically the double
2 inchers can flow almost 20% more air. Of course the triples give you a
straighter shot to the ports, I suppose there is a reason older high
performance engines like Ferrari and such often had an intake carb barrel
for each intake port.
But it doesn't look like you lose anything, and may actually gain, with the
double carb set up.
Maybe some of the racer boys can pop in with some real world experience.
Greg Lemon
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