boosting at altitude

[WS6]

Well guys, now that I am a mile up I won't be able to make any descent power without boost. So the question is what do I need to adjust? I guess what is really confusing me at this point is how much more boost do I need to now plan for? My original plan back when I was lower was simply 8-10psi on a fairly simple build for my LT1. I estimate that could easily put me at 500rwhp. What about now that I am at 4500ft? How much more boost do I need for this level? I'm surrounded by mountains that hit 8-11000 ft. I will be going into these mountains though I won't be looking to make power up there. I just want to make sure I don't damage anything if I go up there. I'll be running an LSx computer more than likely so I can compensate. I guess what I should care about more is what will happen when I come down out of the mountains? If it's long term I'll just change the spring in the waste gate. For a road trip though, would i have to change the spring then too?

just looking for general answers at this point since the car is still in GA. If you'd rather point me to a resource for me to read that would be great too. What can I say? I'm bored since my car isn't here. I figure I might as well plan and learn.



I'm also thinking I'll need to go twins so I can build boost very quick ie coming in strong by ~2k but building power through 6k minimum. The idea is to make power as if I was a built NA engine in good air.

[RSX302]

That's the luxury with turbos. You throw a 10 psig spring in and it will achieve that boost. (Density might bring you down a little in hp, but I don't think it's something to worry about) The turbo will need to work harder to produce the 10psig at higher altitudes as its inlet is less. Air charge temps will be a little higher. Intercooler? Are you looking to see how much hp you're lossing to compensate density? maybe a pound or two of boost? Dunno? I would think you'd be just fine with the 8-10psig boost.
The map sensor will adjust fueling wherever you go if the system is properly tuned.

or I could be way off here, but that's how I understand it..can't say that I've taken my car up to 5000' to find out the power difference.

[Wicked]

For the wastegate control, if you have a 8-10psi spring you will get 8-10psi. The turbo will just spin different rpms to achieve the same boost due to decreasing density of the air at higher altitude. That's why turbocharging was so effective on old planes. The turbo was generally oversized for low altitude flying but would be perfectly matched at high altitude.

Basically at low altitude, the wastegate will open more to maintain the a certain boost level. The wastegate will stay more closed to spin the turbo faster and make the same boost at the higher altitude.
You just don't want the turbo to be at its upper rpm limit at low altitude or it will potentially overspin trying to achieve boost at higher altitude. Worst case of course.

Like RSX said, properly tuned, the engine will sense any variation in boost/load situation and will inject the proper fuel. Basically you would tune on the dyno for low boost and higher boost and the engine ecu will interpolate and put in the proper fuel no matter what the actual level is.