[stanman13]

There's more to efficient exhaust than just big volume. Well-designed exhaust systems have a strong scavenging effect. That means that they are designed so that the flow of gases out the pipe actually helps to suck gases out of the next cylinders in the firing order. Pipe volume, bends, expansion ares, etc. all contribute to keeping the gases moving as quickly as possible so this scavenging effect can occur. What you've done is just add volume but the flow of gases is much slower and less steady than it was with the stock system. Motorcycle riders notice the same thing after they've put open pipes on their rides. They have great performance at WOT, because they're just forcing a lot of gases through there, but at any other throttle position they're dead.

What exactly does a tune do on a NA engine? Just remap the ignition curves and maybe force a little more fuel down there? I don't see how they would increase volume of intake air at all. And if you haven't increased intake volume, there is nothing whatsoever to be gained from increasing exhaust volume. In fact, as you've seen, it works against you.

(edit: because words are hard)

 

[inssane]

Short ram intake is on the car.

Not sure the details of the tune beyond more 'aggressive' performance mode and regular "eco" mode.
Eco smooths out the shifts (truly a solid benefit over stock shifting) and in perf the shifts are more aggressive.
There is alot they can do (a trifecta tune) but details, no clue.

My gut feeling is that going back to 2.25" and a nice smooth flow straight back one side exit will improve things once again.
The 'top end' gain is not worth the bottom end lost.

Thanks for the explanation @stanman

 

[Steelmesh]

Road kill tested a V8 and kept denting the headers and it didn't hurt performance. I think they poked fun at people justifying small exhaust pipes by saying it gives them more torque; but we should use this as a data point and not the gospel.


A suggestion if you haven't already: did you reflash the stock tune and test it?

I noticed in my 4.3 silverado, when I went to a custom flowmaster system (with off road cat delete) it seemed my low end power was reduced because crusing on the offroad highway based soley on more downshifts to maintain speed on slight inclines. When I went back to a more quiet stock type exhaust, it seemed to pick up a little torque down low, again confirmed on the offroad highway again where it doesn't downshift as often to maintain speed on slight inclines. I did not suffer such a big power loss, I could still easily do a burnout and tow car haulers.

 

[spaycace]

There's definitely a fine line as to which exhaust size is the right fit for the car. 2.5" seems a bit large to me on a 1.8L 4 banger, considering my 6.2L V-8 came from the factory with a 2.5" exhaust system. An exhaust system that's too small will cause performance issues, just the same as an exhaust system that's too large will. I've had significant mods on my G8, and my tuner suggested a 3.5" exhaust upgrade, instead of a 3" upgrade. If I were you, I'd try to find out what other 1.8-2.0 liter engine size owners have had success with on their vehicles as far as exhaust diameter. Then again, N/A vs. turbo is another consideration to take into account, as N/A won't require as much air flow out as a turbo set up.

 

[NickD]

Ha, my 7.0 L engine is using 2 1/2" pipes, but has two of them, that's 3.5L each, and you are concerned about a 1.8L?

Fooling around with cats can cost you $40,000.00 if you get caught, but kind of like immigration, nobody is checking.