A while back I was looking for the amplifier output specifications for the NSCR-06 radio/stereo used in the later model Suzuki Alto's (also sold as a Nissan Pixo), but couldn't find them.
The amplifier in this radio is a Toshiba TB2926BHQ. It's apparently a 4x45W audio amplifier.
That means there are 4 channels and each channel is capable of delivering about 45 Watts.
Suzuki seems to use the same stereo in all of their later model Alto's, even in the cars with a 6-speaker sound system. In vehicles with the 6-speaker sound system, the front right dash speaker and front right door speaker will be wired in parallel, and the same applies for the left speakers. In the 6-speaker configuration, the speakers in the dash are only tweeters.
The factory tweeters look reasonable, but the speakers in the doors are junk (paper cone speakers).
In the models with the 2-speaker sound system, the dash has two paper cone speakers and no speakers in the doors. Suzuki didn't even bother putting in the wiring for the door speakers, which is annoying.
As you've probably guessed, one of my cars is a Suzuki Alto (I own 3 of them). The one I drive daily is a 2010 model, and it's the cheap model without any extra features. I've installed some Sony Xplod 4" 3-way speakers into the dash. No modifications to the dash were required.
The sound is much better. I can set the radio on Max volume and there is very little clipping or distortion. It can be quite loud and the music still sounds crisp. The speakers cost around $50.00 AUD (model: XS-GT1038F) for the pair. They were easy to install and no vehicle modifications were necessary.
On another note, I have my doubts that the power wiring in the car will be able to supply the stereo with all the power is needs to drive 4 speakers @ 45 watts per channel. I might test it soon and update this article with my findings.
During another mission to improve the sound quality in the vehicle, I installed Sony Xplod speakers into the front and rear doors. The front speakers are wired in parallel and I decided to keep the Sony speakers I'd previously installed in the dash connected as well. As such, the load on the amplifier in the stereo is probably about 2 ohms instead of 4 ohms on the front speaker channels. I decided to take the risk. If you read the datasheet for the power amplifier IC, it's a pretty rugged amplifier and it should be able to handle overload conditions as its own built-in safety circuits should kick in to prevent damage.
The additional speakers were a good improvement and overall it was worth doing. The stereo will clip at about volume 42 when playing music from a CD, radio stations can be turned up louder before clipping sets in. The type of music you're listing to also has an impact on the volume it'll be happy to run at. If there is little bass, then louder volumes will be OK. For instance, classical music can be listened to quite loudly, where as normal pop music will clip at about volume 42 due to the bass.