Difference between petrol and diesel silencers

MikeT · Post by **MikeT** » 01 Oct 2007, 20:16

Is there any difference, physically or materially, between a silencer fit for petrol engines and one fit for diesel engines on the same model of car?

deian · Post by **deian** » 01 Oct 2007, 20:30

Which silencer do you mean? Some cars have 2 silencers, some have 3, I would assume the front silencers are different as petrol exhaust is hotter than diesel, but further back I wouldn't think there is much difference.

I think diesel has quieter exhaust gases/flow than petrol. Someone will probably follow along to confirm, or not if the case may be.

After saying that, if one is cheaper than the other (petrol vs. diesel) then there must be a difference.

I'm also assuming diesel is denser (soot) but cleaner (chemically) than petrol so maybe the baffles are made of different material in diesel car.

jeremy · Post by **jeremy** » 01 Oct 2007, 20:46

Silencer construction isn't that sophisticated. Certainly for the BX the Turbo diesel and the 16 valve use common rear silencers - they simply can handle a large gasflow - and would be wasted with smaller engines.

Exhaust noise is due to the rapid expansion of gasses - so diesels and petrols will produce much the same under equal conditions - but remember than in a naturally aspirated form a diesel must be about 1/3 bigger in capacity than a petrol engine to produce the same power. Turbochargers will do a good job of mashing up the pulses in the exhaust ( as well as extracting energy from them) and so do quite a good job as silencers.

MikeT · Post by **MikeT** » 01 Oct 2007, 21:46

I should of said "my silencer" as I've only got the one at the end. I'll buy the one listed for the petrol car then, I like the price

Hmm, if I can gut the cat, can I gut the silencer too? 8)

DickieG · Post by **DickieG** » 01 Oct 2007, 22:23

MikeT wrote:I should of said "my silencer" as I've only got the one at the end. I'll buy the one listed for the petrol car then, I like the price

Before you jump at buying a cheaper petrol exhaust, bear in mind that 1.8 petrol exhausts are smaller than TD one's. I'm not sure of the 2.0 petrol ones size wise.

Post by **admiral51** » 01 Oct 2007, 22:30

hi mike

i would say yes as an uneducated guess about the rear silencer and from memory yours has half the work already done with that nice split down the seam 8) 8)

i would imagine it would be a bit "raspier" when resealed but not that loud as to attract to much attention from mr MOT tech

colin

MikeT · Post by **MikeT** » 02 Oct 2007, 07:33

It's the 2.0 petrol one I'm thinking of buying, seems they are the same as the 1.9TD but the listing only showed it as petrol version, hence my question to you guys.

Colin: I can't understand how my silencer has lasted all this time to be honest with you. When it eventually comes off, it'll give me a good idea of what's inside (probably looks like a gutted fish now

) and whether it's worth removing opening up the new one and removing some restrictions.

MikeT · Post by **MikeT** » 02 Oct 2007, 12:12

I got this reply when asking about modding the exhaust... "you could try fitting a 'T' piece in your exhaust pipe and fitting a QD cap so you can get a bit loud and more powerful now and again"

Does anyone know anything about this option?

MikeT · Post by **MikeT** » 02 Oct 2007, 20:36

I bought the silencer today from Euro car parts (£32 but it looks it too

) They first gave me one that had a tailpipe far too long and narrow (perhaps the 1.8 one DickieG referred to?) that was incorrectly "labelled". Luckily they had another in stock.

I still can't get a definitive answer re the MOT - there is no rule stating a silencer needs to be fitted at all! It just says "must not be excessively louder than similar vehicles etc. All I can say, is there must be some very deaf MOT testers around if these little hatchbacks and scooby's are anything to go by.

Post by **Peter.N.** » 02 Oct 2007, 21:51

Diesel silencers tend to rust from the outside inwards because the slightly oily exhaust rustproofs the inside, on petrol engines, the acid produced by combustion/condensation eats the metal away from the inside - hence the much longer life on a diesel.

MikeT · Post by **MikeT** » 02 Oct 2007, 22:22

I wish I knew this before I bought a new one - If I got a "scrap" silencer from a petrol xantia, it would have already been internally de-restricted? 8)

bxbodger · Post by **bxbodger** » 03 Oct 2007, 13:02

I have virtually no silencing on the Triumph at all-it's a straight-thru 2" pipe with a token can on the end but no tester would fail it as the straight six just sounds so good! Wether they'd do the same for the BX is another matter however....mine doesn't have a turbo, and getting a back box wasn't as easy as I thought it'd be: most places couldn't do one.

The kwik-fit catalogue is good as it gives all the dimensions and diameters of the various pipes and boxes available for any given range of cars, and that's how I found out that not turbo BX's have a smaller diameter pipe than turbo'd ones. Luckily I found a kwik-fit with a helpful manager who let me look through the book- even though I was just buying the box from them without fitting. The diesel systems are different- one curiosity of the BX thati found was that the front pipe is actually double-skinned. I cut it in half to get rid of it and that explained why it's so heavy! I think the petrol systems may be single skinned but i don't know, it's certainly the first double skinned pipe I've come across.

I got this reply when asking about modding the exhaust... "you could try fitting a 'T' piece in your exhaust pipe and fitting a QD cap so you can get a bit loud and more powerful now and again"

Does anyone know anything about this option?

In my dim'n'distant youth, you used to be able to get a cable operated valve for doing just this thing- it was cheaper than a cherry bomb! I havn't seen one for years now, kids just buy a full sports type system instead.

LOAC · Post by **LOAC** » 03 Oct 2007, 14:47

It's a good point, about petrol and diesel exhausts. May explain the problem I have:

http://www.frenchcarforum.co.uk/forum/v ... hp?t=22768

LOAC

MikeT · Post by **MikeT** » 03 Oct 2007, 17:19

bxbodger: I love the Triumph straight-six lump. I used to own a 2.5PI (ex-police car) that I converted from auto to manual with overdrive.

Thanks for letting me know about the exhaust passing the MOT, I had a feeling a silencer isn't a requirement. I remember those cherry-bombs, clever idea. I've since found out the guy meant I could fit a plumbing "T" with a screw-on cap (that he removed for racing) - he must have asbestos hands

LOAC: Did you measure the length of the box (not including the pipes out the ends)? I haven't checked mine yet but the one I just bought seems very long. Having said that, I have zero experience of Xantia exhausts.