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MG MGB Technical - Cooling down the engine bay

On very hot days, (we had a few this past week in Philly), with the a/c (Moss) on, my engine bay ('67 BGT) gets very hot. Car is not overheating, but it feels like vapor lock symptoms are happening - loss of power on throttle, general rough running. I have a heat shield, but perhaps it is not effective enough.

In the archives I have read about wrapping the fuel lines in aluminum foil. Does this work?

Does anyone in MG-land have creative suggestions to cool down the carb. side of the bay?

Bernie
Bernie Lowe

Hello Bernie,

As you have a non-smog B it is easy to have few things to check.

First is there still insulation( read asbestos) under carb. heat shield?
Does your thrermostat is a summer one: 165F?

Is your cooling system in good order with a radiator clean?

To improve heat exchange you can add a blank sleeve at thermostat level and use water wetter.

Do you use cold spark plus like N9Y?

Is your timing adjusted correctly?
These are some main points to check.

Cheers,

Jean G.
P.S. A/C is a major power consuming and heat generating device, may be a big part of your problem is there
Jean Guy Catford

A radical solution is to cut holes in the wheel wells to let hot air escape - like the v8 guys.
jlaw

You can't get vapourisation in the fuel lines, as soon as any gas reaches the float chamber it would simply be pushed out of the vent to be replaced by liquid fuel. For vapourisation to be an issue it would have to occur at faster than the rate the pump could operate, which is about 2 Imperial pints per minute for *liquid* fuel. Vapourisation *can* occur on HS carbs in the jet pipes though, between float chamber and jet. This is normally caused by a faulty heat shield, but anything that results in extra heat being put into the engine compartment will exacerbate it, as will many other causes where something isn't working as it should be, like timing, mixture, cooling system efficiency etc.

In an MGB almost all the radiator and engine heat has to exit *downwards*, and at a standstill especially with a slight tail breeze a significant portion of this is pushed forwards, to be recyled through the radiator, which doesn't help. 'RV8' holes in the inner wing can help, raising the escape point to the bottom of the front arches at least, as can louvres in the bonnet.
Paul Hunt

A manifold blanket always works.
James

Bernie-
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 a 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 5.4 degrees Fahrenheit (3 degrees Celsius) decrease in temperature that the air ingested by the engine is lowered by, power output is raised by 1 percent. 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 continue being conducted into the cast iron, the insulation factor will be such that heat cannot escape from a wrapped cast iron exhaust manifold. Consequently, both the cylinder head and the exhaust manifold will run hotter. The heat will continue to 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 the thermosensitive 1 ½” 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 a more effective scavenging of the cylinders. The increased velocity of exhaust gases produced by its 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 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 travel onward 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/ .
Steve S.

Remove the middle section of the bonnet (hood) seal, between the hinges. This helps air flow though the engine bay and gives an escape route for hot air when you are stationary. If you have a tubular exhaust, wrapping the headers cuts down underbonnet temps drastically. But make sure the heat shield is in good order.
Allan

Unfortunately that lets engine bay fumes into the heater intake.
Paul Hunt

Is anyone opposed to the header blanket? It isnt wrapped around the manifold and keeps the heat from the exhaust deflected downwards out of the engine bay.
James

Your underbonnet is full of fumes! Something wrong there then. Most V8 people use that dodge, without having to resort to a gas mask!
Allan

I spoke to the driver of an immaculate B V8 at the last GOF and asked him what he thought of his louvred bonnet (hood). His answer was although it did not make a visible mess he could not drive more than a few miles with the top down without feeling dirty.








Denis4

Being in Phoenix, I'm very familiar with the hot shut-down and rough re-start problem. You can't really do much to lessen the heat under the hood after shut-down (even running an electric fan doesn't do much) but you can take a good look at your floats and float bowl fuel level so that the fuel does not expand out your vents or jets. If you haven't replaced your floats in a long time, just do it, needle valves (or Grose jets) as well, Its all cheap and easy to do. Adjust the jets with shims to get the fuel level down a bit in the bowl (but not too low!) and I'll bet your "vapor lock symptoms" will greatly lessen. Keep in mind that newer floats are non-adjustable and newer Grose jets can be problematic. Many people have found joy in Joe Curto's needle valves (check archives).

'69 BGT

Mike King

Lowering the fuel level in the float chambers, is not to be rcommended. The low pressure venturi has to pull the fuel from the float chamber. For the carb to work properly it needs fuel at the correct height in the jet tube. Only set the float within the, manual stated, tolerences. Both HS's and HIF's are adjustable either with shim washers or bending the brass tab. Louvres are alright, but they do let water in when the car is standing, hence I don't like them. If you want louvers put them in the inner wing!
By the way I drive a V8 modified B Roadster, with rear bonnet seal removed, haven't noticed a "fume" problem with hood down (which is most of the time). But there again my crankcase ventilation is properly sorted AND I have RV8 style header exits!
Allan

This thread was discussed between 01/05/2009 and 08/05/2009

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