Designed in the late 1980s, Nissan's twin turbo, DOHC, 4-valves per cylinder
powerhouse is one of the all-time classic turbo engines. Surprisingly
sophisticated both for its time and cost, the engine was fitted to the 300ZX
twin turbo sports car. Nissan engineers were aiming very high with the car - the
background to the engine development makes this very clear.
In this article we'll cover the technical nitty gritty of the engine and its original development goals.
The Basic Anatomy
The VG30DETT is 2960cc in capacity, with an over-square bore/stroke
relationship of 87 and 83mm, respectively. The combustion chambers are of a
pent-roof design, with the spark plugs positioned close to the bore centre. The
four valves per cylinder are operated by double-overhead cams working with
zero-lash hydraulic tappets. The compression ratio of this twin turbo engine is
8.5:1 and the original specs show an SAE NET output of 300hp at 6400 rpm and a
peak torque output (again SAE NET) of 283 ft-lb at 3600 rpm. Almost a cube in
physical dimensions, the engine is 710mm long, 775mm wide and 695mm high.
While based on the much lower-powered VG30DE engine, major changes were made
for the twin turbo engine. These weren't just limited to strengthening the
engine and installing the twin turbos; instead the opportunity was used to
design completely new intake and exhaust systems. The engineers made much of the
fact that twin intercoolers, twin throttles, twin plenum chambers, twin turbos,
twin exhausts, twin catalytic converters and twin mufflers were adopted.
Intake System
The intake system design had to balance two opposing outcomes:
- the smaller that the intake runner diameters were made, the greater the
frictional losses (and so pressure drops)
- but the larger the intake runner diameters, the slower the airflow speed,
resulting in a decrease in cylinder filling, especially at low rpm
In addition, simulation and testing showed that long intake runners resulted
in better torque development at low engine revs - however, fitting long runners
into an already crowded engine bay was going to be difficult. Runners that were
360mm long gave peak intake efficiency at 4400 rpm, while lengthening these to
480mm dropped the peak intake efficiency revs to 3600 rpm. Since one of the
goals of the engineers was strong bottom-end torque, the longer runners became a
requirement. Further testing showed that a runner diameter of 48mm worked well
with the 480mm long design.
[It's interesting to note the major amount of development that occurred in
tuning the intake system in this turbocharged engine. Many turbo engines -
including Nissan's own RB26DETT Skyline GT-R engine - have no intake resonance
tuning at all.]
Once 480mm (nearly 19 inch!) long intake runners had been decided upon, the
next question was how they'd be fitted under the bonnet. The previous model
VG30DE had placed the plenum chamber centrally on top of the V6, with relatively
short but direct runners connecting the plenum to the intake valves. The
measured pressure drop with this arrangement was 85 units.
[The units used are not completely clear - they may be mm of water at 4.4 cubic metres/minute
flow.]
The first prototype VG30DETT intake system design placed a plenum chamber
above each bank of cylinders, with the intake runners for that head connected to
the plenum above it. This required that each runner go through nearly a U-turn,
and so was called the 'U-turnport' design. The pressure drop of this design was,
however, very high - being measured at 105 units, or nearly 24 per cent higher
than the original VG30DE design with the centrally-located plenum chamber.
A 'crossport' design was then built, where the plenum chamber feeds the
opposite cylinder bank. This design allowed the retention of the long intake
runners but gave a measured pressure drop of only 80 units - better than the
VG30DE design, despite the use of intake runners nearly twice as long. The
change from a U-turnport to a crossport design resulted in a 5 per cent increase
in peak power.
The intake system ahead of the intake manifold was also extensively
developed. The airfilter housing used two filtering elements to provide
sufficient filtering area within the tight confines of the engine bay. Only a
single airflow meter was used, but the junction where the duct splits to feed
each turbo was extensively developed. The final design used a very long radius
inner bend reducing measured pressure drop by 77 per cent over some of the
designs trialled. Together with the use of a bellmouth at the entrance to the
airflow meter, these flow improvements increased peak power by 2 per
cent.
Twin Turbos
Since the VG30DETT was launched, we have become used to high performance
engines being fitted with twin turbos. However, at that time, it was daring
move. In terms of the cost penalty alone, doubling the number of turbos must
have been prohibitive.
The engineers justified the use of twin turbos with some careful study. To
avoid the response penalty of fitting a single large turbo with a commensurately
large rotating inertia, the engineers used two hybrid turbos, each with a Garret
T25 compressor and a Garret T2 turbine. Using these hybrids reduced rotating
inertia by 20 per cent over conventional T2 turbos. And then of course there was
the use of two smaller turbos, rather than a single large one. Compared with a
single turbo of equivalent flow performance, the use of twin turbos reduced the rotational inertia by
30 per cent.
However, the Nissan engineers suggested that the use of twin turbos only
became advantageous at peak power outputs of over 250hp.
Variable Inlet Valve Timing
NVCS (Nissan Valve Timing Control System) is used to vary the timing of the
inlet camshaft by 20 degrees. This table shows the inlet cam timing:
| |
Inlet Valve Opens |
Inlet Valve Closes |
NVCS on |
19 degrees BTDC |
49 degrees ABDC |
NVCS off |
-1 degree BTDC (ie 1 degree after ATDC) |
69 degrees ABDC |
The timing of the exhaust valves is fixed, with opening occurring at 59
degrees BBDC and closing at 9 degrees ATDC.
At idle and low loads the NVCS is off, while at medium to high level loads at
less than 6100 rpm the NVCS is on. At all loads above 6100 rpm, NVCS is off. The
action of NVCS makes a substantial difference at engine speeds below 6100 rpm,
adding as much as 30ft-lbs to the torque output.
The 28 degrees of overlap when NVCS is on is high compared with what would be
used without having variable valve timing. The similar era Nissan RB30ET SOHC
turbo in-line six (as fitted to the Holden VL Turbo) can be compared - it has a
cam overlap of only 18 degrees. Note that the graph shows that more torque is
obtained everywhere under 6100 rpm with the NVCS 'on' valve timing;
however the 8 degree overlap that occurs with the NVCS off helps give a very
smooth idle.
Engine Internals
Upgrades made to the VG30DETT over the VG30DE include the use of pistons with
fully floating gudgeon pins, piston oil squirters and pistons equipped with
cooling channels, "autothermatic" pistons with insert steel struts, and better
conrod big-end bearing materials.
The exhaust manifold was cast from Ni-resist D5S material rather than
conventional high silicon cast iron, giving an improvement in tensile strength
from 3 kg/square mm to 9 kg/square mm at an expected maximum exhaust temperature
of 900 degrees C.
The con-rods were also made from an upgraded material - micro-alloyed steel.
This also enabled a reduction in conrod mass of 93 grams. The crankshaft was
also made from micro-alloyed steel, strengthened at the webs. The VG30DETT crank
is 10 per cent stiffer than that fitted to the VG30DE.
The sump was also reconfigured, with an oil guiding plate designed to return
oil scooped up by the crankshaft counterweights back to the deep section of the
pan.
Conclusion
While the RB26DETT engine of the GT-R Skyline is usually awarded kudos when
Nissan's turbo engine line-up over the years is considered, the VG30DETT - with
its variable valve timing and carefully tuned intake manifold system - is the
more sophisticated of the two engines. (But isn't it amazing to consider that
the one company developed - at the same time - two such brilliant twin turbo six
cylinder engines?)
At the end of the development the engineers said: "We are confident that this
engine has achieved the utmost in power output together with high torque and
high response worthy of a new generation sportscar."
And lots of modifiers over the years have agreed with them...
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