Manufactuer's run basically the smallest size turbo they can on there engine's to make it feel more like a non turbo car so that it flows better. As the majority of buyers are non performance nuts and are after a smooth driving car. Hence you always run a turbo that a factory would fit to something bigger on a smaller engine.
And 6,000 on a 4k with your pissy little ct9? Sorry but you'll be way out of efficiancy by then easily. A t25g standard on an s13 sr20det. Read 2.0L engine will start to run out of puff at around 5-5,500rpm on a 5k and that's only a 1.5L so on a 4k would prob start dropping of around 5.5 to 6 grand. And I'd recommend a t25g at the smallest. My mrs has a t25/t28 turbo on her 4k. Basically your stupid for wanting to use a turbo of a 1.3L unless you only want like 50-60rwkw and like to waste money.
And tax_rx well he would be one of the most experianced guys on here when it comes to turbo k motor's and you just blew him and wouldn't listen to his actual FACTS from his own turbo 4k he had and now turbo 5k he has. A t25g 0.48 rear turbo on a 4k will spool quick and eat your ct9 powered 4k on the same boost all day everyday and have potential for more power also. And shot yourself in the foot is being nice. I'd say shot yourself in the kneecap buddy.
If you want a stock 1.3 turbo go buy a stock charade. No point in putting effort into turboing an engine just to choke it with a ct9 lol. Unless you want less top end then a stock n/a 4k cause that's what the ct9 will do. It'll choke the shit out of it up top.
Matt.