Mark J Bradakis writes:
> 2- Has anyone had any experience with the triple weber carb set up ?
>
>Weber carbs are certainly interesting. First off, I'll say that it is quite
>likely you don't want to install them yet.
From my reading I'd agree. Even the old factory TR Tuning book showed
little difference below 4500 or 5000 rpm.
>The constant depression type carbs like Stromberg CDs or SUs have some things
>in favor of them. For around town driving, and as the engines wear out, they
>are quite acceptable.
They're also quite easy to set up (by comparison to Webers).
I suggest anyone considering Webers read Vizards How to Modify the
A-Series Engine book (even if you have some entirely different engine).
There's tons of generally-applicable information in there, as well as on how
to get the most out of Stromberg/SU. It's not cheap, but it's big and full
of interesting info. He believes strongly in measuring results and testing
assumptions. He's also written a book on some American V8-type engine, as
well as How to Modify Your Mini.
>At low revs when tooling around town, this results in reduced gas
>velocity through the intakes, which yields poor atomization of the fuel, rough
>running, excessive fuel use, poor throttle response, and a general feeling
>that Webers are a pain in the toolbox, and with luck some poor schmuck on the
>Triumph list may want to rid me of these things.
This is almost always a problem with engines with a lot of
chokes/barrels. In fact, the dual-carb setups for A-series engines (sprites,
midgets, minis, etc) can actually be worse than a single larger carb. (See
Vizards book for details.)
>You will no doubt hear about Good's triple SU setup. He claims to get as much
>power as Weber DCOEs, but I wonder about this. With the DCOE carbs, you get
>one throat per cylinder. With the triple SU (or Stromberg) setup, each carb
>feeds two cylinders. This is fine for the middle pair, which have their intake
>cycles 180 degrees apart. The two front cylinders share a carb, but there is
>a 240 degree phase difference betwen cycles. The same holds true for the two
>back cylinders. What this phase difference means is that one of the cylinders
>will be getting robbed of a full charge by the other cylinder, so one runs lean
>and one runs rich. In a year or so when I have a dyno in my shop I'll see what
>sort of difference, if any, this really translates to at the wheels.
This is true, though very careful manifold design can minimize this
problem. (See Vizard again.) Still, the SU's may come very close to the
DCOE's at full-tilt/high-rpm, and be better at mid/low-range and part throttle
(see charge velocity and Vizard again). Also, SU's can be modified in some
ways to flow better (and with less turbulence), along with well-selected
(full-radius) ram pipes.
>So the bottom line is that with the proper care and an understanding of what
>one is getting into, the $1,000 to $1,500 required to get three Weber DCOEs
>working on a Triumph straight six can be money well spent.
Can? Yes. For most people who aren't going full-race, the money
may be better spent on other things, though.
>Oh, and the downdraft DG* carbs are completely different. I still don't see
>how people can add all those extra bends and restrictions to the induction
>path and claim a performance increase. Another thing to check out on the dyno!
Compared to worn-out, leaking and misadjusted Strombergs: certainly.
;-)
--
Randell Jesup, Scala US R&D, Ex-Commodore-Amiga Engineer class of '94
Randell.Jesup@scala.com
#include <std/disclaimer>
Exon food: <offensive words no longer censored - thank you ACLU, EFF, etc>
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