Verified answer

A turbo is not going to give you better gas mileage.

I can give you some real answers...

The amount of boost the b18c1 can withstand is not important; how much power can the stock motor hold is what you want to ask. Power output is directly related to how hard the motor is working - boost pressure equates to how hard the turbo is working. A larger turbo may only need 8psi to make the same power number as a smaller turbo at 14psi. With that out of the way, around 350whp is pushing reliability on these stock motors. Some people make as much as 400whp but their risks are higher as well. On the safe side, most tuners try to keep stock b18c1 around 300whp. With stronger head studs, they may push it a bit more.

Even so, that doesn't mean that you will be safe and your motor will not blow up. The reliability of the build depends greatly on your choice of parts, quality of work and skill of your tuner. Of course, this is considering that your motor was healthy to begin with (something you should check out before going down this route).

For gas mileage - you can actually get real good mpg......if you can stay out of boost. I'm not going to go into the exact details of why that is but in a nutshell, when out of boost, the turbo helps the motor burn fuel more efficiently. In boost, tuners dump fuel to prevent detonation when tuning for max performance. Depending on what turbo or setup you go with, it may make it easier/harder to get good mpg. Smaller turbo with quick spool is going to be harder than a larger turbo with slower spool. Of course, each comes with other advantages and disadvantages on the performance end.

My mpg when stock w/ bolt-ons was 35 hwy (40 a few times), 32 mixed and 28 city. Now, I've been boosted for 3 years at 350whp w/ a small quick spooling turbo. I get consistent mpg of 38 hwy, 34 mixed and 31 city when I play nice. If I don't, I've seen as low as 25 mpg around town and roughly 8-9 mpg at the tracks.

With no modifications, I say about 5-6 psi. Just so you know, a turbo is gonna add a lot power to your motor. And that is not gonna help you save gas. If you want it to run safely & be good on gas, then keep it stock.

The selonoid like all people said, one more factor not said is that if the knock sensor is sensing knock(even supposing there is none) it is going to disable vtec process. Then you've gotten a motor that's clearly no better then a b18b. So ensure the knock sensor is safely working.