Bore x stroke - affects exhaust sound?

[RomanV]

Hi there, I have been thinking about why subarus make that distinctive sound.
Love it or hate it, you have to admit it is completely unique.
I have heard numerous times that it is due to the boxer motor configuration, which I have no reason to doubt.

However, I have been wondering if there are other factors too.
I was thinking about how the gas gets pushed out the exhaust with different motor configurations.
Say you had two different engines, both 2 litre capacity.
One had a long stroke and a narrow bore,
and the other had a short stroke and a wide bore.

The engine with the longer stroke takes longer to push all of the gas out, correct?
And the short stroke engine would push out more gas, quicker.
Which would lead to more of a quick 'pulse' of gas being expelled, rather than a longer flow of gas from the longer stroke motor.
Does this contribute to the subaru sound? I heard that these engines have a short stroke compared to the bore size.
I am sure that other boxer motors do not make a similar sound? eg. Porsches, Volkswagons.

So is this sound a product of the bore x stroke size also?
Or have I confused myself.
I am only theorizing, I have nothing to back this up.
So if I am on the completely wrong track, I would love to hear why.
Any thoughts?

Fake edit: Hmmm I dont think it would take 'longer' for the long stroke engine to push the gas out, (assuming both engines running at same RPM) but at a lower, more constant pressure perhaps? Rather than rapid rise and falls in gas pressure.

[sopachrga]

I don't know about the different effects it causes, but the difference in stroke makes no difference what so ever to the speed or pressure the air comes out of the cylinder.

Remember you are still going to be pushing X Volume of air out in Y Time.
This will not change between the engines.....

[jeremiahnz]

I have heard rumors to that effect.

Specifically that mitsi's employ a longer stroke and lesser bore and therefore develop a deeper sound where as toyota with their 4a series have a shorter stroke but larger bore and therefore have more of a bark.


Here are a couple of examples.
4age 81 mm bore 77 mm stroke

4G93 16-valve SOHC 1.8 liter specs.
Bore x Stroke: 81.0 x 89.0


Although interestingly:
engine - type 4G92
type: transverse in-line 4
bore x stroke: 81.0 x 77.5
capacity: 1597cc
valve train: dohc 16v with MIVEC
compression ratio: 11.0:1

As I said, it was just something I heard so shouldn't be taken as fact.

Someone may be able to back it up or shoot it down??

[vvega]

bore and stoke affects the amout of gas passed
bore x storke = cc's per cycle
increase ehither one and the passed volume increases
so ye therotically thought the sae exhaust the sound can change
but it wouldnt be a lot
and most probably barly audible


simple physic's really...isn5t it ??


v

[sopachrga]

vvega, you have missed the point of what is being asked.

It is plainly obvious what you have just stated, but the question was aimed at two motors both 2L in capacity, with different stoke and bore measurements.

And changing the capacity of the engine would make a very signifigant change in sound and volume not, a difference that you refered to as "most probably barly audible "

[Dr-X]

It seems obvious to me that changing the very dynamics of the engine is going to change the sound of it.

[RomanV]

Aaah! Ive found a way to explain myself more clearly. (perhaps)
Imagine if we plotted a graph with exhaust pressure on the vertical axis, and the time span of one exhaust stroke of the engine along the horizontal.
The two different engines would have (roughly) the same amount of area under the graph (amount of gas being pushed out) but the shape of the curve would be different.

eg. The big bore engine would have a peakier curve than the small bore.
The small bore engine would have a lower peak, but the pressure would be more spread (Less pressure variation) across the range.
Hence the 'pulse' of exhaust gas from the big bore, rather than a more constant flow.
Make sense?

[crshbndct]

subarus sound the way they do because they have a low number of cylinders and yet have 2 banks of cylinders.

if subaru made a boxer8 motor it would sound just like a v8.

also might have something to do with the fact that ona subaru one set of exhaust runners is about 2x the length of the other set.

[Emperor]

Boxer8.. Mmm

[CozmoNz]

also might have something to do with the fact that ona subaru one set of exhaust runners is about 2x the length of the other set.

you got it

If you really want to engineer yourself a set of Subaru exhaust runners, you CAN have the option of making it sound like a mitsi.

reember, 1,3,4,2 (i think its that way for subaru), so left bank, right bank, left bank, right bank.

Think about this.

Exhaust travels (length wise), 30mm, 90mm, 30mm, 90mm (for example).

Obviously at higher revs this changes from a whob to a scream / subaru yell sort of thing.

Funny aye, porshe have a boxer engine, but sound absoutly wicked. the key id go with is exhaust runner length.

[vvega]

vvega, you have missed the point of what is being asked.

It is plainly obvious what you have just stated, but the question was aimed at two motors both 2L in capacity, with different stoke and bore measurements.

