After Market Exhaust ?

I'm talking about a 2017 Legacy 3.6R .

I know that, but different exhaust systems are going to have different air flow rates.

I.m talking about a Nameless with 5" mufflers

No matter what you put downstream, the catalytic converter remains the primary bottleneck in the Legacy/Outback exhaust system.

Bigger is not always better. Modern OE exhaust systems are carefully engineered, and are subjected to many hours of flow-lab testing and refinement ... to the point that OE exhaust systems these days can sometimes be more efficient than straight pipes would be. The relatively small aftermarket for Legacy/Outback exhaust systems just doesn't support that level of testing and development by aftermarket manufacturers.

FWIW, I'm always amused to see quad exhaust tips on a car with a 6-cylinder engine. Think about it.

FWIW, I'm always amused to see quad exhaust tips on a car with a 6-cylinder engine. Think about it.

Points taken, but I can't help but think of the NA 6-cylinder cars with quad tip OE exhausts: base model 911, NA 300ZX, NA 3000GT, E46 M3, etc.

FWIW, I'm always amused to see quad exhaust tips on a car with a 6-cylinder engine. Think about it.

Thanks, your comment got me thinking so I checked with Dr. Google and came up with a couple of interesting hits ...

" in almost all cases, they are just just for looks. If they are quad exhausts all the way to the engine head(s) via separate headers, then they could be for performance, but realistically all that piping would just add more weight with little to no performance gain (unless it was a mid/rear engine car and the pipes were very short). I can't think of any cars that run quad exhausts like that (definitely no non-exotic cars)."

" Depends on the car, the Lexus IS-F just had fake twin tips on each side of the rear bumper that are not even connected to the exhaust...."

" Not exactly just for looks in every case. I'll use the Corvette NPP exhaust as an example, since it's the one I'm most familiar with. While they're piped like dual exhaust until the mufflers, each muffler has two exit pipes, both are perforated within the muffler. One of the exit pipes is pretty much a straight-through pipe. There's a valve on this pipe after the muffler that can close it off. When that valve's closed, the exhaust has to travel through the perforations, through the muffler, and out the second pipe. This causes it to be much quieter. When that valve is open, most of the exhaust makes it through that straight pipe and it's a lot louder...."

OK ,I.m talking about a Nameless with 5" mufflers

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FWIW, I'm always amused to see quad exhaust tips on a car with a 6-cylinder engine. Think about it.

How many cylinders would a quad-tip be OK for?

How many cylinders would a quad-tip be OK for?

Fair question. Answer = N x 4: e.g. 4, 8, 12, or 16 cylinders ... which assumes that the rest of the exhaust system is properly designed to take optimum advantage of multiple outlets. The single catalytic converter and intermediate pipe in Legacy/Outback makes any technical benefit from multiple tips highly questionable.

... your comment got me thinking ...

Thinking is good. I can think of only one technical justification for 4 tips on a 6-cylinder exhaust system: Four small-diameter tips may provide better noise suppression than a single larger-diameter tip with the same net flow restriction. There are probably alternatives (e.g. fishtail tips) that would perform even better but are perhaps less acceptable aesthetically.

Fair question. Answer = N x 4: e.g. 4, 8, 12, or 16 cylinders ... which assumes that the rest of the exhaust system is properly designed to take optimum advantage of multiple outlets. The single catalytic converter and intermediate pipe in Legacy/Outback makes any technical benefit from multiple tips highly questionable.

N X 4, where N = # of cylinders? So, where there are 4 cylinders, N = 4, so the number of tips should be 4 X 4 = 16?

As you point out, Subaru exhausts combine into one pipe very soon after the header, so, maybe the N here would be 1 and 1 X 4 = 4.

We should split the difference and go with 10 tips for now, until we can figure out why this would even matter, other than to look cool:p

N X 4, where N = # of cylinders?

Nope. The question you asked was:

How many cylinders would a quad-tip be OK for?

N = 1, 2, 3, 4, ...

'4' = specified number of exhaust tips (i.e. "quad-tip")

Subaru exhausts combine into one pipe very soon after the header, so, maybe the N here would be 1 and 1 X 4 = 4.

