Carb's are interesting things. To your points - dyno's don't lie. And top
engine guys, of which I only know of two or three, can always get more HP
out of an ultimate racing engine on HS2's than they can on a single 45 DCOE
Weber. The local magician is John Dodd of BVR Cars of Oakville, and his
computerized Superflow dyno is highly accurate, so I believe him.
But the point is that when working with such an engine, SU's are very hard
to tune. And the engine runs like crap off the power. Webers can get
within 1 - 2% out of the box, using off the shelf parts to do the tuning,
rather than a flow bench, grinder, polisher, welding to reshape parts, etc.
SU's are, compared to Webers, quite forgiving of being mis-tuned, so you can
get close to right and have very acceptable results while with a Weber you
have to get very close to right to have acceptable results. That's because
an SU has an automatic feedback loop to self-correct some problems through
the action of the vacuum driven dashpot. And there's a lot more to screw up
with a Weber, so they are harder to tune if you don't know how to do it.
But on a very high performance engine, this seems to reverse itself, and I
know that with a Weber I can get a race 948 or 1275 to run well out of the
box, and tune it with just changes to main and air and idle jets to have
good power and run well very easily (within two practice sessions).
On the point about how your friend was able to use your carbs on his engine,
this brings up an interesting facet of carb lore. Did you know that the
stock 948 in my 1961 Midget has the same needles specified as a stock 1275
Cooper S Mini? (RB needles, I think, although that's from memory) What
lets this happen is what I think of as "signal". Signal is the amount of
vacuum at the jet orifice that sucks the fuel out of the jet. The small
engine can suck less hard on the jet, so there's less signal at the jet
orifice than a large engine which can suck harder on the jet. so a large
engine sometimes needs a small jet (or large needle, if it's an SU) because
by sucking harder it pulls more fuel out of that jet than a smaller engine
can. Bizzare. All I know is when I ran a 40 DCOE on my street MGB, with
an 1800 cc engine, I used almost identical jetting to the same carb on my
race 948. All I changed initially was idle jets - ran everything else the
same. Chokes, mains, airs, emulsion tubes, everything. And both cars ran
great.
Cheers, Brian (who loves figuring out puzzles like how carbs actually work)
Cheers, Brian
At 03:17 PM 2/3/98 -0500, you wrote:
>Brian Evans <brian@uunet.ca> writes:
>>>Webers don't drift out of tune, are less prone to leaking
>>>and produce more HP and they sound a whole lot better :-)
>>
>>most serious engine guys can get more power out of a race engine with SU's
>>than a single weber. Comptune is an example. But it takes a long time, a
>>dyno, several thousands of dollars, and a skilled magician. A webers big
>>advantage is that you can tune it easily to suit a modified engine, and it's
>>better than a stock SU.
>
> Huh? Sounds pretty backwards to me. From everything I've heard,
>Weber's are a bitch to set up right, but make a bit more HP on highly
>modified engines. From my experience, SU's are far easier to set up,
>though getting the last few percent out of them takes some work, but
>they're more tractable than webers, doubly so if the setup isn't perfect.
>Getting the needles exactly right requires dyno time and swapping or
>careful filing (followed by selecting a needle that matches the filed
>one(s)), compared to Weber swapping of chokes, jets, etc, etc and dyno
>time.
>
> I had a friend (Dr. Bobwrench) who took a set of HS2's (originally
>from a 1275) I'd tuned on a modified 948 Mk II Sprite. He put them on his
>racing GT6 and much to his surprise (and mine) they worked well with no
>adjustments. He probably was losing a few percent due to non-optimal
>needles (and they're a bit small for 1 liter of engine apiece), but they
>worked well enough to do a driving school with. They didn't even need
>resynchronization apparently.
>
> IMHO.
>--
>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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