Technical Engine sound discussion

Posted by someone many years ago:

The classic Ferrari 'sound' comes from a number of interacting elements that line up n a synchronized chorous.

First comes the flat plane crankshaft. This gives each cylinder bank an even firing order LRLRLRLR whereas the typical american V8 has an off kilter firing order LRLLRLRR. The even firing order means that the intake and exhaust pulses are also synchronized and evenly spaced.

Next comes the headers. These headers are tuned for the power band of the engine, not too big, not too small, not too long not too short. The headers cause much of the low frequency rumble from these engines by setting up a standing wave pattern of pressure pulses.

Next come the intake system. Here the air path is essentialy straight (e.g. no turns) that enables the air to flow into the cylinders with minimal resistance. The straignt air flow and low resistance enables the velocity stacks to be tuned with the header to broaden the power band.

As the intake valve opens (with the still open exhaust valve) air begine to travel into the cylinder even before the piston starts dwonward from the negative pressure wave from the header. As the intake valve closes, there is considerable momentum in the air flow. When this reaches the closed intake valve, pressure builds until a positive pressure pulse runs up the intake and velocity stack finds air at atmosphereic pressure and sends a negative pressure wave back down. This negative pressure wave runs into the intake valve and sends a negative pressure wave back up the intake and VS, causing a subsequent positive pressure wave. much of the high frequency music of these engines comes directly from this process.

Just under the resonance point of the header there are an even number of intake cycles so that when the intake valve opens there is already positive pressure and momentum to take the fresh charge into the cylinder (also) even before the piston starts downward. Just above the resonance point of the header there is another even cycle count in the intake puls train. There are generally 8 or 10 cycles on the low intake resonance and 8 or 6 on the high RPM intake resonance. These broaden the powerband of the motor.

Finally, the throttle plates are positioined at the center of the intake path and at anything less than WOT damp out even order harmonics in the intake resonance so one hears the primary, 3rd order, 5th order,... of the air movements turned into that sonorous chorous. Even order harmonics end up sounding like an amplifier with cross over distortion, while odd order harmonics sound like an amplifier cliping the peak. Any music buff will tell you that odd is much better than even in sound quality.

The modern V8 engines also employ a 2 stage set of resonators. The air box covering the intake tracks is tuned such that at header resonance a positive pressure wave is above each intake velocity stack just before the intake valve opens. The great rush of air into the cyclinder reinforces this 'standing' wave resonance. These are known as helmholtz resonators.

At the end of this resonator is a smallish tube leading to the largish air filter box. The size and length of this tube damp out the resonator standing wave such that the mass air flow sensor gets a nice smooth flow of air and can thereby be used to give precise control to the FI system.

The difference betewwen Ferrari systems and other manufactures, is that Ferrari works to get the sizes, volumes, and resonance points to actually line up and sing. Whereas others just get them close enough to deliver the power.

 

Not all multi-bank F engines have flat crankshafts (only the 90 deg V8s), but what they do all have (that is the largest contributor to the sweet engine sound in multi-bank configurations) is "alternate bank firing". The reason most auto manufacturers don't do this in their multi-bank engines is:

1. the peak loading on the crankshaft main bearings is higher (so needs a more stout bottom end), and

2. every cylinder has the same performance vs RPM (so you get more in a certain RPM range from reasonance effects, but less outside that range) -- i.e., engines without alternate bank firing have a lower, but wider performance vs RPM (because resonance effects happen at different RPM ranges for different cylinders).

 

That would be me.

 

My understanding is that the F355 has 5 valves per cylinder and that's why it sounds so good. Why did they stop the 5 valves set up?

 

360 has five valves per cylinder as well.

 

Cooling the heads. While a 5 valve motor has better airflow than a 4 valver, the cooling of the heads becomes critical, so the designer has to reduce compression and retard timing to the point that it makes no more real power than a 4 valve head.

Also note: Yamaha that developed the 5-valve technology that worked with Ferrari on the F355 heads have also switched back to 4-valve heads while litre bikes continue to see their power levels increase.

 

Our 1300 HP 259 inch Chevrolet L6 powered Bonneville streamliner HP was limited by valve temp. Kept siezing the valves in the guides on the dyno.

 

Perhaps it should be mentioned that there are many other factors in "engine sound" other than just the number of valves, and crankshaft/cylinder layout:

* Intake system - carbs versus f.i., length and size of intake runners, air filters, etc.
* exhaust system - length, size of headers, muffler resonance, position of exhaust exit.
* turbocharger(s) if present
* mechanical issues - valvetrain sounds, resonances
* in air-cooled engines, the cooling fan

And now for my rather controversial notion - I was somewhat disappointed to hear what I take to be a rather ugly "rattling" resonance in both the 599 and FF cars, when revved up past 5000 at full throttle... The Enzo does not seem to have this - I take it to be an exhaust system tuning issue.