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[Datsun 1200 encyclopedia]

Heat Riser

From Datsun 1200 Club

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Category: Manifold

Datsun 1200 uses a traditional heat riser in the Exhaust Manifold. This heat stove valve is on the exhaust manifold. It deflects exhaust heat to the underside of the Intake Manifold during the first 30 seconds or more of starting a cold engine. Result: quicker starts, more efficient running. However, the A12 GX Engine does not use one, so it requires extended choking on cold days.


The standard Datsun 1200 exhaust manifold features a Heat Riser to deflect hot exhaust towards the bottom of the intake manifold. Since it bolts to the intake, it requires a matching Intake Manifold.
standard exmani

By contrast, the GX exmani for Datsun 1200 is a divorced type -- not connected to the intake manifold
GX exmani
Divorced type is used with a water-heated Intake Manifold. Heating the intake plenum is important for part-throttle performance/driveability and fuel-economy.

Garyp said: If you have the A15 intake manifold with water heat I would use it. Drill the water passages in the head and hook up the return from the manifold to the heater return.

Without manifold heat the car will not run as smooth or be as responsive at low rpm. As an example, I have 2 1200's, one has the stock exhaust manifold with the warm air induction working, the other has an extractor and does not have either manifold heat or warm air. Both cars are running modified and re-jetted stock Hitachi carbs. The car with the extractor had to be jetted richer on the primary to run correctly and even at that, it's not as responsive or smooth and does not pull from low rpm as well as the car with manifold heat and warm air. Also, the car with the extractor gets lower gas millage, 28 mpg VS. 32 in mixed driving.

The plate is the "stove heater". If you use headers (extractors) it doesn't work -- the Engine will be more 'cold blooded' (doesn't run as good when very cold). But will be OK when warm.
post 13657.jpg
The ridges go down (into exhaust manifold).


The heater works with the thermostatically controlled flap ("heat control valve") in the Cast Iron Exhaust Manifold. After a few seconds of running, the flap opens. When the flap is open, the exhaust doesn't flow over the heater, instead goes straight down the exhaust outlet.

So it doesn't restrict exhaust flow except for few minutes of starting cold engine.

. 22135.jpg


A heat riser is a mechanism in the exhaust manifold to divert the exhaust up to the bottom of the intake manifold. This is done when the engine is cold for the first minute or so of operation. The hot exhaust causes the cold air/fuel mixture to vaporize for smoother operation and for lower emissions. The heat riser butterfly valve is controlled by a bi-metallic (thermostatic) spring, and as the engine warm, it will move the valve so that no exhaust is hitting the bottom of the manifold. It's totally automatic and works very well.


The intake and exhaust are bolted together in engines with a heat riser:


Check for free movement of the Heat Riser. If it is stuck, it can cause the engine to run poorly -- or even not at all! (depending at which position it is stuck).

If it doesn't move freely, lubricate the shaft with free-up oil/penetrating oil. In some cases you may need to replace the manifold.

If the spring is rusted out, a replacement can be fitted.

Heating Types

1. The flap type (Heat Riser) Exhaust Manifold in conjunction with heater plate on the bottom of the Intake Manifold properly vaporizes the air/fuel mixture in the first minute of engine starting. Then the heat riser valve closes but still a bit of exhaust keeps the manifold vaporizing.

2. The wet type Intake Manifold helps after the engine warms up using engine water to heat the plenum, thus vaporizing the air/fuel mixture. Used by Datsun 1200 A12GX Engine. Water comes from the cylinder head Water Ports and exits the manifold into the water pump Suction Pipe.

Water exits through tube at front, enters via small ports in the cylinder head

Late A-series manifolds (starting with late B210 sedan/coupe) used a water-heated standard manifold

3. The Heated Air Cleaner (option in most countries) helps prevent carburetor icing at colder temperatures.

4. The Automatic Temperature-Control Air Cleaner (A.T.C. Air Cleaner), standard in USA, keeps the intake air at a constant temperature, allowing for more precise jetting thus increase fuel economy and lower emissions. It creates maximum Power because at full throttle, the ATC allows cold air into the engine.

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This page has been accessed 11,639 times. This page was last modified 08:54, 6 May 2016. Content is available under Datsun 1200 Club.