[BMRLVR]

As promised here is as an explanation of how resonance tuning is used in naturally aspirated engines to broaden a torque curve and increase power. Before I discuss resonance tuning though, I have to explain a few principles first.

The reason some engines produce lots of torque and not a lot of horsepower is due to the fact that they have a very narrow operating range and are unable to maintain torque at higher RPM. This is the reason why many engines have lower redlines. Variable valve timing, variable valve lift, and variable length intake manifolds were all developed to allow engines to have a wider operating range or in other words a broader torque curve. Horsepower is Torque x RPM / 5252, and you can't increase power without increasing torque. So realize that any modification the increases horsepower either increases torque or allows the engine to make torque higher in the RPM range.

1 Lb-Ft at 2000 RPM equals 0.38 HP, the same 1 Lb-Ft at 5252 RPM equals 1 HP, and everywhere after 5252 RPM horsepower is being multiplied by RPM. In the S65 at it's 8300 power peak every Lb-Ft equals 1.58 HP. I hope I am making my point on horsepower and it's relation to torque clear. (If you would like to find out how much torque any engine is making at a given HP output you can use the equation HP / 5252 x RPM = Torque).

Every naturally aspirated engine has a natural resonance where pressure/sound waves aid in filling the cylinders is greatest. The reason for this is as the intake charge is travelling down the intake manifold runner, the intake valve suddenly slams shut this column of air is stopped dead. This sudden stop of the column of air creates a pressure wave that travels back up the intake runner at the speed of sound and enters the plenum. Once the wave hits the walls of the plenum it reverses direction and flows back down the intake runner and all of the other intake runners in the engine still at the speed of sound. This pressure wave (also described as the Helmholtz resonance effect or acoustical intake tuning) is above atmospheric and creates a supercharging effect to the engine when it is timed properly with the opening of the intake valve.

In older engines the intake runner length and plenum volume was decided upon based on where the engineers wanted the peak volumetric efficiency (torque peak) to occur. The reason for having to carefully decide runner length was because valve timing was not variable and the rate of opening and closing depended upon RPM. The pressure waves, however occur at a static frequency dependant upon the speed of sound and the runner length/plenum volume.

The reasoning for developing variable valve timing and variable length intake manifolds were invented so that engines could better take advantage of this supercharging effect at more than one specific point in the rev range. With a variable length intake manifold there are two or sometimes three lengths of intake runners which allow the engine to have a much broader torque curve since this supercharging effect can now occur at more than one point in the rev range. The reason for this is as the runner length and or plenum volume is changed the frequency of the resonance also changes. This allows the pressure pulse to be occurring when the valve is open at two or three specific points in the RPM range. The variable length intake manifold is also used in conjunction with variable cam timing to actually be able to flatten the torque curve somewhat too (since the intake valve can either be opened later or earlier depending upon when the pulse/pressure wave is occurring.

The majority of vehicle engines are not nearly as advanced as the S65 from a computing standpoint. Since the DME on the S65 is able to do 200 million calculations a second, and the cam phasing can be carried out extremely quickly and over a wide range of advance and retard it is able to use the resonance tuning to its advantage strictly by using the cam timing. Basically at low RPM's the cam timing is retarded quite a bit but as revs increase the cams are advanced at a rate that allows the engine to take full advantage of the pressure waves. The fact that the torque curve is nearly flat means the DME and cam phasers are doing a very good job of adjusting the cams over the extremely wide operating range. By the way the plenum volume and trumpet diameter was just as important in the whole design phase of the engine as anything else. I am quite sure that many smart Germans did a lot of dyno testing, math, more dyno testing, and more math to come up with the optimum volume for the plenum and the optimum length, diameter, clocking and air horn radius of the trumpets. To answer your question above about the trumpets and the resonance effect, the trumpets act just like an intake runner in a conventional manifold.

The thing is, this engine may have been able to make more torque, more peak horse power or both, but the compromise that the engineers gave us was, good peak torque for the displacement (the S65 actually makes more torque per litre than a Z06 corvette, C63 AMG, or Lexus ISF to name a few), an extremely wide operating range (extremely broad torque curve) and a very high specific output (103.5 HP Per litre)




I hope that I didn't ramble on too much, but basically, next time you look at torque curves of naturally aspirated engines, the flatter the torque curve the better a job that that engine is doing to maximize volumetric efficiency to broaden the torque curve through resonance tuning.