BCNUL8R

iTrader: (2)

I found some old sound clips and put this video together. Literally the only difference is a fresh rebuild with custom dished pistons to lower compression. 11.4 to 1 static vs 9.5 to 1

stockA4

That's very interesting, I wonder if a different exhaust setup would make any more of a difference in the note, I recently swapped out my 9.5:1 4.8L for a 11.1:1 6.2L and I'm re-using the same cam from the 4.8 (212/218-115lsa) the sound is very similar but the overall note is much deeper. Thanks for posting the video

AwesomeAuto

The pulses will sound faster with higher compression.

Darth_V8r

iTrader: (4)

Can absolutely hear it. At 12.5, you can hear each pop. Unmistakeable

G Atsma

If any of us remember the bad old 70's and early 80's, hearing an 8:1 compression engine with open headers had no "crack" to the sound. More like a lazy rolling "cough".

98CayenneT/A

true story
High compression has a nice sharp, crisp bark as opposed to the lazy rolling cough as G Atsma described.

dreadpirateroberts

iTrader: (1)

Subtle, but there is definitely a difference. Wow.

BCNUL8R

iTrader: (2)

Exhaust on these clips was stainless works 1 3/4" primary long tube headers, off road mids with X pipe, slp cat back. I think back then it was called the power flow which was their quiet catback. I'm guessing the difference would be even more noticeable if it had a louder or open exhaust. I thought it was worth sharing since this literally is the exact same setup cam and everything except rebuilt with dished pistons after I hurt a piston with nitrous and decided to lower compression on the rebuild for a future pump gas procharger build. Seat of the pants wise it was a noticeable loss of low end torque and throttle response as well, but never dynoed on the low compression motor.

Darth_V8r

iTrader: (4)

Here was the first start on my 428 build with 12.7:1 compression. Ignore the WD40 burning off the headers, etc. You can definitely hear the compression difference vs your normal cam only stuff. Too bad this motor only lasted thirty minutes

BCNUL8R

iTrader: (2)

What happened to it?

Darth_V8r

iTrader: (4)

I effed up one intake pushrod and it bounced out of the seat, broke the lifter link bar, spun the exhaust lifter on #6, metal everywhere, etc, etc.

I gotta sleeve the block it got so scratched up.

wannafbody

iTrader: (2)

Lower tone vs higher tone

AwesomeAuto

The exhaust pulse width is actually less with more compression. It comes out with a nice quick snap or pop.

Darth_V8r

iTrader: (4)

Yeah. Its a very distinctly sharp sound in person.

BCNUL8R

iTrader: (2)

Yep, I'd say it's more noticeable in person. I love the super high compression race gas motors at the drag strip. Not only the idle sound, but how fast and snappy they rev.

TimsLS1

Thanks for posting I was surprised,

The deeper tone and pitch of the low comp in video makes it sound more angry.

The high comp is like a kid with a high voice trying to mimic it's Dad with a low voice.

G Atsma

The high comp sounds like it does because it is faster burning, putting more energy into pushing the piston down.
Low comp has a hotter, slower burning, lower pressure exhaust going out, having used less energy to push the piston down.

TimsLS1

Thanks for that.
I find it interesting because it's a natural bias for a performance enthusiasts to say anything the equates to more performance sounds nicer. If it was a blind test I wonder if the results would be the same.

It's the similar with a lumpy idle, I love the sound, but to those unaffiliated it sounds to them like a bucket of loose rattling bolts. Then there is a 3rd catagory who don't like a lumpy idle sound but forgive it because they know it's usually an indicator of high performance.

It's all subjective but interesting.

Darth_V8r

iTrader: (4)

I like hearing them loaded up at the gate, pushing against the trans brake. Even though I Ams three pedal guy through and through

stockA4

Dynamic vs static compression via the Intake Valve Closing point determines whats trapped at low speed so in the video with the cam being the same all the compression lost was also dynamic. An early exhaust opening point makes it sound tougher and then there's that overlap lol