I just wrote something for the Z Cars mailing list: an introduction to
SU carburetors. Since that applies here as well, I thought I'd send
it along.
[. . .]
Okay, my credentials first:
Ten years of hacking SUs, on M.G.s, Austin Healeys, and lately Volvos.
(VOLVOS?) I've used SUs in street, autocross, and road race preparations,
on engines that ranged from stock to limit-of-the-rules Solo II Prepared
motors. I've read every reference I can find on SUs, and written a pile
of articles for the net and a pittance for paper. Finally, I actually
*like* SU carbs because of their tunability, their reliability (yes,
assuming you know what you're doing with them), and their drivability.
Basics of SU Design
The SU is a constant-velocity carburetor, meaning that air flowing over the
fuel orifice is maintained at a relatively constant speed regardless of
throttle opening or engine load. This is accomplished by a suction-operated
piston that doubles as the fuel metering device; the piston slides up and
down in the throttle orifice, and is connected to a tapered needle that
fits in the fuel jet. When air-fuel demand is low, say at idle, the
piston occludes all but the bottom fraction of an inch of the air flow,
and the metering needle's widest part occludes most of the fuel jet.
When air-fuel demand is high, say at full-throttle and redline, the
piston is pulled all the way to the top and the needle's thinnest part
is still in the fuel jet.
The SU's flexibility arises from the ability to adjust the carburetor's
individual air-fuel ratios by changing or modifying the needle. This
means that in the hands of a skilled tuner, the SU can eliminate flat
spots on acceleration, can provide good fuel economy at part throttle
running, and can provide far better throttle response than virtually
any fixed-orifice carburetor, including Webers and Dell'ortos. Full
power at top end occasionally suffers compared to the Italian carbs,
but drivability and response throughout the RPM range is generally
superior, once you know what you're doing.
Basic Tuning
For basic road use, the SU carburetor needs several things:
- Cleanliness. Dirt, both in the float and in the dashpot, is the
enemy of the SU. Don't run them without air filters. The piston
is a precision fit in the dashpot, so any dirt or grime can cause it
to stick in some position (usually closed). Because this is the
air-fuel metering device, your car will not run (or at least the
cylinders served by that carb will not run) at any RPM/load
combination except the one at which the carb is stuck.
- Lubrication and damping. The dashpot has an oil-filled cylinder
that acts as a vibration damper (and also to some extent as an
accelerator pump), as well as a spring that acts to help define the
orifice opening curve. For stock cars, use the oil viscosity and
piston spring provided by the manufacturer. For modified cars,
these parts can be changed to suit your engine's air-fuel needs.
- Balance. Most vehicles use multiple SUs, because their single-
throat design limits the amount of air that a single carburettor
can pass. This means that the carburetors need to be balanced,
or synchronized, so that air flow under all circumstances from
idle to full-throttle is even across all carburetors. This means
adjusting the throttle stops so that the throttle plates start
out even, adjusting the inter-carb throttle linkage so that the
butterflies open at the same rate, and adjusting the initial
richness so that the two carbs begin at the same mixture. The
balancing of SU carbs is somewhat complex but not difficult once
you get the knack, but it is beyond the scope of today's posting
(read: I gotta get back to work :-).
[Besides, for the Britcars list, there's Roger's guide, which can be
found via ftp on hoosier, I believe.--SF]
For street-driven cars, this suffices. For competition cars, there are
a few other steps to take.
1. Measure the rise/fall rate of the pistons and match them. To
measure, you need to find the suction hole in the piston, block
it off with something like plasticine (modeling clay), and then
time how long it takes for the piston to drop out of the dashpot.
(Hint: don't do this over the garage floor!) The object is to
make sure that both (or all, for 1962 BT7 tri-carb Big Healeys
and some Jaguars, with three SUs) dashpots rise at the same
rate, but it's easier to measure how rapidly they fall. If you
get one carb that's much slower than the other, you are permitted
to use a fine polishing compound to smooth the inside of the
dashpot bore. Don't remove any metal, just smooth it out.
2. For Solo II cars or other cars that run exclusively in short
sprints, use a thicker oil in the dashpots. The thicker oil
slows the rise of the piston, which enhances the accelerator-
pump effect for reasons explained below. You can also change
the springs, though that is a coarse adjustment; the oil is
a finer adjustment. For reference, I've had good results
running 20W-50 (the same oil I use in the sump), for both
street use and competition use.
If you use a thicker oil (or a stiffer spring for road-racing), then
when you open the throttle suddenly, the piston won't rise so fast.
This increases (temporarily) the velocity of the air flowing over the
jet, which drops the pressure (remember your Venturi-Bernoulli effect).
More gas rushes into the air because of the temporary pressure drop,
but the actual volume of air is the same because the piston hasn't
risen yet. This enriches the mixture, which aids acceleration -- it
performs the same function as an accelerator pump on a Holley or Q-jet,
but without the sudden drop in airflow that you get when the secondaries
open up. Neat. Then as the RPM rise, the piston still climbs up the
dashpot, which lets more air in to meet the demands of the motor (well
actually, the driver's right foot).
