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703MGB
Senior Member
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I could not resist the temptation to storm up the discussion with one of my favorite articles.
https://www.sterlingbritishmotoringsociety.org/files/mgbtunings.PDF
33,000 words long
Here is the extract about webers:
The use of the Weber DCOE 45 carburetor on street MGBs came about as a result of their use on the factory team's racers.
This fact, of course, produced a "monkey see, monkey do" mentality amongst those seeking more power for their street MGBs.
Why did the factory race team choose the Weber over the tried-and-true SUs? It has to do with the design differences between the two. The SU is a Variable Venturi type, which makes for smooth although slightly slow throttle response and excellent fuel
economy. The Weber DCOE 45, on the other hand, is a Fixed Venturi type. It has the advantage of having an injector pump to shoot raw gasoline into the venturi when the throttle opens rapidly and thus makes for very fast throttle response. This was a definite advantage on the race track, so that's part of the reason why the factory race team chose it over the SU. Remember that on a race track, smoothness and economy must be subordinate to responsiveness, as its responsiveness that makes aggressive
driving possible. Victory is what counts on the track, and nothing else will substitute.
This fast throttle response produces the illusion of more power and so purchasers of this unit tend to experience what Psychologists call the "Halo Effect": they've paid out the big money, sweated the installation, spent more money to convert their ignition system to a centrifugal advance distributor (Weber carburetors don't have provision for a vacuum takeoff for working with vacuum advance ignition systems: read the fine print!) and so they're already predisposed to feel the power increase even before
they drive. When the quick throttle response creates the illusion of more power, they're like religious converts! In reality, all other factors being equal, there is no worthwhile difference between them in terms of power output on the dynamometer readouts unless a radical camshaft is being used. Should you decide to use this carburetor, you would be well advised to use a Soft Mount kit to protect it from the effects of vibration (APT Part # SMW-45).
Unfortunately, the Weber's intake manifold imposes a major drawback: In order to facilitate the mounting of an aircleaner with adequate flow capabilities, its 9.5 cm length is short. This shortness forces the use of a very curvaceous path between the carburetor and the intake ports, which in turn causes the fuel charge to be biased towards the ports for the outer cylinders (#1 & #4). The result is that the outer cylinders (#1 ) tend to run richer while the inner cylinders (#2 & #3) tend to run leaner, the differential between the two increasing with engine speed due to the increasingly greater inertia of the fuel. The Weber 13 cm swan-necked intake manifold, or the similar one offered by Oselli, will reduce this tendency while being more appropriate to camshafts whose designs are oriented toward producing more low rpm and midrange power at the expense of high rpm power, but to fit an efficient aircleaner you will need to rework the inner body panel with a soft mallet. This was never a problem for the
factory race team, but many private owners will take exception to the idea of hammering away at their engine compartments.
Consequently, the combined inlet manifold, carburetor, and air cleaner assembly should not exceed 13 3/4" in depth as this is the maximum allowable for inner fender clearance.
It should be understood that torque characteristics are not determined by the length of the intake manifold or of ram pipes.
Instead, they are determined by the camshaft. The main function of ram pipes is merely to reduce turbulence in the incoming fuel/air charge. If you look into the mouth of your Original Equipment aircleaner boxes you will see what is called a "stub stack." They are there specifically to reduce turbulence. A Weber DCOE carburetor has a fair amount of turbulence at its mouth, so a ram pipe is used to reduce it. It is of significant importance to have the appropriate length of the intake tract for the characteristics of the camshaft. A camshaft that produces a powerful low end torque output functions best with a long intake tract, while a camshaft that produces a powerful horsepower output at high engine speeds functions best with a short intake tract. A Weber DCOE 45 can use different length ram pipes to achieve this rather than forcing the owner to spend more money for different length intake manifolds. If the racer is going to drive on a slow, twisting track where low and midrange power output is critical to victory, he can change his camshaft and tappets, change the metering in the Weber DCOE, and change to a longer ram pipe. If he is going to race on a faster track, he can change his camshaft and tappets, change the metering in the Weber DCOE, and change to a shorter ram pipe to obtain higher output at high engine speeds. There is, however, a major drawback to the use of
ram pipes: the carburetion can be very sensitive to small errors in metering, running rich or lean if the adjustment is off by only a small amount. As such, it is not quite as good as using a longer or shorter intake manifold, but for an amateur racer, it is much more affordable. For professional racers who do have the optimum length intake manifold for the track that they are racing on,
they can fine tune the intake tract by experimenting with different length short ram pipes during practice laps. The availability of different length ram pipes is one of the reasons that the Weber DCOE is so popular with racers. However, while these factors tend to make the Weber DCOE carburetor the most popular choice for racing applications, they are largely irrelevant when building a streetable engine.
