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MG MGB Technical - manifold wrap

I have fitted a stainless steel performance manifold to my MGB, however, from reading various technical info it sounds as if the new new manifold will run much hotter than the old cast one and that I may need to wrap it in insulation tape. Does anyone have experience of these manifolds and can comment?
p savage

Unlikely to be an issue in the UK, even if stainless radiates more heat. But I'd have thought shiny stainless would radiate *less* than rusty cast iron, not more. That may well make it hotter to the touch, but that won't affect things like more radiation will i.e. fuel vapourisation. But MGBs have run in desert states for many years without vapourisation being a problem, as long as the carb heat shields are in place and nothing else is contributing to excessive heat.
Paul Hunt

Boh P.,

Look at the archives, I read horrible things about wrapping.

http://www.centuryperformance.com/exhaust-header-heat-wraps-do-not-use-spg-138.html

for eaxample.

There is some coated headers with better resistance
Jean Guy Catford

Paul-
Reducing temperatures inside of the engine compartment is beneficial for power output, as well as presenting less potential for detonation. Although most carburetors do not have a ‘brain’ or sensor to measure temperature, all carburetors function similarly in that it is pressure drop across a venturi that is the mechanism that pulls fuel into the venturi. The pressure drop itself is a function of air density. Air temperature also plays into the equation. In operation, carburetors are quite effective at adapting within the limited range of operating conditions that we subject them to. Without cold air induction, a carburetor must accommodate a significantly wider range of air temperatures than if you supplied air from outside the engine compartment. For every 3o Celsius (5.4o Fahrenheit) decrease in temperature that the air ingested by the engine is lowered by, power output is raised by 1%. In view of this scientific fact, although wrapping the exhaust manifold in insulating tape (sometimes called “‘lagging”) may seem to be a good idea in principle, it is a very bad idea in practice. Peter Burgess mentions this problem in his book “How to Power Tune MGB 4-Cylinder Engines.” Why is it a bad idea? Heat from the exhaust gases still being conducted into the cast iron, the insulation factor will be such that heat cannot escape from a wrapped cast iron exhaust manifold and both the cylinder head and the exhaust manifold will consequently run hotter. The heat will just build up and up, far beyond what the factory engineers designed the exhaust manifold to handle, with the result that the exhaust manifold will warp, often resulting in a leak at the manifold gasket. In addition, the additional heat collected in the iron of the exhaust manifold will also be transferred into the cylinder head, heating the walls of the intake ports and thus reducing the density of the incoming fuel / air charge. Even worse, the coolant passages in the cylinder head were not designed to remove such an excessive amount of heat, thus preignition of the fuel / air charge can become a problem and valve seat life can be shortened. In extreme cases, due to the fact that the exhaust valves for the middle two cylinders share the same central exhaust port, the cylinder head can actually warp between #2 and #3 cylinders, resulting in a coolant leak or a blown cylinder head gasket. In the case of tubular steel exhaust manifolds, the metal will become so hot that it will often spall and form flakes that will eventually disintegrate to form a hole in the area where the heat accumulation is greatest, usually at the junction of the runners. The lagging tape also becomes a moisture trap, accelerating the rusting process that can plague cast iron exhaust manifolds.

Instead of wrapping the exhaust manifold, get it Jet-Hot coated. Jet-Hot coating is a ceramic coating that can be applied to coat both the interior of the exhaust manifold as well as the exterior. The heat will have nowhere to go except out through the exhaust system, thus it will greatly reduce underhood temperatures. This is a significant factor as exhaust manifolds often reach temperatures above 400 degrees Fahrenheit (204.4 degrees Celsius). Thus, the cooler air being inhaled into the engine being denser, more fuel can be mixed with it to result in a more powerful fuel / air charge. Another benefit is that the setting of heat-sensitive 1½” SU HIF4 Series carburetors can remain more consistent. At the same time, it decreases deceleration of the exhaust gases, the sustained gas inertia thus enhancing the pulsed-vacuum effect, resulting in more effective scavenging of cylinders. The increased velocity of exhaust gases produced by higher exit inertia not only clears each cylinder more quickly; the improved vacuum effect inside of the combustion chamber draws in the next fuel / air charge more efficiently. Jet-Hot coating does not contribute to hydrogen embrittlement, a condition associated with chrome plating and other coatings in which microscopic cracking can lead to premature part failure. One word of warning to those considering Jet-Hot coating or any other type of ceramic coating: Be sure that the entire surface of the exhaust manifold, both the interior as well as the exterior, and that of the flanges is coated so that the heat of the exhaust gases will pass on through the system instead of being absorbed and trapped in the metal of the exhaust manifold, otherwise the absorbed heat will create the same problems as in the case of lagging the exhaust manifold with insulating wrap. Warping of the exhaust manifold would become something to be expected, and warping of the cylinder head would also become a distinct possibility. Jet-Hot has a website that can be found at http://www.jet-hot.com/ .

Should you decide to use a tubular exhaust manifold that is not Jet-Hot coated, be sure to use a synthetic rubber gasket (Moss Motors Part # 296-375) and the later rear tappet chest cover as cork gaskets used in this location tend to fail under prolonged exposure to the extreme heat radiated by such exhaust manifolds. Use of the more warpage-resistant rear tappet chest cover from the 18V-883-AE-L, 18V-884-AE-L, 18V-890-AE-L and 18V-891-AE-L engines will assist in this as well. Always use the thin cork gasket on the front cover - it lays flat and is actually the right size. Both the synthetic rubber gaskets and the thick cork gaskets are too small to fit well.
Steve S.

P.,

'Wrap' is an anagram of 'warp'... I've wrapped a mild steel manifold and had to bin it.

I think it will be hotter on the outside simply because it is thinner than a cast manifold. But there must be tens even hundreds of thousands of Bs and midgets running tubular manifolds with no problem. You need to use a heat shield though or you'll never get the engine started from hot!

I'm not convinced by the gel coat - simply because if the manifold is insulated then there can be no heat transfer between the exhaust gas and the manifold. Which means hotter cylinder head (hello pinking) and hotter exhaust pipe (grass on fire!).

N


Neil

Many thanks for all the comments. I think I'll 'suck it and see'. If there are any problems in the summer then I'll certainly post further comments. As Neil says, there must be loads of B's running tubular manifolds without problems otherwise there would be much more heard about it.
p savage

I had one for some years with no problem winter or summer. I only swapped it for a cast when I wanted to fit a stainless exhaust system, and at the time couldn't get one to interface with the tubular. No problems since either, come to that.
Paul Hunt

This thread was discussed between 16/12/2008 and 19/12/2008

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