VE and HP/L are closely related, not synonymous.
Fair enough. =)
Your cause and *effect argument that I'm incorrect clearly refutes the idea that they are synonymous, as you explicitly state they are two functionally different metrics.
Sortof.... I can split hairs on this subject a little: Timing advance does affect VE of the engine - they aren't totally seperate from each other, although the change in VE isn't as much as when you do something like porting, cam modification, manifold upgrade, etc. Retarded timing can increase the VE as more exhaust gas energy will be blown out, creating a greater scavenging effect to aid the next cylinder in its exhaust operation. However, the positive effect of retarding timing in this example is a far cry from the impact on power output it will have: you are basically giving up combustion energy input into the piston and putting it out the tailpipe.. Although you may be moving more air through the motor, you aren't making effective use of it. Even though, this particular variable in the engine operation would need to be configured to offer the same balance of performance and knock resistance in both engines to try and keep that variable from affecting the test. Just about every element of an engine's operation is in some way or another affected by the other conditions of operation. Trying to seperate all of those conditions into single independent metrics is difficult to do since they are so intertwined with each other. 350Zs look like ass. I never took issue with your conclusion, just how you stated it. I agree with your assessment that the VQ is probably more volumetrically efficient than the NA VG. Was the initial VG designed to maximize turbocharged application efficiency? If so, could they have made up for that by designing the intake tract specifically for a NA application? It is my impression that the VG30 was originally designed in the twinturbo format and the powerplant was mildly modified from there to produce an NA platform while trying to keep as many parts similar as possible while achieving a NA power output according to what they felt would be necessary for the vehicle. I know the cams on the NA are the same as the TT so it would seem to me that there was a compromise there on the NA platform given that turbo cams and NA cams are designed with notably different profiles. It would be interesting to see what a performance set of NA cams would do in the VG30DE (I think Harry: ZTUNER did this recently to yield one of the highest NA dyno pulls) The design of the intake manifold with its long runners doesn't appear to me to have been designed for the twinturbo setup though. Given the small size of the stock turbochargers and their excellent spoolup response, it seems counterproductive to use a manifold with such long runners as this primarily benefits bottom-end torque production. On the same token, you can see where the NA's torque falls on its butt above 6000RPM, which would also benefit from shorter intake runners. I'm really at a loss as to why our intake manifolds use such long intake runners. Why haven't they figured out how to get more power out of 3.5 liters? BMW makes 333 hp with less displacement in the current generation M3. The individual throttle bodies help, I'm sure. I think the real question is: Why hasn't Nissan figured out how to make more hp/L gain than 7hp/L in over 17 years of engineering. Hard to say.. Your guess is as good as mine there.. Horsepower isn't the final goal, and I think a few other metrics should be considered. Emissions at the tail-pipe restrictions are more strict, gas mileage requirements are higher, gearing is different as well (moot for hp, but matters for other variables). Focusing just on the hp/L of an engine, again I think leaves out so many variables, that it's a worthless measurement. A comprehensive overview of at least the items in this paragraph would give a better picture of what's really going on in an engine comparison. FWIW, I could go either way with the argument. I do see your point about emissions and other variables that would have to be considered in order to make a final conclusive statement one way or the other but unfortunately neither of us have the data necessary to do that. Looking at the system as a whole though, VE still plays every bit a role in the other variables that you bring up as variations in intake, cams, manifolds, exhaust, etc ALL solely affect the VE of the engine. Different emission control systems, exhaust systems, intake systems, etc, all directly affect the VE of the engine. The BMW folks that are making some 100HP/L in an NA setup have taken a lot of VE factors into account with their design - that's the only means they have to make that possible: they still use premium fuel, which puts the compression, A/F ratios, and timing advance on the same playing field as everyone else. It would be interesting to see dynocharts of those engines though so we can get a better idea of what they are doing to achieve that. You could design an NA engine with small displacement to produce an comparably insane amount of power but in most cases it is only going to do that in one small range of high engine RPM and with limited peak torque. Here's three 350Z dynocharts of three different cars with different levels of modification: Green = stock
Red = JWT, plenun, headers, highflow cats, exhaust
Blue = Mods listed in paragraph (swapped highflows for testpipes) 
And a stage 5NAZ32
 Just after 6KRPM on the Z32, the torque falls on its face. This is mostly due to the intake manifold design with its long runners combined with the use of turbo cams. The 350Z was designed as an NA platform so the plenum design has shorter runners and designed with NA camshaft profiles. You can definately see the advantage after 6KRPM on the 350Z. Like you said though, there are other variables in the system that one can consider when trying to make an accurate comparison between two engines: the VQ35 was designed from the factory as an NA powerplant whereas the VG30DE was originally designed in the twinturbo format. The camshafts and intake manifold design do not lend themselves as properly designed components for the VG30DE in terms of producing the best HP/L, and those components also directly affect the VE of the engine as noted in the dynochart above 6KRPM in the VG30DE. Had Nissan designed different cams and a different intake manifold for the NAZ32, I think it would have made up the small difference in HP/L we see between the two engines.
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