luv2golf

I understand how a cam works, it just changing the valve opening point and duration. But why does that make the motor louder?

The NNIAL8R

It doesnt...people just usually change the exhaust when they add a cam package

Silverspeed

New cam = engine pumps more air = louder exhaust

cranky

cam controls the valve action and timing. better cylinder filling = more compression, bigger BOOM sound. engine is louder. simply put.

Vet

The only thing that might make any sense, in theory: it's possible that due to longer exhaust valve duration, the combustion chamber is effectively open longer per cycle allowing more sound to escape. And due to a greater overlap of a typical larger cam, at higher rpms you may actually hear extra combustion sound escaping through the intake as well.

Bigger cams may make for improved "cylinder filling" at higher rpm, but do not make for more compression. Bigger cams actually reduce effective static compression due to a later intake valve closing point (before the compression stroke)... there's less actual physical compression taking place.

Bigger cams also make for more effective overlap, overlap causes the intake charge to be diluted with exhaust, as well as allow some intake charge to be spit out the exhaust before combustion, thus less potent combustion and thus less peak combustion pressure. So, theoretically, the combustion itself might only be less loud.

Overall, if an engine with a "big" cam seems louder than the same engine (same exhaust) with a smaller cam, then the only thing that would make any sense is that the valve ports (both intake and exhaust) are simply open longer per cycle, allowing more sound to escape from the combustion chambers, per cycle.

But I think the reality is, in most cases, when guys swap cams, they also swap on headers at the same time etc, and that's why the switch to a bigger cam seems to make the engine louder.

Dif

Ratchet Thunderbolt

A cam can make an engine louder. I've done extensive testing in an anechoic chamber with a motoring dyno driving a non firing engine with working cylinder heads but no pistons. When you change cams you have probably changed the angle of the grind on the cam. Sometimes this will affect closing velocities of the valves. Stiffer springs are added sometimes. The impacts of the valves on the seats make noise. Steeper angles and stiffer springs, louder impacts, louder valve train. I as always amazed at how much the valve train contributed to the over all engine noise.

Vet

Wow, very interesting. Cool!

Previously I had not been thinking about mechanical noise, but only combustion noise.

It surely makes sense that steeper lobes and stiffer springs are going to make those valves slam closed a lot "harder", and when you're dealing with 16 or 32 valves all closing quite a few times per SECOND, that noise is surely going to add up.

brian90505

You guys are right about mechanical noise. Especially in the older engines when changing from a hydraulic to solid-lifter cam.

But I have a feeling that the OP refers to sound out of the pipe.

LS2-Zomb!e

cranky

ever kick started a harley engine over after a big cam change ? ever taken a compression reading ? you might be surprised at the result.

z51vett

The sound you are hearing is the combustion process taking place because of the value overlap. Meaning its open while cylinder is still firing. Thats a very simple way of putting it.
z51vett

Vet

That's interesting.

I have swapped many cams on American V8 engines and have taken cranking compression pressure readings before and after. Any cam with a later intake valve closing point yields less cranking compression pressure than one with an earlier intake valve closing point. And typically (but not always), a "bigger" cam will have greater intake duration and typically a later intake valve closing point (than stock).... this depends on the lobe centers too of course... all grind specific.

It's possible that the high performance Harley cam you worked with has a "short" intake duration (early intake valve closing point) specifically to boost compression (for more low-end power), but a larger exhaust duration to improve high rpm torque etc.

Or, it's possible that the Harley cam has larger intake duration BUT tighter lobe centers which would effectively place the intake closing point RELATIVELY earlier even though the duration is greater.

I am not familiar with custom cam grinds for Harleys. I know those engines (with offset crank pins) are unique beasts and I can imagine they may "require" a less than typical cam grind for optimum performance.

Did you by chance advance the new cam?... if so, this alone may have placed the intake closing point early enough to yield greater cranking compression pressure than the cam that came out (as installed).

I've done extensive testing with V8 cams back in the day trying to specifically REDUCE static compression pressure so that high compression ratio engines could be run with 93 octane pump gas. I was swapping and adjusting cams and checking compression pressure over and over.... I think I wore out three compression gauges... and several batteries.

Using a "larger" cam with later intake valve closing points always reduced pressure / reduced detonation tendencies... or you could even just take the existing "small" cam and retard it for the same effect. (But ultimately, the "best" way to deal with an overly high compression ratio engine (when only 93 octane is available) is to simply reduce the compression ratio... then you can run an "optimum" cam grind and optimum spark advance etc without detonation.)

With all this in mind too, you may remember that in the late `70s (once compression ratios dropped into the bucket) lots of guys were specifically advancing their existing cams and this would yield a nice little noticeable kick in the pants in the lower rpm range... why?... effective increase in static compression pressure... pretty much the same as raising the compression ratio a small bit. Advancing the cam of course places the intake closing earlier.

I had come up with some general formulas for big cubic inch (big stroke) V8 engines based on all my first hand measurements... if I remember correctly, making the intake valve closing point 4 degrees earlier increased cranking compression pressure by about 5 psi, and raising the compression ratio by about 1 full point raised cranking compression pressure by about 20 psi... or thereabouts... averages.

With these above very general formulas in mind, if you were to swap in a "larger" cam with an intake closing point 10 degrees later than stock (as installed), you would drop cranking compression pressure by about 10 psi... this would almost be the same as dropping your compression ratio by about a half point in terms of effective pressure.

Side note... this above info is NOT based on Chevy smallblock engines and very well may not apply accurately to our Corvettes, but just mentioning all this as a general example of compression pressure factors.