QOTW #27 – Discuss exhaust silencer bypass valve failure

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Welcome to BamfordRose and another question of the week. This week it’s exhaust bypass valve failure. What is it? What happens if you have an exhaust bypass valve failure?

So, this is off a V8 Vantage but the silencer design layout construction pretty much the same DB9, V8 Vantage DBS V12 V, all are the same. So, this is positioned as an intermediate pipe from the engine coming into the silencer. This goes into the [inaudible 00:44] exits the car. This valve stops gas from going through the silencer because gas from the engine is going to come into here. With this opened, it’s just going to flow out and through. With this closed, the gas cannot exit here, so it’s forced to go inside the silencer and then back out through this pipe and then out.

So, why they do that is noise drive by. There’s a certain decibel to pass noise drive by regulation, typically, 77 DB and on control bits tarmac there’s a test with microphones measuring sound and start point is something like 30 mile-an-hour, third gear at start gate something like 55 mile-an-hour.

I think you can be in any gear you want but it’s obviously much more advantageous to keep changing gear and keep the engine reps as low as possible to exit the gate in the highest tallest gear. There’s some clever cheats you can do for this or there used to be. You can use cruise control, not thinking the rules to stop the manufacturer doing that. Anyway, these are little nudges towards the line that the maker can do to pass the particular test.

So, in the period that is measured this 30 mile-an-hour 55 mile-an-hour condition, this valve is closed, so gas is forced to go through the silencer and then out. At idle, low engine speed, higher engine speeds, importantly not within this test window, this valve opens up and you get the glorious sound that an Aston makes.

So, what happens over years of operation of this going cope and closed off of a vacuum feed. It is sometimes this sort of rose joint which is held in the spindle arm can break off or the spindle itself can crack inside and then the butterfly is doing whatever it wants. So, you could get the condition where the sound throughout the rev range is variable just because that butterflies flapping around in a breeze. Or if it breaks, this outside locator internally through spring pressure, the valve is default open, so customers will say that it’s never going quiet.

Well, I said that at the start, this orientation is key because if you are inclined at home a little bit of a welder to repair this for yourself, then get this arm here in the same sort of orientation as this is which means the valve is default open and then well back on somehow that actuator. If the fix isn’t this and internally is broken, then there’s a weld line here and there which is the complete valve assembly, so in those cases what we do is remove it from car and we’ve got stock of this particular valve from the supplier and then we reweld this back into the pipe. So, that’s the more expensive fix because that one was an easier fix. A franchise dealer, obviously, they’re just going to throw this whole apart because they don’t repair stuff. Yeah, it’s about 1850 quid plus VAT for a new silencer as opposed to time and materials to fix this which is probably going to come in about 400-500 quid.

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