exhaust system back box question

[whensparksfly]

hi all

I am going to build a new exhaust system my problem is its on a rally car that where the back box fits at the rear will be to close to the road I have plenty of room in the middle of the car but it will leave the tail pipe from the box around 1 metre long.dose the back box in a exhaust system need to be at the rear or could it be in the middle?

 

[steviec_lj]

On an exhaust the boxes are silencers. You can do away with them on a rally car to be honest.

Also, the more bends you can put in will also quieten it down too..

 

[Spark plug]

You could also replace it with something slimmer like a cherry bomb.

 

[711jrp]

There will be a Db limit if it is a rally car so you can't just delete the silencers. In the middle is fine.

 

[piman]

Hello Whensparksfly,

the old fashioned way used to put the box across the back of the car alongside the rear valance. Can you do that?

Alec

 

[bump]

The Mazda 323 also had a sideways mounted backbox due to clearance issues.

 

[Richard.]

Is it a staged rally car where it's used on the public road between stages? If this is the case you would be better speaking to the guys who carry out your MOT as to whether your exhaust ideas will pass. I'm not clued up on what will pass an mot regarding exhaust mods on a rally car.

 

[whensparksfly]

I can fit the box in front of the rear axle it just is this going to hurt the exhaust performance have a meter tail pipe

 

[steviec_lj]

Not at all.

 

[hotrodder]

As far as MOTs are concerned it's all subjective... basically the exhaust must be secure, adequately supported, not leak and shouldn't be obviously louder than a standard vehicle of a similar type in average condition

The internet suggests 98dB for historic rally cars which is the same number used for the IVA/BIVA iirc although the method used for testing can make a huuuuuge difference* which brings us to plod... http://assets.dft.gov.uk/publicatio...oise-testing/trlreportexhaustnoisetesting.pdf and http://www.the-ace.org.uk/loud-exhausts-cost-cash/

Having a silencer** a metre away from the tail pipe won't strangle the thing but exhaust performance (noise levels, back pressure etc) is a lot more complicated than simply how many silencers there are and where they are sited...


* pointing the tail pipe at the ground and/or to the offside if the regulations involve a drive by of a microphone on the nearside for example
** straight through, reactive with resonance chambers etc... http://www.acoustics.asn.au/conference_proceedings/AAS2005/papers/34.pdf

 

[BrokenBiker]

as long as the car meets mot emmisions standards, meets decibel standards for any event you enter it in and the exhaust doesn't wobble about it will be fine.

a mate runs a road legal skyline drift car, has straight through centre section leading to an after market back box, but because of how low the car is, the simply hammered the bottom of the box until it was flat enough and fit so that it didn't touch the floor. no drop in power on the dyno, no extra noise (although it is bloody loud anyway) and doesn't affect how the exhaust hangs

 

[whensparksfly]

I can fit the box in front of the rear axle it just is this going to hurt the exhaust performance have a meter tail pipe

 

[BrokenBiker]

the tail pipe does nothing except guide the gasses out. the tail pipe doesn't have to end at the rear of the car, it just makes sense as it directs the gasses out the back and away from the car, if it stops in the middle it may cause a louder exhaust note when tested by microphone.

the restriction is all in the manifold/cat/silencers, the tail pipe is just a pipe

 

[malcolm]

The tailpipe length is actually quite important. Silencing is more about reflection and expansion than restriction. On a 4 cylinder engine if tailpipe length is below about 200mm you'll tend to get a very boomy exhaust that hurts your ears. The longer the tailpipe the better for really low frequency ear hurty booming. The ideal length tends to be around 300mm to 400mm (including tailpipe extending into the silencer) but there is not often space. Longer than that you tend to start getting tailpipe resonances which will give you a boom somewhere in the rev range. I'd guess a 1m length will give you a boom about 2000rpm or 2500rpm, then another one maybe 4000rpm or 5000rpm. You'll probably get away with it for rally car noise limits, but it might be annoying on a road car. If they test at a certain rpm then you can move the boom by making the tailpipe longer or shorter.

 

[Paul G]

I have made thousands of exhausts for all sorts of cars and the one thing that I have learnt is that if you move a back box from the back of a car to the middle of a car it becomes a middle box.