EBC's, ELEVATION AND TURBO SLEDS.... A GOOD TECHNICAL DISCUSSION.

Nice chart GM
Impressive smart-phone skillz!

Your chart shows, well, that with a functioning "EBC" the turbo is actually not making any more boost at altitude.... Charge-side pressures remain consistent.

I've heard/read in some presentations on this and other forums that "An EBC will increase boost at altitude to maintain power levels"...which is not accurate... you are not making more boost.

Although you are NOT making more boost at altitude with a functioning EBC, the system is spinning the turbine/compressor assembly faster in the less dense air at higher elevations to maintain charge pressures. (see the 'bucket' example above). Your typical boost gauge will however READ higher boost, but the charge pressure has not changed.

The engine 'sees' the same boost at all elevations with the EBC as it keeps the MAP-sensor pressure as consistent as the system design will allow.

gmustangt said:

Simply put if you are running "6psi" at sea level. With an altitude compensating feature. Rough numbers:
Typed on my phone .. Hopefully it lines up.

Elevation. / Atmosphere PSI. / Turbo PSI. / PSI Absolute

0ft ...... ...... 14.7 .................. 6 ............ 20.7

1000ft. ...... ...... 14.2 ...... ...... 6.5 ...... ...... 20.7

2000ft. ...... ...... 13.7 ...... ...... 7 ...... ...... 20.7

3000ft. ...... ...... 13.2 ...... ...... 7.5 ............ 20.7

4000ft. ...... ...... 12.7 ....... ...... 8 ...... ...... 20.7

5000ft. ...... ...... 12.2 ...... ...... 8.5 ............ 20.7

6000ft. ...... ...... 11.8 ...... ...... 8.9 ............ 20.7

7000ft. ...... ...... 11.3 ...... ...... 9.4 ............ 20.7

8000ft. ...... ...... 10.9 ...... ...... 9.8 ............ 20.7

9000ft. ...... ...... 10.5 ...... ...... 10.2 ............ 20.7

10000ft. ...... ...... 10.1 ...... ...... 10.6 ............ 20.7

Click to expand...

.

Someone is entering the dark side...once you go there, there's no going back...

Be careful...

A lot of this is "right" but not exactly widely accepted.

1) Boost pressure is the difference between ambient atmospheric pressure and charge side pressure. In car world, sled world, etc, most are not talking in kPA but in PSI. Its standard. If we are talking aircraft, sure.

2) If the turbocharger is working harder, I consider that "making more boost". Skin the cat however you want, but the compressor has to spin faster and compress more air to bring things to a constant MAP. Put another way, an EBC running on MAP or kPA can compensate for altitude, but the wastegate is staying closed longer, and the turbo making ***more boost***

thanks for the post lots of info.. gives a good idea on how and why the turbo works. hade to read some of that twice but its pretty clear the turbo is working harder at 10000ft to get the same HP the engine once hade at sea level but that's all that is... that makes sense!!! I do see why u say its making ***more boost*** it is right? but it needs to do to lack of atmospheric pressure its just replacing what is missing. so do all turbo work like this and up the boost as you go up in elevation?

sumtinwong said:

thanks for the post lots of info.. gives a good idea on how and why the turbo works. hade to read some of that twice but its pretty clear the turbo is working harder at 10000ft to get the same HP the engine once hade at sea level but that's all that is... that makes sense!!! I do see why u say its making ***more boost*** it is right? but it needs to do to lack of atmospheric pressure its just replacing what is missing. so do all turbo work like this and up the boost as you go up in elevation?

Click to expand...

No. Its all about how the wastegate is controlled, be it electronic or mechanical.

Sidekick is an electronic boost control that keeps MAF more or less the same (within the range of the wastegate's capabilites)

Silber's current setup keeps the pressure differential between atmospheric and charge tube static. That is to say the amount of boost doesn't change regardless of elevation, but your MAF will change.

