Apologies if this has already been discussed, but as I work to bring my '71 Series 2 back to original, I remain puzzled by the coolant line running through the intake manifold.  I've now reconnected it, but only for appearance purposes.

When a car is first started, the coolant is, as name suggests, cool.  So how can it have any effect on smoother running when still cold?  But on the other side of the equation, once the engine warms up why would we want to be heating the intake manifold?  Having the coolest intake charge is fundamental to performance is it not?  The cool fuel mixture is more compact and will there expand to a greater measure at the point of ignition.  

Any thoughts and/or wisdom would be appreciated.  

Ive asked myself the same question long ago..  But apparantly it's better to have some heat in the carbs.. it makes the fuel evaporate better?

There is a short loop through the engine and intake (and inside radiator in your car) at first, and later on when the engine gets warmer the thermostat opens to cool via the big radiator in front of the car.

I have heard that in warm/hot country's they do not use the carb heating lines..  They leave them off as they don't do much..

 I've never understood the reasoning behind heating the carbs unless it was to keep them from icing up in cold temperatures like my 521 used to do. The fuel is already pre-heated in a Z thanks to the routing of the fuel lines to the carbs.

My 10/1971 240z has the stock Assy-Thermo Manifold part # 14100-E8850 which I think is basically a thermostat that remains open until around 150 degrees then shuts off the coolant through the carbs.

 That makes sense. What I've never understood is the reasoning behind heating the carbs.

I also believe it was to prevent iceing in the carbs

Mark,

Thanks for responding. I'm with you on not understanding why do they want to warm the carbs.  The other side of that is that until the car warms up, you're running cold water through the line. so what possible purpose can it serve?

ps  When I first posted this I was afraid I might be asking a dumb question.  Doesn't look like it was dumb after all, as several smart guys echoed by confusion.  

The circulation of coolant through the intake manifold was to reduce emissions.

A cool, more dense fuel/air mixture does indeed produce more power, but with more power also comes higher exhaust emissions. So warming the incoming fuel/air mixture lowers the exhaust emissions.

I removed the passages and plumbing on the intake for my race Z and never have had issues with carburetor icing, so I doubt the system was originally intended to prevent the icing.

 

I remember at school... teacher said: there are NO dumb questions, there are only dumb students that don't ask!

 

2 hours ago, Racer X said:

The circulation of coolant through the intake manifold was to reduce emissions.

Not sure that I agree.  Once the engine gets up to normal operating temps, the carb heating loop is closed (as in: inactive, does nothing, might as well not be there).  Any emissions-related benefits would be restricted to start and warmup conditions, where the presence of the manual choke would overwhelm any contribution that carb heating might make toward reduced exhaust emissions.   Remember, too, that the federal exhaust emissions standards (and related compliance testing) at the time were performed only at ~ room temperature ambient air conditions and with the engine up to operating temperature (i.e. choke off, main thermostat open).

It was always my understanding that the carb heating circuit was introduced in an effort to inhibit drivability issues caused by carb icing.  Not all carbureted engines suffer from this problem.   As well, it apparently isn't specific to constant-vacuum carbs, nor to inline engines, nor to water-cooled engines.  Nor to cold-weather operating conditions.  It seems to occur only with certain combinations of engine, carburetor, and engine compartment layout and under certain air temperature/humidity/engine speed/throttle-opening conditions.  Evidently, Nissan decided that it had heard enough reports of problems with early Z's to warrant spending the (not-inconsiderable) amount of money required to install the carb heating loop (complete with revised castings for the carb bodies and the thermostat housing).

There's a useful discussion of the Z's carb heating system on Hybrid-Z, here.    And if you use the search function here on the CZCC site, you'll discover that several discussions about the system have taken place over the years.  

1 hour ago, Namerow said:

Not sure that I agree.  Once the engine gets up to normal operating temps, the carb heating loop is closed (as in: inactive, does nothing, might as well not be there).  Any emissions-related benefits would be restricted to start and warmup conditions, where the presence of the manual choke would overwhelm any contribution that carb heating might make toward reduced exhaust emissions.   Remember, too, that the federal exhaust emissions standards (and related compliance testing) at the time were performed only at ~ room temperature ambient air conditions and with the engine up to operating temperature (i.e. choke off, main thermostat open).

It was always my understanding that the carb heating circuit was introduced in an effort to inhibit drivability issues caused by carb icing.  Not all carbureted engines suffer from this problem.   As well, it apparently isn't specific to constant-vacuum carbs, nor to inline engines, nor to water-cooled engines.  Nor to cold-weather operating conditions.  It seems to occur only with certain combinations of engine, carburetor, and engine compartment layout and under certain air temperature/humidity/engine speed/throttle-opening conditions.  Evidently, Nissan decided that it had heard enough reports of problems with early Z's to warrant spending the (not-inconsiderable) amount of money required to install the carb heating loop (complete with revised castings for the carb bodies and the thermostat housing).

There's a useful discussion of the Z's carb heating system on Hybrid-Z, here.    And if you use the search function here on the CZCC site, you'll discover that several discussions about the system have taken place over the years.  

Good points.

But.

The passages for coolant only pass through the intake manifold, not the carburetors. The carburetors are also insulated from the intake manifolds by the phenolic spacers.

So how can the coolant, which is heating a portion of the intake stream downstream of the carburetors, warm the area in the carburetors to reduce or eliminate carburetor ice?

I doubt it can, as the heated incoming air/fuel mixture has already passed the carburetors.

In airplanes with reciprocal engines equipped with carburetors, the intake airstream can have warm air drawn from a heat exchanger over the exhaust manifold, to warm the incoming air and prevent carburetor icing.

Also, in other carbureted automotive applications, there is a duct to direct warm air from the exhaust manifold into the air cleaner housing ahead or the carburetor, to reduce carburetor icing.

Indeed, even the Z has this same warm air duct directing heated air into the carburetor inlets.

Heating the intake manifolds after the air/fuel mixture has been drawn through the carburetors does little, if anything, to warm up the carburetors to reduce or eliminate carburetor icing,

 

 

 

 

 

 

Edited August 1, 20222 yr by Racer X

https://www.ztherapy.com/technical_stuff/spotterguides/zcar/240spotter.htm

The 72 intake was water warmed to help atomize the fuel faster when the engine was cold.