A12GX Engine

Revision as of 01:07, 26 December 2008; view current revision
<-Older revision | Newer revision->

The Datsun A12GX Engine is legendary. While it may not be a powerful as an A14, this was a stock package offered in Japanese Domestic Model 1200 GX Sedans and Coupes and has more horsepower than the Standard A12 engine. The GX engine featured a matched pair of Hitachi HJE38W carburetors, larger cam, giant intake ports, and streamlined exhaust.

Also see: Hitachi Twin Carb engine

A12 GX Engine

GX Cylinder Head

The GX cylinder head has the largest ports of any factory A-series cylinder head. The ports are larger than A14/A15 ovals. This makes it suitable for extra-high RPM usage. However, the valves are standard A12 size, smaller than A14/A15 valves. The latter is a better head for regular-RPM usage (under 4000 RPM).

GX cylinder head and intake

 Main article: A-series_Cylinder_Heads

• GX has intake ports shaped differently for flow and swirl, however it is said that a ported and polished standard A12 head with larger valves fitted flows about the same as a standard GX head. Also see head trivia.
• The GX head bolts on an A14/A15, but is very similar to the oval port A15 head. However, the 1974-up heads provided oil to the rocker shafts in different locations so you may need to drill the oil passage in the newer block
• There is apparently an A14 Oval Port GX head which flows more than the A12GX head.
  reference: GX Head or A15
• GX head uses standard A12 size valves. No bigger than stock. The Nissan A14 Competition head was used by Nissan for racing is similar but has the larger A14-size valves.

Identification

To be a genuine GX head, it must have these features:

1. Extra-large oval ports 30x35 mm (larger than A14/A15 28x34 mm)

   

2. Four large-diameter water holes (5-6mm), one below each port (regular engines have three small-diameter, or none at all). These send coolant to the water-heated GX inlet manifold.

   

3. Heart shaped combustion chambers

   GX: Type B

4. Extra hole drilled for rocker oiling

   

5. Valves: 35mm inlet, 29mm exhaust (smaller than A14/A15)

CAUTION: Some A12T heads also have the same features. How to tell the difference between Cherry Coupe (E10) A12T engine and A12 GX engine? Unknown. See discussion'Virgin Pone'.

Also note:

• 1200 GX heads usually (always?) have no casting numbers at all
• A genuine head may have had the dual-valve springs removed. But it will probably still have the dual-step valve spring seats

  Dual-step valve spring seats

Engine Block

• Balanced crank<li>Flat top pistons for 10:1 compression ratio in some engines. Standard dished pistons have been identified in some other engines<li>Slightly longer duration cam than stock 1200.<li>Water pump inlet pipe at front of block has additional hose fitting to recieve manifold coolant outlet hose. </ul>Otherwise the block crank and rods are the same as the standard A12.

  Engine Block

  The engine block is a standard A12 part. However, it has some extra parts attached to it:

  • Balanced crank
  • Flat top pistons for 10:1 compression ratio on hi-gas models (some GX engines have standard 9.5:1 dished pistons)
  • Slightly longer duration cam than stock 1200, for 6800 RPM usage (up 400 RPM over stock cam)
  • Water pump inlet pipe at front of block has additional hose fitting to recieve manifold coolant outlet hose.

  The block crank and rods are the same as the standard A12.

  
  

  Induction

  See: Hitachi Twin Carb

  
  

  <li>This featured a matched pair of left- and right- hand Hitachi HJE38W carbs. Twin semi-sidedraft Hitachi carbs (Variable Venturi/SU-type) -- 38mm bore (~1.5 inches)<li>Oval port inlet manifold with or without water jackets to match the cylinder heads<li>Intake manifold has 2 studs at the top of each pair of inlet ports (cylinder head matches this). Standard A12 has only one stud per pair of intake ports.

  British vehicle SU carburetors (H4 or HS4 1.5 inch carbs) will fit the manifold, with some linkage creativity.

  Exhaust

  main article: Exhaust Piping

  • Dual outlet exhaust manifold that bolts to dual pipes that blend into into one pipe further down. See Photo: Twin exhaust downpipe

  Word has it this works as good as common extractors (headers), though not as good as the best tuned-length extractors. The factory 1200/B110 Competition Tune-up manual recomends the GX manifold for Rally competition use instead of extractors.

  Extractors may not clear the GX inlet manifold.

  
  

  Rocker Cover

  The Rocker cover is unique to the A12GX engine:
  

  • accelerator clip is on front pad, not rear
  • fuel line clip boss is on rear left side, not front left side
  • Datsun logo (not Nissan)

  
  
  Arrows: Accelerator cable clip

  • GX: center feed
  • A12: rear feed accelerator cable

  
  Circles: Left side threaded boss for fuel line

  • GX: rear
  • A12: front and rear

  Specifications

  • Nominal Rev Limit: 6800 RPM
  • Actual Rev limit: reported fine at 8,000, float at 8,150 rpm

  references:

  • FIA Amendment March 1970

  Datsun 1200 group 1 FIA recognition

  Comparison

  part	A12GX Engine	A12 Engine
  Carb	Hitachi HJE38W side-draft (two carbs)	Carb Hitachi DCG306-1 down-draft
  Carb Venturi	?	Primary: 26 mm, Secondary: 30mm
  compression ration	10.0	?
  combustion chamber	29.1 cm3	36.6 cm3
  valve opens	BTDC 20 degrees +- 7 degrees	?
  valve closes	ABDC 56 degrees +- 7 degrees	?
  valve opens	BBDC 58 degrees +- 7 degrees	?
  valve closes	ATDC 18 degrees +- 7 degrees	?
  In. Valve dia.	29 mm 1.14 in	?
  valve lift	9 mm 0.35 in	?
  exhaust ports	27w x 29h +- 1.5 mm	?
  intake ports	30 mm circle with 4.5 mm elongated section (30 x 34.5 oval?)	?
  exhaust ports	26w x 28h with 3 mm radius +- 1.5 mm	?
  center ex port	69w x 28h	?
  intake ports	28/27 mm circle	?
  Flywheel weight	?	9.5 kg
  Flywheel with all turning parts	?	13.1 kg
  hp	83ps @ 6,400 rpm	hp 68ps JIS @ 6,000 rpm
  max rpm	6,800 rpm (80 ps)	hp 68ps JIS @ 6,000 rpm
  max torque	10.0 kg-m at 4,400 rpm	9.7 kg-m at 3,600 rpm
  max speed car	160 km/hour	150 km/hour

  
  

  B210 A12 GX Engine

  The B210 GX engine was basically the same as the B110 twin carb (1200 GX Engine), but based on the newer cylinder block. The head was no longer drilled for the oil gallery that was unique to the A10 & 1200, & a revised induction was used. This involved the flat top [or square top if you prefer] carbs & the inlet mnifold had proovision for a few more fittings to allow compliance with newer smog laws. There was also provision for a vacuum takeoff for the brake booster.

  The B210 GX induction seems to be the most commonly encountered twin-carb A engine setup in Australia.

  The exhaust manifold was the same as the 1200 one, but the fuel pump was of course the newer type for the new cylinder block. GX pumps had a slightly stronger spring under the diaphram to boost fuel delivery pressure a smidgen.

  The air filter housing base plate was revised a little to accomodate more fittings too, but the main cover piece was unchanged, other than for the change in colour from red to blue. It seems that L series SSS air filters went from red to orange.

  In the USA, a B210 "GX" car model was offered, but this referred to the car only and did not include a twin-carb engine as did GX models sold in Japan.

  Photo Index