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MG MGB Technical - Exhaust for Weber DGV

I plan to replace the Zenith-Stromberg with a Weber DGV i my 78 MGB. What exhaust systems will fit? I have a cast iron exhaust manifold but not sure I will use it.

George
G.E. Bulwinkle

The stock pre '75 cast iron stock exhaust manifold is hard to beat. In just about every test done, it outperforms the after market tubular headers that tend to rust out in a few short seasons. RAY
rjm RAY

RAY,

Then just use a 68 to 74 exhaust system from the manifold on back?

George
G.E. Bulwinkle

George-
If you use the Weber DGV, don't expect performance. The intake manifold shape for the downdraft Weber DGV carburetors has the airflow characteristics of a bathtub with a hole in each side. This is due to Pierce Manifolds, its distributor, bundling their own poorly designed intake manifold with the carburetor and selling the resulting package as a kit. As a result, virtually every example of this combination that I have encountered or ever heard of had a “flat spot” in the powerband from 1,500 RPM to 2,500 RPM where throttle response was poor due to a weak fuel / air mixture. This “flat spot” can be eliminated by instead using a Cannon intake manifold that incorporates a pair of fittings that cycle the hot coolant from the radiator heater hose (flexible pipe) to the front of the intake manifold and from the rear of the intake manifold onward to the heater box. The result is smoother running and elimination of the infamous “Flat Spot”. However, this will not eliminate the problems imposed by the restrictive airfilter that Pierce Manifolds supplies with it in its kit. Note that the Weber DGV 32/36 is a progressive-type carburetor. This means that the 32mm primary bore is opening with the throttle first, but the 36mm secondary bore (hence the 32/36 designation) does not open until the accelerator pedal is about two-thirds of the way to the floor. Since 1 1/2” approximates 38mm (1.496”), there are two considerations. First, due to the smaller bores of the Weber design, the two bores of a Weber DGV 32/36 Series carburetor together cannot possibly flow as much fuel / air mixture as twin 1 1/2” SU carburetors can at full throttle. Second, when at less than full throttle, the progressive opening of the 32mm and the 36mm bores of the Weber design do not flow as efficiently as the constant-velocity airflow of the 1 1/2” SUs. It is possible to convert the progressive throttle plate linkage to simultaneous throttle plate linkage which would provide a carburetor that would drive very much like the twin 1 1/2” SU carburetors. However, with slightly smaller venturis than the 1 1/2” SU carburetors, it is simply impossible for a Weber DGV 32/36 to perform as well as either twin 11/2” SU HS4 Series carburetors or twin 1 1/2” SU HIF4 Series carburetors. If your goal is performance, remember that the potential of the Weber DGV 32/36 is inferior to that of the twin 1 1/2” SU Series carburetors. Its cousin, the Weber downdraft DGES 38/38, has twin 38mm synchronous butterflies. It is larger than the standard 32/36mm DGV, and thus has greater airflow capacity. It mounts onto the same intake manifold and gives more torque at low engine speeds, but can make the engine difficult to start in cool weather and has developed a reputation for troublesome running at low and moderate engine speeds. Both of these Weber carburetors require rejetting in order to function properly on the B Series engine.
Stephen Strange

Ok I got the exhaust manifold back from the machine shop. Old studs all removed. I plan to use new studs and brass nuts to connect the manifold to the early down pipe. What should I torque the nuts to? Should I use Lottie? Lock washers?

George
G.E. Bulwinkle

Should I use Loctite?
G.E. Bulwinkle

George. As you have noticed, the studs tend to "lock tight" themselves to the exhaust manifold. The liquid thread locking compounds tend to break down at high temperatures--exactly what you find in the exhaust system. Thus, they are of no value in such situations.

Inatall the studs to the exhaust manifold. Measure the thicknesses of the manifold's flanges. Compare the thickness of the intake manifold flanges to the exhaust manifold flanges, then see if/what thickness of spacers may be required. (The flanges must be of almost equal thickness if they are to properly seal the manifold gasket to the cylinder head.)

I always install the exhaust manifold to the cylinder head, then come in from the underside and install the header pipe to the exhaust manifold. If the engine is in the car, I have never been able to fit them as an assembly. If the engine is out, it is easier to bolt them together outised the car and fit as an assembly. Use good quality lock washers on the flanges between the exhaust manifold and down pipe. Check the tightness again after your first trip or two to make sure the bolts are remaining tight on the studs.

Les
Les Bengtson

Thanks Les. You answered my questions.

George
G.E. Bulwinkle

George, I just made this conversion on my 78 and it is GREAT. You will love it. Some will tell you that there are better preforming carbs and I am very sure there are, but this is going to be such a great step forward for you, you will love it. I got mine from Brit-Tec and they were very helpfull with the instalation information.
Good Luck.
jimmy chew 1

Jimmy,

I too got my Weber DGV from Brit-tek. Excellent instructions!

George
G.E. Bulwinkle

This thread was discussed between 27/04/2013 and 25/05/2013

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