I don't know about Myvis, but for an engine like the 1.8L 4G93 with an autobox which spends 99% of its time at 2500-4000rpm, a good exhaust upgrade that maximises power at those rpms (I don't care about sound) will be a straight flow muffler with an internal diameter of roughly 1.7" (which is more or less the same with the stock piping). The stock piping is actually sufficient but the problem is that there are too many kinks, restricting flow. Change that stock piping to mandrel bent piping of the same 1.7" diameter, and remove the resonators.

The end effect will be a direct flowing exhaust of 1.7" inner diameter all the way from the extractor to the exhaust tip... ie. no increases or decreases in inner diameter (ie. volume) from one end to the opposite end. That should maximise the output of the 1.8L engine between 2500-4000rpm.

Yes that is indeed my opinion. A free flowing exhaust with no back pressure is good for any car, automatic or manual.

An engine+auto is no different to the same engine+manual trans. The only difference is the rpms where the driver spends most of their time.

Like I said, it is the rpm which is important.

For example,

1L engine @ 3000rpm produces n m3 of exhaust gas & needs 1" exhaust piping (no mufflers etc) to vent the gas out most quickly.

1L @ 4500rpm -> n+1 m3 & needs 1.2" piping.

1L @ 6000rpm -> n+2 m3 & needs 1.4" piping.

So, piping must suit the volume of gas produced at the relevant rpm.

So, what is the "relevant" rpm?

For manuals, that's whatever rpm you want it to be.

For automatics, it's usually between 2000-4000rpm. Rarely anything out of that range for 99% of the time. So, in order to have the best piping for this 1L auto, you'll need something between 1" to 1.2".

So, why do most people think autos need "backpressure" and restrictive exhausts or additional resonators?

Simply put, because they go listen to the exhaust shop and install piping which is too big, for eg. 1.4" which will be fine for high rev 1L manuals but not for autos which don't high rev very much.

What happens when they go back to the exhaust shop complaining about low rev sluggishness? Will they replace the piping with a smaller one? No, because that confirms they are not very good in exhaust tuning and they can't charge you $$ for the additional work. So, they make up a fairy tale that "autos need backpressure" and recommend restrictive exhausts or additional resonators to make the entire system more restrictive. For eg, adding a restrictive muffler to 1.4" piping will make it flow as if it is 1.2" piping. On top of that, they make $$ selling you mufflers and resonators.

So again, "backpressure" ie restriction is NEVER good. You simply need to choose the piping size that flows best for your engine's capacity and rpm range. If 1.2" piping is best for your auto, then make sure your free flow exhaust and resonators also have internal diameters of 1.2".