Well they are optimised, just often not for power.

Still, I've never seen an after-market exhaust make a significant difference on a modern car, personally; at least not to outright power.

Scavenging efficiency will be reduced by back pressure, irrespective of pulse tuning. You also lose power through extra pumping losses.

Reducing back pressure without making any other changes can have an adverse effect on an engine, especially carburettored or early EFI ones that are unable to adapt to the changed fueling requirements.

Yep that's it exactly. But only n/a motors. Good exhaust tuning can introduce another cycle into the combustion process.

With forced induction setups the intake charge is already under greater than atmosphere pressure, so you get no gain from scavenging. Which then makes it all about flow rate.

I think this depends on 2 key points.

1. What car
2. What do you mean by exhaust

If taking point 2, a person means a back box or a cat back. Then no, you'll not see massive gains. If by 'exhaust' you mean fitting a tubular manifold (optimised for exhaust scavenging on a na car), free flowing cat back, high flow metal matrix cats and suitable bore sizing and suitable tuning. Then ANY car will see significant gains.

Taking point 1. If the car you are tuning makes relatively low HP, even fairly large percentage changes will still yield small actual HP gains. e.g. a 3-5% gain from a 143hp motor is still not a lot of HP gained.

But 3-5% gained on a 325hp motor suddenly are much larger gains. Indeed there are many dyno tests of LS1 V8 cars gaining in the region of 20rwhp with the addition of long tube exhaust headers. Total exhaust tuning on these engines can yield 30-40rwhp total gains over stock. In percentage and raw HP terms I'd say that's fairly significant.

The thing I love about "NA engines need backpressure" is that race cars, like F1, have enough pipework for pulse tuning and nothing else.

Either F1 cars would be faster with a silencer or pumping loses are always bad.

Is that after remapping the ECU to optimise fueling and ignition for the new exhaust?

I can see two logical explanations.

1. Not all high flow cats are created equal. I think David Vizard suggest 2.2cfm per 1hp for a resistance free exhaust system. Almost all ceramic core cats, even high performance ones will fail to attain this at higher hp levels. I also suspect many are not 3" inlet/outlet and core dimensions.

2. A cat will affect the exhaust pressure wave. This is a very hard thing to tune (imo) and usually way beyond the amateur or enthusiast (and certainly me!!) to resolve without expert intervention.

Yes, this touches on the exhaust pressure wave I mentioned above. I think David Vizard also suggest the use of a resonance chamber in some of his designs to allow tuning of the wave.

lol

Nah it's easy to make gains. To make the most gains and the most efficient setup is the difficult part.

Breather mods on my brothers VVC MGF made a world of difference. We never had it dyno'd, but it self SOTP significantly quicker, revved freer, drove better and certainly performed better against other cars. And this was without optimising the tune either.

In relation to why, even after a remap, a "less restrictive" system might give lower power - if you reduce the back pressure, you reduce the pressure in the primary pipe, changing the resonant frequency of the tuning effect.

The resonant frequency is dependant on diamter, length and air density (good luck doing some quick hand calcs on the average density of a pulse of gas cooling from 950 degrees down a few feet of pipe, but it'll change a bit with backpressure).