JJ posted it made mention about how the turbo will spool up more just from the lack of atmospheric pressure and gain boost ,,well I think that's what it said???
I think a lot of this is getting lost in definitions...
"more spool up" I'm reading as "making more boost".
EG, if atmospheric pressure is 10psi compared to sea level 14.x psi and you have your sled setup to run 5psi.
If you are using a mechanical setup (no ECB) iit effectively works as a "pressure differential regulator" in that it'll always add 5psi back into the system, regardless of atmospheric pressure (difference between charge tube and atomsphere will always be "atmosphere + 5". So at sea level this means you have a MAP of 19-20ish whereas at 11,000 feet you'll be closer to 15. (14.7+5=19.7 - sea level pressure; 10+5=15 - 11,000 feet). Though output will be very different (and so will your clutching and fueling requirements), the turbo will react very similary, in that the way it "builds boost" and how much lag there is won't change too much (but it'll still change...in fact, your charge temps will even go up despite the net drop in power...but lets leave that out for now).
Now if you are running an electronic boost controller with a MAP sensor, the turbo's wastegate is controlled far differently. At "5psi" you are going to see a MAP pressure of 19-20 at all elevations (assuming the turbo can keep up, which again, it should be able to)
So lets walk through this scenerio...
At sea level, everything is the same. Literally nothing changes.
At 11,000 feet, the turbo has to compensate for the 5ish psi loss in air density. It does this by "working harder", "spinning faster", "compressing more air". At 11,000 feet the turbo has to put double the psi back into the manifold to arrive at the same output (10+10=20). In this case the turbo will have to "spool up" more to arrive at the same output. There will be more heat (charge temps rise), more lag (takes longer to compress more air). And yes, the turbo is compressing more air as the system can't just rely on ambient atmosphere (no compression) for its baseline air density. Depending on how much more heat it is making, your output will drop. As your air density went down. (hotter air makes less power). This may be a moot point as we are running in snow and 10psi is still relatively low amounts of boost compared to 4 stroke car world....but yeah, your charge temps are still going UP.
As tdbaugha mentioned, going to a lbs/min or any sort of true density/flow calculation is going to be more accurate as it takes into account the variable of rate. Never thought about CFM vs lbs/min but he's right. Density is going to be more important than volume, though CFM still is an indicator of this, its not as precise i suppose.
For this example, go get a crappy $50 Harbor Freight compressor. Then go get a $2,000 gas powered compressor. Set both to 80psi and try to blow your backyard sprinkler system out. Both were set to the same psi, so what happened? In this case, its all about flow. One is akin to a kitchen faucet the other a fire hose. Pressures being the same, one is doing a lot more work than the other.
How much air you can get into a motor is what we are trying to alter. More air = more power. This is why the whole idea that thinner air somehow makes it "easier" on the compressor wheel is so wrong. For that, I again turn to my snow analogy (heavy snow is easier to compact than light powdery snow...and in this case, we want the heaviest snow possible)
Again, I'm just an enthusiast. Not a real pro. YMMV.
! still wondering about the wright up JJ posted it made mention about how the turbo will spool up more just from the lack of atmospheric pressure and gain boost ,,well I think that's what it said???
