Tom 400 CFI

This is coming out of nowhere, but driving around in my truck caused me to think about it.

What is the primary CAUSE of induction noise?

I used to think it was caused by cam duration; when the intake valve was open beyond BDC, the rising piston pushes some air back into the intake. That would make noise when the throttle is open. Why I'm questioning that is this; My Corvette make very little induction noise, yet has a ~205-210 duration cam. It also has zero intake noise baffling and a very short/simple intake. The only "muffler" on the intake is the filter, which is a K&N, so not much noise cancellation there. Again...VERY little intake noise at WOT.

On the other hand is my Truck ('96 Vortec 350) which has a LOT if intake noise. It has a 194/196 cam (IIRC) which holds the intake open past BDC very little. It las a slightly longer intake tract, with two 90* bends, a MAF, an accordion tube, and a "box", "T'ed" off the intake tract to cancel noise. The air box itself also has a network of ducting to cancel noise, and then there is the pre-airbox segment that runs through the fender, and up under the headlight (multiple bends, cavities, shapes, length, etc. to cancel/absorbe noise.

The truck with everything installed makes mroe intake noise than the 'Vette, and with the air box open (but the other stuff all in place), it is 5x louder (to my ear) than the 'Vette. Intake noise only, I'm talking here. Both cars have stock or stock-like exhaust, which is to say, virtually silent.

I also had an '06 'Vette, similar intake design to the above ('92) but with a MAF, and much larger volume and TB...but it had virtually no intake noise either. Then I have a Caddy CTS-V (LS6) and it makes more intake noise than the '06 'Vette did...it has a slightly larger cam, but multiple 90* bends in the intake that you would think would quiet it down.

Last is carb'ed cars. With a dual plane intake, you should get nothing but smooth, even pulses from a WOT carb or TBI. Yet IIRC (it's been a while) hearing that same "pulsey" sound that you get from a single plane/EFI intake. Each side of the dual plane should get evenly spaced intake pulses.

What do you think about all this...besides that I'm geeking out on this stuff? Mainly, I don't "get" why my small cammed truck has SO MUCH more intake noise than my straight-forward intake equiped, MAP/LT1 'Vette. Doesn't make sense to me.

Damon

Having grown up with nothign but carbureted engines until I was a teenager I can tell you that the secondaries opening up (especially the huge ones on a QJet) made things dramatically louder. Buh...waaaaaAAAAAAAA! Call of the wild.

Cam timing and intake reversion had nothing to do with it. They all made the same sound. Chevys, Pontiacs, Fords, Dodges. Mild or wild, if it had an open element air cleaner on top of a big 'ol 4bbl, it roared. And I can't say I noticed much difference between single planes and dual planes, either. Both receive "even" intake pulses, just a matter of how frequently they hit the plenum area.

Near as I can tell, and I'm no engineer, it's simply a matter of how far the throttle plates are from the intake valve and how big the throttle bores are. If it's got big, honking throttle plates it's got more area for pressure waves (sound) to come out of. Like having bigger speakers. A long, torturous intake path like TPI engines have may dampen it somewhat as well. But generally if it's bigger, it's louder.

I also suspect that where and in what direction the sound is "pointed" can make a difference from the driver's seat, even if the sound pressure level is identical at the point of exit from the intake system.

Beyond that comes the "weird" stuff that deals mostly with the QUALITY of the sound coming out. Nothing to do with volume, but more about how the ear perceives it. That's where you get into the weird resonance tubes and such that are often installed in the intake plumbing of modern engines. Cancel certain "offensive" frequncies while amplifying others that are generally more "pleasing" to the ear. For example, later model Miatas have sound tubes that are "tuned" to send certain frequncies into the passenger compartment while denying others. Different intake designs are natively going to have different characteristics to start with so they "tune" the sound with different intake plumbing and resonators. And I'm sure in a truck application they are, in general, much less concerned with the sound levels than they are in car applications.

Tom 400 CFI

Good thoughtful post. Thanks.

I completely agree...which is why it seems odd to me that the truck does have SO much baffling/tuning in the intake...and why it's SO loud when only some of that removed. Then the 'Vette has a straight, smooth...even "trumpet shaped" intake tube...yet is so quite. That could be from what you pointed out below though...




The truck, all the runners enter the plenum pointing straight up and the TB is directly above 4 of them -offset to the front of the rear 4. Also, it has something like a 74mm TB which forms a pretty "gapping" hole, when open. The 'Vette, the runners face perpendicular to the TB and of course there are two smaller 48mm openings.

HOWEVER....then I think back to the CTS-V and the C6; both have similar intake manifolds (same design) and TB layout, but the C6 had a larger 90mm TB, a straight-shot induction tract that terminated under the hood, where the Caddy has the smaller 80-something mm TB, 90* intake, an air box, then piping out the front to behind the headlight for CAI...yet is still perceivably louder to the driver when you open it up, then the C6 was.

I think in general, you are right on with the idea of the TB size and orientation to the runner openings. That was an excellent point/thought, that I had missed. I was "stuck" on cam timing.

TreeFiddy

Fwiw, years ago a friend of mine years ago had a nice quiet V6 Commodore, removed the stock intake tract from the air cleaner to the tb, and stuck in a god-awful fat straight chrome bling-pipe with a K&N cone stuck on the end.

The result was a really loud annoying drone at all rpm above idle, but peaking at around 2k at cruise, right where u didn't want it. I'll go with Damon's tuning theory - the length of the intake was altered and straightened out, resulting in the resonant frequency of the tube in exactly the wrong spot. Just like a musical instrument.

Sure sounded like it had a trombone stuck up it, but he thought it was great.