The answer is: Nobody can determine what is "safe" vs. "unsafe" horsepower or boost for the motor. The real risk factor is always there. It can be minimized, but never eliminated.
There are two risk factors that we have to look at. First one is the compressive strength of the rod once fatigued, which we don't know simply due to the shear number of cycles a rod sees (the rod would have to sit in a fatigue testing cell for a couple years 24/7 to come close to modeling the cycles a rod sees in real life). The only thing we can do is base it on real life experience. This means how many engines have snapped a rod due to installing forced induction.
Back in the days of the 350Z, this answer was simple: 400whp was the limit...anything more, and you would need a built motor. Another thing I'd like to throw in there (I will cover in more detail later) is that it has been established is that supercharger cars had a longer life expectancy at higher power levels than twin turbocharged cars. The reason is simple: turbo cars make more torque and do so abruptly thus shock loading the rods and snapping them like so many toothpicks. Fast forward to the VQ37VHR. We noticed that the rods are beefier and it's almost as if Nissan was compensating for the more powerful higher reving engine. Here is a pic below showing the transition from the VQ35DE to the VQ35HR to the VQ37VHR.
Arguably, we have installed more forced induction kits on the VQ HR and VHR motor than anyone in the world. So far, these engines have shown a great track record. I think the reason for this success is a combination of a few things that have been improved over the older DE engine.
1) Better Internal Engine Components
2) More stable Main bearing caps
3) Better Combustion chamber design
4) Better cooling system design (cylinder heads are the first to be cooled down vs. the old DE engine which had the block being cooled first)
5) Better Head Bolts
6) Better Dynamic Timing Control
Finally and most importantly is TUNING. This is the key ingredient to maximizing the longevity of a forced induction build on a stock engine. Tuning is an art as well as a science and requires intimate knowledge of the components of an engine management system and how they all relate to each other. For example, the Nissan ECU has a Temperature Compensation table to alter the Ignition Timing Table based on what the MAF sensor sees. So, if the MAF sensor sees cold air, the ECU will advance the timing. We located the MAF sensors where we did for precisely this reason, so that we can give the ECU a real time indication of how hot or cold the air entering the combustion chamber really is. This is a critical part of saving the engine. We all know that when supercharging, our biggest challenge is IAT (Intake Air Temperature). Since the supercharger is a compressor, the air is heated up substantially and it is important to have the proper ignition timing for hot air/fuel mixtures. Here's a sample of how a Temperature Compensation Table is supposed to look (note that the hotter the air gets, the more timing gets pulled):
Another thing to keep in mind when it comes to these engines, is that although it is an 11:1 compression...that's static compression. Dynamic compression takes into account volumetric efficiency, so that means that the effective compression ratio changes throughout the power band. I can tell you from experience that there is no way in hell that we could run the timing table we use on the VQ37VHR on an earlier VQ motor.
That all said, I want to give you a visual guide to torque curves to help you all understand why you can have more horsepower on a supercharged car than you can on a turbocharged car. The graphs below show different twin turbo setups compared to the 523hp supercharged setup. Pay special attention to the peak torque and how hard torque comes on. Understand that peak torque represents peak cylinder pressures. So the less peak torque you have, the less peak cylinder pressures your engine is seeing and the less stress there is on the motor overall. Although the supercharger engine has to work harder to make the same peak power as the twin turbo car, is still has less internal stress as the torque grows in a linear fashion.
If you ask me what is the single most important thing that anyone can do to keep their forced induction system running as long as possible without building their engine, is to focus on the tuning aspect of the build and eliminate any detonation. Detonation is an uncontrolled explosion rather than smooth expansion of burning gas. Eventually, detonation will beat up the engine bearings, leading to excessive clearance and spun bearings. Even if the rod or piston doesn't break, you will still have premature engine failure.
In closing, the things to watch out for are as follows:
Peak Torque
Detonation
Air Intake Temperature Compensation
Fuel Quality
IMO a well tuned 500 RWHP has a lot better chance surviving then a 400RWHP that is not tuned correctly
Sam