2007 QPV 4.2 Auto Exhaust sound

Ebenezer

Member
Messages
4,445
Please help me out. I'm aware that for the QPV 4.2 auto there are no external valves in the exhaust box. However, thanks to previous members gutting their boxes we know that there is a spring loaded baffle inside which is forced open by exhaust pressure. Under low revs the baffle is closed and the exhaust is routed through the box and subject to attenuation by the wadding etc and exits through one of the two exhaust pipes. Under higher pressure the baffle is forced open allowing the exhaust to bypass the attenuating path and exit through the other exhaust pipe.

So my question is, why does the volume increase pleasingly when accelerating or under load, but then tail off even though the revs are up?

Eb
 

Zep

Moderator
Messages
9,110
It’s because the throttle is like a valve, it restricts the flow of air into the engine. Engines are basically air pumps with a bit of extra petrol, so even at high revs, if the throttle isn’t open very far, a lot less air is going in, so a lot less gas is coming out. The pressure drops and the exhaust valve closes.

Only exactly the right amount of air goes in to match the load. Diesel engines on the other hand, always pump the same amount of air per stroke, but the amount of fuel injected determines the power produced.
 

Alan Surrey

Member
Messages
990
Ah....Another learning experience for me.
I did not know that mine has an internal mechanically operated baffle bypass system, so thank you Eb and Zep for explaining that. :)
It does explain two things:
Why it sounds as it does, beautifully described by Eb.
Why the two tail pipes in each pair are not equally dirty on my car.

So if I have understood correctly and put two and two together.....
I recall from my 6th form days, something called Brake Mean Effective Pressure- a measure of the pressure in the cylinder during the power stroke, and therefore of the pressure in the moving wavefront in the exhaust pipe during the exhaust stroke. It strikes me that the operation of this noise valve is to do with BMEP.
Big BMEP comes with big throttle settings, like during acceleration at low or mid revs.
Big BMEP at high revs is the stuff of racing drivers and mad dogs. Firmly in the lunatic zone to my way of thinking. Not tried it yet. Noise valve closes at high revs :)
 

Zep

Moderator
Messages
9,110
Ah....Another learning experience for me.
I did not know that mine has an internal mechanically operated baffle bypass system, so thank you Eb and Zep for explaining that. :)
It does explain two things:
Why it sounds as it does, beautifully described by Eb.
Why the two tail pipes in each pair are not equally dirty on my car.

So if I have understood correctly and put two and two together.....
I recall from my 6th form days, something called Brake Mean Effective Pressure- a measure of the pressure in the cylinder during the power stroke, and therefore of the pressure in the moving wavefront in the exhaust pipe during the exhaust stroke. It strikes me that the operation of this noise valve is to do with BMEP.
Big BMEP comes with big throttle settings, like during acceleration at low or mid revs.
Big BMEP at high revs is the stuff of racing drivers and mad dogs. Firmly in the lunatic zone to my way of thinking. Not tried it yet. Noise valve closes at high revs :)

Kind of, yes. BMEP is a measure of pressure exerted on the piston, which is a function of throttle position and volumetric efficiency (and plenty of other gubbins). It is at its highest at low RPM (less than 5252 rpm) because cylinder filling is at its highest efficiency. Efficiency (and so BMEP) reduces at higher rpm because cylinder filling reduces, but because it happens more often, it makes more power. Torque (~BMEP) is capacity to do work, power being the number of times it happens. So the gas flow at high RPM and throttle is also higher, although BMEP is lower, because it’s happening more often - when throttle is open of course.

The valve isn’t opened by a pressure wave, but by a pressure differential, as flow goes up, so does pressure drop across the silencer and at a certain point the pressure difference is sufficient to overcome the spring tension on the valve, so it opens. Throttle closes, pressure diff reduces, valve closes.
 
Last edited:

Ebenezer

Member
Messages
4,445
So by design, the non valved exhausts only produce sound under load as the baffle only opens up then. Once you reach a steady state, regardless of revs, the baffle closes and you lose the sound. So the only way to get the sound remove the baffle which is what I think was done.

Is there anyway to fit an actuator to control the baffle manually....?

Jealous of following Strads as they sound glorious all the time!

Eb
 

Oneball

Member
Messages
11,075
It’s because the throttle is like a valve, it restricts the flow of air into the engine. Engines are basically air pumps with a bit of extra petrol, so even at high revs, if the throttle isn’t open very far, a lot less air is going in, so a lot less gas is coming out. The pressure drops and the exhaust valve closes.

