My mate and i where just wondering what makes that flat 4 rumble noice from the boxer enginge?
we came up with an idea of what could make that "boom, boom, boom,boom noise - either it was the layout of the pistons or an unusual crankshaft?

any ideas?

P.S. i hope this is in the right section of the form?

 

I think its the unequal length headers.. but could be wrong

Phil

 

The exhaust manifold (headers) are unequal in length which means that the exhaust gases have different distances to exit the exhaust system creating a pulsing effect.

As near as I can get being non technical.

 

Yep sounds like the right answer and what a noise eh? absolutely fantastic aint it makes young girls lose the knickers at three miles LOL

 

so does half of Lager in Newcastle

Phil

 

Must not be local girls then Mdgrip lol that normally takes a double vodka and coke.

 

 

Anycase a deep throaty exhaust is still cheaper than a Vodka and Coke least it is on a fri and sat night in the toon.

 

As well as the un-equal length headers, I think it has something to do with them being cast as well..... (waiting for techie nerds/guru's to shoot me down in flames though )

Matt

 

thought it had something to do with the pistons being horizontally opposed or something???

 

Lol, the fundamental reason is as stated up top, the unequal length of the exhaust headers is the main thing responsible for the signature Subaru engine note.

It's because the engine's a flat design with the turbo off to one side that it ended up with unequal manifolds in the first place, so while the horizontal configuration isn't directly responsible for the sound, it probably is fair to say that it played an indirect role.



[Edited by greasemonkey - 1/27/2004 8:17:58 PM]

 

Is that why Sport's don't have the same sound?

 

no - sports do...

you just have to change the zaust to hear them...

 

woo-hoo, so i'm maybe right in what i said?????

 

Lol! It all depends on how loosely you define "something to do with"!

BTW, regarding the Sports, I don't know whether the 2.0 litre cars have the same exhaust manifold layout as the American 2.5RS or not, but the RS has equal length manifolds and doesn't make the signature "burble".

 

greasemonkey

the gl series also have the rumble although not as pronounced as the turbo , the exhaust headers enter the cat in an lopsided y configuration, giveing the odd exhaust note,

another method is the 4 into 1 single feed aka saab method

this also gives a characteristic burble

quick Q though

if alfa,s have the flat four configuration, why do they have a
sharp crackle sound to there exhausts?

Mart

 

Put an equal length manifold on and gain 20 bhp. But lose the burble just like the sti8 spec c

Cheers

 

...and as i was kindly informed last week on here, very soon all new Imprezas in the UK will have equal length headers. Therefore, no more flat four burble in the future.

 

If anyone wants a really techy explanation then I'll post one (with graphs and all sorts) but in a nutshell greasemonkey is spot on, it's all to do with how the standing waves (acoustic) are modified by the unequal length geometry. Does anyone care that much?

 

Not the end of the world. WRC cars don't "burble" either, but I don't hear anyone complaining about their engine note.

Thanks Mart for the heads(no pun!) up on the N/A cars. Always wondered what the arrangement was there.

BTW Matt, I was considering unleashing a really techy explanation, but didn't think that anyone who'd posted so far had done anything unpleasant enough to deserve it!

[Edited by greasemonkey - 1/28/2004 12:07:21 PM]

 

No probs

Mart

 

Do your worst!!!

Matt

[Edited by mutant_matt - 1/28/2004 1:20:39 PM]

 

I ahve herd that the burble is the un-equal length manifolds! so they are firing like 1 then 2 then 3 then 4 for example unlike all together on normal cars!
i think!!

Got any mechanics out there that could help to be more precise/accurate answer! please! it might help commin' from an expert in the matter, or even better a subaru mechanic would be better still!

maybe!.....

 

if my memory serves me correctly,

a conventional 4 in line vertical engine has the following firing order

1342

i believe the scoob is the same, but i cannot confirm if 1 and two cylinder are on the same bank, or opposed.

The exhaust pulses will then have differnt timings/ distance to travel to the turbo/ cat system. this will also have an effect on the return pulses to the cylinders (this has been explined before)


hope this helps

Mart

 

Will post a techy reply then somewhen but I'm too busy having fun in the snow at the mo!!

 

The path from the four cylinders to the tailpipe is even more complex on the twin turbo versions of EJ20. Think two up-pipes, a cross pipe between the turbos, and two downpipes that join under the car before feeding the centre cat.

Still makes a nice uneven burble though

Anyone know what the twin turbo six cylinder one sounds like?

Rich.

 

Someone said that equal length gives 20 odd hp increase - if that's the case (is it?) - then why didn't subaru do it in the first place? Is it a lot more expensive?????

 

The firing order on the production Subaru flat fours is 1-3-2-4, and the cylinders are opposed.

There are probably multiple reasons behind the original decision to use an unequal manifold. The main ones were probably cost and ease of packaging/manufacture. Of course, the ease of packing and manufacture also influence costs, so it's all down to money at the end of the day.

[Edited by greasemonkey - 1/29/2004 4:34:29 PM]

 

greasemonkey,

I've heard loads of US/Canada spec 2.5RS's with the flat four burble. Not all of them had big exhausts either...

 

Greasemonkey,surely if the pistons are opposed then two cylinders fire at the same time and then the other two(1-3 & 2-4)thus the need for less counterbalance on the crank ,it's what was called the 'big bang' on gp two stroke bikes. So with unequal length headers the exhaust pulses are grouped together in pairs ,just slightly out of synch,with equal length headers the sound is soley down to two pistons firing together making it sound more like(but still different than) a v twin no?