Ping me if this doesn't makes sense...

In car world, some ECUs actually turn the car's turbo DOWN at elevation on account it has to work harder in thinner air, causing your charge temps to rise, sort of entering a negative fulfilling loop. This is why bigger turbos (higher CFM) and better intercoolers help with respect to higher boost situations...which is essential at higher elevation.

MAF (Mass Airflow sensor) on a sled???

JJ_0909 said:

No. Its all about how the wastegate is controlled, be it electronic or mechanical.

Sidekick is an electronic boost control that keeps MAF more or less the same (within the range of the wastegate's capabilites)

Silber's current setup keeps the pressure differential between atmospheric and charge tube static. That is to say the amount of boost doesn't change regardless of elevation, but your MAF will change.

Ping me if this doesn't makes sense...

In car world, some ECUs actually turn the car's turbo DOWN at elevation on account it has to work harder in thinner air, causing your charge temps to rise, sort of entering a negative fulfilling loop. This is why bigger turbos (higher CFM) and better intercoolers help with respect to higher boost situations...which is essential at higher elevation.

Click to expand...

.

sumtinwong said:

thanks for the post lots of info.. gives a good idea on how and why the turbo works. hade to read some of that twice but its pretty clear the turbo is working harder at 10000ft to get the same HP the engine once hade at sea level but that's all that is... that makes sense!!! I do see why u say its making ***more boost*** it is right? but it needs to do to lack of atmospheric pressure its just replacing what is missing. so do all turbo work like this and up the boost as you go up in elevation?

Click to expand...

Great discussion above from all!


It is true, the turbo is spinning faster to keep the charge side pressure Constant in the sled. With an EBC, charge side pressures are not increasing though. See the Bottle-Example in the first and second post above.

Some of that is due to the waste gate being held closed longer... some is due to the fact that the lower ambient pressure also spins the compressor faster because the pressure inside the exhaust stays the same, but the pressure outside is dropping.

According to a conversation that I had with an engineer at Borg Warner, turbocharger shaft speed increases 1 or 2% with every 1,000 ft of elevation increase with same waste gate open/closed cycle. So... at 10,000 feet, the turbo will be spinning 8-16% faster at elevation with the waste gate operation remaining the same.

On these lower 'boost' systems with "EBC"s, systems that are designed to keep the charge side pressures consistent... and are essentially 'fooling' the engine into believing that they are working at a constant elevation... If the system is designed well, and the mfg takes the time to incorporate the correct components... pumping efficiency is not run "on the ragged" edge and charge pressure/temp levels are maintained at respectable levels. Levels that the consumer benefits from in terms of consistent operation, longevity and flexibility at altitude.

When you "crank up the boost" at elevation.... meaning actually increasing the charge side pressure and not the gauge reading and INCREASING the HP output of your engine... you will get further away from a given systems design goals... and run that system on the edge.

Sure, you can make a system that is adaptable and allow for higher HP levels ... but if you design a system to run OPTIMALLY at a given charge pressure/temp level... It will perform the most consistently for the owner at those levels. This is what makes the current offerings so user friendly. They are designed to run best at recommended charge pressures and HP levels.

If your goal is to have power levels that a stock engine can handle, and give the owner a good balance of reliability, lack of tuning needed, and great 'rideablitly' (throttle response and flexibility at all intended elevations)... then 'low boost' (aka reasonable consistent charge-side pressures/temps).... Then a low boost kit is just the ticket... get greedy for more HP than say, 180 TRUE hp output, then you are asking for less durability and less flexibility from the system.

Even at 12,000 feet, if the turbo system is working within it's efficiency curves.... temp increase really is not a major concern for a given system with reasonable boost levels... intercooled or not.

Because the air is less dense at higher elevations, heat gain due to friction is not that huge even though turbo-shaft-speed is increasing. Higher elevations often mean lower ambient air temps. Friction is caused not by the volume of the air being pumped, but by the number of air molecules of air 'rubbing' against the surfaces of the compressor assembly.

