Cylinder Heads Overview

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Datsun 1200 engines came in two basic forms: standard round-port manifold for 5,000-6,000 RPM operation, or GX oval-port for 6,400 RPM operation.

A Engine Cylinder Heads Cylinder Heads Overview Heads By Type (H89, H72, etc) Head by engine (A12, A14, etc) Part Numbers by Datsun model Head Features & ID Changing Head Gasket more: Index

Overview

There are two basic cylinder heads used by 1200s:

GX - extra high-RPM large oval-port inlets
Standard - high-RPM round-port inlets

If you enjoy revving the engine up high, the GX head is preferable. If you like to keep the RPMs "reasonable" (under 4,000 RPM) then you will like the standard head better.

Two types of combustion chambers were offered:

A/B: Closed chamber (quench head): Best power & fuel economy
C/D: Open chamber: A touch less power, but far lower NOx emissions

The difference was 1 hp. However the closed chamber ran better on the same grade of gasoline with less tendency to [[ping]/knock, so the performance difference was noticeable. Even though the open chamber was used with lower compression pistons, it still had more tendency to ping because of the less efficient chamber design.

Compression Ratio

All 1200 cylinder heads had the same Compression Ratio. The pistons set the final ratio:

GX flattop pistons: 10:1 compression ratio
Low-dish piston: 9:1 compression ratio (69hp, most countries)
Deep-dish piston: 8:5 compression ratio (68hp, 1972-1973 North America)

Higher compresion makes more power, but requires better petrol.

10:1 requires Supreme/PULP (high octane)
9:1 can use regular grades of petrol

Newer A-series engines used a similar size combustion chamber. Generally speaking, if you want to change the CR, change the pistons instead of the head. Alternatively your existing head can be milled or a spacer used to vary compression. See Compression Ratio for more information.

Choosing

Which the best head to use -- A15, A14, or A12? and what are the benefits or disadvantages of each?

Which cylinder head is best depends upon what you mean by "best"? Better fuel economy? Better acceleration?

• A12: The head is best for convenience (if your 1200 already has it and is running well). Best for fuel economy, yet still provides sprightly fun and revs hard to 5,000 RPM (6,000 RPM with a carburetor update).

• A14/A15 round port cylinder head is not a noticeble change from the standard A12 head, although performance in the 5,000 to 6,000 RPM range is slightly better due to the larger valves that all A14 heads carry.

• A14/A15 round oval-cylinder head makes more power in high-RPM use (in the 4000-6000 RPM range)

• A12GX large-oval cylinder head is best for most power (in the 4600-6400 RPM range) when used in conjuction with a larger duration cam, such as the GX camshaft (6400 RPM cam).