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MG Midget and Sprite Technical - Restoring a Manifold

While we're enjoying driving around on a Maniflow 1 5/8" that's too close to the chassis rail, I want to refurbish the car's 1980 Derrington manifold, which I believe might perform better on a 948, and probably won't hit the chassis rail.

Here it is. I could just lengthen it with a 1 1/2" pipe, which would slide over the existing spigot and slide into the Maniflow exhaust pipe that's on the car, but I think it needs more work than just that.
(see next picture)

Nick and Cherry Scoop

Here's a close-up of the bottom branch junction. At "A", where it's still slightly oval from the fork above, it looks as if the manifold has been cut at some time, and a new bend pushed in and brazed. Surely not an original feature. Looking down with a camera I can see not only the end of the pipe but also that it goes in far enough to leave a pocket all round for catching the gases and breaking up the flow.

Also, at the back end where the pipe has been enlarged, and two cuts made for clamping, the joint would be back to front of good practice, wouldn't it? With a potential obstruction to the flow.

Or am I worrying about nothing? Maybe it was common for tuning shops to build joints like this.


Nick and Cherry Scoop

Hi Nick

In my view, for road use, the clamping arrangement would not make too much difference. The real benefit is the LCB which helps extract the exhaust gases.
Bob Beaumont

I agree, Bob. But the first joint is the one that really worries me. Looking down inside the centre branch, I see this.

My idea was to un-braze that joint and try sheathing on the outside. I could get a 1 1/2" 90 degree mild steel bend, cut it at the correct angle and squash/stretch/cut it to fit over.

Why was it brazed? Is it OK to weld manifolds?

Nick and Cherry Scoop

Oh I see, yes that's not so good.Yes your idea will work providing you keep a shallow bend. I would weld it as its stronger. All the manifolds I have seen have been welded.
Bob Beaumont

Lots of braze on WER's original exhaust manifold. You have to remember even the chassis (pl.) of Formula One cars were brazed in the 1950s: in an era when chassis flex was expected - sometimes still even welcomed - braze was preferred as being more flexible than 'brittle' welding.

I've never discovered why, but it was traditional practice that the downpipe of a tubular manifold would go over the exhaust system pipe (Vizard mentions this somewhere). The best-made ones had a flange to increase the size of the manifold pipe, so the gasses saw the same internal diameter.

The downpipe could have been shortened to bring the exhaust system closer to the body (a known rally mod), but I think it's most likely that the downpipe has been damaged by hitting something - as so many were - and what you see is a repair.

An amount of back-pressure was regarded as essential (and it is said the physics of shock waves travelling back up the pipe are exceedingly complex) but your drawing certainly looks more like manufacturing expediency.

Kind of makes me (even me) want to go running to Maniflow rather than use an enviably period item. Didn't we agree this was a Speedwell manifold?
Tom Coulthard

"Didn't we agree this was a Speedwell manifold?"

I believe so.
Dave O'Neill 2

You're right. We did, but the lugs at the port face are a slightly different shape from the Speedwell illustration, and then I found, in the first owner's notes, that the manifold was in fact supplied by Derrington, and their catalogue illustrates it perfectly.

So, 1980 I'm afraid, and not from the illustrious Speedwell shop ten (?) years earlier.

This is a winter project, so the first job will be making a jig to locate the important bits.

Nick and Cherry Scoop

Correction:- not 1980, but July 1966 that the manifold was fitted.
That allows a few years for the Derrington to fail, and a Speedwell to be fitted, so you could still be right.
Nick and Cherry Scoop

This thread was discussed between 08/09/2018 and 14/09/2018

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