It’s very rare that I have driven over a couple of thousand feet so I haven’t tested this for myself; I understand that normally aspirated cars are pretty shoddy at altitude due to the lack of oxygen. However, I would have thought that one with forced induction would be able to get just as much oxygen in as at sea level; furthermore, at speed there would be less wind resistance, which is usually the limiting factor - and so it should be able to go faster.
Is my reasoning correct? Are supercharged cars faster on the tibetan plateau?
Say car A is turbocharged, and car B is naturally aspirated, and both cars run 14.0 in the quarter at sea level. Say car A uses 8 psi boost to make peak power (22.5 psi total manifold pressure); car B will have 14.5 psi total manifold pressure.
If car A has a relative-pressure sensor (i.e. it allows up to 8 PSI boost above atmospheric pressure), it’ll continue running about 14.0 in the quarter at 10,000 feet. It’ll have 18 PSI or so in the manifold, leaving it at about 80% power, but it only has 66% of the aero drag it did at sea level. It will not get out of the gate as hard but it may post a higher trap speed. However, car B will be struggling badly with 10 psi in the manifold; it’s down around 66% power. That’s worth over a second in the quarter mile.
If car A has an absolute-pressure sensor (i.e. it allows 22.5 PSI in the manifold, no matter the outside air pressure), it will have just as much power as it did at sea level, and will be pushing thinner air - maybe a few tenths faster in the quarter, all gained on the top end. On the other hand, 66% density air may not be enough to keep the engine and intercooler cool enough to not have to back off…
I’ve lived in Colorado most of my life, so I don’t really know how cars perform elsewhere. What I can tell you is that (a) most cars don’t show any major effects under 10,000 feet, and (b) based on conversations I’ve had, a turbocharger does better at true high altitude than a supercharger.
Even a car with a carburetor, once the carburetor is adjusted to the new altitude, will do reasonably fine at any altitude you’re likely to encounter. People have been driving cars to the top of Pikes Peak (14,110 feet) for 70+ years.
This is correct, with the proviso: You’re still sucking in less dense air, which means the ultimate power output will be similar, but building boost will be slower.
An additional discovery on my part: I ordered a slightly smaller diameter pulley for my supercharger, so that I could produce the 8psi boost at Denver’s altitude that my supercharger is supposed to make at sealevel.
It was a bad idea.
The gas we buy up here won’t support 8psi without recalibration of the ECU.
I would think that the fact that the car is having to compress the air more would make the air hotter and thus would make significantly less power.
Also, adding to what you said, superchargers don’t have a way to adjust themselves to maintain a certain pressure despite less ambient pressure so they’d be in the same boat as the normally aspirated car. You could put on a smaller pulley but then you’d have more parasitic drag plus what Unintentionally Blank said.
I have a Thunderbird Supercoupe and from the factory, it makes 12 psi of boost. Usually boost is measured in psi and vacuum is measured in inches of mercury.
I live in Northern NJ and race at 2 different tracks. One is at an elevation of 531 ft above sea level, the other is 131 ft above sea level. I see a consistently better ET and trap speed at the lower altitude track (about .2 seconds and 2-3 MPH). So even a 400’ difference is noticeable. I’ve been told that it’s due to the more dense air being crammed into the engine. I DO have to compensate by adding more fuel to my tune at the lower elevation track.
My car is a turbocharged '87 Buick Grand National running 21 PSI boost, BTW.
It (98 Corvette with an LS1, Magnuson with Intercooled supercharger) runs 6 psi in Denver with a 4" Pulley. The 3.8" pully brought the boost up to 8 psi, but the motor was NOT happy. It built a lot of heat and pulled a lot of timing due to knock.
“Major” is pretty subjective. Here’s a plot of pressure versus altitude in metric units; conveniently, since sea level pressure is 101 kPa, the pressure in kPa at any given altitude also represents the approximate percentage of sea-level power you can expect out of your engine. In Denver (5280ft/1600m), your engine has just under 90% of sea-level power available. Every few years I take a motorcycle trip from Michigan out to Colorado and Utah, and the power difference is noticeable, even in Denver.
Vehicles do get up Pikes Peak, but they don’t go quickly. At the summit (14,000ft/4267m), you’ve got about 60% of sea-level power available. My bike was pretty peppy at sea level, but near the summit of Pikes Peak, it was a dog; couldn’t launch without bogging, downshifts were screwy (difficult to rev-match because engine revved more slowly), even engine braking was compromised (reduced engine pumping losses). The typical family sedan, which would have a considerably lower power-to-weight ratio than my bike, must be really sluggish up there.
[QUOTE=Machine Elf;14836888 "Vehicles do get up Pikes Peak, but they don’t go quickly. At the summit (14,000ft/4267m), you’ve got about 60% of sea-level power available. My bike was pretty peppy at sea level, but near the summit of Pikes Peak, it was a dog; couldn’t launch without bogging, downshifts were screwy (difficult to rev-match because engine revved more slowly), even engine braking was compromised (reduced engine pumping losses). The typical family sedan, which would have a considerably lower power-to-weight ratio than my bike, must be really sluggish up there.[/QUOTE]
Do you think that this was caused by running rich? Too much fuel to achieve the proper stoichiometric ratio due to the decreased air charge?
I’d heard 3% per 1000 feet…so Denver would be about an 18% loss in power. I CAN say that the few times I’ve had a Vette closer to sea level, 2nd gear felt like it pulled as hard as 1st. (Normally aspirated.)
The Hotrod was built to be a mid 12’s car at sealevel, up here it struggles to break into the 13’s. If you use all the correction math NHRA uses to equalize times for comparison, it DOES map out to mid 12’s.
:smack: I shouldn’t have trusted some random chart I found on the internet. I have an Excel spreadsheet that contains models of atmospheric pressure, temperature, and density from well below sea level up to about 500,000 feet. I just took a look at that spreadsheet, and you’re right, the atmospheric pressure in Denver is 82.4% that of sea level.
No; my bike used closed-loop fuel injection, and also had ambient pressure/temperature sensing for open-loop operation, so the fueling would have been automagically adjusted for stochiometric operation at any given altitude. The engine was just plain gasping for air.
All I got say is wow that is a lot of pressure. Been away from gear heads a long time but last I knew a boost of 28 inches (yes inches) was high.
By the way Vacuum can be measures in inches of Mercury and pressure can be measured in inches of of a water column. I know I take pressure readings every day.
It’s box stock (I wanted a fun, but RELIABLE, daily driver). AWD 300 hp in a 4banger is more than enough. Sure, more power is but an exhaust and a fuelmap away, but then things get more peaky and less stable.
Well 21 psi is about 1.45 bars. Unintentionally Blank’s Subaru has about the same pressure as my Supercoupe.
ETA: I’m guessing the MAP sensors are rated by absolute pressure whereas boost is relative so that makes sense. The 0.8 bars would be about 1.8 absolute.
FWIW,
In 1994 Volvo introduced the then new 850 Turbo at the annual dealer meeting in Colorado Springs Co. The altitude there is about 7,900 feet IIRC.
The engineers calculated that the car had lost about 3% of it’s sea level performance due to altitude, but you have to remember that the injection system on that car was designed to compensate for altitude.
