effect of altitude on superchargers vs. turbos
12-04-2009, 10:12 AM
#26
It would seem I was not mistaken. I must say I didn't know if wastegates had atmospheric pressure on the backside of the spring or a fixed pressure. Some of the dual port wastegates may use atmospheric on the back so a drop in atmospheric would result in a decrease in psia boost and maintain a constant psig boost.
12-04-2009, 01:50 PM
#27
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I was a bit vague in my wastegate explanation but it will give you a set boost pressure rise in psi over ambient rather than based on a fixed ratio. In other words if you had 9 psi at sea level on a standard day (23.7 psi), compared to an altitude where the outside pressure was 11 psi, you'd still have 9 psi over 11 so your gauge still says 9 psi. However if you compare this pressure to sea level reference you can see that it's not the same. You are actually down 3.7 psi from sea level even though your gauge reads the same. You still lose 16% of your power from sea level. However comparing this to the supercharger from my previous post, that car lost a total of 26% of it's power. Let's say that we had a naturally aspirated car. Air pressure at 14.7 psi vs 11 psi at altitude is still a 26% power loss. The percentage of loss between a supercahrged engine and a naturally aspirated engine is the same. If the supercharged engine were 50% more powerful at sea level than the n/a engine, it's still 50% more powerful than the same engine if both were at higher altitudes. Even a turbocharged engine loses power as altitude rises. It can only compensate for so much. It just loses less than other options.
12-04-2009, 03:20 PM
#28
Note that the Pr increases at altitude which actually makes a turbo more efficient (up to a point) as the altitude increases.
You have to go waaaaay out there on a waaaaay undersized turbo before it will not deliver at-altitude performance equal to sea-level performance.
Now, one thing typical homebrew FI people forget is the PSIA/PSIG differential increase at altitude that can put new stresses on your piping. Its not uncommon for a setup to suddenly come apart at altitude because someone's weld or coupler was on the edge at sea-level.
12-04-2009, 03:28 PM
#29
So RG says the reference pressure on a WG is atmospheric. And I assume MM is saying it's fixed. Might this come down to WG design and whether it is dual port or not?
Maybe I am interpreting the statements incorrectly but digging around I had seen both single and dual port WGs which had different designs on the backpressure side of the spring. I do know on other similar equipment with reference pressures set to atmospheric that lowering the atmospheric reference will cause a lower absolute threshold to trigger, but that's pretty elementary stuff.
Maybe I am interpreting the statements incorrectly but digging around I had seen both single and dual port WGs which had different designs on the backpressure side of the spring. I do know on other similar equipment with reference pressures set to atmospheric that lowering the atmospheric reference will cause a lower absolute threshold to trigger, but that's pretty elementary stuff.
12-04-2009, 04:06 PM
#30
I said nothing about the wastegate. I was simply pointing out that when you increase the Pr to achieve the target absolute manifold pressure on a turbo, you are usually moving into a more efficient range on the compressor's flow map.
That said, a single-port wastegate actuator derives its signal from the turbo output and is driven against spring pressure, so it operates independently of atmospheric influence.
So, yes - the WG opening pressure is fixed.
That said, a single-port wastegate actuator derives its signal from the turbo output and is driven against spring pressure, so it operates independently of atmospheric influence.
So, yes - the WG opening pressure is fixed.
02-05-2011, 08:51 PM
#31
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A supercharger giving you 9 lbs boost at sea level will not give you 9 lbs more pressure at altitude. It would seem like that but you need to remember that it is giving you a set percentage more air from a ratio standpoint and not a set amount of pressure in psi. If we put a supercharger in space where we have no pressure in, we'd get no pressure out. We wouldn't get 9 psi.
If we had a standard 14.7 psi reference pressure at sea level and our supercharger was setup to give us 9 lbs of gauge pressure, that's going to be a total of 23.7 psi or a ratio of 1.61. However if we were up in the mountains and our ambient pressure was 11 psi, our same supercharger is still going to give us the same 1.61 ratio which equals 17.71 psi or a total of 6.71 lbs over our starting pressure. However pressure as compared to sea level would only be 3 psi!
If we had a standard 14.7 psi reference pressure at sea level and our supercharger was setup to give us 9 lbs of gauge pressure, that's going to be a total of 23.7 psi or a ratio of 1.61. However if we were up in the mountains and our ambient pressure was 11 psi, our same supercharger is still going to give us the same 1.61 ratio which equals 17.71 psi or a total of 6.71 lbs over our starting pressure. However pressure as compared to sea level would only be 3 psi!
How much boost do I lose per 1000' elevation? None. Our supercharger always produces it's rated boost over atmospheric pressure. Approx. .5 psi per 1000' of atmospheric pressure is lost due to elevation. Boost plus atmospheric pressure is called absolute pressure. 14.7 psi at sea level +5 psi boost = 19.7 absolute psi.
http://kennebell.net/KBWebsite/FAQ_p...q-answers6.htm
02-06-2011, 12:25 AM
#34
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i've herd announcers on speed channel get this wrong claiming superchargers and turbos are effected by altitude just like N/A engines.
i ment to say ARE
i ment to say ARE
Last edited by nate340; 02-06-2011 at 12:19 PM.
02-06-2011, 05:13 AM
#37
Not a MAF based car, not directly anyway, street tuning the fuel VE map at a high altitude would be potentially a problem, though the baro adjustments should keep it inline, I've never tuned a MAF car at 3-4000 ft ASL...so I am not 100% sure.