Only exactly the right amount of air goes in to match the load. Diesel engines on the other hand, always pump the same amount of air per stroke, but the amount of fuel injected determines the power produced.

A lot of diesels now have throttle bodies to control exhaust pressure and help with emissions equipment work.

Interestingly (or not) early petrol carburettors controlled engine speed by changing fuel flow rather than moving a throttle disc.
 
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Zep

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9,110
A lot of diesels now have throttle bodies to control exhaust pressure and help with emissions equipment work.

Interestingly (or not) early petrol carburettors controlled engine speed by changing fuel flow rather than moving a throttle disc.

Every day is a school day, I shall read on! :)
 

Gazcw

Member
Messages
7,700
So by design, the non valved exhausts only produce sound under load as the baffle only opens up then. Once you reach a steady state, regardless of revs, the baffle closes and you lose the sound. So the only way to get the sound remove the baffle which is what I think was done.

Is there anyway to fit an actuator to control the baffle manually....?

Jealous of following Strads as they sound glorious all the time!

Eb
You could put an electric bypass valve before the boxes which when open exits below the car. That is the simplest to make extra noise. That way you have standard valve for bursts of sound or prolonged unsilenced. This seems the method used in the US a lot.
 
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Zep

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You could put an electric bypass valve before the boxes which when open exits below the car. That is the simplest to make extra noise. That way you have standard valve for bursts of sound or prolonged unsilenced. This seems the method used in the US a lot.

Yes, cutouts are a good shout. Might be very loud though.
 

Oneball

Member
Messages
11,075
You could put an electric bypass valve before the boxes which when open exits below the car. That is the simplest to make extra noise. That way you have standard valve for bursts of sound or prolonged unsilenced. This seems the method used in the US a lot.

After spending a fortune trying to get the Vette quiet I’ve realised why wherever you are in the world you can always hear an American talking loudly….they’re all deaf!
 

Ebenezer

Member
Messages
4,445
With the cut out you'd probably lose the quality of the sound though. That's the problem.I presume the back boxes still shape the sound.
Eb
 

Gazcw

Member
Messages
7,700
With the cut out you'd probably lose the quality of the sound though. That's the problem.I presume the back boxes still shape the sound.
Eb
Not so sure. If you empty them and wedge the valves like mine was, it is just a big box. I guess a good rest is just to slide the boxes off and see what it sounds like.
Mine was fine with the unsilenced x and empty boxes. It was quietish when cruising helped by the double glazing.
 

Ebenezer

Member
Messages
4,445
Not so sure. If you empty them and wedge the valves like mine was, it is just a big box. I guess a good rest is just to slide the boxes off and see what it sounds like.
Mine was fine with the unsilenced x and empty boxes. It was quietish when cruising helped by the double glazing.
I was never sure from the pics posted what the different paths were within the box though, between baffle closed (quiet) and open louder - temporarily... It would be good if someone could draw a diagram of whats going on in side

Separately, for the valved back boxes, does the valve actually mean that the exhaust bypasses the back box altogether?
Eb
 

Oneball

Member
Messages
11,075
I was never sure from the pics posted what the different paths were within the box though, between baffle closed (quiet) and open louder - temporarily... It would be good if someone could draw a diagram of whats going on in side

Separately, for the valved back boxes, does the valve actually mean that the exhaust bypasses the back box altogether?
Eb

Stolen photo from the other place. 104121

Outer shell is missing but exhaust in on left out on right.

Quiet it goes through the perforations in the smaller tube does a whole loop and out the small exhaust on the right.

Loud exhaust pressure pushes the valve open it goes in the top end of the big pipe does half a loop and out the larger exhaust on the right.
 

Gazcw

Member
Messages
7,700
Stolen photo from the other place. View attachment 104121

Outer shell is missing but exhaust in on left out on right.

Quiet it goes through the perforations in the smaller tube does a whole loop and out the small exhaust on the right.

Loud exhaust pressure pushes the valve open it goes in the top end of the big pipe does half a loop and out the larger exhaust on the right.
Correct. The valve is seated inside the large bore which is simply pushed open by the gases coming in from the left.
 

Gazcw

Member
Messages
7,700
I was never sure from the pics posted what the different paths were within the box though, between baffle closed (quiet) and open louder - temporarily... It would be good if someone could draw a diagram of whats going on in side

Separately, for the valved back boxes, does the valve actually mean that the exhaust bypasses the back box altogether?
Eb
Not sure on the gts boxes whether it is simply a straight through pipe or whether it still goes through some sound deadening and makes it's own way out. I guess no-one has ever needed to open one up.