Dual 2 Barrel carburetors
to Single 4 Barrel Carburetor Conversion
for the E-3 and E-9.
By Marty Roach
Before reading this, interested parties should
take the time to read some hot-rodding books as well as introductions
to general engine performance. I can personally recommend Power
Secrets by Smokey Yunick and How to Build Horsepower,
volumes I & II by David Vizard.
OEM 4 barrel Solex Carburetors
The four barrel Solex carburetor was installed on
European 2.8 and 3.0 liter engines as well as some Mercedes Benzes
of the period, replacing twin 35/40 INAT Zeniths on dual manifolds.
The Solex four barrel can be found on the following Euro BMW models:
E12: 525, 528, 530. E24: 628, 630, and E23: 728 and 730. An E21
320/6 may also be equipped with a version of the Solex. Neither
E9 Coupes nor E3 sedans were originally equipped with a four barrel.
The OEM Solex 4A1 used on Big Sixes has 38mm primaries,
electric choke, and vacuum actuated 58mm secondaries. I have been
told that Solexes 4A1's are good carburetors when there are set
up correctly. Not surprisingly, these are the same savants who can
tune a Zenith 35/40.
I once drove a Euro 728 sedan and the while the car
drove well enough under firm throttle, it had sticky secondaries
that never opened (I tied them shut to verify), and a miserable
idle, both hot and cold. I dosed it with carburetor cleaner, high
octane fuel, checked the ignition timing, etc. - same thing. Should
I have adjusted the valves and done a complete tune-up? Certainly.
This particular Solex was a year old and was purchased new, not
rebuilt, from BMW-NA. ($700?).
If you find a car with a Solex 4A1 that drives
well, and the parts are cheap, buy it and take a chance. The Solex
has an advantage against most aftermarket American four barrels
since you can continue to use the stock fuel mechanical pump. If
it is a BMW, grab the intake manifold, throttle linkage, and black
OEM air cleaner assembly.
My choice of carburetor
Holley 4160/ model 8007, 390 CFM, p/n 8107. Dual 47mm
mechanical primaries and 47mm vacuum actuated secondaries.
(Note: Tom Van Gunten has been running four barrels
on 3.5 liter Big Sixes for several years. Read his FAQ and decide
for yourself. The spreadbore carburetors are much easier to use
on the stock manifold than the 4160/ model 8007)
The 4160/ model 8007 is designed for use with
small V8 and V6 engines and the airflow capacity is 390 cfm (cubic
feet per minute). Virtually all of the motor press that I have read
to date specifies that the carburetors should flow between 1.8 and
2.0 cfm per cubic inch of engine displacement. Since a 3 liter BN|BMW
engine has 182 cubic inches of displacement, the carburetor should
be able to flow 364 cfm. The Holley carburetor is easily able to
supply this amount of air to the motor, even if the BMW engine has
a volumetric efficiency 7% higher than the robust 2 CFM per cubic
Airflow requirements for c.i.d and rpm's. (per the Holley
Carburetors Sales Brochure, ed. 11/96 )
Where: required CFM = (engine displacement (c.i.d.) * maximum
rpmī's) / 3456
Note: 1 liter = 61.3 c.i.d
3 liter motor = (182 * 6500) / 3456 =
3.3 liter motor = (200 * 6500) / 3456
= 376 CFM
3.5 liter motor = (213 * 6500) / 3456
= 399 CFM
Some readers will note that I chose 6500 rpm as a maximum
rpm versus the more typically quoted 7000 rpm. Given my
normal driving pattern, even 6k is probably still too high:
I rarely rev over 5500.
Using the above calculations, the Holley 4160/ model 8007
is acceptable for engines up to a street driven 3.3 liters.
In my opinion, it would not be a good idea to put a larger
capacity carburetor on a straight six BMW engine. Proper
fuel atomization depends on a strong intake signal at the
cylinder and the resulting high velocity of air through
the carburetor venturis. Carburetors are designed to work
best within certain airflow parameters. If a carburetor
is too big for an engine, it won't adequately meter and
atomize fuel because the carburetor doesnī't receive a strong
enough intake signal from the engine.
