Big deal about keeping the underdrive crank pulley when running an M90. YOUR TURNING THE ROTORS ON A SUPERCHARGER, but your worried about parasitic loss from not being able to run an underdrive pulley??? What you lose in underdrive power gains your making up for in belt grip on that larger crank pulley... Because the larger the crank pulley the larger your supercharger pulley can be for any given amount of boost. The larger the charger snouth pulley is means that you have more belt grip which means less boost dropoff in the upper rpms. Without proper belt grip for that supercharger you will suffer much larger losses of power than the underdrive crank pulley could make up for. Even an Eaton @ 8-9 psi will slip if the crank to snout pulley ratio is not sized properly, especially with a 6 rib serp setup.
Jake, I would not mind analyzing two dyno graph comparison made of a before & after scientific test of stock manifolds vs. headers.
The comparison of charts you present to me do not show before and after on the same vehicle with similar conditions. Even when comparing two VERY similar vehicles with similar mods there are just too many variables remaining to really make any scientific assesment of whether (and how much) power was gained at peak or under the curve. Analyzing graphs on the same vehicle under slightly different atmospheric conditions will play havoc with a graphs relative accuracy, let alone with the mountain of variables presented there.
With all that science talk out of the way here is my take on these things. Plain and simply headers will give a vehicle more average power/torque, whether it be peak or under the curve, than one with stocker manifolds. If a header company does not see any gain from a header configuration then they will not market it. There WILL be a power gain if a header company as respectable as JBA offers a header package for a vehicle. However, the gains may not be large, they will be there. Jake, don't expect a mountain of more torque but expect something.
