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MG MGB Technical - ZS to SU 1979 MGB help!!

Hello all,

Is there a problem associated with maintaining the original 1979 electronic distributor when used with an HS4 carb conversion...other than lower performance associated with the advance curve?

Does anyone have recomendations on what catalytic (make-part number) converter to place beneath the car when using a pre-1975 stock exhaust system? It will need to be small to fit beneath the car without dragging.

Thanks for the help.

Jay
Jay

Jay. The stock distributor should work fine. It may not be ideal, but should not cause problems that can damage the engine. As to the cat, I have a good muffler shop and simply let them handle it. The aftermarket cat they supply is a flattened oval shape, wider than it is thick. That should not cause a problem with your car. Les
Les Bengtson

Jay,

While you'll see some performance gains (due to increased breathing/fuel intake, particularly with the use of the earlier type manifolds), the work you're performing will be limited when using the original emissions-spec distributor. It probably won't break anything, it just won't perform as well as you'd desire. Moreover, without cylinder head work, the increases associated with these modifications will be more modest than such an investment of time and money should otherwise dictate.

On the cat converter, the early (pre-'75) exhaust was not designed to work with a cat in-line. However, I know Doug Jackson at www.mgbmga.com describes an in-line converter you may wish to look into (not in any detail on the site, but he does acknowledge the existence of a freer-flowing, in-line cat, so he can probably help out). I would imagine that tuning will be complicated (requiring the use of leaner-than-stock metering needles) should you attempt to use HS4s with a converter (hard to tell what needles you'd need...twin SUs with a converter would probably be uncharted territory, requiring some experimentation on your part). The converter itself will reduce your breathing sufficiently so as to reduce airflow, requiring leaner needles. Then, you'll also need to consider clearance at the brake booster/master cylinder...with the HS4 external float bowls (versus the HIF4 integral bowls), you may find this pretty restrictive. I suppose you could always convert to non-servo-assisted brakes, using a pre-74 1/2 master cylinder. If you keep the servo, you'll need to use the conical filters (which aren't a very good fit and do not provide the best breathing characteristics) or have a custom air cleaner assembly fabricated. On the other hand, your breathing will be restricted anyhow, due to exhaust restrictions (the converter) and the absence of the cylinder head work necessary to bring the '79-spec head up to par. While the twin carbs will look nice, I believe you're going to be a bit disappointed with the outcome unless you have the head work, go with a better breathing air filter setup, and lose the cat converter. Moreover, the distributor will not complement the carbs, so you may experience only limited gains over the stock ZS setup (this is not entirely correct, as the change in manifolds alone WILL produce a significant increase [oddly enough, more than the carbs themselves, when contrasted against the ZS, if it's in good shape], just not what you would otherwise enjoy if considering the entire system).

Lots of coverage on this and related issues in the archives.

Barry
Barry Kindig

This thread was discussed on 16/04/2002

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.