As rpm's increase this will be less of a problem but at
low engine speeds the low airflow will result in bogs and
sags. Large diameter throttle plates and large diameter
venturis look impressive when the air cleaner is removed
but they are designed for engines more than twice as large
as a Big Six. It shouldn't be surprising if a big carburetor
performs poorly until the Big Six was revved high enough
to generate the same level of airflow as a larger displacement
The aftermarket carburetor market developed to fill the
needs of V8 engines that are 5 liters and larger and existing
products reflect this premise. In the past ten years, fuel
injection has replaced the carburetor on virtually all production
cars. These factors combined severely limit the variety
if carburetors that older BMW owners can use now, and will
preclude any future development of aftermarket carburetors.
Necessary manifold Alterations
The 4160/ model 8007 has a squarebore base, meaning that
the venturis are both the same size and equidistant from
each other. The BMW manifold is a spreadbore pattern and
the holes for the secondary venturis are larger and wider
apart. The holes for the 38mm primary venturis are quite
far part and the 58mm secondaries are separated by only
10mm of metal. To put it mildly - the Holley squarebore
carburetor doesn't fit on the stock manifold: mounting holes
do not line up nor do the profiles of the carburetor base
to the manifold deck.
I took the cheap and easy way out and purchased a squarebore
to spreadbore adapter plate from Jeg's automotive mail order
house. It cost about $35. It was manufactured by Edelbrock
though I am sure that other less expensive brands are available.
Do not buy a cheap adapter that has one large opening rather
than four separate openings (see below). Some matching was
necessary and was done using Prussian blue and rat tail
files. Since the adapter plate created a very sharp turn
into the manifold underneath the primary venturis, I removed
as many of the casting imperfections as possible. Reorienting
the carburetor so that the primary venturis are over the
part of the adapter plate with more friendly contours would
involve changing the throttle linkage.
Single Plane versus dual plane manifolds
The stock BMW 4 barrel manifold is divided into two halves
and each set of three cylinders is fed by one primary and
one secondary throttle. It may be easier to think of the
left and right side of the carburetors. This kind of divided
manifold is usually referred to as a dual plane or divided
The goal is to keep the cross sectional area in the manifold
plenum as close to the original design as possible. If the
area in the manifold under the carburetor is increased,
there is a greater tendency for atomized fuel to linger
since the air velocity would be decreased: less suction.
Conversely, a single plane manifold has an open area at
the base of the carburetor where all of the cylinders can
be fed by the whole throttle capacity of the carburetor.
In the case of the BMW manifold, think of the manifold with
the center rib removed. Dual plane manifolds usually
outperform single plane manifolds at part throttle and offer
better low rpm drivability and torque.
With an unmodified BMW dual plane manifold, using the 390CFM
Holley as an example, each cylinder can be fed by a total
of 195cfm of carburetor flow capacity. (One primary barrel
at cruise plus one secondary barrel at full throttle). Referring
to the CFM per c.i.d. requirements I included above: (182
c.i.d. / 2) * 2 = 182 CFM of carburetor capacity for 3 cylinders.
If the manifold center were routed out and the manifold
became a dual plane manifold, each cylinder could have fuel
supplied by the full 390cfm of the carburetor. As a result
of reading several books on performance engine modifications,
I think that a dual plane manifold would be better than
a single plane on an M30/ Big Six engine.
Most drivers are at full throttle only a few minutes per
week of driving. The rest of the time, the combination of
the carburetor and manifold will be too much for the engine
and drivability will suffer. When you are starting from
a stop, at low rpm's, or cruising at part throttle, it reasonable
to assume that using a dual plane manifold would cause:
lower airspeed in the manifold and a weaker intake signal,
resulting in poorer fuel metering and atomization, in layman's
terms: sags, bogs, and flat spots.
|Pierburg electric fuel pump / distributed
by Hella USA. Cylindrical pump that fits neatly in the trough
that surrounds the fuel tank in the CS trunk. It generates approximately
5.5 psi. Holley Technical Service advised that this carburetor
works with this fuel pressure. Nevertheless, I need to keep
the warm idle at about 1200 rpm so that the smell of gasoline
does not enter the passenger compartment. I think that the floats
begin to overflow slightly if the car sits at slower idle. A
fuel pressure regulator is in order.
|Secondary Spring Quick Change Kit,
Holley. 8 color-coded springs that allow you to vary the engine
vacuum at which the secondaries will open. I have only gone
to the next lightest spring tension that stock and the results
were immediate. I will try a lighter spring in 2001 ' at this
point I am happy just to drive the car and leave well enough
|K&N Drop Base 14" air cleaner.
