High altitude boost question
Staging Lane
Joined: Mar 2007
Posts: 55
Likes: 0
I would think that the same kpa would not make the same power at altitude as at sea level just because there is less oxygen in the air being compressed at altitude. Or am I in left field?
Restricted User
Joined: Mar 2014
Posts: 7,192
Likes: 109
From: Ohio
Lol. Stop looking at your damn boost gauge and start looking at your MAP sensor reading. The fact you're comparing boost 10 PSI from a gauge to 10 PSI from a gauge at sea level is absolutely absurd, because it becomes a relative term or an indirect factor and is useless.
~170 kPa on a MAP sensor is 170 kPa whether its at 10000' or sea level.
~170 kPa on a MAP sensor is 170 kPa whether its at 10000' or sea level.
Thread Starter
On The Tree
Joined: Jan 2011
Posts: 184
Likes: 1
From: Texas Hill Country
Thats exactly what im saying. Two identical engines at the same kpa are under the stress/load and should make the same hp regardless of what the boost gauge reads reguardless of altitude.
as mentioned in my previous- at altitude it will make less than sea level at the same manifold pressure.
the increased deltaP across the turbo compressor is one reason.
Restricted User
Joined: Mar 2014
Posts: 7,192
Likes: 109
From: Ohio
The power loss isn't that extreme. Its not going to be proportional to DeltaP like it is in an N/A car. A turbo car at altitude is similar to the difference between a turbo car with and without an air filter at sea level. You are basically decreasing the pressure at the compressor inlet, which causes a higher pressure ratio which can decrease compressor efficiency a few percentage. This is why SAE corrections for turbo cars at altitude shouldn't be trusted.
LS1 Tech Stories
The Best V8 Stories One Small Block at Time
6 Common C5 Corvette Failures and What's Involved In Repairing Them
Pouria Savadkouei
Retro Modern Bandit Pontiac Trans AM Comes With Burt Reynolds' Autograph
Verdad Gallardo
Top 10 Greatest Cadillac V Series Performance Models Ever, Ranked
Pouria Savadkouei
Top 10 Most Powerful Chevy Trucks Ever Made!
Hennessey's New Supercharged Silverado ZR2 Has 700 HP
Verdad Gallardo
Coachbuilt N2A Anteros Is an LS2-Powered C6 Corvette In Italian Clothes
Verdad Gallardo
Awesome K5 Blazer Restomod Comes With C7 Corvette Power
Verdad Gallardo
10 Camaros You Should Never Buy
10 LS Engine Myths That Refuse to Die
Verdad Gallardo
This is incorrect. two engines making the same kpa will not make the same power, because at altitude that amount of oxygen in the air is less.
The power loss isn't that extreme. Its not going to be proportional to DeltaP like it is in an N/A car. A turbo car at altitude is similar to the difference between a turbo car with and without an air filter at sea level. You are basically decreasing the pressure at the compressor inlet, which causes a higher pressure ratio which can decrease compressor efficiency a few percentage. This is why SAE corrections for turbo cars at altitude shouldn't be trusted.
Thread Starter
On The Tree
Joined: Jan 2011
Posts: 184
Likes: 1
From: Texas Hill Country
The oxygen content is the same, the pressure is whats different. Two engines at the same kpa will see the same internal loading. The higher altitude engine will be seeing more gauge pressure to build the same internal koa though.
for example you boost both engines to 20psi absolute pressure. At sea level you start with 20% oxygen content, and at elevation you start with 10%. at 20psi sea level there is still 20% oxygen in the compressed air, and still 10% oxygen in the compressed air at elevation.
the only thing different is that at elevation you start at 12.2 atmosphere psi, and are adding 7.8 psi of boost to get to 20psi overall. At sea level you start at 14.7 and add 5.3psi, oxygen content does not change.
I hope this makes sense, not sure how else to explain it. Every thing being equal the same absolute pressures will always make less power the higher the elevation.
Your saying the in a 1 cube foot container the oxygen contain is the same everywhere in the world? Base on your logic cars at sea level and mt. everest will make the same power....
I'm not trying to argue with you, just trying to explaining twhy you won't make the same power as a sea level engine with the exact same absolute boost pressure.
I'm not trying to argue with you, just trying to explaining twhy you won't make the same power as a sea level engine with the exact same absolute boost pressure.
Thread Starter
On The Tree
Joined: Jan 2011
Posts: 184
Likes: 1
From: Texas Hill Country
Ive worked on fuel cars and tuned alcohol cars for close to twelve years, been drag racing my entire life, and tune throttle stop cars and bracket cars by DA. Im pretty sure i have the bases covered concerning why we turn the wick up at altitude orwhy we change tune ups to run specific et for weather change.
At 5000' your percentage of o2 is still 20.8 you have less pressure to condense that 20.8% into one cubic foot of air thus giving you less volume of oxygen and all other componets that make up our atmosphere. They will not make the same n/a power because there is less pressure assisting that engine to breathe.
What i am saying is that 1 cubic foot of air pressurized at 14.7 psia at sea level will indeed have the same oxygen volume as 1 cubic foot of air presdurized at 14.7 psia at 5000'.
Theres not less oxygen, just less pressure.
At 5000' your percentage of o2 is still 20.8 you have less pressure to condense that 20.8% into one cubic foot of air thus giving you less volume of oxygen and all other componets that make up our atmosphere. They will not make the same n/a power because there is less pressure assisting that engine to breathe.
What i am saying is that 1 cubic foot of air pressurized at 14.7 psia at sea level will indeed have the same oxygen volume as 1 cubic foot of air presdurized at 14.7 psia at 5000'.
Theres not less oxygen, just less pressure.
Thread Starter
On The Tree
Joined: Jan 2011
Posts: 184
Likes: 1
From: Texas Hill Country
So yes on mt everest, if u were to pack 1 cubic foot of air into a container and kept packing in air(just like a compressor) until it reached 14.7 psia, you would have the same volume of oxygen as you would at sea level.
Thread Starter
On The Tree
Joined: Jan 2011
Posts: 184
Likes: 1
From: Texas Hill Country
The makeup of the air you breathe is always 78% nitrogen 21% oxygen, the volume of those componets are decided by the pressure your at. You get less volume at altitude because theres less pressure to assist and compact that atmosphere. If you were to run around at 10000 feet with a bubble around your head and a little compressor compressing that air to 14.7 psia you would have no issues breathing. The air is thin because its less dense, not because of oxygen percentage.
Barometer is the number one contributing factor in DA tuning. We add and subtract fuel out of our tune ups due to da change. This is why et changes throughout the day. Theoretically your engine is going up and down in altitude all day. This is caused by changes in pressure, not oxygen percentage going up and down.
Barometer is the number one contributing factor in DA tuning. We add and subtract fuel out of our tune ups due to da change. This is why et changes throughout the day. Theoretically your engine is going up and down in altitude all day. This is caused by changes in pressure, not oxygen percentage going up and down.
You must account for increased back pressure of the turbine wheel and compressor discharge temp. 200 Kpa at sea level will make more power than 200 kpa at 10,000' elevation. Sea level would be around 15 psi and 10,000' would be 20 psi.