Be advised that neither the Weber nor the Oselli intake manifolds have a balance tube to modulate pressure fluctuations between the two intake tracts which is necessary to prevent "robbing." This unmodulated pressure fluctuation, which is aggravated in the individual intake tracts by the uneven breathing resulting from the 180 degree opposed throws of the crankshaft, is the reason that these manifolds have no provision for a vacuum advance takeoff. The advance plate in a vacuum advance
distributor would be rattling back and forth so violently that consistent ignition timing would be all but impossible to achieve. This is turn forces the use of a pure centrifugal advance distributor. Expect poor part-throttle response, high engine temperatures, a tendency to burn valves, a tendency to preignition under heavy loads, decreased fuel economy, and a ragged idle. On the other
hand, the Cannon 801 intake manifold has provision for the installation of a primitive balance tube.
There is, however, a considerable difference between the Weber and the SU in the process of setting them up. The SU has only one needle and one jet, so you can modify it in your driveway. The Weber, on the other hand, has a multiple choice of replaceable venturi sizes, six jets (starter air correction jet, starter jet, idle jet, main jet, accelerator pump jet, and air correction jet),
plus an emulsifier tube! As Peter Burgess rightly points out in his book, carburetors are rarely properly set up as delivered (but people rip a Weber out of its package and slap it on their engines in sheer ignorance of this fact). This multiplicity of jets and venturi sizes does, however, make it almost infinitely adaptable, even to practically any exotic camshaft lobe profile, and this is
another reason why the factory racing team used them. They could more easily tailor the engine's performance characteristics to the type of track that they were about to race on.
However, unless you're using a radical camshaft, have access to a dynamometer, and you really understand how a carburetor works, take my advice and use the 1 1/2" SU! The bigger 1 3/4" SUs
might make for a bit more power at high engine speeds (above 6,000 RPM) due to their higher flow capacity, but unless you're mounting them to meet the demands of either a 1950cc engine with ported heads or a smaller bore engine with a Piper 285 camshaft and ported heads, you'll get it at the price of less power at low engine speeds (which is where a street engine spends most of its operating life), a lumpy, vibrating idle, and difficult cold weather starting. If you do choose to use them on the aforementioned engine types, mount them on the Special Tuning intake manifold available from Burlen Fuel Systems. On a standard displacement engine they will sacrifice as much power below 4,000 RPM as they will gain above that point.
I rest my case! /ubbthreads/images/graemlins/hammer.gif
https://www.sterlingbritishmotoringsociety.org/files/mgbtunings.PDF
33,000 words long
Here is the extract about webers:
The use of the Weber DCOE 45 carburetor on street MGBs came about as a result of their use on the factory team's racers.
This fact, of course, produced a "monkey see, monkey do" mentality amongst those seeking more power for their street MGBs.
Why did the factory race team choose the Weber over the tried-and-true SUs? It has to do with the design differences between the two. The SU is a Variable Venturi type, which makes for smooth although slightly slow throttle response and excellent fuel
economy. The Weber DCOE 45, on the other hand, is a Fixed Venturi type. It has the advantage of having an injector pump to shoot raw gasoline into the venturi when the throttle opens rapidly and thus makes for very fast throttle response. This was a definite advantage on the race track, so that's part of the reason why the factory race team chose it over the SU. Remember that on a race track, smoothness and economy must be subordinate to responsiveness, as its responsiveness that makes aggressive
driving possible. Victory is what counts on the track, and nothing else will substitute.
This fast throttle response produces the illusion of more power and so purchasers of this unit tend to experience what Psychologists call the "Halo Effect": they've paid out the big money, sweated the installation, spent more money to convert their ignition system to a centrifugal advance distributor (Weber carburetors don't have provision for a vacuum takeoff for working with vacuum advance ignition systems: read the fine print!) and so they're already predisposed to feel the power increase even before
they drive. When the quick throttle response creates the illusion of more power, they're like religious converts! In reality, all other factors being equal, there is no worthwhile difference between them in terms of power output on the dynamometer readouts unless a radical camshaft is being used. Should you decide to use this carburetor, you would be well advised to use a Soft Mount kit to protect it from the effects of vibration (APT Part # SMW-45).
Unfortunately, the Weber's intake manifold imposes a major drawback: In order to facilitate the mounting of an aircleaner with adequate flow capabilities, its 9.5 cm length is short. This shortness forces the use of a very curvaceous path between the carburetor and the intake ports, which in turn causes the fuel charge to be biased towards the ports for the outer cylinders (#1 & #4). The result is that the outer cylinders (#1 ) tend to run richer while the inner cylinders (#2 & #3) tend to run leaner, the differential between the two increasing with engine speed due to the increasingly greater inertia of the fuel. The Weber 13 cm swan-necked intake manifold, or the similar one offered by Oselli, will reduce this tendency while being more appropriate to camshafts whose designs are oriented toward producing more low rpm and midrange power at the expense of high rpm power, but to fit an efficient aircleaner you will need to rework the inner body panel with a soft mallet. This was never a problem for the
factory race team, but many private owners will take exception to the idea of hammering away at their engine compartments.
