The 350Z duct should not cause any losses. The 350Z duct is the same as a G35 duct, except without the big box in the middle to deaden sound. I would guess that most if not all the claimed gains by the aftermarket intakes are also from eliminating this box. Other than the fact that the air has to travel all the way around to the back of the plenum and then come forward (an aspect the aftermarket stuff doesn't fix), the stock intake set up is excellent. It's one of the things the Mustang guys comment on when I leave my hood open at the strip.

It does not make any logical sense to me that Injen's set up would be superior. Whatever piddling gains are made in access to cooler air should be defeated by the two big bends in the duct and the extra distance the air has to travel in the duct (which will retain more heat than using no duct until after the second bend). You can tell that air doesn't flow smoothly through it due to the stalling problems. And if you're traveling at speed, the temperature difference should be even smaller. With all the air rushing in through the front of the car, heat from the engine isn't going to travel towards the front of the car unless it has a solid surface to go through (like a duct). The second bend in the Injen intake is in the exact same place as the entrance to the stock intake, so, assuming they're made from similar materials, they're going to be the exact same temperature from that point onward. So in order to provide a benefit, the air in the Injen intake, taken from 6(?) inches lower than the stock intake's air, needs to get through two bends in a duct in an engine compartment at a lower temperature than that of air 6 inches away that hasn't touched any part of the car. Does that sound likely? It's certainly more likely with the car sitting on a dyno in a room than it is in reality. A ram-air design isn't terribly useful on a motionless car, so don't use the stock design unless you'll be driving the car forward.

My advice: get the 350Z duct to bypass the G's sound deaden-er. I bet that accounts for at least half the hp difference in the two cars. Brand spanking new, that's $75, around $40 used. Then, if you want, spend $45 on a performance air filter. All the extra $165 is buying you is 6 inches of s-pipe and some stickers. That's my opinion anyway.

-Jack
Obsidian '03 G35 Sports Coupe
Titanium '03 MX-5 Shinsen #532

 

I did not say the 350Z duct I was refering to the 350Z intake pipe.