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 inch calculation.

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 = 342 CFM
  • 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 engine.

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 plenum manifold.

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.


Needed Accessories

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 alone.

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.



Hidden Cost

Brake Modifications: 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.


Final Results

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 manifold vacuum.

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 system.


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.

Other things 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 barrel manifold

Fuel distribution

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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