I have noticed that many misunderstand what is safe and what is not. Therefore, here is some info straight from the SCT "the basics of tuning" book:
A "SAFE" Tune
What is a safe tune with a blower or nitrous? A safe tune is a tune where the timing is not aggressive and adequate fuel is provided, such as contained in the SCT value files. Does this mean that the engine is totally safe? In many cases this is not true. Customers must be made aware that blowers increase cylinder pressure on a stock bottom end or with high boost levels and high horsepower levels can sometimes result in a mechanical failure. Metallurgical failures within engine components and simply stressing parts beyond design limits can result in catastrophic damage.
To add to this, here is a question you will hear fpretty often from a customer: Will I harm my engine with 13# of boost with a "safe tune"? Here is the answer you should give: there is a thermodynamic equation that states PV=nRT P is pressure, V is volume, T is temperature. This formula is used by engineers to calculate pressure.
The following example is a blown V6 mustang. It is running about 11 PSI, non-intercooled. Lets assume that the V (volume) is basically constant (engine displacement is the same, and air consumption is about constant). As tumperature increases so does pressure. But, if the air temp rise is less, the pressure is less. So, if we were to have an intercooler, that 11 PSI would drop down by about 2 PSI, giving 9 PSI. Any intercooler has a flow loss. It can be estimated in this case to be about 1 PSI. Now, it is running 8 PSI with the same setup after adding an intercooler (this all assumes that you are measuring boost in the manifold). As you turn the blower faster you will increase boost back to 11 PSI.
As a result you have more air flow than non-intercooled blower. More airflow is more cylinder pressure. To much cylinder pressure can and will break ring lands or rods, even if no knock is present. Some engines have hypereutetic pistons and powdered metal rods. These engines can handle peak cylinder pressures of about 1000 PSI, probably less if you want it to last long term. A typical blown engine develops about 2.65 PSI per HP produced. This is a general guidline. If you are making 300 RWHP, this is about 340 crank HP. With a blower you give up about 70 HP in friction, so the motor makes 410 HP, but you only get 340 of it, the rest is lost in heat and used to spin itself. 410 times 2.65 is about 1100 PSI. The Gen 2 Lightning motor is just under 1300 PSI and it has forged pistons. You can make in the 350 RWHP range with 13 PSI with an intercooler on a V6 if tuner right. This is about 390 crank HP, plus about 80 HP in friction (remember you are spinning the blower faster), this is 470 HP. 470 times 2.65 puts it in the 1250 PSI range. This will break pistons or rods. Can you run 13 PSI? Yes. Can you keep cylinder pressure lower? By taking timing out of the engine. You will make more low end torque due to more air flow. You will also have the exhaust valves glowing with the timing so retarded.
There have been several near-stock big boosted engines that spit out rods with ZERO detonation. A 4.6L owner will rarely run 13 PSI with a stock bottom end and make it last. And, if go to WOT a few times a day, your motor life will be shortened, probably by a lot. The rods in a Lightning are at serious risk when you start turning the power way up. Does this mean that you should not do it? No. But be aware that the more power a car makes, the more stress you put on the motor and shorter it's life will be. This is a non-arguable point. The basic issue is how much do you use the power. Done correctly, nitrous is safer since it does not add any friction to teh motor like a blower does. But, if a solenoid failes, you melt the motor. If you take timing away up top and then add fuel to keep the valves cool, many think that if the motor never knocks it will last forever. It won't.