02-06-2011, 05:24 AM
#38
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I understand...kinda ...cause the MAF sensor is still taking into account all the air that is coming in and going out of the engine @ any given point....
02-06-2011, 06:54 AM
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Yet another question...lets pick an altitude arbitrarily...say..6035ft... what percentage power loss can you expect from a turbo engine if any? Something tells me that you cant as easily account for the (loss) due to the compensating effects of individual turbos. Mainly the different efficiencies....
02-21-2011, 09:17 PM
#42
In other words, at higher altitiudes, air molecules are reduced per volume of intake. The SC or Turbo would still compress the available air according to their drive ratios, but with few air moleculeds avialable, the end resut would be lower overall aid intacke vs. lower atlittued. That'a why NHRA dragsters don't make the E/Ts at Denver as they do at New Jersey. You're still compressing the avialable air, it's just that there is less of it to compress at higher altitudes. I've heard 10hp/1000 ft altitude is a good rule of thumb, NA or FI.
Now if you start ovedriving the F/I, you can match lower altitude figures, albeit at the expense of extra scavenging losses by the faster S/C or Turbo ratio.
Now if you start ovedriving the F/I, you can match lower altitude figures, albeit at the expense of extra scavenging losses by the faster S/C or Turbo ratio.
02-22-2011, 05:35 AM
#44
02-22-2011, 08:44 AM
#45
No.
The wastegate requires an absolute pressure to open.
If it is set for 9 PSI at sea level (23.7 PSIA), it will open at 11 PSI at 4000 feet of altitude where the air pressure is 2 PSI lower (still 23.7 PSIA).
The Pr increases, which (on a properly sized turbo) will put the compressor into a higher adiabatic efficiency range.
The wastegate requires an absolute pressure to open.
If it is set for 9 PSI at sea level (23.7 PSIA), it will open at 11 PSI at 4000 feet of altitude where the air pressure is 2 PSI lower (still 23.7 PSIA).
The Pr increases, which (on a properly sized turbo) will put the compressor into a higher adiabatic efficiency range.
02-22-2011, 10:27 AM
#47
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Turbo's can only compensate where they have room to spare: where the wastegate is open.
I lower rpm areas and other areas where the wastegate is 100% closed, they are also negatively affected by altitude.
If you wish to truly be unaffected by altitude, you must size your turbo appropriately.
I lower rpm areas and other areas where the wastegate is 100% closed, they are also negatively affected by altitude.
If you wish to truly be unaffected by altitude, you must size your turbo appropriately.
02-22-2011, 09:32 PM
#48
Proper turbo sizing is the single most important aspect of kit design.
Which, when the turbo is properly sized, is almost all the time.
02-22-2011, 10:18 PM
#49
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and is this why turbocharged cars dyno graphs use uncorrected numbers since they aren't unrealistically overly inflated by using correction factors since they don't suffer at altitudes?
02-23-2011, 05:56 PM
#50
A supercharger giving you 9 lbs boost at sea level will not give you 9 lbs more pressure at altitude. It would seem like that but you need to remember that it is giving you a set percentage more air from a ratio standpoint and not a set amount of pressure in psi. If we put a supercharger in space where we have no pressure in, we'd get no pressure out. We wouldn't get 9 psi.
If we had a standard 14.7 psi reference pressure at sea level and our supercharger was setup to give us 9 lbs of gauge pressure, that's going to be a total of 23.7 psi or a ratio of 1.61. However if we were up in the mountains and our ambient pressure was 11 psi, our same supercharger is still going to give us the same 1.61 ratio which equals 17.71 psi or a total of 6.71 lbs over our starting pressure. However pressure as compared to sea level would only be 3 psi!
A turbo is setup so the wastegate opens after a set amount of total pressure is reached. It doesn't matter what level you started at. When it gets high enough, it bleeds off. This means that as air thins out with altitude, the turbo will spin faster and faster to hit the required pressure. While you should theoretically have 9 psi at sea level or in the mountains, keep in mind that your temps will be different between them so while a turbo does much better than a supercharger at altitude, it still has some loss. It is the best option for altitude though.
If we had a standard 14.7 psi reference pressure at sea level and our supercharger was setup to give us 9 lbs of gauge pressure, that's going to be a total of 23.7 psi or a ratio of 1.61. However if we were up in the mountains and our ambient pressure was 11 psi, our same supercharger is still going to give us the same 1.61 ratio which equals 17.71 psi or a total of 6.71 lbs over our starting pressure. However pressure as compared to sea level would only be 3 psi!
A turbo is setup so the wastegate opens after a set amount of total pressure is reached. It doesn't matter what level you started at. When it gets high enough, it bleeds off. This means that as air thins out with altitude, the turbo will spin faster and faster to hit the required pressure. While you should theoretically have 9 psi at sea level or in the mountains, keep in mind that your temps will be different between them so while a turbo does much better than a supercharger at altitude, it still has some loss. It is the best option for altitude though.
Depending on the time factor and how quickly full boost is needed, altitude does potentially hamper the performance of the turbo depending on turbo size, intake volume, and wastegate arragngement. It can't be helped. And actually, I'm starting to see the advantages a turbo has over a supercharger, at least in this case. I think I'm going to rig a turbo to pressurize my supercharger intake. Then maybe some NOS and a set of JATO rockets
