Exhaust advice

Post Thu May 21, 2020 8:06 pm

Need some advice on my exhaust please guys. Until last week my car consisted of standard manifold, standard downpipe and centre box (all 1 piece) and a scorpion back box (including pipe from centre box) It really made a terrific noise.

I've recently had my centre box custom made in stainless and my scorpion back box opened up and the baffles replaced with stainless and new packing material. So I figured with it being stainless from the gearbox back it would sound even nicer.

When they removed the old centre box they said it was empty.

It sounds good now, a lot more refined like a 325i should, but it has lost its howl from 3k rpms, it doesnt sound like the revs are as high when at the same rpms. I need that sound back. I honestly didnt mind how loud it was before.

So do you think that it was due to the empty centre box? What should I get them to do with it? Remove the packing from the new stainless centre or remove the centre completely and have straight pipes through to the back box? I've also been looking x pipes, but cant seem to find any in 45mm. Would they be a good idea if deleting the centre box?

Post Thu May 21, 2020 11:02 pm

This probably isn't ultrah helpful but I've always found the standard 325i exhaust that's gone a bit ropey to be one of my favourite exhaust notes. Maybe just let it rust for 20 years....

I have heard about cutting out baffles from a stock muffler (rear) to increase the raspiness but can't say I've got any recent/first hand experience with this.

I would imagine that without a centrebox full stop it would likely get a bit droney, at a guess.

Maybe the centre box hasn't left the skip yet if you want to ask the exhaust place for it back and play around with it yourself

Post Fri May 22, 2020 5:31 pm

The distance to the first expansion point in the exhaust (ie. the front silencer) is carefully calculated to ensure that the best power is extracted from the engine.

If you remove it you mess that up and lose power.

Not a problem if all you are worried about is the noise it makes, but I wouldn't do it.

The most likely thing that has changed the note is the repacking of the back box.

Ben

Post Fri May 22, 2020 7:45 pm

Thanks for the help lads, and Ben it sounds like you are pretty clued up on this. I've had another look at the photo of the new centre box compared to the old one and they havnt copied it very well. It looks as though the silencer is now fitted further forward, as you can tell by the length of the pipes coming out the rear. And I'm actually not too pleased with the pipes going in either, surely bending them and having them entering the silencer straight would be a better way to go.

20200516_113602.jpg

This has made my concern over the sound less important and now thinking that it may not be of optimal design? I'm sure if I get onto them they will make the changes I want, what do you think?

Post Fri May 22, 2020 8:11 pm

The replacement is not ideal. They should have kept the length of pipe in front the same.

The original looks in reasonable condition. Why did you change it?

Ben

Post Fri May 22, 2020 8:45 pm

Thanks Ben, it started with a resonance in 5th gear at 3.6k rpms precisely, shortly after the exhaust developed a rattle which went away after a while, but the resonance stayed. I thought it was more likely to be the centre box as the rear box was stainless. So I got the new centre box installed, and after 20 min driving a high pitched rattle developed. I went back and the rattle was traced to the back box, one of the baffles had broken loose. So they replaced both of the baffles in the back box and fitted new wadding. I'm definitely considering getting them to alter the routing into the centre box more like the stock one, and move it back a couple of inches.

Post Fri May 22, 2020 10:24 pm

The front box is the necessary X pipe. Many years ago (more than most of you have been alive) I was into exhaust design for a while, and produced a few manifolds to my own design, and the idea basically is that when the negative pressure pulse (caused by an exhaust valve closing and the inertia of the gas in the pipe) reaches the X point, it travels back up the other pipe, just in time to help suck the exhaust gasses from another cylinder. The time the pulse takes to get there depends on the length of the downpipes, and the timing to get there just in time to suck out the cylinder depends on the engine revs. In simple terms, shortening the downpipes pushes peak engine power to higher revs, and lengthening them brings the point to lower revs.
Stainless steel silencers are good in terms of corrosion resistance, but because they resist corrosion, they also get made out of thinner steel than they would be if the steel was not corrosion resistant. In short, they might not get holes in them, but they do fall apart.

Post Sat May 23, 2020 12:38 am

Thanks Brian, so the new centre box full of packing material wont perform as well as the empty box that was on there before in that respect? I'm assuming when they said it was empty, that meant no perforated tubing or packing.

If I was to get the centre box removed, would it be beneficial to place a H pipe as close to where the old box was? Just wondering what my options are as I do want to get them to adjust it in some way.

Post Sat May 23, 2020 1:19 am

Brianmoooore wrote: ↑
Fri May 22, 2020 10:24 pm
The front box is the necessary X pipe. Many years ago (more than most of you have been alive) I was into exhaust design for a while, and produced a few manifolds to my own design, and the idea basically is that when the negative pressure pulse (caused by an exhaust valve closing and the inertia of the gas in the pipe) reaches the X point, it travels back up the other pipe, just in time to help suck the exhaust gasses from another cylinder. The time the pulse takes to get there depends on the length of the downpipes, and the timing to get there just in time to suck out the cylinder depends on the engine revs. In simple terms, shortening the downpipes pushes peak engine power to higher revs, and lengthening them brings the point to lower revs.
Stainless steel silencers are good in terms of corrosion resistance, but because they resist corrosion, they also get made out of thinner steel than they would be if the steel was not corrosion resistant. In short, they might not get holes in them, but they do fall apart.

Probably the best paragraph on exhaust design that I've ever read.

Post Thu Jun 11, 2020 3:58 pm

A quick update on this, went back to the exhaust fabrications today, luckily they had the old centre section still. They cut it open we had a look inside and there was just two straight perf tubes and no packing. So we decided to remove the packing from the custom centre section like the original and now happy to say it's sorted. Feels much quicker and no resonance on the motorway. Its raspy now, not quite s54 rasp but sounds good. I'm guessing it was down to cancelling the x pipe effect by having all the packing in there.

Post Thu Jun 11, 2020 4:55 pm

Very interesting - don't suppose you have any pics? Glad it's sorted

Post Thu Jun 11, 2020 7:11 pm

20200516_113602.jpg

This is the only pic I have. The standard box was a bit smaller, although I dont think it was a genuine one anyway. As I say there was no packing inside just two perforated pipes.

We checked the position and the new box is fitted 2 inches further forward than the original was, it wasnt worth the time moving it back. I dont think its made a huge difference to the power delivery, but it's possible the torque is slightly higher up the rev range as Brian suggested it would.

All I can guess is having that packing in there was cancelling the x pipe effect which reduced power, sound and didnt help with exhaust resonance. It feels quicker, smoother and I'm enjoying it again which is the main thing.