Originally Posted by rx7i2
No. not really. And yes I've heard an Impreza RS with a fully built exhaust before. The cylinders in the engine are horizontally firing, and they firing rate of the cylinders differ to that of an inline 4.
Rather than having a normal engine that fires off the cylinders in an order, the crankshaft on a flat engine is different because there are cylinders on opposite sides of the block. The only explanation for this, which I'm sure is true is that the firing rate of a flat engine is 1-1-2-2, rather than 1-2-3-4 which is why their sounds are lower pitched than a VTEC honda engine. (It's pretty easy to differ, actually. And I've seen MANY built hondas in my area)
Plus from what I've seen, it looks like they have two separate exhaust manifolds as well. That rumble sound can very well be a result of all of this happening under the hood.
Two seperate manifolds yes since an horizontally opposed engine has two cylinder heads.
Your information on firing order for inline 4 is off. Inline engines do not fire 1-2-3-4. That would create a pattern 1-2-3-4-1-2-3-4... which is very inefficient due to uneven forces on the crankshaft. I know in the ford zetec motor the firing order is 1-3-4-2 which keeps the force on the crankshaft evenly distributed.
"1-1-2-2"? What happened to the other two cylinders? Yes I know what you meant. Look at the following awesome picture:
Assume that 1,2,3,4 are cylinders, "--" connecting rods, and || the crankshaft. Since the opposing piston will extend as the other fires, firing right back would be inefficient due to the momentum the piston is carrying. The firing order making the most sense would be 1-3-2-4 so that no cylinders ever fire against each other. That firing order happens to also be that of a Subaru STI motor.
The sound you hear is a collection of sounds from the exhaust cycle for each cylinder.
Take this simple example. Take an object, any object that can make some noise (this represents one cylinder). Now hold it upright and make it make a noise. Now hold it at an angle and do the same. Repeat holding horizontally. Did the tone of the sound change because of position? Of course not. Placement of cylinders has no bearing on sound.
Therefore one can only infer that sound out of an open cylinder would sound the same in any position as long as everything about that cylinder remained constant (e.g. volume of cylinder, number of exhaust valves). By that being taken into account, it is only plausible that the sound you hear is modified between the cylinder head and the opening at the end of the tail pipe, therefore proving what I stated in a previous post:
"Muffler, resonator, catalytic converter, piping material, piping length, and bend type all contribute to sound"
Throw in exhaust manifold since I neglected to mention that.
Another consideration is engine RPM. As RPM increase, so does sound pitch. Honda engines generally have to rev higher to make power due to their lack of displacement, hence why when you hear a Honda zip by it has that high pitched pissed of mosquito sound. The Impreza RS has a displacement of 2.5 liters with is a half liter more than a civic SI. The displacement alone would produce a lower pitched sound due to the greater cylinder volume. Now couple that fact with that of how a Subaru motor would not need to rev to 7000 rpm to make any kind of noticeable power and there you have it.
So my point is, its not the positioning of the cylinders that create the deeper sound you hear. It is not the "engine" but rather components external to the exhaust side of the heads.Edited by xtascox - 12/14/10 at 11:40pm