Right, I have a SWB manual twin turbo, it currently has a decatted exhaust which it had when it came in from Japan, it's a 3" bore exhaust and both sides meet in a centre box then come out the other side to two exits, now, the box got rattly as the welds must have gone inside, so I today had the box replaced with two seperate pipes to replace the box, So having done this the car lost a lot of compression due to lack of back pressure caused by the both sides of the exhaust not meeting which equalises the pressure, am I on the right tracks?

 

So I went back to the garage who did the work and explained to them what was going on and that the both sides of the exhaust should meet in the middle somewhere, so the guy that did it says to fetch it back sat morning to sort out, which is fair enough.

 

What he's going to do is to cut a hole in the two new pipes opposite each other and fit another 3" pipe between them to equalise the pressure which will induce the back pressure that I've lost, so it would look like a 'H', know what I mean?

 

Will this surfice?

Will this do the job as he says it will?

Right, I have a SWB manual twin turbo, it currently has a decatted exhaust which it had when it came in from Japan, it's a 3" bore exhaust and both sides meet in a centre box then come out the other side to two exits, now, the box got rattly as the welds must have gone inside, so I today had the box replaced with two seperate pipes to replace the box, So having done this the car lost a lot of compression due to lack of back pressure caused by the both sides of the exhaust not meeting which equalises the pressure, am I on the right tracks?

 

So I went back to the garage who did the work and explained to them what was going on and that the both sides of the exhaust should meet in the middle somewhere, so the guy that did it says to fetch it back sat morning to sort out, which is fair enough.

 

What he's going to do is to cut a hole in the two new pipes opposite each other and fit another 3" pipe between them to equalise the pressure which will induce the back pressure that I've lost, so it would look like a 'H', know what I mean?

 

Will this surfice?

Will this do the job as he says it will?

 

I'm not an expert on exhaust systems but when I had my system custom made I did some reading-up on the subject.

 

Basically, the bore of the original TT exhaust is 2.5-inch. If you up that to 3-inch you are allowing the exhaust gasses to flow more freely but at the same time you are slowing the flow down (I think that's what it was). Anyway - the crux of the story is that a bigger bore exhaust will reduce the back pressure (as you've somewhat correctly stated - albeit for a different reason). The central box you removed most likely acted as a flow restrictor allowing sufficient back pressure to build up.

 

Running two fully separated exhaust pipes (ie one per bank and not joined) is not what is causing the drop in back pressure. The fact you've had the box remove is the root cause of this. By adding a "H-pipe" (as suggested by the garage) you are allowing the pulsating exhaust gas flow to equalise - this has nothing to do with back pressure. You can also make the two pipes cross over in what's called an "X-pipe" - which has the same equalising effect.

 

So, in summary, you want to increase the bore of the exhaust pipe to allow the exhaust gasses to flow more freely but not the negative effect it has on back pressure (the two are inversely related - as in small diameter pipe = higher back pressure). Finding a happy "middle-of-the-road" is the difficult bit.

 

My suggestion is to see if it is possible to either reduce the bore of the exhaust pipes to 2.5-inch (upto the back boxes) or introduce some sort of a restriction in the pipes.

 

I hope the above makes sense and that I am quoting correct information. Remember, I'm no expert and am going by what I can remember from reading various technical publications.

 

Good luck!

 

Dan

so the pipes come from the turbos each side then go into one box then out of the other side of the box comes the two pipes again to your two tail pipes?

 

i had a custom on mine m8 straight through to the back ,now when i was talking to the bloke who did this he said cant be any more that 2.3/4 inch bore,he may be correct or wrong but i no his reputation,car seems to boost now at lower revs by about 500rpm and with a definate increase in power.

by putting in the pipe so it looks like a H wont that part of the exahust - get very hot.