The SR-series of Nissan engines (perhaps the best-known is the potent and
easily upgradeable SR20DET turbo 2-litre) was designed by Nissan as a
replacement for the earlier CA series of engines. Here’s how the engineers went
about their task.
The development aims stated by the engineers for the engine was:
power output and ample torque in the middle to high engine rpm ranges
good balance between high output and fuel economy
“clear, linear sound” up to the redline
Oddly enough, except for a throwaway line (“good adaptability to future
emission control regulation”), objectives relating to emissions were missing
from the original discussion – perhaps one reason why after a production
lifetime of about 13 years, emissions performance killed the design.
Unlike many contemporary engine designs which were being constantly upsized
(for example, from 2 to 2.2 or 2.4 litres), the engineers stuck with the
‘traditional’ 2-litre swept volume, gained from a bore and stroke each of 86mm.
To keep engine mass low, an aluminium block was used. This design used a closed
upper deck and a skirt which extended well below the crankshaft centreline. The
head used compact, pentroof combustion chambers (cross-flow, of course), with
the sparkplug located in the centre of the combustion chamber.
The straight intake ports were designed with what Nissan terms an
‘aerodynamic port shape’, that is, a port that decreases in cross-sectional area
as the runner/intake port gets closer to the intake valve. The ports were also
positioned high in the head, so reducing the angle which the air had to
negotiate before entering the combustion chamber. In addition to improving flow,
the greater length of the port served to improve torque through giving a longer
tuned length to the intake port/intake runner combination. In fact, a combined
intake port/runner length of 450mm was used in the SR20DE engine. Compared with
a low port design, the high port design improved measured torque by 4 – 7 ft-lb
at engine speeds of over 4400 rpm.
A narrow valve angle of 29 degrees was selected – this can be compared with
the 45-degree angle of the CA engines.
Over the CA18DE, the SR20DE engine provided a 2.7 per cent improvement in
its brake specific fuel consumption at the same torque output (one which
corresponded to 60 km/h road speed, presumably in top gear).
The high ports required that the camshafts worked the valves through Y-shaped
rocker arms pivoted between the intake ports. The rockers featured reduced
contact area with the cam followers and also reduced inertial weight over other
designs, so reducing valvetrain friction.
The cams were chain driven.
The exhaust side of the head was designed last. To reduce exhaust
interference effects, the exhaust manifold used a design where cylinders 1 and
4, and 2 and 3 were each combined into a dual manifold – standard practice on
most four cylinder engines. The dual part of the exhaust extended as far
rearwards as the rear part of the sump.
Nissan engineers very carefully considered the structural options open to
them before settling on a DOHC, narrow valve angle head with the valves operated
through outer pivoted Y-shaped rockers. Other options considered included:
- a similar design to the final iteration, except for the use of one
rocker per valve and the use of inner pivots for the rocker arms (the negatives
were greater mass – and so friction - in the valvetrain, and a very wide
direct-operated valves with a wider valve angle (negatives: wide
head, couldn’t use roller rockers, built-in hydraulic lifters would add to
masses, valve lift restriction because of bucket size)
- direct operated valves with a narrow valve angle (negatives:
couldn’t use roller rockers, built-in hydraulic lifters would add to masses,
valve lift restriction because of bucket size)
The SR20 engine was developed around the time of the 4.5-litre VH45 V8 engine
used in the Infiniti luxury car. Both engines used Low Pressure Die Cast alloy
blocks and drew on the experience gained by Nissan with their first alloy block
engine, the 1-litre MA series. The technology jump was a large one, although the
weight saving of the SR20 block over the CA18/20 iron block was only 9kg.