I used an adapter plate to fit the Holley onto the stock manifold.
The Holley is taller than the stock Solex carburetor and the
adapter plate raises the carburetor another 1.5". As a result,
there is only about 2" of space between the hood and the air
horn on top of the Holley. Having to make a custom filter out
of K&N components defrayed much of the savings that I had
realized up to this point. I looked in vain for a less expensive
alternative but found nothing. One advantage of the K&N
filter is a great intake sound.
|Squarebore to spreadbore intake
manifold adapter. The footprint of the Holley and the Solex
are vastly different, as mentioned. I needed to use this adapter,
as well as match the profiles of the manifold and adapter. This
did require time and patience. Chinese copies of name brand
adapter plates cost only $15-17.
|Used 320i brake booster. The four
barrel manifold WILL NOT FIT in the E9 coupe with the bigger
brake booster that was used on virtually all BMW six cylinder
cars before the advent of hydraulic brakes. This smaller brake
booster means that you will need more pedal pressure to match
the braking force with the larger booster.
|Primary Jets. The stock jets produced
a lean surge at cruise and I needed to massage the throttle
in order to keep the engine happy. Moving up one jet size has
smoothed out mid throttle operation and overall power has improved.
I may experiment with the jets at some point in the future but
at this point I am happy to have the car run.
|Thermostat to manifold inlet: 11
53 1 256 478
|E24 / E12 manifold outlet to heater
core: 64 21 1 364 769
|728 Euro hose: manifold to coolant
reservoir: 11 53 1 265 112
|Total Cost for Accessories
Adding in the cost of
the carburetor ($125), my total cost is: $412.
Please note that I got the manifold for
free and the normal price of the new Holley is approximately
$250. Buying a new Holley raises the total cost to over $500.
Adding in a used manifold raises the total cost in the vicinity
of new downdraft Webers. If you need to do the pedal
box modifications mentioned below you should think hard before
ordering a new Holley carburetor.
In order to have the manifold fit into the coupe (and I stress
coupe) engine compartment with room to spare I needed to cut
and re-weld the tower that holds the brake booster. Contact
Tom Van Gunten to check the underhood dimensions of the E3/Bavaria.
I needed to move the mounting surface about 1.5" inches towards
the inner fender. If you look closely at the photograph, you
can see a "hook" in the middle of the tower. This work was performed
by Mr. Jim Shank, who also helped me to combine the brake booster
rods of an E21 320i and an E3 Bavaria. To be honest, I couldn't
have done this as well and as quickly without his help.
Car starts and idles
perfectly when warm. It is a bit "cold blooded" for the first
few minutes after start up on a cold day. Pick up from a standing
stop is brisk and part throttle acceleration is also first
rate. There is a mild "hitch" at part throttle but I am not
sure whether this is due to the ackward angle that air has
to take before entering plenum or if it due to the influence
of the accelerator pump timing as influenced by the shape
of the cam that actuates the pump itself' this will require
more investigation. It has been many years since I drove a
car equipped with a carburetor ' fuel injection spoils you
with consistent operation under all conditions. If I could,
I would make the operation of the secondaries progressive
and mechanical rather than have them triggered by load and
My coupe is equipped with a 5 speed close ratio
and 3.64 rear end. Fuel mileage is 17.8 mpg with most of the
driving done on the highway. I am confident that when I reduce
my final drive ratio 19% in fifth gear my highway mileage
will improve significantly.
When you step firmly on the accelerator, the
secondaries open and the engine develops a deep throated resonant
hum. Car pulls hard in fifth gear to 5,000 rpm (and over)
with plenty of pedal left. It bears mentioning that the Holley
carburetor is easy to work on. It is quick and painless to
change the jets and set the float levels ' while the carburetor
is still on the car.
