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MG MGB Technical - Exhaust Manifold

The exhaust manifold on my 74B is very nice looking and appears to have equal length sections coming down from it. If it were removed and the inside smoothed out in a clean way wouldn't it then become a header? Or at least with similar driving characteristics? Does anybody know of anyone who does this type of thing? If it would not work...Why not? Thanks Bob
Bob Ekstrand

Bob,

I think some people have tried to extrude honed them.

I do not have Burgess' book infront of me, and I am paraphrasing, but the header design from Peco, is copy of the original manifold. Burgess did not seem to give a benefit of the Peco header, other than the ease of fitment to rest of the exhaust.

I think the others will chime in. I have fitted a Peco header, and the initial quality was not that great. A lot of time was spent matching the ports. It also is difficult to fit to the car because the outer branches are not in true position (after all it is sheetmetal).

I think your idea will work. It should have the same characteristics, so long as the flange to down pipes was smooth joint; it should be same as a header.



Shareef Hassan

Shareef,

Curious how you went about machining the ports on the header.

James
james

Bob,

My memory is that Peter Burgess did speak well of the stock double drop manifold. At least it is not the first place to attack for power improvement.

I have found that it is quite easy to shorten the steel collector pipe (after the manifold) a few inches and weld on an extension to turn the rest of the exhaust into a 2 inch system.

James, I think you misread. Shareef said matching, not machining.

Charley
C R Huff

Shareef how did you go about matching the ports
James

James,

I’m not Shareef, but I’ll go ahead and give you my opinion for the exhaust manifold / head interface matching. I didn’t figure this out, I read it in David Vizard’s book for the A-series engines, though he might have been talking about the intake.

An exact match between the head and manifold is very difficult to achieve, but it is not too hard to make sure that the exhaust doesn’t hit a “wall” on its way out. So, you just need to insure that none of the manifold flange covers any part of the exhaust port. If the overlap goes the other way, it doesn’t make much difference as long as the direction of flow doesn’t hit a wall.

You can tap out a gasket that exactly matches the face of the exhaust manifold, and put it on the head for a check, or you can tap one out for the head and put it on the manifold. Alternatively, you could try getting a mark with engineer’s blue (Prussian blue). Don't forget about the slop between the studs and manifold.

If the manifold overlaps the ports, grind off the interference with a die grinder, dremmel, file, or whatever. Also check that the manifold gasket doesn’t cover any part of the exhaust ports.

Charley
C R Huff

Thanks Charley. Ive used engineers blue to check. Just was wondering exactly how much everyone is modifying and grinding. Its one thng when its a meaty cast, another thing when its tubular headers.

Thank you,
James
James

James,

When I was modifying the tubular header for my Sprite, I had to grind the face plate so much that I also had to grind through the tube. Then when everything was big enough I welded up the gap so that it formed a taper from the face back to the original tube.

Charley
C R Huff

Bob-
The standard pre-1975 factory exhaust manifolds, of which there were two models, are surprisingly good performers. The exhaust manifold used with the 1½” SU HS4 Series carburettors’ intake manifolds (BMC Part # 12H 911, 12H 1397, 12H 2571, 12H 2575, and 8G 767) have a mounting flange thickness of 9/16” (14.2875mm) and can be readily identified by its external casting number of 12H 709 (BMC Part # 12H 709). On the other hand, the exhaust manifold used with the 1½” SU HIF4 Series carburettors’ intake manifold (BMC Part # 8G 774 ) has a mounting flange thickness of 7/16” (11.1125mm) and can be readily identified by its external casting number of 12H 3911 (BMC Part # 12H 3911). While its designers allowed for the turbulence created by the roughness of the interior surface of the casting, I highly recommend electropolishing to improve the airflow capacity of the cast iron exhaust manifold.

Electropolishing is an electrochemical process used to smooth metal, usually prior to plating. It is commonly performed on a precision casting (such as a window winder handle) or on prepolished sheet metal after it has been formed to shape (such as a bumper) prior to plating it. The item to be electropolished is thoroughly cleaned and subsequently immersed in a chemical bath. A current is then run through and the highest points on the surface of the metal are removed. In a sense, it is the reverse of plating in that metal is removed instead of deposited.

The benefit of electropolishing a cast iron exhaust manifold lies in the fact that because the item is completely immersed into the electropolishing bath, the process can get inside the runners of the exhaust manifold, reaching into remote areas and otherwise inaccessible curves where human hands and mechanical tools cannot reach so that it will polish the interior of the exhaust manifold quite nicely. This produces a smoother surface that makes for reduced turbulence in the exhaust gas flow, just like the smooth walls of an exhaust manifold that has been constructed of tubular steel. Be sure to instruct the firm doing the electropolishing to protect the gasket surfaces with plater’s tape as an overly smooth mating surface may give sealing problems when used with some gaskets.