Nope. The question you asked was:N x 4 = number of cylinders, where:

 

N = 1, 2, 3, 4, ...

 

'4' = specified number of exhaust tips (i.e. "quad-tip")

 

The point is that with a single catalytic converter and intermediate pipe, any number of exhaust tips greater than 1 is almost purely cosmetic.

I see no correlation between the number of cylinders and the number of tips. Your equation makes it seem as if an engine must have 8 cylinders before a second tip is warranted (and obviously assumes you'd have to round to next whole integer for those which don't equate to one; e.g., 6 cylinders = 1.5 tips...)

I guess I'm asking where you came up with your logic for the number of tips? Tips don't really matter anyway, except as they affect sound. The only time the number of exhaust pipes would matter, and need to be increased, is if the exhaust flow requires a larger diameter pipe than is practical, and a second pipe must be designed to fulfill this design need, as well as the space constraints, as they relate to interference, including ground clearance, suspension geometry, etc.

Also, for a muffler, the number and design of the outlets will affect the sound amplitude and tone. Even if the majority of the exhaust apparently exits one or two of the four during normal driving, the other 2-3 outlets, if functional, will change the sound of the exhaust at higher flow rates. The same would hold true for the number of exhaust pipes throughout the exhaust, not just the tips (e.g., pre-cat, intermediate post-cat, pre-mufflers, etc.). All of these affect how the exhaust sounds, including both the good sounds (e.g., the previous, desirable Subie rumble, though due to an inefficient design) and the bad (e.g., drone at idle and or highway operation)

So, even if there is some practical logic into why Subaru designed their muffler or mufflers with quad or dual-tips, respectively, your last sentence is most likely the real reason for the design - it looks cool.

Are you people really that serious about how many exhaust tips should be allowed or is this all just friendly banter?

This old fart sees it this way: back in the day of the common V-8, it just made sense to provide a separate exhaust system for each bank of cylinders. Sometimes they ended up at the rear bumper and sometimes they ran along the rocker panels. High performance V-8s all had twin exhausts so it became a kind of high-performance signature. Sedans with V-8s might have single exhaust outlets but the fun stuff had DUALS.

Fast forward to the late 90's and 00's and car manufacturers, trying to capture some of that dual exhaust fun, started putting dual exhaust tips on everything from muscle cars to sedans to 4 cylinder SUVs. This included 4 cylinder inline engines that would not likely ever benefit from a dual exhaust system. I know; my 2008 Miata has a factory muffler that splits into two exhaust tips. I don't find it ridiculous. It's just styling. And I like the balanced look, frankly.

If any 4 cylinder engine deserved a dual exhaust, it might be a boxer. But I have a suspicion the engine engineers know there would be little advantage. So our Subarus have an exhaust that collects all four cylinders into one pipe that is then split into two mufflers and two exhaust tips - for styling only.

If someone wants to put quad tips on a Legacy - why not? We all know it's just for looks anyway.

Are you people really that serious about how many exhaust tips should be allowed or is this all just friendly banter?

he he Man look what I started .

I didn't mean to start anything. If you refer back to my original post, all I said was:

"... I'm always amused to see quad exhaust tips on a car with a 6-cylinder engine."

I see no correlation between the number of cylinders and the number of tips.

It's a matter of exhaust dynamics, specifically overlapping exhaust pulses. Early high-performance cars employed one full-length exhaust pipe per cylinder ... a design feature still employed on some high-performance motorcycles ... but engineers quickly realized that individual exhaust pipes could be combined with little loss in efficiency ... as long as there was minimal overlap in the exhaust gas pulses.

(What follows assumes a real-world 4-stroke engine with uniform cylinder timing.)

The power strokes (and thus the exhaust pulses) in a 4-cylinder engine do not overlap significantly. As a result, a well-designed exhaust system in a 4-cylinder engine can channel all four cylinders into a single pipe with little loss in efficiency, saving weight at the same time.

On the other hand, the power strokes (and exhaust pulses) in engines with six or more cylinders do overlap, which in part accounts for their smoother power delivery than a 4-cylinder. But the exhaust system in a 6-cylinder engine can easily be divided into two groups of three non-overlapping cylinders each. Carried all the way to the outlets, this arrangement would dictate two exhaust tips. Similarly, the exhaust system of an 8-cylinder engine can be split into two groups of four non-overlapping cylinders each, also resulting in two tips.