However, as Nissan engineers of the time suggested, “attention was devoted not
merely to reduce weight, but to assure functional reliability and to improve NVH
[noise, vibration, harshness]
The block alloy was heat-treated JIS AC2A, a material already used by Nissan
in cylinder heads. A closed-deck design was adopted for these reasons:
head gasket sealability
permanent bore distortion
To test the reduction in NVH of a closed deck, Nissan engineers had a CA20
block cast in aluminium (the CA being an open deck design) and then tested it
back-to-back with a prototype closed-deck SR20. In the closed-deck design the
greater stiffness of the water jacket wall reduced radiant noise by 2-3dB over
the whole engine rev range.
Other Nissan testing showed that cylinder bore distortion of the cast-in iron
liners could reach 0.05mm in an open deck design, causing piston slap noise and
oil consumption problems. This permanent distortion was reduced to 0.04mm in the
closed deck version.
A deep skirt block design was adopted to improve powertrain rigidity –
but to give reduced NVH, rather than improved engine strength. To prove this,
the engineers tested a modified VG30 3-litre V6 Nisan engine in deep skirt and
half skirt block configurations. In deep skirt guise the engine showed a clear
increase in frequencies, indicative of a stiffer powertrain. In addition, the
connection to the gearbox was improved by the use of a two-piece oil pan, with
the upper half an aluminium casting. This was needed particularly because of the
Pulsar GTiR application, where the engineers were concerned with the mass of the
four-wheel drive transfer case and differential being mounted on the side of the
engine and transmission.
A cast-in-place dry iron liner was used within the aluminium block. However,
unlike many other engines of this construction, the iron liners were ‘buried’
beneath the surface of the block. This was done for two reasons:
improve cylinder head gasket sealability
improve the life of machining tools (the resulting gain was 5 -10
Nissan engineers said that they placed great importance on both reducing
generated engine noise and also creating “improved sound tone and quality”. To
this end, they concentrated on the rigidity of the engine (and of the
engine/trans combination, which used 10 connecting bolts) by using a main
bearing girdle, the two-piece sump and the closed-deck block. (It’s interesting
to consider that it’s quite likely that much of the engine’s legendary
durability came about through technologies aimed at improving the quality of the
sounds made by the engine....)
Reliability and Maintenance
Nissan bucked the trend of going to cam belts by retaining chain drive for
the twin cams. The single roller chains used a tensioner applying force through
the use both of oil pressure and springs.
Unusually for the time, platinum tipped plugs were used which were said to
not require adjustment or replacement for 100,000km. Also aiding reliability
were cylinder head bolts which were tensioned such that they stretched an
appropriate amount and a “large size distributor
secondary voltage for up to 7500 rpm” to be available.
The 16-bit ECU controlled bottom-feed injectors with a high heat resistance
and a hot film type airflow meter was fitted. In addition, the engine management
system included control of the air/fuel ratio (with learning control from an oxy
sensor), knock control and self-learning idle speed control.
Nissan spent about 3 years developing the SR series of engines – including
the SR20DE, SR20DET, SR20DI, SR18DE and SR18DI.
Said the engineers: “We believe that the goal of this development, namely a
good balance between high performance and fuel economy combined with a pleasant
engine sound, has been attained by the design technique of the 4 valve DOHC
engine which Nissan has been cultivating over many years, and by the solid
reliability built into this car
by Nissan’s production technology.”
Water cooled, 4 cycle in-line 4 cylinder|
||Cross flow, pentroof type|
||DOHC, 4 valves per cylinder, chain drive|
|Bore x Stroke:
||86.0 x 86.0mm|
|Crankshaft journal diameter:||
|Con rod length:||
||In: 34.0mm, Exh: 30.0mm|
||685 x 610 x 615mm|
140hp at 6400 rpm (SAE net)|
||132 ft-lb at 4800 rpm (SAE net)|
SAE paper 901714 – Development of the Four Cylinder SR Engine
SAE paper 910431 – Nissan’s New V8 and L4 Aluminium Cylinder Block – Design
Note: Unfortunately Nissan engineers did not release a paper specifically
on the development of the SR20DET
turbo version of the engine. Perhaps they thought that with that particular
version, they were too far ahead of the field to give away company