Duey,
you are correct that the twin pack is derated for the Griffon. The PT6 is a good engine. It is used on numerous a/c around the world, such as the Dash series a/c, etc. I'm not sure if the other readers understand the exact way a helo gets lift, i.e. can take off and fly. So, I will try to explain it simply. The rotors are actually airfoils (wings) spinning around. The only way you can get lift is to have air flowing over the airfoils (rotors). In a helo, this is done by spinning the rotors around. This causes the air to go over the rotors and because of their shape, lift is produced, and the helo can take off. The thing that affects lift the most (lift varies with the square of the velocity while the other terms only vary to the first power) is the velocity of the air going over and under the blades. So, why don't we speed up the rotors? Well, a problem comes up where the rotor blade tips (end of rotor) could actually go supersonic if the rotors were sped up too fast. That is really bad and could cause the blades to break apart, stall, etc. Generally put in the bad category. Another way to explain it is to tie a rock to a string and spin it around. The rock has to be going faster than the string near your hand in order to cover the larger distance the rock travels in the same time that the distance the string near your hand travels. So, if we can't speed up the blades to get more lift, what do we do? Well, the answer is to increase the surface area of the blades. You can do this different ways but the two general ways are to increase the number of blades or increase the width (i.e. chord) of the rotors (blades). However, this causes another problem. This will increase the torque that is felt by the mast. This torque has to be overcome by the engines (through the combining reduction gear box, then through the transmission, and then through the mast). A helo's engines are almost always turboshafts, such as the PT6 is (smaller helos actually use piston engines). Turboshafts don't give a high velocity of air out the rear end, like turbojets do. Most of the power a turboshaft produces goes into turning the compressor and then the remainder goes into the shaft that goes into the reduction gear box. As for the mast, the thicker the mast, the more torque it can take.
Now, for the Griffon as compared to the Twin Huey or the new Yankee class "Super Huey" that the Marines have planned. The Griffon was originally designed to have a larger payload than the Twin Huey. It does have a greater all up weight (aircraft plus payload, essentially) than the twin. One other major difference is that the Twin never had what is called a torque tube. The Griffon does. This measures the torque that the mast is feeling. I venture to say that a lot of Hueys were being overtorqued and nobody knew about it. The Griffon has a small mast as compared to the Black Hawks, etc. Therefore, it is torque limited, thus the engines must be de-rated.
From above, one can see that you can't just throw in more powerful engines in a helo and hope to get more lift and thus be able to take off with more mass. It ain't going to happen. In combination with the more powerful engines, you need a beefed up RGB, tranny, and most importantly, mast. That is what the Super Huey (Yankee model) has, in combo with the more powerful GE engines.
On a side note, unless my material is date (been away from Tac Hel for one year now), the Super Huey has not entered into full production due to a lack of older models to refurbish (read war in Iraq causing these probs). Duey, do you know if this prob has been fixed?
Duey, when did you get to 427? I don't remember seeing you there when I was there (I saw your pic in front of the Chinook in a different post).
Hope this helps and doesn't confuse anyone. I'm available for questions if you have any. I was at Tac Hel and I'm an AERE (Aerospace Eng).
