Manifold Heat

The original manifolds generally had some form of heating; either from the exhaust gas or engine coolant. It's pretty standard practice to isolate the heat source with any performance manifold, and to run the manifold as cool as possible in the interests of maintaining the highest possible charge density. For those running downdraft carbs however, there may in fact be some advantage to supplying a controlled amount of heat to the manifold. Unlike a V8 manifold which has a large surface area with which to absorb heat from the engine (even without the exhaust crossover) straight six manifolds are mostly out in the breeze and tend to run very cool. A bit of heat applied to the manifold floor can improve the mixture quality dramatically, and the resulting improvement in performance can more than make up for any small drop in charge density.

Most downdraft carbs tend to produce large droplets or strings of fuel when compared to say an SU or Dell'Orto, and a fair bit of this fuel will tend to drop out of suspension. To make matters worse the manifold will almost certainly have some long runners with significant bends. The end result is that at small throttle openings hesitations and stumbles are common, and/or extra fuel has to be supplied to compensate for the unmixed fuel. It's not a problem at bigger throttle openings where there is sufficient air volume and velocity to carry and mix the fuel, but for applications where clean running off a closed throttle (eg. out of corners on a circuit car) is important then experimentation with manifold heat could certainly pay off. Wet fuel flow and pooling will be much reduced and the car will likely feel sharper and run more cleanly. Engine coolant is the best source of heat, being a fairly constant temperature. Exhaust heat on the other hand tends to leave the manifold too cool at light throttle when you really want it hot, while at full throttle when you want the manifold to be cool it will be much too hot and reduce the engines output. If the coolant flow is controlled (eg. by an orifice) enough heat will flow to maintain light throttle mixture quality without reducing full throttle power. At wide open throttle, the sheer volume and velocity of air flow will be enough to prevent any significant heating of the charge. Welding a length of aluminium half-pipe to the underside of the manifold provides a coolant flow passage that has the coolant in direct contact with the manifold floor and is easily done. I realise this goes against popular performance practice, but for any street application as well as many competition cars a bit of manifold heat will very likely improve the in-car performance with downdraft induction.