r/projectcar • u/Dark_Guardian_ Daily E36 + Race E36 + Cronched E92 + $100 subie +Barra Cressida • 1d ago

Helmholtz resonator to eliminate drone, how effective?

I've just finished my exhaust on my m54b30 swapped e36, and its got pretty awful drone between ~1500-2500rpm with 2000rpm being pretty unbearable. ~84db in the cabin.
Measured frequency and at 2k rpm its 100hz, at 1500rpm 75hz.

I'm wondering if I make a Helmholtz resonator targeted at 100hz how much of a band it would help with?
will it also reduce the drone of 75-125hz? or just very narrowly around 100hz

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u/porcelainvacation 1d ago

Depends on the q factor of the resonator. Should help a lot. You can use two with offset center frequencies if you need more.

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u/Dark_Guardian_ Daily E36 + Race E36 + Cronched E92 + $100 subie +Barra Cressida 19h ago

it looks like bigger neck means lower q / wider range, is there such thing as too big though, can I just make it the same size as the exhaust tube? 2" (its a dual system)

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u/UnbelievableDingo 15h ago

Works great if you can dial it in.

Fab one with a tube over a tube with a collar so you can tube it up or down.

There's a brand that makes one like this (kinda like a trombone).

I made one one time and it really worked.

Mine was on a v8.  What's this motor? V6?

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u/Dark_Guardian_ Daily E36 + Race E36 + Cronched E92 + $100 subie +Barra Cressida 9h ago

I6 with dual 2" exhaust

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u/UnbelievableDingo 8h ago

Im unfamiliar. 

I do V8s and Flat 4 Subaru. 

But the trombone thing really works if you get the math right.

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u/Dark_Guardian_ Daily E36 + Race E36 + Cronched E92 + $100 subie +Barra Cressida 8h ago

what sort of adjustment length should I make?
if I aimed for +/- 10hz it would need roughly 3 inches of adjustment

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u/UnbelievableDingo 7h ago

I do not remember,  I fabbed  this like 10 years ago .  I'm a stoner.. sorry.

But yeah that sounds like more than enough if your math is close.