What happens when you upgrade to a larger inducer while retaining the same exducer? The most notable change is more airflow capability; since the turbo is taking a bigger "bite" of air in every revolution, it can obviously "spit out" more air as well. Gee, more airflow aounds great... so why not go to the biggest inducer you can find? Because that creates two main problems, one much more important than the other. The smaller problem--really it's just a nuisance--is the turbo will now have a little more lag during spoolup (because the bigger wheel weighs more, plus it has to do more work with each revolution, etc). While this extra lag might not be noticed on a dyno--all the bystanders will be oohing and ahhing at the huge top-end horsepower such a turbo would produce--it would make for dissatisfaction in your day-to-day drive and could even cause you to lose a drag race to a car with less peak horsepower but more area "under the curve" due to his turbo that spools sooner. The real trouble with a large inducer increase but no exducer increase, though, is it makes the turbo much more likely to surge. Surge is the situation when the compressor "spits out" more air than the engine can swallow, which causes a backup of air at the intake and it actually creates reverse-flowing pressure waves that can be very damaging to the turbo. You want to avoid surge at all costs.