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MG MGB Technical - DCOE40 or 45?

I'm looking for opions re: Webers, either a 40 or 45. I will be rebuilding my 70B engine this winter, reboring, port polish,balance, etc.etc. I am thinking about a 270 cam, and that brings me around to the carb. All the research I have done tells me that a DCOE 40 is marginal while a 45 is great if you are at the top end all the time. I have a chance to buy a 40 and manifold,linkage etc. for a good price. So, I am looking for advise from our experts.
PS: The car will be used for daily driving, plus weekend club activities.
Thanks
David
David W

David-
The venturi size on both carburetors is controlled by a removable venturi plate. Either carburetor can be set up with the venturi size appropriate for a Piper 270 camshaft. However, the Weber DCOE 40 is a tad fragile when compared to the Weber DCOE 45. In any event, while there's no really worthwhile advantage to installing a Weber DCOE when paired with a mild camshaft such as the Piper 270, there is the disadvantage of inertia effect in the intake manifold causing the outer cylinders to run rich. Instead, rebuild your SUs.
Steve S.

Steve,

I know you have much more experience with the different carb setups on MG's than I have, but with SU's the outer cylinders run leaner than the inner cylinders due to the firing order. How does the Weber reverse that tendancy?
George B.

George-
It's the inertia factor. The fuel suspended in the incoming fuel/air charge is denser and heavier than the air, so its inertia causes it to go towards the outside of the curved intake manifold. This biases the fuel towards the intake valves of #1 and #4 cylinders, thus creating a richer mixture for those cylinders. The intake manifold for the downdraft Weber DGV carburetors is even worse when it comes to inertia effects.
Steve S.

Back to the original question... typical setup on a B uses 36mm chokes, so the 40 is plenty of carb. If you've got a sweet deal....

However, for the kind of build you're doing, the SUs are more than capable, and a lot less trouble.
Baxter

I would recommend the 40, especially if you can get your car on a dyno to tune the engine once it's complete. I do not think you would notice a very large increase in top-end performance with the 45. For a street car, the performance package you've outlined so far is a good "match" in my opinion.

Sean
Sean Brown

Baxter-
Quite so. Setting up a Weber DCOE without the help of a dynometer is an expensive exercise in frustration as the money pours out for different emulsion tubes, replaceable venturi sizes, experimenting with different versions of the six jets (starter air correction jet, starter jet, idle jet, main jet, accelerator pump jet, and air correction jet), and there's no worthwhile advantage over a well-sorted set of SUs when used with a mild street camshaft. If a Weber DCOE 40 is to be mounted on an MGB engine, I consider a soft-mount system to be a necessity to protect it from vibration. The @#$%&! thing is just too fragile for long-term street use.

David-
If after hearing all this you're masochistic enough to insist upon using the Weber DCOE 40 on your MGB, you really should get a soft-mount system from APT. They have a website at http://www.aptfast.com/
Steve S.

For the record, I was very excited about my latest B, which came with a 40. Properly tuned, weber FEELS faster, and it can take advantage of high performance mods better than SUs, or at least to a greater extent, but still, the SUs are probably easier to live with, and can give a lot of power.
Baxter


Thanks everyone, it was a question because of the price. I'm very happy with SU's (5 cars with them). Any research I have done indicates that the weber is great for out and out racing which I'm not doing. I have a midget with an 1 3/4SU and it gives me lots of torque and power, I might consider a pair of those later on.
Thanks again, nice to talk to fellow MGB ers.
David
PS Steve I received your info and will get back to you about it.
David
David W

This thread was discussed between 12/11/2002 and 13/11/2002

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