What's a good rule of thumb for max boost psi at high altitudes (4000+)?

...running 91 octane pump gas

One mechanic in my town said 10 psi. I want to know what you've heard.
I know theres a few people from Colorado and Utah lurking on the FI board.

 

Depends on the motor you're running? From what I've seen on turbo honda/acura's nobody really makes changes in boost levels to compensate for altitude here. I may just be dumb, but to me 8psi is 8psi no matter what altitude you're at. The turbo might have to work harder at different altitudes to generate the same pressure, I think that's the only difference. Somebody correct me if I'm wrong...

Justin

 

sorry, It's a built B16A with a T3/T4E Turbo, .63 A/R exhaust, .50 A/R compressor

 

Boost is not boost and altitude has a big difference on boost.

 

I think I heard somewhere for every 1000 feet in altitude gained, it's like losing 0.5 psi in airflow, cause the air gets less dense. I dunno.

I think I'm going to stay at 0.8 BAR (~11.7 psi) just to be safe.

 

Ok, so what's the difference? Please explain cause I'd like to know. I just don't understand how 8psi at sea level is any different than 8psi at 5280ft. Obviously the air is thinner up here and there is a noticeable difference w/ an N/A motor. But once the air is "compressed" to 8psi, you still have 8psi just like you would at sea level, no? Maybe I'm just confusing myself more, hehe. Anyone else explain this to me?

Justin

 

I agree with your logic and an curious

 

Just because you have 8psi of "air" the oxygen percentage is not changing. Just like the NA motor you have less dense O2. I noticed just changing altitudes about a 2psi difference with stock ecu and bolt on fuel computer. 8psi at alt and 10psi at sea level. Make sure your baro correction works.