andrew6321

This is probably really ignorant of me, but:

When I'm asked why Subarus deliver that 'offbeat' exhaust note, I've always replied "oh, its to do with the flat four layout." But, anything beyond that and I'm stumped.

So, given that an ordinary in-line four doesn't make this noise, is it to do with the firing order, crank angle, or what?

Let's hear the theories.....

Squizz

Erm, flat four engine, plus unequal length exhaust manifolds...

The Zohan

Squizz is on the money here, i concur (like that counts for much).

Paul

andrew6321

Er, call me 'Mr Thickie', but I'm still none the wiser....

Why would unequal manifolds make it sound like its only firing on two cylinders? (hang on, maybe mine IS only firing on two cylinders...!)


[This message has been edited by andrew6321 (edited 28 February 2001).]

The Woodman

Erm... Crankshaft angles totally different to an inline four... I think? 1 and 2 at TDC together, whilst 3 and 4 at bottom on a flat four. (1 and 4 work together on in-line engine.) Thus also different firing intervals? Could well be wrong though????

logiclee

Having unequal length manifold means the noise from two cylinders is entering the exhaust at nearly the same time, then a gap etc.

The latest WRC cars have an equal length manifold and sound very un-Impreza like.

Then again I could be wrong.

Lee

mutant_matt

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:<HR>Originally posted by logiclee:
<B>Having unequal length manifold means the noise from two cylinders is entering the exhaust at nearly the same time, then a gap etc.

The latest WRC cars have an equal length manifold and sound very un-Impreza like.[/quote]

Lee, that's exactly as I understood it.

Matt.

Squizz

That's what I thought, lads.

I wouldn't have it any other way.

Jacko

Firing order has nothing to do with it...
Unequal length manifold runners governed by the package of the engine underneath.
Power enginnering and others do an equal length, which at first glance look like a bag of worms compared to the OEM exhaust manifolds.
Equal length runners will give approx. 25BHP increase, but bl00dy expensive and you loose that phat scooby offbeat sound

RAZZ

Whatever the explanation, the import scoobies sound the absolute bollox!!!!!!

pat

Hi All!

Just for information.... the Subaru EJ series engines (which the Impreza EJ20X series belong to) are a horizonally opposed four cylinder engines. Looking at the car from the front, the nearest clyinder on the left is No. 1, behind it is is No. 3 (closest to driver), with 2 closest to you on the right, and No. 4 closest to the passenger seat. The Subaru firing order is 1-3-2-4, so you get two cylinders which are next to eachother firing then the other pair fire... Of course the length of pipework between the nearside and offside cylinder heads to the exhaust is different (unequal length) so you get two pops with equal delay between them, then a little pause, then another two with equal spacing, immediately followed by the first two again with equal spacing... the delay between the pops from the No. 1 & 3 head to the No. 2 & 4 head makes it sound like it's missing.

Firing order is also important. Say that it fired 1-4-3-2 then you'de always be swapping cylinder heads on each pop, so you'de get pop, wait, pop pop, wait pop.... it would sound like it's missing more regularly (as it is it sounds like a 5 cylinder with one missing, this way it would sound like a 3 with one missing).

With equal length headers the off-beat sound is indeed almost lost, but it's still there, ever so slightly, due to the locality of the exhaust ports (two and two, not four together).

Regardless of whether the engine is flat or inline four, it uses even firing impulses at 180 degree crank intervals. Oddfire is normally only used when there are an odd number of cylinders or when there is a V arrangement and it is beneficial to crank integrity to use odd-fire.

Hope this helps,

Pat.

The Woodman

I nearly got it right then (well sort of)!!

moonraker

Also note the Scooby firing order is 1-3-2-4 whereas a conventional in line 4 engine is 1-3-4-2.

SC008Y_MAD

iTrader: (5)

I think it is the unequal length exhaust manifolds which create the rumble!

SiDHEaD

Nothing like resurrecting an old thread

Andy

StickyMicky

cos there crap

olliecampbell

iTrader: (3)

"they're" surely?

I thought that the cylinders were numbered 1-4 (say for arguments sake left to right) and it was just a matter of the distributor connecting to the correct spark plug? (if that makes sense?)

Ollie

Danny B

Cos they have a loose HT lead.

greasemonkey

Left to right doesn't really apply on a flat or vee engine Ollie. In the Impreza's case there are two cylinders on the left and two cylinders on the right.

You mean in determining the firing order? Yes and no. You can't just change the firing order by swapping the HT leads around on the distributor! The crank and camshafts will be designed with a certain firing order in mind. In any case, turbocharged Imprezas don't have distributors.