I have learned a few things in regards to the n54 cylinder head and performance.
First, spring pressure is not just about engine rpm. The stock springs have a low seat pressure (approx. 65-68lbs @ 36.5mm), low spring rate (under 200/in iirc), and tall coil bind height. Revving the stock springs 7500 on stock cams is fine if your boost is moderate and turbine housing/wheel is larger. Depending on your turbo setup, once the boost gets higher and the EMAP starts to climb quick you run into problems. It is fairly easy to see 50+psi of EMAP at 30psi of boost (depending on turbo) and 60+psi of EMAP on 35+psi... the smaller the turbo the worse this gets. Ever been on the dyno and for the life of it you cant figure out why the power just rolls over at 6000-6500 no matter what? And is exaggerated worse at you turn up the boost? The EMAP is through the roof and the hard dip in torque is the exhaust valves not fully seating... not from rpm, but because the pressure on the back of the valve it equal or greater than the springs pressure.
The next focus is cam lift and duration. I can tell you that schrick high lift cams are great for mid range power/spool and "good" for modest boost levels at high rpm. They suck for big boost, big power, high rpm even with a fairly large turbo (to the n54 world). If you're planning to go north of 850hp on a decent size turbo the high lift cams are too big. Sounds funny, but I'll explain.
Picture yourself on the dyno working through the tune. You've done everything "right". Built shortblock, ported head, stiffer springs, big cams, big turbo, etc, etc. You're loving the quick spool, improved midrange, and power/psi. Everything is going well and making quick gains at 20, 25, and 30psi. Then 32, 34, 36, etc are making gains, but small and torque is rolling over hard and all your "right" parts still makes peak power below 7k. I see it all the time. It's amazing how many posted dyno graphs look exactly the same after 6500.
In the case of the schrick high lift cams the lift and duration increase is so much the same centerline results in a ton more overlap. This is where all that spool and midrange you're seeing is coming from. However, as the MAP and EMAP get high we need to pull the cams apart and remove the overlap; simple enough. Except the cams have enough lift and duration that even with the cams separated there is still a good amount of overlap. This overlap period during high emap conditions kills power and the emap compounds.
I have modified cam gears to allow more cam movement and get more overlap out. However, I've found that by the time I get the cams far enough apart to remove the overlap they dont fill and exhaust the cylinder very well; I've just substituted one problem for another.
So far the meal ticket has been less lift and less duration for high rpm, high boost, high power. I'm still working my way through all my regrinds, but it's going in the right direction.
It should also be noted that the OTS Ferrea springs arent really much of an upgrade. They offer more cam lift without bind, but seat pressure is close to stock. And if you're running high lift cams, then you dont have room to shim them up.
Just thought I'd share my experiences and workflow. Good luck with the build.
