I grew up in back seat of '75 2000GTV (with Abarth muffler), '77 Alfetta (with Ansa rear twin pipe).
Ain't no cure for that childhood abuse....
Out of all the cars my dad had when I was a kid (and boy he had some good ones) I remember the '77 Alfa sedan the most. It wasn't the fastest but it was a lot of fun. I'm surprised (and frankly relieved) that I didn't get into Italian cars.
Is it initial opening of intake valve (in some cases, well before TDC, while exhaust stroke is cleansing cylinder and there is still >atm pressure in cylinder), where the intake tract "becomes" and exhaust stack (due to cyl press, where piston is between BDC and TDC...etc), or is it the "draw" of postive>increasing negative>back to positive cylinder pressure during intake stroke?
I would defer to someone who actually has real-time experience with camshaft events to a greater degree but I would imagine that both pressure signals translate to sound to one degree or another. Take an FK-41 cam. It opens the intake valve when the crank is at 14* BTDC. That's a pretty short-duration cam and I would imagine that it wouldn't create much reversion. Conversely, a W-125 opens the intake at 26* BTDC. So it creates considerably more reversion at low speeds where the intake charge velocity can't over come it. I know from experience that wilder cams like that are a bit more honky, so to speak. Even more extreme, an FK-89 opens the intake valve at a whopping 36* BTDC and we all know what kind of sounds engines with THOSE cams make at low-mid-range speeds.
I would also imagine that the closing event has something to do with it. The air doesn't just stop moving when the valve closes. It bounces off the back of the valve and proceeds back up the manifold and carburetor.
Think about how an organ pipe works. The 'resonance' results from pressure waves bouncing back and forth in the pipe. Take an empty beer bottle and repeatedly smack the opening. It'll make a soft 'boink, boink, boink' sound. That's close to the resonance created by rapid changes in pressure.
Or is it the combination, along with the power/exhaust cycles, and the (sometimes) open path from intake stack to tailpipe that "sings"?
Well if you look at it, both the intake tract and the exhaust system are technically tuneable. In our cases it's not very feasible--the intake tracts would be long--like 20 to 22 inches long. That's the basis for ram tuning--you tune the resonant frequency of the intake to match the resonant frequency of the engine at its peak torque. That's why intake runners on modern injected cars are so long--they're tuning for a peak torque at like 3,000rpm.
Most of us are pretty familiar with exhaust tuning (though few people actually exploit it). The pipes resonance should match the engine's peak torque.
Both tuning methods exploit the elastic nature of air and resonant frequencies. Get them all right and you can make a lot of power. It's just really tough to do.
My current 94 x 78 with 271' (0.050), advanced 4 deg, and 9.9:1, I think sounds even sharper and more vociferous though.
Well advancing the cam opens the intake valve sooner so that creates the potential for reversion at lower speeds. That could be one. And the cam also shuts the intake valve sooner by duration AND advance. Because the intake shuts before the piston moves too far up the cylinder then chances are that the column of air moving down the intake tract is moving at quite a velocity when it bounces off the back of the valve.
Of course now I'm just pulling things out of my ass but it sort of makes sense if you think of the dynamics.
Like I alluded to pages ago, to me, more important than 1/4 e.t., mph, is how the car reacts when I put a demand on it, and its "voice." I'd trade explosive throtle repsonse at low-mid rpm for a limb, though I want that "come on the cam" nasal howl @ 4000-5000rpm as well. This motor (cam, mostly) does that.
Well here's something to ponder. Where does that cam make its peak torque? I noticed that my w110-cammed 2110 honked good from 3,000 to 4,000rpm. The W110 makes peak torque at about 3,500rpm (mine certainly did). It wasn't super loud but it was good and growly almost as if you could hear each induction event. After that the engine sort of screamed up to 6,000rpm.
Naturally a longer duration moves the peak torque to a faster engine speed. And at those speeds the engine noise is likely to drown out the induction noise, at least from inside the car. I distinctly remember what a big-cam engine sounds like from a block away at the street races, though. At lower speeds the intake howls more than the exhaust.
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