Exhaust bypass valves explained

[AndreiD]

Hi friends,

Can anyone please explain to me how the exhaust bypass valves or exhaust modes works on our GT350s?

To my understanding there is an electronic valve that closes and opens on normal or sport. But from where such a big difference in sound?

[ame="[MEDIA=youtube]D10ItE4Dgqs[/MEDIA]"]To my understanding they work similar to this![/ame] I have yet to see the GT350 in action.

My question ponders me since even with a Corsa X-pipe installed it's just a day and night difference between normal and sport mode. I even renamed those modes to civil and honnigan mode.

Any pictures or videos are greatly appreciated!

Thanks in advance!

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[jacknifetoaswan]

The GT350 has a valve that either allows sound to come from two of the four exhaust outlets (muffled, quieter) or all four (no muffling, louder). The PCM or another module has programming to open and close the butterflies, based on what the car is doing, I believe. So, if you put the car in Race, I think it opens the butterflies fully, and lets the exhaust sing. In Sport+, the butterflies likely open based on RPM and throttle input, and in Normal mode, the butterflies are likely full closed.

This is similar to what the Roush Active Exhaust does, though the Roush has no connection to the PCM, so it can't read RPM or throttle input, it's just based on user input from the control module/switch.

JR

 

[Synyster06Gates]

The GT350 has a valve that either allows sound to come from two of the four exhaust outlets (muffled, quieter) or all four (no muffling, louder). The PCM or another module has programming to open and close the butterflies, based on what the car is doing, I believe. So, if you put the car in Race, I think it opens the butterflies fully, and lets the exhaust sing. In Sport+, the butterflies likely open based on RPM and throttle input, and in Normal mode, the butterflies are likely full closed.

This is similar to what the Roush Active Exhaust does, though the Roush has no connection to the PCM, so it can't read RPM or throttle input, it's just based on user input from the control module/switch.

JR

Yes it does. It taps into the OBD2 data

 

[AndreiD]

The GT350 has a valve that either allows sound to come from two of the four exhaust outlets (muffled, quieter) or all four (no muffling, louder). The PCM or another module has programming to open and close the butterflies, based on what the car is doing, I believe. So, if you put the car in Race, I think it opens the butterflies fully, and lets the exhaust sing. In Sport+, the butterflies likely open based on RPM and throttle input, and in Normal mode, the butterflies are likely full closed.

This is similar to what the Roush Active Exhaust does, though the Roush has no connection to the PCM, so it can't read RPM or throttle input, it's just based on user input from the control module/switch.

JR

Wait, I thought it's more of an on and off thing. Either fully open or fully closed. You mean to tell me they can be partially open/closed?

If that's so, what exactly doest the exhaust button do? Force them fully open or fully closed?

 

[cking]

pic 350 exhaust valve

Closed exhaust goes through muffler, open lets it go straight through the perforated in the muffler

 

[Stuntman]

Wait, I thought it's more of an on and off thing. Either fully open or fully closed. You mean to tell me they can be partially open/closed?

If that's so, what exactly doest the exhaust button do? Force them fully open or fully closed?

You are correct. Open = Loud, Closed = Quiet (for the oem GT350 exhaust).

 

[rb92gt]

Simple, four pipes in back, two (one on each side) have exhaust exiting through the mufflers all the time when the valves are closed (quiet) or open (loud). The other two are essentially straight pipes within the muffler essentially bypassing it when the valves open (loud). Ckings pic shows the valve open, would like to see the internals of the muffler as would explain it best, but amazingly google can't find a pic.

 

[AndreiD]

Simple, four pipes in back, two (one on each side) have exhaust exiting through the mufflers (quiet). The other two are straight pipes through the muffler (bypassing it, so loud). Exhaust always exits through one pipe on each side in both modes.

Ok but if that is so then defatted headers + corsa xpipe + loud mode = straight pipes from engine to final? :eyebulge:

Cuz in that scenario I don't see anything canceling noise! Does it make sense?

 

[TheDeadCow]

Stock mufflers have 1 input and 2 outputs.

One of those outputs is always open, the other has the butterfly in it.

