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MG MGB Technical - su or webber

HI
IS ANYONE RUNNING WITH THE WEBBER CARB SET UP IF SO
IS IT BETTER THAN THE SUs
thanks
ron
R C Reed

Hi - this is very subjective for me as it is not my car, but friend has same car as i do - he changed to a webber. In my opinion, there is little in the way of performance difference, the webber makes more induction noise and he thinks he uses more fuel than he did with the su's. My feelings (based on, well feelings!) is that unless you have a bigger bore, wilder cam, stage 2 head type of engine, and wish to drive it hard, then su carbs are just the job for anything else. I don't think that changing out one component will do a lot ie if the cam and head remain the same, then since SU's are adequate, I cant see having a webber making a lot of odds. But depends on your set up I guess.
Someone who knows what they are talking about will be along in a moment to tell me what a load of nonsence i am talking!
mick
mick

This is a common subject in the threads so be sure to look into the archives. Most will urge you to never change from SUs to a Weber. There is more than one Weber model, the DCOE and the DGV. The DGV is the most commonly used and won't offer the performance of twin SUs. The DCOE might work well, but only with other performance modifications to the engine like you mentioned. The DGV is a good replacement for the single Zenith carb used from the mid to late 70s and 80 models although the twin SUs would work even better, but require more work to install in place of the Zenith. I have an 80LE with the Weber DGV (changed by an earlier owner) and it will never offer the performance (not even close) to the 70B that I drove for years. The 70 had no performance modifications, ran the entire smog setup, twin SUs and would still outrun any later model that I've been in. It's hard to improve on British Leyland's engine design from the late 60s through about 72.
Rick Penland

thanks i get the point i will stick with the sus and get them setup
thanks
ron
R C Reed

The SU's are an elegant and efficient design...and in my opinion, well worth keeping......As long as the shaft bushing is good...the rest is effortless to tune and maintain......
Pete

Hello,

My two cents advice, keep SU if in good shape. As having read many post where you need three post-doctoral studies in caburation to deal with Weber adjustments and jetting. Keep things simple. I am foung of HS4 and easy to restore and adjust.


Cheers,

Jean G.
Jean Guy Catford

The prevailing theory seems to be that the SUs are original and are correct for a British car. I rebuilt my first SU and was surprised by how easy it was! The design was much more efficent than the Hollys I was used to. The SU design eliminates the accelorator pump and circit. Also it eliminates many parts thus reducing the complexicty and making the carb simple. Webbers while provide a simple solution I rather like the SUs and have restored my Zenith/Stromberg as it is original with the MG. In addition, I've seen several upgrades using the Austin Marina manifold with the SU which would be much diffrent than most MGs.
S.L. LaPaugh

If you rely on garages for your servicing, forget Webbers, most will not have a clue. SUs are so simple, and if possible choose the old HS4 and not the HIF.
c cummins

Hey RC,
I have 72/79 mostly original MGB's.

Conversion on both to Webber. No problems and performance is very satisfactory. Both are electric automatic choke. Push the pedal once and go except when they sit for a long time, then it takes more pumping and about 30 seconds and they START!

James
A Huggins

i have a weber on my mgb,and it is a bit lumpy on tickover ,so i put twin su's on to smooth things out, it was smooth ok,and it was slow as a boat,so i have put the weber back on and put up with the lumpy tickover, but it is a 1950 stage 2 engine.
da wright

Ron-
I use Weber DCOEs on my MGB with its crossflow head, SUs on my midget, and Delorto DHLAs on my MGC.

When it comes to the subject of carburetion, many people tend to opt for items that they perceive as being exotic, such as the Weber DCOE or the Dellorto DHLA. The Dellorto DHLA atomizes fuel to a much finer degree than the Weber DCOE does. However, the finely atomized fuel droplets combust more quickly, producing maximum pressure earlier in the combustion stroke, and thus force the use of a richer fuel / air mixture in order to slow the combustion process. The larger amount of finely atomized fuel thus takes up a greater volume in the induction tract than the larger, slower-burning droplets produced by the Weber DCOE, displacing air and making for an effectively smaller fuel / air charge, and thus less power output. However, due to its production of a finely atomized fuel charge, combustion becomes more efficient, creating better fuel economy and less pollution. Conversely, the less-finely atomized fuel droplets produced by the Weber DCOE design requires a more generous supply of oxygen in order to combust properly. This is the explanation for that carburetor’s reputation for being sensitive to changes in altitude. However, the slow burn characteristic resulting from these larger fuel droplets do result in maximum cylinder pressure being attained later in the combustion stroke, thus reducing the amount of heat that is lost through the roof of the combustion chamber, and thus enhances power output. This is the secret to its marginal superiority in power output. All other factors being equal, if you switch from a properly set up Dellorto DHLA carburetor to a properly set up Weber DCOE carburetor, you should easily get a 5% increase in power output across the entire powerband.

The use of the Weber DCOE 45 carburetor on street MGBs came about as a result of their use on the factory team’s track race cars. This fact, of course, produced a ‘monkey see, monkey do’ mentality amongst some of 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 racetrack, so that is part of the reason why the factory race team chose it over the SU. Remember that on a racetrack, smoothness and economy must be subordinate to responsiveness, as it is responsiveness that makes aggressive driving possible. Victory is what counts on the racetrack, 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 have paid out the big money, sweated the installation and calibration, spent more money to convert their ignition system to a centrifugal advance distributor (Weber carburetors do not have provision for a vacuum takeoff for working with vacuum advance ignition systems: read the fine print!) and so they are already predisposed to feel the power increase even before they drive. When the quick throttle response creates the illusion of more power, they become 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 that produces its maximum power output at elevated engine speeds is being used.

There is, however, a considerable difference between the Weber and the SU in the process of setting them up. The SU has only one fuel-metering needle and one fuel jet, so you can modify its metering in your driveway. The Weber, on the other hand, has a multiple choice of replaceable main and auxiliary 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 are using a radical camshaft, have access to a dynamometer, and you really understand how a carburetor works, take my advice and use the 1˝” SU! Its 130 Cubic Feet per Minute airflow capacity is quite adequate for the majority of streetable small-bore B Series engines, plus it can readily be modified to increase its airflow capacity by approximately 30% to 169 Cubic Feet per Minute should the need ever arise.
Steve S.

Ive used them both on different cars. The SU's are great in both instances. I think the ultimate combo would be a nice rebuilt set of SU's with a o2 sensor and the mega jolt system. You can adjust your needles that way when viewing rpm and air/fuel to really dial your engine like it has never been done before.
James

When I bought my GT it had a twin choke Dellorto and a stage three engine.
It was a dog belwo 2000 rpm and so difficult to drive in traffic. It was OK for continuos higfh speed running but the cops could still get you!
I changed to twin SUs and the car was transformed. It was tractable across the rev range and used less fuel. Admittedly full hard acceleration was greater with the Dellorto but how often do you need that unless you are in the habit of robbing banks.
Conclusion use SUs unless you intend to race or drive at high speeds (what is the federal limit now?)
Bob Marshall

This thread was discussed between 21/01/2009 and 26/01/2009

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