One cubic foot of air at higher elevations will have less molecules of air (nitrogen, oxygen, argon, C2O, etc) in it than one cubic foot of air at lower elevations. The space between the molecules increases, but not the number of molecules.

Of course, there is a tipping-point where increased shaft/wheel-speed in the turbo will cause temperatures to rise on the charge side of things... where that tipping point is, well, that is how "well designed system" comes into play.... avoiding drastic changes by operating inside of certain operating curves.

And YES, you can increase charge side pressures by 'cranking it up' but you will then have to deal with all of the variables that accompany it. Those variables include finding a new balance of increased fuel octane and burn properties, increased

A larger turbo will not give you the same throttle response as a small turbo unless that smaller turbo was not sized properly... it's just physics.
Sure, it may give you more HP, but what is the trade off??

It's not just a matter of using Higher Octane fuels when you want to increase HP levels of your turbo system.

More fuel for conversation. (pun intended)





.

mountainhorse said:

MAF (Mass Airflow sensor) on a sled???











.

Click to expand...

Sorry that was supposed to say MAP.

mountainhorse said:

It is true, the turbo is spinning faster to keep the charge side pressure Constant in the sled. Some of that is due to the waste gate being held closed longer... some is due to the fact that the lower ambient pressure also spins the compressor faster because the pressure inside the exahust stays the same, but the pressure outside is dropping.

According to a conversation that I had with an engineer at Borg Warner, turbocharger shaft speed increases 1 or 2% with every 1,000 ft of elevation increase with same waste gate open/closed cycle. So... at 10,000 feet, the turbo will be spinning 8-16% faster at elevation with the waste gate operation remaining the same.

On these lower 'boost' systems with "EBC"s, systems that are designed to keep the charge side pressures consistent... and are essentially 'fooling' the engine into believing that they are working at a constant elevation... If the system is designed well, and the mfg takes the time to incorporate the correct components... pumping efficiency is not run "on the ragged" edge and charge pressure/temp levels are maintained at respectable levels. Levels that the consumer benefits from in terms of consistent operation, longevity and flexibility at altitude.

When you "crank up the boost" at elevation.... meaning actually increasing the charge side pressure and not the gauge reading and INCREASING the HP output of your engine... you will get further away from a given systems design goals... and run that system on the edge.

Sure, you can make a system that is adaptable and allow for higher HP levels ... but if you design a system to run OPTIMALLY at a given charge pressure/temp level... It will perform the most consistently for the owner at those levels. This is what makes the current offerings so user friendly. They are designed to run best at recommended charge pressures and HP levels.

It's not just a matter of using Higher Octane fuels when you want to increase HP levels of your turbo system.

More fuel for conversation.


.

Click to expand...

Honestly, a lot of what you are saying is over complicating things. A few of these things aren't taken as gospel either...

1) Exhaust pressure does not stay the same. At elevation, exhaust pressure actually is less (off idle/in no boost situations). This is because the motor isn't running as "strong". (less air in, less air out)

2) Turbocharger speed does change with altitude, as far as that 1-2% number, that's *highly* variable dependent on turbo design, compressor size, temperature, exhaust design and even humidity. In sled world, I'm really not sure what that number is but I can promise its going to be "highly variable" across the board. Good news is most turbos were designed, built and tested in the mountains, so its a bit of a moot point.

3) Wastegate duty will change with elevation, be it mechanical or electronic. Air is thinner. Simple as that. Either it is "held closed" longer as it takes longer to build the pressure differential (measured in charge tube PSI) or it is held closed longer both due to less exhaust pressure, thinner air and an ECB "upping" the output of the turbocharger to compensate for barometric pressure differential.

4) Even with a good EBC, its not going to keep things totally the same. Charge tube temps will change, the way the turbo spools will change and the sled will react differently. Yes, "net" output on the top should be the same but how the torque curve feels at 10,000 feet will be completely different than 1,000 feet. I don't care how well the system was designed... You'll even feel the difference between 5,000 feet and 10,000 feet.

