Hi, need info on the standard turbo exhaust

[ross255]

Hi,

i have an '87 turbo, and I think I have a standard rear silencer, as it has the black trim. But when you shine a torch down the end of it, its a straight through box i.e. i can look straight through it and see the bend at the other end, are they all like this?

Also, being an '87 it has no cat, but a silencer instead, has anyone had this off and looked in it to see if it a straight through pipe as well, I would just take it of and look, but then i will probably need gaskets.

If you can help, Ta very much.

 

[Diver944]

My standard rear silencer was just the same - shine a torch down it and you can see straight through to the bend [&:] I've never understood how silencers work [&:]

I don't know about the front silencer as I've never seen inside one.

 

[ross255]

Cheers paul, that does help.
I was sure it was the stock item, just wanted to be sure. Just need to find out about the front one now

 

[mik_ok]

My standard rear silencer was just the same - shine a torch down it and you can see straight through to the bend  [&:] I've never understood how silencers work [&:]

Janspeed back box is the same. The pipe inside the box is covered in perforations however that allow gases to "leak" out into the packing material to quieten things down.

 

[ross255]

Just goes to show that theres no point buying a "performance" back box, as its already a striaght through pipe, unless you going to a bigger diameter i suppose.

 

[sawood12]

Are you certain that the bend you are looking at is the bend at the other side of the backbox? I know from experience that it's very difficult to judge distances down tubes by shining torchlight down them. If it is then I guess there is some feature like holes in the side of the pipe that allow the gasses to expand in the larger volume of the backbox. The majority of the sound that comes out of the exhaust is the supersonic expansion of the hot exhaust gasses rather than the actual sound of the mechanicals of the engine, so the scilencer/backbox is designed to allow some expansion before the gasses exit the exhaust. That is the basic principal although in practice it's a little more complicated, as always. I suppose the pipe must go straight through though as that is exactly how the scilencer on a gun works and of course bullets don't go round bends too well!

 

[ross255]

yes, the bend is definately at the other side of the box, I stuck a broom handle in to check the depth. All the pipe has is lots of little holes for the sound to be absorbed by the baffle material surounding the pipe ala most other kind of performance boxes. If you have a look at some other na cars, you dont see straight through boxes, I guess either the front box does a lot of silencing, or it very true what they say about the exhaust turbine greatly reducing the exhaust noise as it chops it up.

 

[sawood12]

Thats right, the exhaust turbine extracts energy out of the exhaust gas flow to drive the compressor by allowing it to expand in a controlled way which in essence is exactly what an exhaust baffle is doing.

 

[Guest]

ORIGINAL: ross255

Just goes to show that theres no point buying a "performance" back box, as its already a striaght through pipe, unless you going to a bigger diameter i suppose.

Agreed. Though, the stock cat-back section is pretty heavy (mainly the stock muffler/back box is ridiculously heavy) so you could shave a pretty decent amount of weight off by going with a more conventional stainless aftermarket muffler. That's one benefit at least. Besides, even on 944 turbo's changing just the muffler/back box section normally won't give you much if any increase in power as opposed to removing the cat and/or increasing the overall diameter of the entire exhaust.

 

[Fen]

ORIGINAL: ross255
I guess either the front box does a lot of silencing, or it very true what they say about the exhaust turbine greatly reducing the exhaust noise as it chops it up.

The Turbo exhaust is very loud if the tailpipe and a bit of the surrounding end of the silencer falls off, so that box does a lot of silencing.

 

[944Turbo]

And when the wastegate opens some of the exhaust gas is bypassing the turbo completely,
Tony

 

[Guest]

Its the packing material in the silencer that does the quietening of the exhaust.

I've done quite a bit of dyno work on my supermoto race bikes with baffles in/out and different packing materials. Remove the packing material and it releases power but does increase noise quite alot.

On my bike at least and i would expect on cars too the biggest improvement is in the front pipe/manifold.
On my bike i have a micron serpant system. Its called th serpant because at every required bend in the pipe it flattens out a bit like a snakes body. Similar theory if you spray water side on at a wall. As it hits the wall the water changes shape and becomes a flatter fan shape, the anifold is more efficient because of this shape. Hope that makes sens?. Cars usually use a larger diameter manifold rather than using hydroformed pipes probably because of price but the serpant manifold on my bike gave me 4.6bhp and the bike only makes roughly 50hp standard. Quite an improvement i think you'll agree. The shape/size of the silencer and baffle in/out didn't change power more than 1-2 hp and thus for the extra noise just wasn't worth it imo.

There is quieter packing material that is used over standard stuff but on an exhaust that is several years old the packing material would have deteriorated enough to release extra power and make the exhaust louder

 

[ross255]

Well even though its pouring with rain, I could resist no longer.

I took the exhaust off, and can now say for sure that both silencers on non cat turbos are straight through boxes with just a perforated pipe. So theres no point me cutting the front one out and welding in a pipe, its only an advantage if you have a cat.

The bad news, I though it may have been a little blocked, explaining why i dont seem to notice any power difference going from 14.5 to 18 psi.

 

[mik_ok]

I don't understand your logic Ross ~ why is there no point in replacing a box if it's a "perforated pipe" design. By definition, this isn't like a straight through pipe, even if it appears to be.

 

[Diver944]

Hi Ross, I had my front silencer replaced with a straight pipe about 10 days ago and it was noticeable straight away, it gave an extra psi of boost without adjusting anything else.

I notice on your Rennlist profile that you have the Vitesse MAF and Turbo. Do you also have wideband monitoring? Could you tell me how the air fuel ratios were affected going from 14.5 psi to 18psi, were they still in the same region?

 

[ross255]

mik-ok, what I am saying is there is very little in it, from the front box to a pipe, and certainly no way as restrictive as the usual boxes that have different 'chambers'

diver944, yes i have a wideband, and a/f ratios areb always ok, i can basically do whatever i want, its allways right. Its quite simple really with a maf, its measures how much air so knows hos much fuel to add, even if you increase the boost, then it can read the extra air that is used and still maintain the correct amount of fuel.
You have probley read this being covered, or on rennlist, but itrs all true it does work.