There are several things choking this engine at the upper RPM range. But the major component that gives a graph it's shape is the cams. These are fairly small cams for good low end torque and to make to car feel faster than it is. The timing of the cams can be altered (advanced) to move the curve upwards a bit but really it's the small cam with very little overlap that is causing this. It's great for emissions and low end torque but hurt's high end HP.

The other two factors are the absolutely crappy exhaust manifolds coupled with the single exhaust valve, they are going to be restrictive at the upper rpm range where the engine is trying to push a large volume of air.

The next factor is going to be the intake manifold. A dual runner manifold like we have is a comprimize, the long runners aren't long enough for a true torque monster, and the short runners aren't optimized for high end HP.

Also tuning has a lot to do with it. The ignition advance is severly retarded (as in late during the combustion cycle) on this engine to move the flame front to the end of the combustion process to increase EGT's, and thereby light off the front cats sooner, to (again) lower emissions. I believe I read in MB tech specs on this engine that they retarded the ignition timing 10deg across the board to do this. They were able to retard the timing that much and still get complete burn by having the dual spark plug setup, with a single plug setup they said they couldn't have done this. (also, the next factor is going to limit the amount of advance you can put back into the tuning)

The last factor is the absolutely tiny fuel injectors on this car. They are speced very small (19lb), and the reason being is because this car is spec'd to run a very lean mixture (someing they can do, again, because of the dual spark system). The lean mixture helps to raise EGT's and again light off the cats faster...as you see another emissions concern.

Go to http://www.rceng.com/technical.aspx and you can put in 240hp (our guestimated crank HP), our BSFC (good guess of .45), 6 injectors, 56psi, and .80 max duty cycle and see it comes up with a 19.xxlbs injector. We really don't have the fuel available to go higher safer without larger injectors. If you increase the airflow to the engine and thereby the HP all we are doing is running our injector's past .80 duty cycle, and at that point most injectors spray pattern is getting wonky and the injector will wear out faster causing leaks and the possibility of one sticking open.

If we got more fuel we could run more ignition advance and gain a bit of power there, but the cams/intake/exhaust will be the real limiting factors in the upper RPM range. You can try www.crower.com ($920 for our cams + the cost of a set of factory fresh cams from MB) or www.webcamshafts.com (unknown cost, but they have a pn for MB of 42-020 for MB V6 cams) for a set of reasonably priced custom re-grinds (re-grinds are getting better and better and I wouldn't really be that concerned about them any more). Also, a REALLY good custom shop can build you a set of headers and an intake manifold (I got a quote of $1800-2500 for headers from a shop I had look at our manifolds). And lastly you would really want your exhaust ports fully worked to increase flow.

All that should bring this engine on par with an early vq35de (350z) engine and put you at around 280ish crank HP. But you're talking about lots of money to get there. And the limiting factor is going to be your tuning, these Bosch ECM's are tough to crack. A few companies claim to be able to tune them but I have yet to see definitive results...just claims.

So as you can see all the factors hurting the HP and torque are because of engineering trade-offs to lower the emissions. There's a lot of things coming together to rob this engine of power... But the damn thing sure does burn clean!!!!