5) QUOTE: "When you "crank up the boost" at elevation.... meaning actually increasing the charge side pressure and not the gauge reading and INCREASING the HP output of your engine... you will get further away from a given systems design goals... and run that system on the edge."

****Boost is boost****. I don't care how you skin the cat, but if you are going for a higher total net output or going up in elevation and looking to keep your net output the same, the turbo has to work harder.

This is why a system designed to run at sea level will struggle, no matter how good the ECB is or what the intercooler is doing, at altitude (10,000+ feet) and vice versa. The turbo is having to add 4+ psi into the system over what it did at sea level plus whatever the original design goal was (say 5psi). That's basically doubling the turbo's job.

This isn't really any different than a guy at 10,000 feet looking for his 5psi differential system to work at 10psi levels. Can the system do it? Probably. Will he need to make changes and will it feel different? Absolutely.

I honestly think we are making all of this WAY too complicated.

There are currently two systems, one electronically controls the wastegate (either internal or external) to try and keep net output of the sled basically the same across a range of elevations thus keeping fueling and clutching demands relatively constant. The torque curve will change, but again, top end stays relatively constant.

The other utilizes a mechanical gate to add an unchanging amount of air back into the motor regardless of elevation, temperature etc. In this case, net output, fueling and clutching will need to be monitored as the rider goes up and down in elevation, much like a stock sled. The way the turbo spools will feel fairly similar at most elevations however.

I agree... the conversation has gone off topic... the original topic was related to gauges and what they are actually telling the pilot.

So, to keep this on topic... I've moved this "complicated" discussion here.

JJ_0909 said:

****Boost is boost****. I don't care how you skin the cat, but if you are going for a higher total net output or going up in elevation and looking to keep your net output the same, the turbo has to work harder.

This is why a system designed to run at sea level will struggle, no matter how good the ECB is or what the intercooler is doing, at altitude (10,000+ feet) and vice versa. The turbo is having to add 4+ psi into the system over what it did at sea level plus whatever the original design goal was (say 5psi). That's basically doubling the turbo's job.

This isn't really any different than a guy at 10,000 feet looking for his 5psi differential system to work at 10psi levels. Can the system do it? Probably. Will he need to make changes and will it feel different? Absolutely.

Click to expand...

Here's my 2¢... my opinion.

I agree... this is complicated...from both of us.

But heck...it's a complicated topic.

We disagree on what is meant by the turbo "working harder"...and that is ok. Your presentation is still "solid".

****Boost is Boost****, for me, ONLY if you have a level field of monitoring what the engine is receiving. Certainly 10lbs of BOOST read at the boost gauge at 0' elevation is not the same as 10lbs of BOOST read at the boost gauge at 10,000'. One has a Manifold Pressure, MAP, of 24.7 while the other has a MAP of 20.1.

This is why I feel it is better to talk about "BOOST" in terms of MAP... that keeps things LESS complicated... no need to "factor" in anything when discussing the kind of pressures your motor realizes at the intake.

To pump the charge side up to a given relative level, requires the same amount of 'work' at different altitudes ... yes the shaft/wheel speed is increasing to move more cubic feet of air...but that air is less dense and is easier to pump. The resistance that it has to work against is less.

Analogy: Just like riding bike up a hill will give you less wheel RPM than the same bike riding on level ground.

All of this depends on DESIGN and component selection.

How much the wastegate is open in the first place. Struggling is vague word. If it is spilling (bypassing) most of the exhaust before it hits the turbine... then turbo is not working hard/efficiently either. If, at low elevation, the bulk of the exhaust stream just does not make it to the turbine to build the 4psi, mentioned above, and keep the MAP at 18.7 (and not go higher)... then it is not designed to work well at that elevation...and going to a higher elevation could mean that it operates more within the intended design specifications without 'struggling'.