If I could do it all over again?
Instead of using an adapter plate to mount the
carburetor, I would opt for having the manifold deck lowered
and making a square bore adapter plate made. The adapter that
I chose allows the carburetor to fit but there is a nasty
angle that the mixture has to negotiate before it enters into
the main plenum. I would not be surprised if this had a lot
to do with the slight bog that I experience at low speeds.
In addition, by having the manifold decked, you would be able
to use the OEM air cleaner assembly, which is much better
looking than the chromed K&N components, which are frankly
hideous. The stock filter will fit on top of the Holley, but
it does need to be massaged (Billy-bashed) in one spot.
I spoke to Pete McHenry a few years back and
he mentioned that he could have the manifold decked and a
custom adapter plate machined for $175. What really appeals
to me is that Pete's modifications don't increase the internal
volume of the manifold very much. More importantly, the air/fuel
mixture has a much more direct path to the cylinders: air
moves more directly at lower rpms, and, as a result, there
is a stronger signal at the venturi ' an important factor
at lower airflow speeds. Given another chance, I would probably
do this modification.
The relative performance of the Holley
in comparison with the other induction options listed is difficult
to assess because my engine is equipped with components that
boost performance on their own: Stahl exhaust headers, 008
European mechanical distributor, and a Crane electronic ignition
Is the Holley a worthwhile modification
or a Boy Racer hack job? It depends on whom you ask. While
the Holley conversion is totally functional, it is clearly
unusual. Gearheads think it is a great idea to use a Holley
on a BMW. Many BMW purists will recoil in horror that you
have subjected your BMW engine to the indignity of using an
American four barrel. If you are thinking of selling your
car, this should be taken into consideration: double and triple
Webers are more authentic and palatable to wider range of
people. The range and depth of needed modifications were well
beyond what I anticipated.
to think about
|Getting fuel into
the Big Six motor, ranked from best to worst:
- timed / multi-point fuel injection with optimization
capabilities ' Motronic
- multi-point fuel injection: L-Jet, D-jet
- triple sidedraft carburetors (3 x 2 barrels)
- twin carburetors (Weber 32/36's)
- single four barrel (Solex 4A1, Holley, Quadrajet)
Put simply, it's always better to have a dedicated
source of metered and timed fuel for each individual cylinder.
Depending how you drive and your budget, you can be happy
with one of the lesser alternatives. I chose a four barrel
carburetor versus the others and I am satisfied.
Disadvantages of the Big Six four
None of the intake runners on the Big
Six four barrel manifold are the same length, shape, and even
their inside diameter varies slightly (different volume).
Cylinders attached to a common plenum scavenge mixture from
each other. What is unused fuel mixture doing floating around
unused in the first place? This clearly affects fuel distribution,
power, and drivability:
All six of the intake charges vary slightly
from one another. Air/fuel mixture velocity is different
in each intake runner because of the differing plenum volumes
and the varying signals present at the jets. Cylinders closest
to the carburetor will get richer mixtures than those which
are further away.
Air/fuel mixture from the carburetors
gets bounced around and lost on its way to the intake ports
because of the greater amount of common area in the manifold
and the intake pulses from the cylinders.
In contrast, examine at a Big Six with fuel
injection or triple carburetors with six independent runners.
Those systems have six identical intake runners, both in length
and diameter. In these intake systems, the air drawn into
the engine may come from a common source but the fuel is introduced
downstream, at the intake of each cylinder. The further our
design moves away from six balanced intakes and individual
fuel meters the worse off we are.
Dyno tests with V8 engines comparing 4 barrels
against multi-point fuel injection and multiple sidedraft
carburetor set-ups show that the 4 barrel lags behind the
others in terms of torque and horsepower throughout the entire
rpm range. Four barrels may produce the highest horsepower,
but they lag behind the others at all other points. The same
is (likely) true for dual downdraft Webers.
The four barrel manifold is clearly a
compromise. Will 95% of Big Six owners notice the difference?
I'm not sure. I simply can't afford to run a conclusive set
of dyno tests that would plot horsepower and torque of the
same engine with the different intake systems.
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