A single side draft on a Spitfire 8 port head is not a really good design.
This is because the manifolds designed to go from the two DCOE ports into
the 4 intake ports on the head leaves a drastic difference in the length of
the inside runners and the outside ones. The result is that either the
inside two cylinders run rich when the outside two are OK or the outside two
run lean when the inside ports are properly tuned.
Normally you will find that single Weber side draft setups are 45 DCOE. If
you can find a pair of dual 40 DCOE with the proper manifold, they will
deliver a much more consistent mixture across all 4 ports.
There is a host of various parts that can (and usually MUST) be used to get
the setup properly tuned to your engine and the altitude you will be running
at. So if you are going to be traveling to various altitudes, you will have
to carry a lot of optional jets to accommodate for the conditions.
SUs are much easier to tune because there are not nearly as many parts that
must be changed to tune them. Basically only the needles.
Cheers,
Joe C.
-----Original Message-----
From: fot-bounces@autox.team.net [mailto:fot-bounces@autox.team.net] On
Behalf Of George Harmuth
Sent: Thursday, October 30, 2008 12:00 PM
To: fot@autox.team.net
Subject: [Fot] Weber carbs
I have a carb question, I was just offered a Weber side draft carb, I don't
know the number off hand, for my 72 street Spitfire. I see lots of
conversions to a Weber downdraft carb but very few for the sidedraft type.
In theory, a sidedraft seems like a more efficient flow into the engine, no
right angle turns. Is there some reason weber sidedrafts aren't that
common? Am I better replacing the single Stromberg with a spare pair of SUs
from my racing spit or the sidedraft Weber? Are there clearance or
mounting issues with the Weber?
thanks
mike
G. Michael Harmuth
Software Engineer
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