Like I said above... for the "Best" system... you should have a target altitude for where you want the system to optimally function at... and go with a kit builder that takes that into consideration... that, IMO, is where the real difference is in perception, by the pilot of the sled, of a "good" kit or a "mediocre" kit.

Yes, the torque curve will change at altitude, you are correct... The degree to which that is noticeable by the pilot is the true test of the kits design and manufacturing abilities to match components to expectations. This is where those custom billet wheels, or variable vane turbos can pay dividends if done well.

Is it possible to have a system that will perform OPTIMALLY at all elevations and intended HP output levels... not really.

A turbo system that can maintain MAP at consistent levels in all conditions and rider inputs will come on top in the eyes of the consumers.

Narrow down where you want to ride, and what character you want your sled to have (ie max power or throttle response in most varying conditions) and find a kit designer/mfg that builds THAT kit = Home-run IMO.









.

To keep things really NOT complicated....The "KISS" mentality.

Ask which turbo systems are running well...WITH the characteristics/conditions you most value... from people that have those systems. Get as many opinions as you can and ask specific questions about how they run.

You will find what works best for you.... as the Good kit builders will thrive or die based on what they build.

Trust in what they have built and leave it at that.

This discussion is intended only for those that want to delve into what is happening inside of the system...
but Certainly NOT necessary to enjoy MILES OF EXCELLENT THROTTLE THERAPY.

BRAAAAAAAP !!!!




.

This is why I feel it is better to talk about "BOOST" in terms of MAP... that keeps things LESS complicated... no need to "factor" in anything when discussing the kind of pressures your motor realizes at the intake.

To pump the charge side up to a given relative level, requires the same amount of 'work' at different altitudes ... yes the shaft/wheel speed is increasing to move more cubic feet of air...but that air is less dense and is easier to pump. The resistance that it has to work against is less.

Click to expand...

with this in mind would you say that a sled at 10000ft with boost. just the right amount to equal sea level map riley has no boost at all or do I need to read this all again for the 5th time..... just trying to know if what I don't know is something that I might be starting to know!!! Oo0 I'm all Fup now

sumtinwong said:

mountainhorse said:

This is why I feel it is better to talk about "BOOST" in terms of MAP... that keeps things LESS complicated... no need to "factor" in anything when discussing the kind of pressures your motor realizes at the intake.

To pump the charge side up to a given relative level, requires the same amount of 'work' at different altitudes ... yes the shaft/wheel speed is increasing to move more cubic feet of air...but that air is less dense and is easier to pump. The resistance that it has to work against is less.

Click to expand...

MH NOTE: edited this to correct spelling and punctuation to what I believe you are saying, correct me if I'm wrong.

With this in mind would you say that a sled at 10000ft with boost, just the right amount to equal sea level MAP, really has no boost at all... or do I need to read this all again for the 5th time..... just trying to know if what I don't know is something that I might be starting to know!!! Oo0 I'm all Fup now

Click to expand...

Garrett, one of the largest turbo mfg's in the world (if not THE largest) defines "Boost"

Q: How is Boost Measured?
A: Boost is measured as the pressure that the turbo creates above atmospheric pressure. Normal Atmospheric Pressure (1 atm) = 14.7 psi.

http://www.turbobygarrett.com/turbobygarrett/How_is_boost_measured

Click to expand...

I'm going to make some assumptions here.
By your "right amount to equal seal level MAP"... I am assuming that you mean a sled that HAS a turbocharger... running at sea level with 14.7 PSI MAP. That same sled, with turbocharger is then taken to 10,000 feet and held, at a consistent 14.7 PSI MAP.

In your example...I believe... Your goal would be to keep the performance of the sled running at 10K ft. with turbo close to that of a Naturally Aspirated sled at sea level. EASY to do !!




