Exhaust

Stiggy

Torque Junkie
Points
202
Location
Worcester
Car
Audi A3 Quattro
Just a quick question on the exhaust tuning page, just removing the the back box gave power gains. would that be an option on my car (1.8 16v ecotec astra) and just extend the pipe to the back of the car. would it give me a resonable rise in BHP or is it a BIG NO NO due to torque
 
Well for one it would be stupidly loud as you will have no baffles, but if you want to gain anything from the exhaust your best bet is a decat pipe and a larger bore back box.
 
In my old Astra I removed the back box and made the first expansion box much larger with a 2inch diameter bore throughout the system - i think we used a Vectra one. It stuffed economy but did seem to give a bit more top end power and was not stupidly loud but it was only a 1.4 Hi Torque.
 
well iv had a look at my exhaust and i dont have a centre box on mine apart from the cat, from the cat it seems to be 3.5" bore (standard) straight through to just before the flange for the rear box to connect of which it then drops down to a 2" bore

(as im confusing sometimes with the way i word things im just putting this) so basicly you have the cat then the cat back pipe that is 3.5" bore which drops so 2" at the end, then it come's the the flange and then the back box connects
 
Is a 3.5 inch bore standard - that seems quite big to me on a 1.8. You could add a cherry bomb 'silencer' on the end - they give a fantastic sound.

You can't really go straight through without any muffling at all (you will attract police attention, wind up the neighbours and give yourself a headache on long runs) - so you will need something to muffle the sound a little - either an expansion box after the cat or better flowing rear box like a cherry bomb. I don't think there will be much in the way of a power gain for you but it will make the car much more fun to drive with that sporty sounding roar!

The 3.5 to 2 inch joing should be a cone shape rather than a step - otherwise the turbulance created will stuff your airflow. So if it is a step - stick a 3.5 inch back box on and that will improve things for you.
 
yea it is a cone shape

from the cat its 3.5" bore for about 3 - 4ft (more likly 3ft) then it funnels down from the 3.5" to the 2" (the funnel length is about 6" long and then its about 1ft of the 2" bore size untill it hit the the flange wear you connect the back box to

hope thats a bit more clearer but i still cant see a box further up the system unless im confusing the box for the cat and the cat is built into the manifold like on the newer toyota's (e.g. the new avensis)
 
Not all cars have 2 expansion boxes - my car was an early 1.4 and the previous owner had tweaked it and played around with it so it might not even have been standard.

The cone shape is good. It just leaves you to do the back box now. Cat is definately not in the manifold on the 1.8 astras.
 
ah thats cool then, so i dont need to bother with a cat back system as the standard one is probably the best one for it
 
If the cat dies it will be worth getting a sports or high flow cat to replace it but other than that a back box is all you need.

New headers (an exhaust manifold) would help but for the price stick with a better back box.
 
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