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More power, less noise??

General engine tech -- Drag Racing to Circle Track

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travis
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More power, less noise??

Post by travis » Fri May 15, 2020 6:50 pm

I noticed something interesting today. Maybe someone can explain...

I’ve got a pair of Pypes M80 mufflers, which are 14” straight through perforated core “mufflers” (they don’t muffle much). Behind the old nearly stock 351w with shorty headers and either dumps or exiting out in front of the rear tires, the noise level bordered on ridiculous. Now, with 1.5 points more compression, much better heads, long tube stainless headers, etc and probably another 150 HP, it is much quieter. Certainly more crisp and “intense” sounding now, but the crazy dull booming exhaust is gone.

I expected it to be much louder now, but quite the opposite is happening. What is going on here? I get the same results with the chambered Flo-pro mufflers as well.

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Re: More power, less noise??

Post by GARY C » Fri May 15, 2020 10:09 pm

Compression would have an effect on ex gas speed and may provide a more complete burn before evo, changing the evo would effect sound as well.
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Re: More power, less noise??

Post by travis » Sat May 16, 2020 4:02 am

GARY C wrote:
Fri May 15, 2020 10:09 pm
Compression would have an effect on ex gas speed and may provide a more complete burn before evo, changing the evo would effect sound as well.
That’s kind of what I was thinking, that maybe with the low compression and the long exhaust duration of the stock FI cam with the wide lsa things are still burning when the exhaust valve opens.

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Re: More power, less noise??

Post by naukkis79 » Sat May 16, 2020 4:48 am

Less compression ratio mean less thermal efficiency - there's more pressure in cylinder when exhaust valve opens. 4:1 compression ratio flathead with straight pipes is loud, like continuous shotgun firing as most of charge energy is released from exhaust.

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Re: More power, less noise??

Post by Schurkey » Sat May 16, 2020 1:43 pm

More energy used to push the piston down, less energy available to disturb the air molecules at the end of the tailpipe.

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Re: More power, less noise??

Post by PackardV8 » Sat May 16, 2020 2:25 pm

A while back I attended a vintage sports car race. In one class, by far the noisiest exhaust was one Porsche 911; also the slowest in his class. The faster 911s were much quieter than this guy.
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Re: More power, less noise??

Post by Orr89rocz » Sat May 16, 2020 2:44 pm

That is strange. But i can see long tubes in stainless also changing the temp of exhaust with more surface area and possibly changing the sound frequency that is heard

But also could the muffler just be more effective when there is more volume flowing thru it?

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Re: More power, less noise??

Post by Tuner » Sat May 16, 2020 7:53 pm

travis wrote:
Fri May 15, 2020 6:50 pm
I noticed something interesting today. Maybe someone can explain...

I’ve got a pair of Pypes M80 mufflers, which are 14” straight through perforated core “mufflers” (they don’t muffle much). Behind the old nearly stock 351w with shorty headers and either dumps or exiting out in front of the rear tires, the noise level bordered on ridiculous. Now, with 1.5 points more compression, much better heads, long tube stainless headers, etc and probably another 150 HP, it is much quieter. Certainly more crisp and “intense” sounding now, but the crazy dull booming exhaust is gone.

I expected it to be much louder now, but quite the opposite is happening. What is going on here? I get the same results with the chambered Flo-pro mufflers as well.
The higher compression ratio has a flip side, it is also a higher expansion ratio. Because of more expansion before exhaust valve opening, higher CR engines have lower exhaust temperature and pressure when the exhaust valve opens, and so less noise.

NASCAR rules idiots had a hard time getting this through their pea brains when they required Bush and Truck engines to have 9.5/1 for several years during the mid-late '90s, as a result the high exhaust temps were responsible for lots of engine failures and the speedy-dry people prospered. If you recall, failures were common after a yellow or a pit stop, when low speeds reduced cooling and light load raised exhaust temps further.

This may seem ironic but lower compression ratio engines benefit more from vacuum advance than high CR engines because the added advance compensates for the slower burning part throttle combustion and reduces the elevated exhaust temperature which results from low expansion ratio.

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Re: More power, less noise??

Post by BOOT » Sat May 16, 2020 10:14 pm

Are the SS headers thicker gauge?
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Re: More power, less noise??

Post by travis » Sat May 16, 2020 10:19 pm

BOOT wrote:
Sat May 16, 2020 10:14 pm
Are the SS headers thicker gauge?
I believe they are the same as the old shorties. Both are stainless JBA products

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Re: More power, less noise??

Post by Cobra727 » Sun May 17, 2020 3:48 am

I understand the higher compression theory, but wouldn't a race motor actually be louder due to the amount of overlap they have..
I would think the more overlap the more noise in total you would hear, because the engine combustion process is wide open longer.

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Re: More power, less noise??

Post by naukkis79 » Sun May 17, 2020 6:12 am

Cobra727 wrote:
Sun May 17, 2020 3:48 am
I understand the higher compression theory, but wouldn't a race motor actually be louder due to the amount of overlap they have..
I would think the more overlap the more noise in total you would hear, because the engine combustion process is wide open longer.
Long overlap only exposes exhaust to intake vacuum - and with enough overlap there's not much of it. But with some overlap you could hear in idle and light throttle bass boom which comes when gas is sucked back to cylinder from exhaust. But that boom is pretty minor noise compared to sound that comes out in full throttle when leftover cylinder pressure purges out.

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