Yes, by the definition above... which I believe JJ is referring to... the sled running at 10,000 ft. IS making boost... it is building MAP above the atmospheric pressure of 10.1 PSIa that the sled is operating in. To get your "equal to sea level MAP" condition... keeping the MAP consistent at the same 14.7 PSI...The turbo will need to produce an additional 4.6 PSI above the lower ambient atmospheric pressure at 10k feet.

I get back to my point above....
"Certainly 10lbs of BOOST read at the boost gauge at 0' elevation is not the same as 10lbs of BOOST read at the boost gauge at 10,000'. One has a Manifold Pressure, MAP, of 24.7 while the other has a MAP of 20.1 respectively. "

Most Pump Gas (PG) turbos will do more than produce 'zero boost' at sea-level... they will make say 2-3 lbs Gauge pressure (16.7-17.7 MAP) at sea level... as a base line, above clutch-engagement RPM. At 10k elevation, that same sled will have no more MAP if the 'EBC' is functioning.



Hope this helps.


...

mountainhorse said:

Here's my 2¢... my opinion.


****Boost is Boost****, for me, ONLY if you have a level field of monitoring what the engine is receiving. Certainly 10lbs of BOOST read at the boost gauge at 0' elevation is not the same as 10lbs of BOOST read at the boost gauge at 10,000'. One has a Manifold Pressure, MAP, of 24.7 while the other has a MAP of 20.1.

This is why I feel it is better to talk about "BOOST" in terms of MAP... that keeps things LESS complicated... no need to "factor" in anything when discussing the kind of pressures your motor realizes at the intake.

To pump the charge side up to a given relative level, requires the same amount of 'work' at different altitudes ... yes the shaft/wheel speed is increasing to move more cubic feet of air...but that air is less dense and is easier to pump. The resistance that it has to work against is less.

Analogy: Just like riding bike up a hill will give you less wheel RPM than the same bike riding on level ground.

All of this depends on DESIGN and component selection.

How much the wastegate is open in the first place. Struggling is vague word. If it is spilling (bypassing) most of the exhaust before it hits the turbine... then turbo is not working hard/efficiently either. If, at low elevation, the bulk of the exhaust stream just does not make it to the turbine to build the 4psi, mentioned above, and keep the MAP at 18.7 (and not go higher)... then it is not designed to work well at that elevation...and going to a higher elevation could mean that it operates more within the intended design specifications without 'struggling'.

Like I said above... for the "Best" system... you should have a target altitude for where you want the system to optimally function at... and go with a kit builder that takes that into consideration... that, IMO, is where the real difference is in perception, by the pilot of the sled, of a "good" kit or a "mediocre" kit.

Yes, the torque curve will change at altitude, you are correct... The degree to which that is noticeable by the pilot is the true test of the kits design and manufacturing abilities to match components to expectations. This is where those custom billet wheels, or variable vane turbos can pay dividends if done well.



A turbo system that can maintain MAP at consistent levels in all conditions and rider inputs will come on top in the eyes of the consumers.


.

Click to expand...

The only reason I keep commenting is the off chance someone is reading this and taking it for gospel. Your analogy is so wrong, as is the idea that "thinner air is easier to pump", when our net goal is to add air back to the system.

If it was "easier to pump" you wouldn't see the turbo opening its wastegate less, spinning faster and generating more heat. If it was "working less" you wouldn't see (in the automobile world) manufacturers turning down boost at elevation (WRX for example) to deal with the increase in heat, and higher likelihood of turbo failure.

The human body experiences this "harder work" too. Just go up in elevation. Run around. The body's ability to deal with less dense air directly effects performance. The body, like an engine, in a way, is an air pump!

I get that you want everyone to talk in MAP. I disagree. I want people to talk about what their turbo is specifically doing and do some simple addition to figure out my net output. I think this is a key thing to understand if you are riding a two stroke turbo snowmobile.

