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VG30DE intake plenum....

Vtec 4 Life
03-25-2008, 12:10 AM
Hi guys, I'm still learning about the "performance" aspects of the 300ZX. I understand that an engine equipped with long intake runners (which I know the 300ZX possesses) helps to yield a good low end torque, but at the sacrafice of high end horsepower. But after looking at the specs of the 300ZX n/a (VG30DE), I noticed that not only does it have good low end torque, but also high end horsepower. Can someone please explain to me how this is possible (how the vg30de motor achieve low torque and high horsepower)? Any help will be highly appreciated. Thanks guys in advance.

Vtec 4 Life
03-25-2008, 02:38 AM
Also, does the VG30DE (n/a motor) come with a variable induction system?

03-25-2008, 04:02 AM
Intake design invariably comes down to a compromise between low and high end power production but much effort was expended on that long runner manifold to make sure it would flow enough air to sustain power output to red line. This article sheds a bit of light on the matter:

"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."


And yes, all 1990-1995 300ZXs came with the VTC system. The stock cam profile is fairly aggressive for a factory cam and would usually give up power in the bottom end. However in the low and mid rpm range the variable intake cam gear retards the timing to allow for better power production. It was dropped in 1996 for reasons of increasingly strict emissions standards and cost if memory serves.

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