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Discussion Starter · #1 ·
I was test driving the Audi A4 3.0 quattro and the Volvo S60 2.4T AWD at the same dealership, and the salesman told me that non-turbo engines lose 15% of its power for every 1000 ft of altitude while turbo engines have the same power regardless of altitude. Anybody heard of this before? This concerns me, since my daily commute ranges from 5300-7400 feet elevation. It makes sense to lose some power, but not that much!?! And why only the non-turbo?
 

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quote:

Originally posted by grumpsy:
I was test driving the Audi A4 3.0 quattro and the Volvo S60 2.4T AWD at the same dealership, and the salesman told me that non-turbo engines lose 15% of its power for every 1000 ft of altitude while turbo engines have the same power regardless of altitude. Anybody heard of this before? This concerns me, since my daily commute ranges from 5300-7400 feet elevation. It makes sense to lose some power, but not that much!?! And why only the non-turbo?
I don't know about the exact percentage of loss of power but it seems about right. It is true that normally aspirated internal combustion engines lose steam at high altitudes. At such high altitude, that seems about right. I am not sure of the exact formula (if there is any). Turbocharged engines are less susceptible to power losses at higher altitudes since the sequence of turbocharging does negate the adverse effects.

Yannis
 

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quote:

Originally posted by grumpsy:
And why only the non-turbo?
Yannis is correct. Just to be clear though, the difference between normally aspirated (non-turbo) and turbo is going to be essentially the same for all makes of cars. This is not just a Volvo thing. As the vehicle goes to a higher elevation the air is "thinner" and therefore the engine has less oxygen to burn with. The turbo overcomes this by compressing the available air and "ramming" it into the engine. At some point the turbo will also begin to lose power as the air begins to get even too thin for it to compress enough but it will maintain a margin over the N.A. engine. I have no idea how high you would have to go before the turbo would start losing power from full rated but my guess is pretty high, higher than your normal drive.

At any rate, since you will be driving at higher elevations it makes a turbo a better deal than it would be for us sea level types.

Eric
 

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Uh, minor clarification, yes the air is less dense but the oxygen isn't the issue that lowers the power--it's actually the mass of air that flows through the engine. What the engine does is when it compresses the air on the compression stroke, the ignition of the fuel/air mix causes the resultant air (including the nitrogen) in the compressed space to heat rapidly. As everyone knows, heated air in a compressed space increases the pressure and this pressure (in pounds per square inch) times the area of the piston face (in square inches) is what causes a force to push on the piston--and that's called the 'power' stroke as the hot expanding gasses perform 'work' on the piston.

So the more air mass you've got in the cylinder, the more pressure will be generated when the ignition occurs.

As you gain altitude the air density decreases (slugs per cubic foot or kg per cubic meter) and power will fall off in a normally aspirated engine linearly with altitude. Hence, a turbo that compresses the air and pumps more air mass through the engine will maintain its sea-level performance merely by closing the waste-gate and allowing the turbine side of the turbo to spin faster to allow the compresor side of the turbo to pack in more air into the intake manifold. Once the waste-gate is fully closed the amount of boost available from the turbo is at its maximum and any further altitude gain will see a drop in air mass flow rate because the turbo can't boost any more. The point where the waste gate fully closes is called the critical altitude.

Oh my, wasn't that fun? Auto Otto cycle thermodynamics 101. (It's OK if you don't understand it. I'll understand.
)
 

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quote:

Originally posted by T5 Dave:
Uh, minor clarification, yes the air is less dense but the oxygen isn't the issue that lowers the power--it's actually the mass of air that flows through the engine.
Well, I can go with air mass, but it is oxygen in the air mass that is allowing the fuel to burn. The less AIR MASS the less OXYGEN. If there ain't no oxygen it don't matter how much air mass! Getting more technical but saying essentially the same thing.

I have never seen any car manufacturer publish what the critical altitude is on any turbo vehicle but that would be interesting information. I think it would vary greatly from model to model. Also, the age and wear on the turbo and turbo system would affect critical altitude as well.
 

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Also,
You lose about 1 lb of boost per 2000 vertical feet. Unlike superchargers that lose 1 psi per 1000 feet, we can actually maintain (most) of our power at altitude, the only noticable effect is slightly delayed turbo lag due to air density like Big E said.
 

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quote:

Originally posted by Big E:
Well, I can go with air mass, but it is oxygen in the air mass that is allowing the fuel to burn. The less AIR MASS the less OXYGEN. If there ain't no oxygen it don't matter how much air mass! Getting more technical but saying essentially the same thing.