Another technique for attaining a smooth interior surface in the exhaust manifold is called Forced Extrusion Honing. Extrusion Honing is great because it can remove metal from areas where it is otherwise impossible to do so, such as inside the long runners of the exhaust manifold. In this technique, a dense mixture of abrasive clay is forced through the interior of the manifold, polishing the surfaces to an even greater degree than can be achieved on a casting through electropolishing. Smaller, more restrictive areas in the cylinder head act like a venturi so that the mixture of abrasive clay flows faster there. This faster flow causes more cutting action and thus the Extrude Honing process by nature removes material where its removal is needed the most. This cutting mechanism is very good at producing runners that flow equally. I have seen a cylinder head in which both the intake and exhaust ports have been subjected to this process and it is very impressive. This service is available from Extrude Hone. Their website can be found at http://www.extrudehone.com/ .

The Cost vs. Benefit factor of these processes is all a matter of personal values. It is true that a decent tubular exhaust manifold might be obtained for less money than that of extrude honing and/or electropolishing, but tubular exhaust manifolds have much thinner walls that can resonate under the fluctuating pressures inside the exhaust system and thus are much, much noisier. At certain parts of the powerband, they can actually resonate into an annoying ringing sound. In addition, tubular exhaust manifolds are welded assemblies, and their welds have been known to crack under the repeated stress of heating and cooling. Further potential problems of misalignment of the exhaust runners with the exhaust ports, and mounting flanges not being on the same plane, all contribute to making the purchase of a tubular exhaust manifold into something of a gamble. The Original Equipment cast iron exhaust manifold has none of these problems. Because of the lesser heat conductivity of the cast iron and the decreased surface area, the electropolished exhaust manifold will radiate less heat into the engine compartment. Its greater mass will also have the side benefit of reducing noise to a level notably less than that attainable with any tubular steel exhaust manifold.

I sincerely believe that an LCB (Long Center Branch) 1 ¾” diameter tubular steel exhaust manifold will not flow any better than an electropolished Original Equipment cast iron exhaust manifold if it has the same basic design. It can also be beneficial to electropolish exhaust ports, thus reducing carbon buildup that results in the creation of airflow turbulence and less heat being conducted into the cylinder head as a result of reduced surface area of the runners.
Steve S.

Bob - If I may comment briefly - the stock manifold IS a header, it is simply made of cast iron. It lasts forever, but weighs like, well, cast iron. Other designs may be lighter, but the stock design optimizes flow for the stock (carb and cam) set up.

I have also heard that polishing (intake or exhaust) can hurt gas flow - it depends on all the elements involved.

When you open up an intake or exhaust, the rest of gas flow system needs to be harmonized, especially as the original MGB engineers had optimized stock exhaust flow from the previous MGA powertrain. I'm no expert (like Steve S., for example) but faster flow exhausts chase to faster flow intakes, carb work (or new carbs), altered cams and usually upgraded valve and port grinds. This can all work - it often does - but you need to work it all out, either by yourself or by skilled craftsman.

Good luck!
John Z

John-
A mirror finish on the intake ports is not only unnecessary, but is actually undesirable because it will eliminate border turbulence, thus leading to fuel condensation and a consequent loss of power. However, as I previously pointed out, a mirror finish can be advantageous in reducing carbon build-up in the exhaust ports. One should not polish either the walls or the roof of the combustion chamber in an attempt to discourage carbon buildup as this will lead to condensation of the fuel-air charge both as it enters the cylinder and as it is being compressed. A glass-beaded finish will produce sufficient border turbulence to do nicely in terms of discouraging not only this problem, but that of the development of surface cracks as well.
Steve S.

James,

Sorry for the delay. I matched as Charley described. I was more conscience of the manifold step. I also pinned the intake manifold, and used the flats sides of the header flange to get a repeatable alignment. The studs location were too sloppy.

I used to work at an aerospace shop, so I brought the head, header, and intake in to the quality department on a Saturday. I verified the results with a boroscope that is connected to a video screen. The GM was not a gearhead, and was not all the impressed, but the machinists were.

Shareef
Shareef Hassan

This question is probably aimed at Steve;
I have a jet hot coated manifold from a 74...I sold that car but kept the manifold to put on my '66 stock HS4 not realizing the manifold flange was a different thickness until I just read your post...will it fit?
thank you -Jeremy
jjralston

Jeremy. When using a mis-matched intake/exhaust system, you have to shim the thinner flanges out to the size of the thicker flanges. This is a fairly common problem. Not only the differences between the HS and HIF systems, but, also a problem with the flange thickness of headers when used with almost any intake manifold.

Depending on the difference in thickness involved, you can either make up shims or, if the difference is very minor, mill the large washers to reduce their thickness on one side.

Les
Les Bengtson

This thread was discussed between 30/07/2008 and 11/08/2008

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