Loud Pop After WOT Acceleration Now Idles Louder Than Before
 

I was driving yesterday and while on the freeway did a pass at WOT. Upon letting off the gas there was a loud pop that shocked me. I looked in my rear view because I thought something may have fallen off the bottom of the car it was that loud. It was definitely a sound created by a large differential in pressure because my ears popped when it happened, maybe a backfire? I immediately did a diagnostics check on the gauges and steering, brakes, etc. Everything looked and felt normal and it drove fine all the way back home.

When I got back to my driveway and with the car just idling I could tell it was louder. Not open header loud, but just a little deeper and throatier than before. However at speed and under throttle I can't tell a difference in exhaust sound. I did a quick check on the bottom side of the muffler and didn't see an holes, but I suspect something in the exhaust system gave out. I am going to have it up on jack stands next week. Any ideas on where I should look/diagnose or what it may be?

Cobwebs?

Baffles in the exhaust system can and do let go- whether you might have detected anything being ejected in the rearview mirror who knows. If the idle sound is different then that almost certainly signifies something has deformed or been ejected.

Reminds me of my student days driving my 650 Triumph one summer evening- I throttled up to climb a steep hill and something like you describe happened and I then spotted a baffle bouncing down the street. Not much I could do but when I arrived back at my student accomodation I could see a big hole in the left hand exhaust pipe where the baffle had been. The solution- I bashed the baffle out of the other side exhaust- sounded great!.An original RMB?

Maybe your cat had a blockage and it then broke apart inside. Can't tell until you take it all apart.

Pound on your cats and see if they rattle.

You've discovered the 928 version of PSE. You just can't ever close the valves now for quiet mode.

I wish! Spider population is down this year. Ants are taking over the house.

I think this makes sense. Something blew out. Whatever it was it doesn't get louder with revs. Nice story on the Triumph. A British Harley after that!

Could be. I go back and forth on doing a cat delete with an after market exhaust or just staying stock. I'm leaning toward staying stock and saving up to get the highest mile/cheap running S4 I can find and turning that into a non-stock higher HP experience.

Will do!

Well it has a little more personality than before, but its never going to be confused for an F-Type. :)

Nate,

That is a really good way to piss off a British bike enthusiast if you really wanted to- a "British Harley" - "clunk" that was one of my nuts falling off from laughing so hard! Almost as funny as following a Harley into a bend and watch the rider do his shorts! Obviously no one told Messrs Harley and Davidson that most of the world has roads with bends in them. In the UK we have roads with bends in the bends [double apex] just to add to the fun. To be fair I believe Harley have come a long way in recent years and I would be proud to own one- especially the model Porsche Engineering "tickled".

My BSA Goldstar did not have the baffle problem- it had no baffles to start with! To be fair I do like the sound of the Harley or any twin cylinder four stroke with a proper non-silencer on it!

^ Lol! I had no idea. I'm not a bike guy myself. I like to look at them, just never had the desire to ride 'em. I just know Harleys are loud. :)

possibly a critter carcass that is now gone.

A WOT backfire will often blow a hole in your resonator or muffler. The split second misfire sends all that fuel down your really hot exhaust system and it ignites with a bang and lots of pressure. I had this happen on a 73 Z28 that I had, peeled the backsides right off the mufflers.

I got my car up on lift bars tonight and did a once over on the entire exhaust system. No obvious big holes or tears. The only thing I found was a very tiny hole on the intermediate silencer. It is a very small hole (~2mm) and there was a water drip comming out of it. I have a hard time convincing myself that this is the cause of the louder idling, but maybe? What do you sound experts think?

https://cimg8.ibsrv.net/gimg/rennlis...ac3924ff25.jpg
https://cimg0.ibsrv.net/gimg/rennlis...a640b65269.jpg

Backfire can result from an exhaust leak at the head or at the 3 bolt flange at the end of the manifold, but usually at the head. Doesn't have to be a big leak, but enough that it allows air into the exhaust, which can mix with unburned fuel causing an explosion. I suspect that you had a small leak and when it popped it now made the leak larger, which might explain the increased noise. Hard to tell for sure without diagnosing the car, but check for signs of exhaust leaks in those areas.

Not sure its even possible but severely retarded timing causes unburned fuel to explode in the exhaust. When I run "Boost on The Line" in my GTR I run up to 7deg ATDC in order to generate 5-7psi without load. It essentially causes the gas to explode in the turbo manifold, which spools the turbo.

Yeah I looked around the 3 hole flanges on both sides and good as I could with the exhaust still installed and didn't notice anything obvious, not to say that couldn't be the source. I know sound can travel in odd ways making it seem like it's coming from somewhere it's not. That being said the increased noise at idle is definitely behind my head and to the left, which made me think muffler at first.