I have never seen any car manufacturer publish what the critical altitude is on any turbo vehicle but that would be interesting information. I think it would vary greatly from model to model. Also, the age and wear on the turbo and turbo system would affect critical altitude as well.
Well, yeah, there's less oxygen, but there's less nitrogen as well. But the ratio of oxygen to nitrogen remains the same no matter what altitude the car is at: air is about 20% O2 and about 75% N2. (And I politely beg to differ, air mass from a thermodynamic standpoint is everything and is the fundamental physics of what's going on in the Otto cycle, but I'd need a lot more forum space to prove it, so I'll let it go at that.)

Besides, not all the oxygen in the cylinder is burned when ignition occurs, but that's another long story that we probably don't want to get into.

We're saying the same thing: the higher the altitude, the less power the engine makes unless the turbo is there to make up the difference and fool the engine to thinking it's at sea level.

Critical altitudes on aircraft turbo engines are around 15,000 feet, give or take. I don't know if that applies to car engines or not. I'd guess they'd be set around 10,000 feet or so for practical purposes--there aren't many roads above that. But that's just a guess.
 

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Discussion Starter · #8 ·
This is an interesting and educational discussion. My salesman's claim of "15% power loss per 1000 feet" still sounds too high. (This would work out to over 50% power loss at Denver's altitude.) So, I searched and found another explanation on the web: http://webnz.com/turbochargersnz/about.html

The description of turbo was very similar to this discussion, but also adds the following:

"With a naturally aspirated engine, horsepower drops off 3 percent per 1000 ft (300m) because of the 3 percent decrease in air density per 1000 ft (300 m)."

3%/1000ft sounds a lot more reasonable than 15%. Anyway, I guess that's why my old Altima is a slug in Colorado. It seemed to have plenty of power when I lived in Phoenix (elev 1100).
 

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This is a very interesting discussion. So it seems, for a 5000 ft elevation change in a NA 300hp V8 engine, there would be a loss of around 45hp. Pretty significant IMHO.

Another interesting thing that I wanted to mention is that Saab's Trionic-7 engine managment system supposedly compensates for altitude in their Aero HOT engine. In effect, the Trionic-7 controller continuously varies boost from 15 psi to 20 psi and opens the throttle even more than the driver requests in high altitude situations, adding more torque.

I know that Volvo's TCM (Transmission Control Module) takes altitude (climbing or descending a slope) into account, but I wonder if the new Bosch Motronic-7 EMS (Engine Management System) in Volvo's have any sort of altitude compensation features. I think it would be something interesting to find out.

-Drew
 

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Drew,
You make an interesting point about the turbo mechanism (computer) taking altitude into account. If that were not the case and the turbo were driven only in relation to exhaust flow-mass, then the turbo would be spinning less at higher altitude as well, providing proportionatly less boost. Then the only way to get the same power would be to drive the engine harder to create more exhaust gas. Of course that would be the same as driving a non-turbo engine harder to get more power.

What I'm trying to say is that if the turbo doesn't compensate for ambient air pressure then a turbo engine will suffer a power loss similar to a normally aspirated engin.
 

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quote:

Originally posted by Freeworld:
Drew,
You make an interesting point about the turbo mechanism (computer) taking altitude into account. If that were not the case and the turbo were driven only in relation to exhaust flow-mass, then the turbo would be spinning less at higher altitude as well, providing proportionatly less boost. Then the only way to get the same power would be to drive the engine harder to create more exhaust gas. Of course that would be the same as driving a non-turbo engine harder to get more power.

What I'm trying to say is that if the turbo doesn't compensate for ambient air pressure then a turbo engine will suffer a power loss similar to a normally aspirated engin.
Actually, even in the old mechanical systems the turbos were controlled with a device called the waste-gate which dumps exhaust overboard if the turbo spins too fast. It was a simple system that sensed intake side pressure, and if it was too high the hydraulics opened the waste gate more. This resulted in less exhaust passing through the turbine, which slowed it down, which lowered the intake side pressure. A low pressure intake reading would cause the waste-gate to close, thereby increasing exhaust flow through the turbine and increasing its spin rate, thereby increasing the compressor spin rate and its output into the intake manifold.

This system was purely mechanical (or hydraulic in the early systems) and just sensed intake manifold pressure. At higher altitudes, the manifold pressure would be lower (for the density reasons given above) so, therefore, the waste-gate would be closed more, the turbine would spin faster, and so the actual intake manifold pressure would be near the sea-level boost value.

I don't know how the turbos are controlled nowadays, but I suspect they're electro-hydraulic where the computer commands the waste-gate electrically and the hydraulic system is used to provide the oomph to run the actuator (though they could be pure electric motor driven, too.)