Now when it comes to engine/turbo management, sure, an ECB is going to beat out a mechanical wastegate especially if you ride in a huge range of temperatures and elevations. If you don't however, its kind of a moot point as a tubo's wastegate does a great job keeping boost constant.

End of the day, I'm not a turbo expert, just an enthusiast. But I find a lot of what you are suggesting to not in fact be true.

JJ,

I too keep commenting for the same reasons you mention.

But then we would have a never ending cycle of this and that takes away from the value of this thread.

I respect your opinion... even though I don't agree with it.... and thats OK.

Note: Wong... I've moved your AV Gas/Octane questions to this thread
http://www.snowest.com/forum/showthread.php?t=220800&page=6


.

mountainhorse said:

JJ,

I too keep commenting for the same reasons you mention.

But then we would have a never ending cycle of this and that takes away from the value of this thread.

I respect your opinion... even though I don't agree with it.... and thats OK.

Note: Wong... I've moved your AV Gas/Octane questions to this thread
http://www.snowest.com/forum/showthread.php?t=220800&page=6


.

Click to expand...

http://www.theturboforums.com/conte...m-complete-Turbocharging-guide#compressormaps

Lots of good stuff here...

The high-altitude performance of a turbocharged engine is significantly better. Because of the lower air pressure at high altitudes, the power loss of a naturally aspirated engine is considerable. In contrast, the performance of the turbine improves at altitude as a result of the greater pressure difference between the virtually constant pressure upstream of the turbine and the lower ambient pressure at outlet. The lower air density at the compressor inlet is largely equalized. Hence, the engine has barely any power loss.
is this saying that theirs no need to turn up bost pressure to maintain HP at elevation??? or I should say "barely" any need...
Oo0 and thanks for the link JJ I'm enjoying the evening education!!!

sumtinwong said:

The high-altitude performance of a turbocharged engine is significantly better. Because of the lower air pressure at high altitudes, the power loss of a naturally aspirated engine is considerable. In contrast, the performance of the turbine improves at altitude as a result of the greater pressure difference between the virtually constant pressure upstream of the turbine and the lower ambient pressure at outlet. The lower air density at the compressor inlet is largely equalized. Hence, the engine has barely any power loss.
is this saying that theirs no need to turn up bost pressure to maintain HP at elevation??? or I should say "barely" any need...
Oo0 and thanks for the link JJ I'm enjoying the evening education!!!

Click to expand...

Few things...

1) Turbocharging is simply compressing air to add back into the motor. Technically you could still be "under" sea level performance (say if the turbocharger is running a 3psi differential and you are at 12,000 feet, you'd be under sea level performance).

2) Thinner air is ***harder*** to compress than dense air. This weekend, go try and make a snowball. First try and make one with really powdery snow. Then try and go make one with really wet snow. Which was easier? Same applies for turbocharging. The denser the air, the easier it is to compress. Turbo sleds work infinitely better than naturally aspirated sleds at elevation but a turbocharger still "struggles" more than it would at sea level.

2B) This is why at altitude you'll get more turbo lag to arrive at the same MAP. Regardless of what Mountainhorse is saying, the turbo ***has to compress more air*** to bring the MAP to the same level as a lower elevation. I don't know how else I can say it. There is less barometric pressure, the turbo has to do more work. To add, the air is harder to compress to begin with as indicated above. To my point, a turbo charged airplane has altitude limitations based off this exact principal. If the turbine worked better at altitude as MH is suggesting, a turbocharged single prop airplane would be able to fly at any altitude. This is not the case. (ignoring principals of lift here)

3) If the turbo is controlled via ECB, and is calibrated to keep MAP the same then yes, you will theoretically have zero top end loss, so long as the turbo can "keep up" (which is should be able to do).

If you have a manual boost controller, or are utilizing wastegate by itself to control boost (such as the Silber system), you'll have to do some math and swap wastegate springs to keep output the same at all elevations, though your torque curve will always get more "laggy" as you go up in elevation, regardless of net output.