Following the same design principles, the exhaust system of a 12 or 16 cylinder engine can be divided into four equal branches, thus four tips. (It would be possible to divide a 12-cylinder exhaust into only three branches, but I don't think I've ever seen it done that way. A straight-12, possibly, but it would be really awkward to implement evenly on a V12 or H12 with six cylinders per bank.)

IMO, it's probably the practical employment of quad exhausts on Italian V12 supercars, starting in the 1950s, that accounts for their association with high performance and exotic sound, and explains their persistent popularity as cosmetic accessories in the aftermarket today. Me? I prefer the SoundRacer myself.

I didn't mean to start anything.

Actually you did start something ....... a discussion which I find interesting ... so thanks for that.

I didn't mean to start anything. If you refer back to my original post, all I said was:

 

"... I'm always amused to see quad exhaust tips on a car with a 6-cylinder engine."

 

 

 

 

It's a matter of exhaust dynamics, specifically overlapping exhaust pulses. Early high-performance cars employed one full-length exhaust pipe per cylinder ... a design feature still employed on some high-performance motorcycles ... but engineers quickly realized that individual exhaust pipes could be combined with little loss in efficiency ... as long as there was minimal overlap in the exhaust gas pulses.

 

(What follows assumes a real-world 4-stroke engine with uniform cylinder timing.)

 

The power strokes (and thus the exhaust pulses) in a 4-cylinder engine do not overlap significantly. As a result, a well-designed exhaust system in a 4-cylinder engine can channel all four cylinders into a single pipe with little loss in efficiency, saving weight at the same time.

 

On the other hand, the power strokes (and exhaust pulses) in engines with six or more cylinders do overlap, which in part accounts for their smoother power delivery than a 4-cylinder. But the exhaust system in a 6-cylinder engine can easily be divided into two groups of three non-overlapping cylinders each. Carried all the way to the outlets, this arrangement would dictate two exhaust tips. Similarly, the exhaust system of an 8-cylinder engine can be split into two groups of four non-overlapping cylinders each, also resulting in two tips.

 

Following the same design principles, the exhaust system of a 12 or 16 cylinder engine can be divided into four equal branches, thus four tips. (It would be possible to divide a 12-cylinder exhaust into only three branches, but I don't think I've ever seen it done that way. A straight-12, possibly, but it would be really awkward to implement evenly on a V12 or H12 with six cylinders per bank.)

 

IMO, it's probably the practical employment of quad exhausts on Italian V12 supercars, starting in the 1950s, that accounts for their association with high performance and exotic sound, and explains their persistent popularity as cosmetic accessories in the aftermarket today. Me? I prefer the SoundRacer myself.

I think we're on the same page here (mostly). The nomenclature used seems to be the disconnect. By 'tips' you've meant overall exhaust geometry and design-build, including the combination of header piping through to the muffler exit. Based on this, I think we can both agree that the number of 'tips' is irrelevant...

Two examples (irrespective of cylinder count):

1. Single intermediate pipe splits to dual axle-back (two mufflers). The number of tips is irrelevant to performance.

2. Single intermediate pipe carries to single muffler, with dual-tips exiting from each side (quad-tips). The number of tips is irrelevant to performance.

In each case, the split occurs after the components with the highest D/P, the turbo (if equipped) and the catalytic converters. Any exhaust pulses will be dampened and, by the time the exhaust flow reaches the 'tips', the previous pulses due to sympathetic or individual firing won't matter.

Quad-tip exhausts from a system with a single intermediate pipe are for aural and aesthetic purposes only, if there are no restrictions in geometry, such as needing to break the intermediate pipe up into more than one pipe due to suspension and ground-clearance limitations.

... a discussion which I find interesting ...

If you can find a copy, I highly recommend The Scientific Design of Exhaust and Intake Systems, by Philip H. Smith. Written mostly in non-technical language, it's easily approachable. The first edition was published more than 50 years ago, but the fundamental principles haven't changed.