And changing the capacity of the engine would make a very signifigant change in sound and volume not, a difference that you refered to as "most probably barly audible "

changing 60cc as the result of a overbore will not change sound significantly
real world experiance tells me this


sorry i dissagree with you but you are painfuly and obviously wrong on both accounts accept iy and get on with it...


the answer is yes it will ....
any change to engine dynamics ... bore vrs storke ..engine layout.. cams...tyoe of induction will all change you exhaust note
again symple physic's
your resonate value(the sound your engine makes) is dependant on the rest of the pulse and flow dynamics for it very makeup ....

like i said simple stuff

[vvega]

Aaah! Ive found a way to explain myself more clearly. (perhaps)
Imagine if we plotted a graph with exhaust pressure on the vertical axis, and the time span of one exhaust stroke of the engine along the horizontal.
The two different engines would have (roughly) the same amount of area under the graph (amount of gas being pushed out) but the shape of the curve would be different.

eg. The big bore engine would have a peakier curve than the small bore.
The small bore engine would have a lower peak, but the pressure would be more spread (Less pressure variation) across the range.
Hence the 'pulse' of exhaust gas from the big bore, rather than a more constant flow.
Make sense?

add to this that more pressure in the exhaust keeps the ex gas hotter = differnd ex temps comming out the tip = differnt sound ..why because air temp alters sound waveforms......

v

[CozmoNz]

so basically, lots of crap makes them sound, they way they do.

but i was told in the new legacys they got rid of that sound.... by changing the headers.

meh >.>

[sopachrga]

vvega, you have missed the point of what is being asked.

It is plainly obvious what you have just stated, but the question was aimed at two motors both 2L in capacity, with different stoke and bore measurements.

And changing the capacity of the engine would make a very signifigant change in sound and volume not, a difference that you refered to as "most probably barly audible "

changing 60cc as the result of a overbore will not change sound significantly
real world experiance tells me this


sorry i dissagree with you but you are painfuly and obviously wrong on both accounts accept iy and get on with it...


the answer is yes it will ....
any change to engine dynamics ... bore vrs storke ..engine layout.. cams...tyoe of induction will all change you exhaust note
again symple physic's
your resonate value(the sound your engine makes) is dependant on the rest of the pulse and flow dynamics for it very makeup ....

like i said simple stuff

At no stage did I say that it would not change the sound, I said that it would not change the speed or pressure of the air coming out of the cylinder. due to the fact that a piston with a larger bore will travel slower than a piston with a smaller bore at the same Revs.....

Once you can get this in to your head and stop making up imaginary arguments, maybe you could learn to read, and spell.

Before you try to rubbish any of my statements in the future I would prefer you to read what I have said and only try to prove me wrong on statements I have actually made.

e.g. Can you please show me where I have said that different bore, and stroke will not make a difference in sound?

As for the sound change on an over-bore, your real world experiance obviously differs from mine, You yourself have just finished trying to argue that any change to engine dynamics will change the sound.

I think maybe you need to think your arguments through a bit more clearly before you start aimlessly typing away.

[to4garret]

also might have something to do with the fact that on a subaru one set of exhaust runners is about 2x the length of the other set.

correct again, thats why my car now sounds like a 6 cyl scooby, longer length ectractor from the rear cyl, shorter at the front.

before the single turbo conversion, just sounded like any other V6.

[TygerTung]

See you will notice that a lot of subaru rally cars have the equal length headers and just have a smooth 4cyl sound.

[CozmoNz]

See you will notice that a lot of subaru rally cars have the equal length headers and just have a smooth 4cyl sound.

bs, you couldnt hear a subaru rally car, youd go deaf!

anti-lag = NOISY AS FUKZ0R

[vvega]

At no stage did I say that it would not change the sound, I said that it would not change the speed or pressure of the air coming out of the cylinder. due to the fact that a piston with a larger bore will travel slower than a piston with a smaller bore at the same Revs.....

Once you can get this in to your head and stop making up imaginary arguments, maybe you could learn to read, and spell.

Before you try to rubbish any of my statements in the future I would prefer you to read what I have said and only try to prove me wrong on statements I have actually made.

.

where do i start....
so your saying quite clearly the 2 pistons of the same stroke but with a larger of smaller bore travel at different speeds @ the same rpm...

due to the fact that a piston with a larger bore will travel slower than a piston with a smaller bore at the same Revs.....

why yes you are
fact: piston size has nothing to do do with piston speed @ a given rpm


fact : stroke length is the only thing that will change piston speed aside from the obvious reving of a engine

my typing skills are not as good as your i have no problem agreeing with that
but whats the point of be able to type when you have no idea what your tring to convay......

a rebore wont affect sound in the real world and is not a sugnificant change in the dynamic where as a larger cam is a considerable differance to both effencency and dynamics
as i have said

maybe you need to go back to school and get a education to go with you typing skills so that you dont end up looking like a total dipshit

i did not do that you did it to yourself,......

v

[RomanV]

Well we were discussing an engine of the same displacement. (2.0L)

So, if the bore increased, the stroke would decrease to suite.
So what he said is correct in the context of the discussion

So the bigger bore piston *would* travel slower.

[JT]

Removed the crap, this thread doesn't need cozmo teaching us how he thinks he can break the law of physics.