Exhaust middle box question

[RaceDiagnostics]

My middle box is aftermarket and does not fit very well, the back end of it rattles against the heat shield.

When it was off the car I checked and found that it was completely empty (is that normal), so I am now wondering what effect it is actually having? If I replaced it with straight through pipes to the back box would it be any louder or just the same?

PS I don't want it to be any louder.

Mid box.

Back box.

 

[cumbriasteve]

It should have perforated pipework and baffle plates, the lines on the outer surface would suggest at least the baffle plates are there, the lines are in the locations you would expect the plates to be.

EDIT: Sorry..was looking at the back box...would think the middle box had something though..seems strange.

 

[RaceDiagnostics]

I stuck in a bore scope and it was empty, I'm sure that's because it was cheap aftermarket thing.

 

[cumbriasteve]

Here you go...looks like there should be some method of the gas having to transfer inside the box from the front pipe to the pipe that goes to the back box, or perhaps just the empty space does the business.

Centre silencer is a part of the exhaust system of the car. It is located in front of the main silencer and has a barrel-like shape. Inside it there is a receiver tube that extends to the rear silencer. The special design of the centre silencer provides a significant reduction of noise in the exhaust system. Therefore, it is easy to determine failure in this area.

 

[the black box]

What car is this on? Looks to me like a balance style silencer designed to balance the two banks of cylinders.

 

[RaceDiagnostics]

450SL, it already has an H pipe towards the front to balance.

 

[p0689109]

The two front pipes look uneven as they go into the front of the box so twisting it to allow to rattle to me. As to is it empty I would go with cumbriasteve on that

 

[james butler]

having an empty box increases back pressure by slowing the gasses down in the void before continuing down the pipe.
i have the same setup on my scirocco, the original mid box has nothing in it as standard.
i cut it out to see if it did anything and replaced with a straight through pipe, all it did was increase the popping on overun out of the exhaust and my mpg got worse so i put it back in.

 

[pedrobedro]

If it's empty it's probably an expansion box not really a silencer.

 

[ibrooks]

"Backpressure"

It's not uncommon to see an empty box in an exhaust. They are a resonator and should be a tuned size/shape.

When you say it's "aftermarket" do you mean a pattern part (copy of an original made by another manufacturer) or something from something else that's been put on the car?

The exhausts job from a performance point of view is to maximise gas speed so it needs to be big enough to flow the waste gas from the engine at max speed with minimal resistance but small enough that that gas maintains speed. The idea with getting the gas moving quickly is that it picks up momentum and when the exhaust valves close the gas tries to keep going and can't slow down and reverse flow. That way when the next exhaust valve opens the flowing gas from the previous cylinder "sucks" some of the gas out before the piston has to start pushing it (using power that could have been going to the wheels). When the piston gets to the top of its exhaust stroke and starts moving down the gas keeps going out you get a little "negative" pressure in the cylinder to help pull the intake charge in. If your cam has some overlap (practically all will to some extent) you may actually get some initial flow where intake is pulled all the way through the cylinder and into the exhaust - popping is the sign as unburnt fuel has been drawn into the exhaust, you know that all the exhaust has been cleared from the cylinder in this case. Ideally you don't quite want the unburnt fuel to have gone that far but it's always going to be a compromise, the exhaust has to work over a huge range of operating conditions so you pick the point where you want it to work best and live with it being sub-optimal everywhere else. The manufacturer has picked their compromise of performance at certain engine speeds and noise depending on the cars intended use (little cars will have an exhaust made to be quiet and give max fuel economy at town speeds whilst big motors with big engines will be more biased to working better at motorway speeds).

A resonator is designed to use pulse tuning to quiet the exhaust without harming gas speed rather than baffles - baffles are an attempt at a Tesla valve that works over a range of gas speeds. When an exhaust valve closes a shockwave will travel down thee exhaust, if you can arrange for that shockwave to bounce off something in the exhaust travel back and then bounce off the back of a valve that's about to open then as it travels back down the exhaust on it's second journey it takes a bit more of the exhaust from that cylinder with it. Try modelling that! Now add any venturi effect at the tailpipe from the cars speed.

Back to help for the OP - it's surprising what you can achieve by turning joints a little to gain clearance so that would be my first try. If you are set on changing the box then try to get an OE part or quality aftermarket. The cheaper aftermarket stuff as well as being lower quality will often have a small range of "boxes" and they will simply pick the one from their shelf that's closest to the exhaust they are trying to copy based purely on shape/size rather than what type of box they are actually trying to replace and the job it's doing.

 

[wookie]

"it's surprising what you can achieve by turning joints a little to gain clearance so that would be my first try.

That's what I always try first, the number of exhaust rattles I've stopped by just tweaking the joints is crazy.

 

[RaceDiagnostics]

You can see better here, the top of the box is touching the heat shield, and completely misses the recess for it. Twisting wont fix that issue.

The above came from Eurocarparts, the original looked more like this.

If the consensus is that straight through will be louder/worse then I will just need to bite the bullet and buy another better part.

 

[Rig Pig]

Chop it out! My car has most of the exhaust silencer bypassed, sounds lovely

 

[wookie]

If the consensus is that straight through will be louder/worse

It all depends on your car/engine, I'd be asking other people with the same car/engine as you as to what they have found works.

I had a tuned up VW caddy that was a bit louder with middle box replaced with pipe and it went a bit better but the mpg stayed the same.

I've had a number of Rover V8's, the one that still had the original cast manifolds desperately needed the back pressure or it lost all torque, so full exhaust with H pipe etc required. The current has a pair of 4 into 1 tubulars then a Y pipe to a single silencer, all large bore so breathes very easily and goes like the proverbial but the volume is definitely up to 11 when you boot it, it's not exactly quiet at idle or light throttle either

 

[ibrooks]

Klarius - suddenly all becomes clear.

They are one of the "pick the nearest size box that we already make and shove it in there" brigade.

They're also slapdash when it comes t the welding.

 

[james butler]

doesnt fix the issue as such but could you bolt one of these to the exhaust https://www.google.com/search?biw=1...k#imgdii=0kFENA5iwK1ZuM:&imgrc=zC7P3mwt21q1hM:
then when it does touch the floor/heatsheild it wont damage it being metal on metal.