Consequently, the combined inlet manifold, carburetor, and air cleaner assembly should not exceed 13 3/4" in depth as this is the maximum allowable for inner fender clearance.
It should be understood that torque characteristics are not determined by the length of the intake manifold or of ram pipes.
Instead, they are determined by the camshaft. The main function of ram pipes is merely to reduce turbulence in the incoming fuel/air charge. If you look into the mouth of your Original Equipment aircleaner boxes you will see what is called a "stub stack." They are there specifically to reduce turbulence. A Weber DCOE carburetor has a fair amount of turbulence at its mouth, so a ram pipe is used to reduce it. It is of significant importance to have the appropriate length of the intake tract for the characteristics of the camshaft. A camshaft that produces a powerful low end torque output functions best with a long intake tract, while a camshaft that produces a powerful horsepower output at high engine speeds functions best with a short intake tract. A Weber DCOE 45 can use different length ram pipes to achieve this rather than forcing the owner to spend more money for different length intake manifolds. If the racer is going to drive on a slow, twisting track where low and midrange power output is critical to victory, he can change his camshaft and tappets, change the metering in the Weber DCOE, and change to a longer ram pipe. If he is going to race on a faster track, he can change his camshaft and tappets, change the metering in the Weber DCOE, and change to a shorter ram pipe to obtain higher output at high engine speeds. There is, however, a major drawback to the use of
ram pipes: the carburetion can be very sensitive to small errors in metering, running rich or lean if the adjustment is off by only a small amount. As such, it is not quite as good as using a longer or shorter intake manifold, but for an amateur racer, it is much more affordable. For professional racers who do have the optimum length intake manifold for the track that they are racing on,
they can fine tune the intake tract by experimenting with different length short ram pipes during practice laps. The availability of different length ram pipes is one of the reasons that the Weber DCOE is so popular with racers. However, while these factors tend to make the Weber DCOE carburetor the most popular choice for racing applications, they are largely irrelevant when building a streetable engine.
Be advised that neither the Weber nor the Oselli intake manifolds have a balance tube to modulate pressure fluctuations between the two intake tracts which is necessary to prevent "robbing." This unmodulated pressure fluctuation, which is aggravated in the individual intake tracts by the uneven breathing resulting from the 180 degree opposed throws of the crankshaft, is the reason that these manifolds have no provision for a vacuum advance takeoff. The advance plate in a vacuum advance
distributor would be rattling back and forth so violently that consistent ignition timing would be all but impossible to achieve. This is turn forces the use of a pure centrifugal advance distributor. Expect poor part-throttle response, high engine temperatures, a tendency to burn valves, a tendency to preignition under heavy loads, decreased fuel economy, and a ragged idle. On the other
hand, the Cannon 801 intake manifold has provision for the installation of a primitive balance tube.
There is, however, a considerable difference between the Weber and the SU in the process of setting them up. The SU has only one needle and one jet, so you can modify it in your driveway. The Weber, on the other hand, has a multiple choice of replaceable venturi sizes, six jets (starter air correction jet, starter jet, idle jet, main jet, accelerator pump jet, and air correction jet),
plus an emulsifier tube! As Peter Burgess rightly points out in his book, carburetors are rarely properly set up as delivered (but people rip a Weber out of its package and slap it on their engines in sheer ignorance of this fact). This multiplicity of jets and venturi sizes does, however, make it almost infinitely adaptable, even to practically any exotic camshaft lobe profile, and this is
another reason why the factory racing team used them. They could more easily tailor the engine's performance characteristics to the type of track that they were about to race on.
However, unless you're using a radical camshaft, have access to a dynamometer, and you really understand how a carburetor works, take my advice and use the 1 1/2" SU! The bigger 1 3/4" SUs
might make for a bit more power at high engine speeds (above 6,000 RPM) due to their higher flow capacity, but unless you're mounting them to meet the demands of either a 1950cc engine with ported heads or a smaller bore engine with a Piper 285 camshaft and ported heads, you'll get it at the price of less power at low engine speeds (which is where a street engine spends most of its operating life), a lumpy, vibrating idle, and difficult cold weather starting. If you do choose to use them on the aforementioned engine types, mount them on the Special Tuning intake manifold available from Burlen Fuel Systems. On a standard displacement engine they will sacrifice as much power below 4,000 RPM as they will gain above that point.
I rest my case! /ubbthreads/images/graemlins/hammer.gif
