anchour33

I am thinking about buying a 2003-2004 A4 for extensive use in Colorado, i.e., driving frequently between Denver at 5,280 feet and Silverthorne at ~ 9,000 feet. Surprisingly, I wasn't able to find very specific information about the effect of altitude on engine performance. Given rough estimates I've found, the 3.0L V6 (220 hp) should be roughly equivalent to the 1.8T 4 cyl (170 hp) in Denver and possibly even slower in the mountains at higher elevations. My initial choice was the 1.8T but I became concerned about its performance after reading the excerpt below from an Audiworld.com review of the 2004 1.8T with the sport package (bearing in mind that the reviewer was evaluating the car in terms of whether it offered a true sports car experience in the tradition of a BMW 3 series). So what would everyone recommend to be driven at altitude? The 4 cyl turbo or the 6 cyl 3.0L? Or should I just scrap the whole A4 idea altogether and just get a Subaru like everyone else in Colorado?

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"I cannot imagine loading this car, as tested, with four to five adults and their gear, and going on any kind of trip. If you are skiing or hiking fan then forget it, the power is just not sufficient for the needs of those who intend on taking this car up to high altitude, fully loaded with passengers and gear. "

https://www.audiworld.com/features/tests/2004sport.shtml
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Polish2005

Well, on a naturally aspirated engine, the power loss equation is very simple. It is the same as the reduction in air pressure, thus O2 concentration. Less air pressure means less air in the cylinder and a roughly equal percentage less fuel, because the air-fuel ratio must always remain in the stoichiometric range ( 14.7:1 is the ideal, though engines usually run slightly richer than that). So a 10% reduction in air pressure is roughly equivalent to a 10% reduction in power. This is an estimate, but is accurate enough for practical use.

In Denver, at 5280 feet, the air pressure is approx. 83% of that at sea level, for a 17% loss. At 8000 feet, the percentages are approx. 76%/24%. So the naturally aspirated 3.0L will put out roughly 183hp in Denver and approx. 166hp at 8000 feet (which I use as a ballpark average for a Denver-Silverthorne run).

Turbo-charged engines also lose power at altitude, but the effect is less pronounced (the power can be brought to near-sea-level levels only with a bigger turbo). The calculation is a little more complicated. Note that with turbos, there are so many rather complicated thermodynamic things happening that the following is a much rougher approximation than the naturally aspirated analysis above.

An example. Let's say that you have a car with 10psi max boost, and that the ECU controls boost by ensuring that the turbo always puts out 10psi at max boost, ambient air pressure notwithstanding (there are different ways of managing boost and altering it for altitude, but that's beyond the scope here.....this is back of the envelope stuff anyways). At sealevel you will have approx. 24.7 psi of pressure in the intake manifold at max boost (14.7+10). In Denver, you will have approx 22.2 psi of pressure at max boost ( 12.2psi+10psi). That's an approximate 10% power loss from sea-level. Better than the naturally aspirated 17%, but it's still there. As you may have figured out, the higher the maximum boost, the lower the altitude penalty. On a car like the Lancer Evo, with 20psi max boost, the loss in Denver is around 7%. A Subaru WRX, with boost of around 12psi, the loss is 9% in Denver.

On to the A4. Let's say 2004 for shi*s and giggles. Looks like two versions of the 1.8 were available in the A4 -- one producing 160hp and another producing 187hp. I'm not overly confident on the boost figures because I couldn't find them quoted in a car magazine review (such magazines usually only report things like boost for full-on performance cars, in the case of Audi, the older S4), but according to one article ( http://www.torquecars.com/audi/18T-tuning.php), the 160hp version puts out 8.7psi and the 187hp version puts out 11.6psi.

The 160/8.7 would put out roughly 143hp in Denver, 135hp at 8000 feet.

The 187/11.6 would put out roughly 169hp in Denver, 161hp at 8000 feet.

Repeating the 3.0, it's 183hp in Denver and 166hp at 8000 feet.

So the 3.0 looks better. I don't feel like looking up air pressure at 9-10k feet and reworking my spreadsheet, but the more powerful 1.8T would put out slightly more power than the 3.0 at these higher altitudes. However, the difference would be negligible, especially considering the margin of error introduced by the many assumptions and simplifications built into the analyses.

On balance, I would go with the 3.0 over either of the 1.8Ts. First, it'll clearly have more oomph around town, and will have roughly the same power as the larger turbo 1.8 at altitude. Also, the turbo cars will be sluggish when below their boost threshold -- so unless you're launching from 5000 RPM (and burning some of your clutch if you do it right, or fuc*ing up your driveline if you do it wrong, which on an AWD car is not hard to do), they'll be quite sluggish off the line, until your revs get to around 2000-3000.

Whatever you decide, make sure you get a stick shift. That's the most important factor in mountain driving. On descents, you don't need to cook your brake bcs you can use engine braking much more effectively, and on ascents you can keep the tranny in a good gear without the auto tranny switching gears on you as you lift on and off the go pedal. In slippery conditions, the worst thing that can happen is adding just a little throttle at the apex of a turn (which by itself is fine), and having an automatic tranny decide that it's time for a downshift, giving you much more torque to the wheels at a time when that can induce a sh*t-inducing slide. With a stick shift, you can smoothly apply power at any time, knowing exactly how much power you're applying.

Plus stick is just more fun. Yeah, that's what she said.

quickaudi:STFA

now go back and refactor the variables if he gets a chip :P

C5 Chas

That's not a small difference to be overlooked. By simply adding a $400 ECU upgrade he'll more than make up for the slight differences in HP and still be lighter than the V6 with the I-4.

I used to destroy larger, naturally asperated engines on I-70 in my '99 VW Passat. The car wieghed in at slightly over 3000 lbs. Less than a Quattro A4 for sure, but I'd take the AWD any day over the Front driver only.
The funnest part of the drive was around 11K feet near the entrance to the "tunnel." (he knows what I'm talking about) The large 6L and above trucks though they could just sit in the left lane and drive to the top without hving to pull over to let anyone pass them. Not the case. I'd have to almost push them up the hill and then they would finally move over once it was obvious that I could out power them to the top. It was too much fun!!!!

Numbers aside, get the 1.8T A4. Put some aftermarket software into the ECU, 2.5 inch exhaust and a new DV and you'll be having way too much fun passing normally asperated cars left and right.

M.S.G.

Chip wins easily.