It was interesting to read the IPD chip article George posted. Apparently the T5 ECU runs the engine at full boost after about 2800 rpm or so and then just adjusts fuel and timing to produce the desired power output. The beauty of this concept (and it's brilliant--I whish I'd thought of it 'cause it never would have entered my mind at all ) is that with the boost at full pressure, the power response is instantaneous becuase the ECU can crank up the fuel flow and spark timing nearly instantly. I.e., it doesn't have to wait for the turbo to 'spool up!' I guess at lower power settings like highway cruising the ECU just backs off the fuel flow and timing to the point where there's very light detonation in the cylinder (and that's OK). I've noticed that at 55 mph when I want to punch it around some clown the power comes on instantly, quicker than I can snap my fingers. This explains why that's so.

Quite clever. I'm impressed.
 

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Very interesting, indeed. Quite a thread, huh?

Dave, what type of T5 do you have?

Yannis
 

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quote:

Originally posted by GrecianVolvo:
Very interesting, indeed. Quite a thread, huh?

Dave, what type of T5 do you have?

Yannis

S60, the only one in the country I think with a Geartronic and no sunroof. (And Cypress Green to boot. Talk about rare.) Ordered TDS so I could get the handling package that wasn't available in November of '00 to order for USA deliveries. I just added the geartronic, touring package, metallic paint, suspension package, rear window sun blind, and the motion sensor. All else is stock. (My T-bird had just been smashed so I couldn't go hog-wild on all the goodies becuase my wife's S80 was only a year old.)

I thought about the bigger wheels, but they looked harder to clean than the stock units, plus the 16's would ostensibly be less prone to damage from potholes, so I decided to save the $500 and not get the 17's. I do wish I'd bought the DSTC, but the car was getting pricey and had to leave it off. Ticked me off no end to discover the T5's stock seats had pieces of vinyl in key locations to burn my thighs when wearing shorts on hot desert summer days. (Grrrr. . . . )

Then Volvo refunded the $$$ for the metallic paint and sunblind and installed the 10-disc CD changer when I got back as part of the AOL promo on the S60 launch. (Well, plus it helped to walk up to the VNA Marketing Manager at the ReVolvolution Tour 2 days before I went to Sweden and tell him my beef that AOL OK'd the package when I ordered the car TDS then reneged when I went to claim it. When he learned we'd just bought an S80, too, he arranged for us to get the promo package by the time we got back from Europe.)

All in all I had a great time. The car has run flawlessly ever since I picked it up in Goteborg. (Ironic since the car is made in Belgium at the Ghent plant and I drove from Sweden to Brussels to drop it off on my way back home.)

I just hope that this car never gets totaled by a hit-and-run like the last one because I most likely won't be able to replace it now that the sunroof is standard on the T5. (Leaves me out of the running for the S60r, too, sadly.) The sunroof cuts out too much headroom for me and I don't think they can be ordered without them anymore.

That could change, you never know, I could always e-mail the marketing manager again and get special dispensation. But I hope to keep this car at least 10 years so hopefully it's a moot point.
 

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quote:

Originally posted by T5 Dave:
Well, yeah, there's less oxygen, but there's less nitrogen as well. But the ratio of oxygen to nitrogen remains the same no matter what altitude the car is at: air is about 20% O2 and about 75% N2. (And I politely beg to differ, air mass from a thermodynamic standpoint is everything and is the fundamental physics of what's going on in the Otto cycle, but I'd need a lot more forum space to prove it, so I'll let it go at that.)
Well, I'm certainly not going to get into thermodynamics and I wholly agree that the ratio is the same so it is a moot point BUT if I get all the oxygen and you get all the other gasses in ambient air guess who can get his car to run. Obviously nobody has a car set up to run on pure oxygen but it COULD be done (might be fun too...) but you ain't gonna run it on the rest of the amibient gasses. At any rate, we are both saying the same thing - go higher and there is less available air to burn fuel with. The turbo can help make up the difference to some point at which it also begins to run out of air.
 

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quote:

Originally posted by T5 Dave:

I just hope that this car never gets totaled by a hit-and-run like the last one because I most likely won't be able to replace it now that the sunroof is standard on the T5. (Leaves me out of the running for the S60r, too, sadly.) The sunroof cuts out too much headroom for me and I don't think they can be ordered without them anymore.

Thanks for the reply! Yes, you do have a rare bird. As far as the future is concerned if you ever need to order a T5 without sunroof, if you push the "right buttons" you will be able to...
Hopefully, you will not need to do that.

Yannis
 
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