In quiet mode the butterfly only opens at 3/4-full throttle. In loud mode the butterfly is always open.

 

[rb92gt]

Ok but if that is so then defatted headers + corsa xpipe + loud mode = straight pipes from engine to final? :eyebulge:

Cuz in that scenario I don't see anything canceling noise! Does it make sense?

Just to be clear, there is ALWAYS some exhaust going through the muffler through one pipe on each side. This pic shows what we are all trying to say, the outside pipes always have flow through the muffler to quiet exhaust. The inner pipes with baffles let exhaust essentially bypass some muffling when open (an internal pic would be best, can't find one).

 

[Zombo]

To make matters even more interesting, the higher the flow thru the exhaust, which is caused by increased RPM and load (throttle opening), the greater percentage of exhaust flows thru the loud side.

For the EEs here, the exhaust is the equivalent of a parallel DC circuit. Pressure drop across the exhaust is like voltage drop, flow thru each leg is like current, and resistance is, well, resistance. Except, in this circuit, the resistance is variable, and is a function of the current. For the quiet side, the resistance increases at a faster rate with flow (current) than it does on the loud side.

Thus, at idle (just throwing out numbers here) maybe 40% of the flow goes thru the quiet side (60% the loud side). On WOT and high RPM, maybe it's only 10% quiet side, generating the sound we love...

 

[rb92gt]

To make matters even more interesting, the higher the flow thru the exhaust, which is caused by increased RPM and load (throttle opening), the greater percentage of exhaust flows thru the loud side.

For the EEs here, the exhaust is the equivalent of a parallel DC circuit. Pressure drop across the exhaust is like voltage drop, flow thru each leg is like current, and resistance is, well, resistance. Except, in this circuit, the resistance is variable, and is a function of the current. For the quiet side, the resistance increases at a faster rate with flow (current) than it does on the loud side.

Thus, at idle (just throwing out numbers here) maybe 40% of the flow goes thru the quiet side (60% the loud side). On WOT and high RPM, maybe it's only 10% quiet side, generating the sound we love...

Mind... Blown. :faint:

 

[nastang87xx]

So what's really your concern OP? I'm confused. Or are you just asking?

 

[krt22]

To make matters even more interesting, the higher the flow thru the exhaust, which is caused by increased RPM and load (throttle opening), the greater percentage of exhaust flows thru the loud side.

For the EEs here, the exhaust is the equivalent of a parallel DC circuit. Pressure drop across the exhaust is like voltage drop, flow thru each leg is like current, and resistance is, well, resistance. Except, in this circuit, the resistance is variable, and is a function of the current. For the quiet side, the resistance increases at a faster rate with flow (current) than it does on the loud side.

Thus, at idle (just throwing out numbers here) maybe 40% of the flow goes thru the quiet side (60% the loud side). On WOT and high RPM, maybe it's only 10% quiet side, generating the sound we love...

I would say its more like there is a current limiting diode on one leg (quiet side of the muffler due to the extra baffling) and standard resister on the other. For the most part the total flow through the quiet side may remain relatively constant and the rest will go through the loud side as total flow increases, so yeah those percentages may be true in terms of total flow

 

[Zombo]

I would say its more like there is a current limiting diode on one leg (quiet side of the muffler due to the extra baffling) and standard resister on the other. For the most part the total flow through the quiet side may remain relatively constant and the rest will go through the loud side as total flow increases, so yeah those percentages may be true in terms of total flow

A current limited diode would be like a pressure relief valve, which I suspect is not present in the GT350 exhaust. No, Fluid Mechanics dictates that each "leg" of the exhaust has it's own flow impedance curve. This curve is characterized by the function: Pressure Drop = Constant x (Flow Rate)^power. The power is usually between 1.7-2.0. For the loud side, the constant and power is lower than on the quiet side (because there is less restriction), so the flow impedance curves have a similar shape, but the loud side is much more gradual, as pressure drop rises with flow rate. Thus, at high flow, the restriction of the quiet side increases much faster, resulting in continuously lower flow. I stand by my initial analysis.

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