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2.5" exhaust Terrible on LS, even with tune - Advice please

2775 Views 5 Replies 5 Participants Last post by  NickD
Hi there

So the short of it is that my exhaust has been piecemeal over time on my 1.8L :)

The final setup, for the past year or so, is deleted secondary cat, replaced with hi flow 2.5" res.
Deleted muffler and stock res, replaced with a Vibrant 2.5" resonator.
Out to a 4" tip

I like the exhaust tone, it's really great and has a growl, not a buzz - but performance is AWEFUL!

I have a Trifecta tune and have a big dead zone between 1k and 3k rpms.
WOTuning has even looked at logs and said the diameter exhaust is probably hurting ,but my top end is great.
It IS a great and throttly highway car...but I did my 0-60 today.... 12.4 best in performance mode. ICK!!!!! Makes me sick, and the lag between 1k and 3k feels like about 2-3 seconds. I can't even peel the tires from a stop at a light ;(

I love the tune overall, but it must be the exhaust.
Once my car is above 3k rpms, it pulls hard.

With that said, if I were to clean it up should I:
Scrap all parts and just do a 2.25 mandrel bend with a resonator (or hifh flow muffler) - I likethe TruBendz simple stainless set ups.
Replace only the existing 2.5" tubing with 2.25" - but there will still be two resonators with 2.5" in/outs

Low end really dropped out when I deleted the secondary cat - combined with more 2.5"

I do want headers in time, but I have to fix this low end situation - it's terrible AND dangerous in traffic when my car just doesn't move at 2000rpms through a turn.

Thoughts would be great - thanks!
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· Registered
39 Posts
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)

· Registered
205 Posts
Discussion Starter · #3 ·
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

· Registered
205 Posts
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.

· Premium Member
824 Posts
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.

· Banned
9,198 Posts
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.
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