MGB: Intake manifold

Can someone explain the function of the coolant passing through the intake manifold? 79' 1500 engine.
Thanks,
Mark

To heat the manifold, which supposedly allows the fuel to vaporize better.

Mark,
As you (and all MG owners) know, carbureted gasoline engines don't run well until they are warmed up.

When the cold engine is "choked", the mixture is richened to the point where combustion can take place, but much of the fuel goes out the tailpipe unburned.

The coolant passage allows some heat to build quickly in the manifold so that the fuel and air can mix properly and the fuel can begin to atomize in the manifold.

Many race engines block coolant flow through the manifold, but this is usually not recommended for street driving.

I don't get it. It seems to me a cold engine=no coolant flow (thermostat won't open for quite a while yet) and even if it were flowing, it'd be cold, how does that heat the manifold? Isn't the manifold one of the first things to get hot? Seems like opposite would be true, the coolant tube would pull heat off the manifold to keep the temp down during normal operating. That's what I thought it was for, to cool the manifold when the engine is hot, not heat the manifold when the engine is cold, but I admit I don't know much, just curious.

[quote=jimmycoco]
To heat the manifold, which supposedly allows the fuel to vaporize better.[/quote]

And better vaporized fuel should burn more completely and thus more cleanly. Supposedly. In theory. Or something like that :I3:

Prevent carb freezing?

[quote=Carolinamidget]
Prevent carb freezing?[/quote]
I'd never thought of that one! I'll get heat flowing when it hits 32 or lower. Dude, it's already happening!:moody:

carb freezing is worse at higher temperatures, it is not ambient temp, but the dew point and the humidity of the air interacting with the big pressure (and temperature) drop just after the venturi



Norm "common misconception" Kerr

This is an emission control technique intended to better vaporize the fuel. Of course it only works when the coolant is hot. I once had an '82 Jeep with coolant running through the intake manifold. The manifold also contained a multi-pronged aluminum heat exchanger, presumably to agitate the air flow, add more heat to provide better vaporization. Cooling the intake improves charge density and makes better power but emission controls were never concerned about that. Just look at all the awful engine designs from '75-'85 - probably the worst years for complicated and cludged engine strangulation systems.

So, which is it? Is the intake cooled or heated by coolant flow? I still think it's cooled by it but what do I know.

The only heat entering the intake manifold is by conduction at the flanges where it connects to the head and by radiation from the ambient air in the engine compartment. The evaporating fuel inside the intake manifold creates a very large cooling effect. I've never measured the temperature of an intake manifold on a hot engine, but you can usually place your hand on it when the engine is hot. The cylinder head and radiator is much hotter so the coolant could not be lowering the temperature of the intake manifold. Also, if the coolant lowered the intake manifold temperature you would see it widely used as a performance enhancement. But I think you will only find it used during those development years for emission controls.

Jeff, it's heated. The lines to the intake should be plumbed as a bypass to the thermostat so the manifold doesn't have to wait until the engine is totally up to temperature before it receives heat from the coolant, same as the water choke on those cars that were equipped that way or the heater, which will begin to give you some heat and demist action well before the thermostat opens. The idea is to get the intake charge warmed up as soon as possible for better drivability. That's not the same as maximum performance of course, but for daily use it's much more friendly. A lot of cars used connections between the exhaust manifold and the intake to transfer heat, either from direct contact or via heat tubes. Many cars from the 70s used heated air risers to the air filters to warm the incoming air as well. All to get better fuel vaporization when running the lean mixtures that low emissions required.
With engine temperature regulated to around 212 F when fully warm the engineers could tune for the resulting mixture density, winter or summer.

K, thanks, that all makes sense. That intake feels dang hot to me when I'm fiddleing around in there but yep, I see how that make sense now. I had on Opel before this and the exhaust manifold mated to the intake, for that very reason I'm sure. Carry on.

"complicated and cludged engine strangulation systems."

What a great line !

"Cludged " ????

:thumbsup:[quote=saaboat]
"complicated and cludged engine strangulation systems."

What a great line !

"Cludged " ????[/quote]
[color=#DC143C]Funny thing......even though we may never have heard of "complicated and Cludged[u][b] engine strangulation systems", we all instantly know what it means![/b][/u][/color]