Remove middle muffler? 1,8trd

axbx · Post by **axbx** » Mon Dec 05, 2011 1:04 am

My middle muffler(135-531) is becoming rusty and noisy, i was thinking of just removing it and connecting the "middle pipe"(786-535) to the rear muffler, is there any reason why i should not do it?

I will of course see a drastic increase of power?

BX Meteor · Post by **BX Meteor** » Mon Dec 05, 2011 3:43 am

You won't see an increase in power, you will probably see a decrease in bottom end torque.

The cam timing and lift has been designed to work with the back proessure that the full exhaust system gives. If you consider a singe cylinder 4 stroke engine desgined to work with a "muffler" exhaust, then the entire system has been tuned so that when the exhaust valve initially opens, there is a slight positive pressure in the exhaust pipe (called back pressure). Initially the pressure within the combustion chamber has to overcome the back pressure, but then the large mass of gas starts to move down the exhaust, which continues to "pull" (scavenge) the remaining gas through the system, so that at valve overlap (when exhaust and inlet are both open) there is now a negative pressure helping to suck in the inlet air (negative pressure caused becaouse the lump of gas has travelled further down the ehaust system). When the exhaust valve shuts, some of the remaining gas in the exhaust will "reflect" back to towards the exhaust port, ready for the next time. This air-mass system has been designed by someone with the intention of reducing the noise created. For a racing car, where noise is not an issue, then very different cams are used. If you alter part of the exhaust system without making any other changes, then the engine will not be able to suck in as much air anymore, particularly at lower revs, because the lumps of mass in the exhaust are not behaving as intended.

If you want to renew the middle box on its own, then it will be very difficult to detatch it from the first short pipe and from the last box. In fact it is a false economy, because you will have to replace these sooner rather than later, meaning that you have to mess about detatching the middle box carefully from the bits you have salvaged previously (if you were lucky to get them off previously).

As you are in Norway, go up to a deserted forest tarck (much easier to find than here) and then remove both boxes and tie up the pipe, and then see how dreadfull the performance is. Loud but very slow.

Way2go · Post by **Way2go** » Mon Dec 05, 2011 4:56 am

The connectors on the middle box are easy, being circumferential clamps they fall off when unscrewed. There is a U tube you can purchase which I believe is called a "devil-pipe" that will then connect the two ends so that the tuned length remains the same (but louder).

BX Meteor · Post by **BX Meteor** » Mon Dec 05, 2011 6:22 am

but no increase in power ?

Post by **mat_fenwick** » Mon Dec 05, 2011 8:37 am

I've done this, but on a tuned rather than standard turbo diesel, where the standard exhaust system sounded quite restrictive at high boost. I didn't feel it gave a noticeable increase in top end performance but it did enable the turbo to spin up quicker. I found that the noise was pretty similar to no silencers at all and although not a problem in the car, was fairly embarrassing from outside!

I think what BX Meteor is referring to holds true for a non turbo engine, where the gas flow in the exhaust helps with engine breathing.

BX Meteor · Post by **BX Meteor** » Mon Dec 05, 2011 9:29 am

Yeah I forgot about the turbo. The turbo presents the main restriction to flow I think (ie back pressure is caused by the turbo), and as you say it might spin up a bit sooner, so could bring in boost a bit lower down, but otherwise no difference. I don't know much about tuning of turbo exhausts, I assume that a tuned header is even more critical to match the turbo, so just removing the silencer restriction won't do much to a non-tuned header system, except just make more noise.

More convenient than fitting the middle box, and will scare all the reindeer out of the way in the forest.

Post by **mat_fenwick** » Mon Dec 05, 2011 10:22 am

That fact that it's forced induction though, dominates any effect caused by exhaust changes, so anything you can do to get the turbo spinning quicker will help performance. You may need to adjust the fuelling to get the best out of it, but I would think there will be some improvement even without.

When my home made system rusted last winter I just replaced it with a standard box (I was having back problems and just wanted a quick and easy bolt on fix rather than having to weld lying down!) and did notice it felt a little flat afterwards. I now have the Devil pipe so will fit that soon.

Dollywobbler · Post by **Dollywobbler** » Mon Dec 05, 2011 3:37 pm

Diesels generally do not require as much back pressure as petrols, so you might get a slight power increase. I can confirm that Mat's car was quite loud! Sounded good though...

Post by **Defender110** » Mon Dec 05, 2011 8:22 pm

Replacing the mid section silencer with a straight through pipe appears to be an increasingly fashionable practice on Land Rovers nowadays.

axbx · Post by **axbx** » Tue Dec 06, 2011 1:20 am

ok so the conclusion is that i can replace the mid-muffler with a pipe and see that i might get boost a little earlier, or i can do this and also open up for some more fuel and get even a bit more power?

thank you

Post by **mat_fenwick** » Tue Dec 06, 2011 11:51 am

Yes, that's basically it.