3. For road-race or endurance use, remove the damping piston altogether.
MGB preparation expert Butch Gilbert echoes the feelings of most
SU tuners that no oil exists that doesn't suffer enough viscosity
change during the course of even a 25-minute SCCA race to have an
effect on throttle response. Butch just pulls the piston assembly
out of the little cap, then tests different spring ratings to get
the throttle response characteristics he wants. I've driven cars
thus equipped, and surprisingly, they work just fine. Of course,
they had also been set up on a chassis dyno by a knowledgeable
tuner of SU carbs, which I think is the trick.
4. For maximum airflow, you can do some porting to the piston edges.
The mid-Sixties practice of "through-boring" SUs -- that is,
removing the bridge at the bottom of the carb -- has been pretty
thoroughly debunked by David Vizard, who instead prefers to smooth
the rough edges of the bridge but leave it in place as a venturi
restriction necessary for good fuel atomization. Vizard also
has found that modifications to the piston, giving it a semi-airfoil
profile, is a significant enhancement to airflow. For the ultimate
in SU flow, Vizard recommends fly-cutting the throttle butterfly
so that the trailing edge is narrower than the leading edge, brazing
the butterfly into place and removing the middle portion of the
throttle shaft. Such modifications typically enhance an SU's flow
capacity by up to 35%.
5. Finally, for good airflow and strong signal to the fuel jet, SUs
like to have wide open spaces and gentle curves at their throats.
Surprisingly, the stock air filters on MGBs have an almost ideal
layout for maximizing airflow into the throat: a big chamber
with 3"-4" between the throat and the inside of the air filter
box, and a nicely radiused entry to the carb formed by the back
plate of the filter. Surprisingly, a short, broad-radius entry
works much better on an SU (up to +7.5% airflow) than a long,
traditionally shaped velocity stack. The 90-degree input that
stock SUs have with no additional shaping actually reduces the
effective throat diameter and drops airflow almost 10% off the
theoretical maximum. Add it up and you get a big (18%?) win
by shaping the intake. (Now, it should be noted that this 18%
is in airflow, not in horsepower; don't expect to peel the tread
off the carcass of the tires just by swapping to a Stub Stack.
Horsepower increases from air filter and air entry optimization
typically run in the 3% -- 8% range, depending on just how bad
the stock unit is. I actually anticipate something over 5% on
the Volvo 122S, because of other problems with the stock filters.)
Conclusion
In short, most of the SU's bad reputation comes from how easy it is
to modify them -- what's the saying about how if you make something
so simple that any idiot can use it, some idiot will mess it up? SUs
are infinitely adjustable, and offer a wide range of settings that
are *almost* right. The trick is getting closer to *exactly* right.
While a dyno is best, especially if it reads the air-fuel ratio at
the exhaust while reading power at the rear wheels (like Charlie
Rockwell's), a Gunson ColourTune makes a good tuning aid for the
home tweaker.
SUs are also prone to wear in the throttle shaft bores, and rebushing
them is a critically precise operation. But having played around
with old SUs and new SUs, I've come to the conclusion that most of
the things people dislike about SUs are simply due to the age of
the particular carbs in question, not to the basic design. A simple
rebuild -- a thorough cleaning, new jets, new gaskets, and a new valve
for the float chamber -- costs about $40 in parts for a pair of carbs,
and will give you a chance to learn how to balance them. But as I
said, I'll have to cover balancing SUs in a later message.
Partial Bibliography:
The Complete Official M.G. MGB with Abingdon Special Tuning Guide,
Robert Bentley Publishing, Cambridge, Mass. Reprint of the factory
shop manual, with driver's handbook at front and the factory's
racing prep manual at the rear. Some on SUs, not much.
Tuning the A Series Engine, David Vizard, Haynes-Foulis Publishing.
Probably *the* best book on engine modifications; particularly good
for owners of Sprites, Midgets, Minis, and other A-Series powered
cars, but lots of good information in general. This is the book that
really taught me to understand camshafts, for instance. It's also
THE reference for modifying SUs for competition. Accept no substitutes.
Buy this if you have SU carbs, regardless of what they're on, or at
least check it out from the library.
--the blue SU book published by the same people who do the Passetti books
on Webers. Can't remember much except the yahoo who wrote it couldn't
tell the difference between it's and its, which yanks a writer's chain
the way confusing amps with volts does to an EE :-). Not much on
modification, a lot on different SU configurations.
The Haynes SU manual. Great pictures, so-so procedures, little or
nothing specifically on modification though the theory is all there.
Good for identifying the differences between varieties of carbs; I
think there's also a needle chart, but I could be wrong.
And for the Britcars list: Roger Garnett's excellent guide to tuning
SUs. Combine that practice with the theory and modification guides
here, and you'll have a first-class reference to everything you wanted
to know about SU Carbs.
--Scott Fisher
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