Tuning Exhaust diameter

[Chris170296]

Hi there, I have a 2007 fiat bravo 1.4tjet 120, I'm just looking to see if anyone knows what diameter the exhaust is, I cant find information about it anywhere online

 

[portland_bill]

Strange question. Not the sort of info that would be published.
You have the car. Do you not have a tape measure?

 

[Chris170296]

I do, but I was just curious to see if anyone knew before i went under the car

 

[portland_bill]

What are you trying to do? Can't see why this is important. Changing the exhaust diameter will change the gas flow, and potentially upset power.

 

[Chris170296]

I'm not going to change it, just going to remove the back box and put in a straight pipe to make it sound a bit better

 

[aurick86]

You have the car. Do you not have a tape measure?

The exhaust has 2 diameters, the last ~1 meter is ~58 or 60mm 2 1/4 inch but all the way from the cat to the backbox it's smaller 52mm-2 inch,

 

[Chris170296]

So if I was to trim it just before the back box a 2 inch pipe with a flange and clamp would fit or would it be 2 1/4

 

[aurick86]

I have no idea what you want to do, i think the pipe does into the backbox right after an angle change , a bend in the pipe, you might not be able to do what you want.
Go underneath the car and check.

 

[Ben D]

Doing what you propose will make it sound far from better and probably look even worse than it sounds. Get a powerflow back box made up if you want more noise and if that is not enough then get the centre box (if the bravo has one) replaced with a straight pipe. By the time you've done all that you'll have spent close to what it would cost for a full cat back system. Most restrictive bit after cat on my Punto was the front pipe which was just under 1 7/8" at narrowest.

If you want to diy it, you'll need a caliper for measuring diameter and something like a curve / graph ruler for measuring the bends and be able to weld whatever material you chose.

 

[Chris170296]

I've had a look under it and I'll be able to cut it a few inches before the back box and the fit the straight pipe, I've heard what it sounds like and I like the noise, it's a lot more throaty sounds very similar to the 595 abarth, and the exhaust is hidden at the moment so I'm probably going to keep it hidden

 

[portland_bill]

The exhaust pulses.
As each exhaust valve opens, the gases rush out, (at about 180mph, if my memory serves). The valve shuts, the gases are still moving, creating a vacuum behind them. This then causes the gases to return a little, a bit like two steps forward, and one back.
The pulse needs to exit the tailpipe cleanly. The manufacturer has taken a lot of effort to get this best for overall performance.

With a larger pipe, the pulses will travel a shorter distance each time. So the end may need to be slightly longer, or shorter. Similarly with a narrower pipe. The silencer, even with a straight-through pipe inside, will have that pipe perforated. That breaks up the gas flow, removing some noise, but replacing with a straight pipe, may change the pulse position at the end of the pipe. You may get more noise, coupled with less power.

This is why so many cars with large tailpipes make a lot of noise, but seem to struggle to pull themselves along.

So good luck with the experimenting. Be prepared to refit a new backbox, so ensure your cuts will allow that.

 

[Chris170296]

It will decrease the restriction which is most likely to increase power, I'm also keeping the pipe the same diameter it's possible that it could cause a slight drop in back pressure which can cause there to be less torque but with both the power and torque it's highly unlikely to a difference of more than 2% or 3% and anything less than 10% your never gonna feel the difference of on the road

 

[aurick86]

The Bravo's back box, is really ****ty.. . It looks like it turns 180 degrees two times:


Being a turbo engine the back box doesn't really make much of a difference, at 2 inch diameter it can take about 140 ho in NA form...turbo much more.

I've also driven a Bravo with no backbox=> straight pipe... it was noisier.. and at constant highway speed it sucked... so much drone (even tho it still had the middle resonator in place.