in2dwww

That's why they use superchargers in light aircraft. Not all, of course, but definitely in some of the fun and fast ones!

I think tire pressure, ambient temps, weight, and fuel quality (octane) play their part. Technique is down to the grip and incline

feralc

The boost that is produced by the supercharger is fixed, a tune increases it to a certain extent and later you need a mechanical modification to produce more boost (stage 2 is a supercharger pulley that produces more boost)
So let's say the boost produced by the stock supercharger is 11.6 psi and at sea level the atmospheric pressure is 14.7psi then the PSIG of the supercharger is fixed at 11.6psi but the PSIA is 14.7+11.6=26.3
Atmospheric pressure will always play a role because the base is different, the higher the altitude the lower the atmospheric pressure.
People from really high altitude places can verify this, when they travel to a lower altitude places their supercharged cars perform better.

forza1976

I can certainly attest to this as I ran my previous completely stock 2011 S4 at Bandimere Speedway in Colorado (6000 ft actual elevation) and when I moved to the East Bay I went up to Sacramento drag way and ran there as well. For example, the S4 at Bandimere ran 13.954@100.67mph with conditions that were pretty much crap (DA of 8564 ft above sea level, link http://www.dragtimes.com/Audi-S4-Timeslip-23129.html) and then ran at Sac (still completely stock on even worse CA gas) with a very very favorable DA of -579 ft and ran 13.027@104.96mph (link http://www.dragtimes.com/Audi-S4-Timeslip-24917.html)

Clearly different conditions but the speed to the car was immediately noticeable when I was driving it out from Colorado back in 2012 and when I was driving down i80 through Sac that morning I couldn't believe how well it pulled down the highway. Anyway it felt like a new car, I am sold on sea level now.

Anyway, those were my findings.

snagitseven

My research shows supercharged car engines will lose power as elevation rises above sea level due to lesser air density, although, perhaps somewhat less than a normally aspirated engine. Turbocharged engines at higher elevations lose much less power in comparison.

From an article:

"The debate over whether or not ‘forced-induction’ (turbocharged or supercharged) vehicles are statistically more efficient than ‘naturally-aspirated’ engines at high altitude remains mostly unsolved. One consistent truth behind this, though, is turbocharging is more efficient than supercharging. The theory behind supercharged engines is that at high altitude, they are no more efficient than a naturally-aspirated engine. This thought stems from the fact that a supercharger is belt-driven, much like an alternator, power steering pump or air conditioning compressor. As such, it utilizes the engine’s power to turn a pulley and belt system which, in turn, winds up the insides of the supercharger to ‘force’ extra air down into the engine for combustion. The advantage of this process is the virtual elimination of ‘lag’ when accelerating, due to the fact that a supercharger’s insides are turning efficiently even when the engine is idling".

and another:

"A supercharger provides a fixed amount of boost at a given RPM, so it will loose power at altitude, unless you adjust the speed i.e. pulleys. Superchargers where used on WW2 planes. They would kick-in with a clutch at altitude.

A turbo will automatically compensate for altitude, if the boost control is set for absolute pressue (most are). So a turbo, will actually over-boost at altitude to maintain the same pressure in the intake manifold as at sea-level. So a turbo car will make the closer to the same hp at altitude as at sea level, if the turbo is big enough. At least, it would provide some altitude compensation".

Finally, I found this post to be very informative:

http://www.e90post.com/forums/showthread.php?t=746486

HaveBlue

I'm new to Audi's but I have experience supercharging Toyotas aftermarket. A sc produces pretty linear boost levels across the rpm range unlike a turbo. Usually 6-8 psi (above 1atm 14.7). This may not sound like much but consider that an unboosted engine actually has vacuum in the manifold, ie less than 14.7psi.

By boosting the engine you can increase the volumetric efficiency above 100%. Although you get the same psi across the rev range, at say 6000rpm 8psi you are still moving twice the air as 3000 rpm 8psi.

All superchargers have a bypass valve which vents air back into the low side of the intake. This recirculates the air and decreases boost into the cylinders effectively decreasing the efficiency (psi) of the sc. Under light throttle this vent stays open basically disconnecting the sc because it's not needed. In mechanical systems this happens using a vacuum signal. High vacuum = low boost.

The Audi's bypass valve is controlled electronically. This allows more precise metering using other sensors. If you look at the HP curve of the 3.0 it is pretty obvious that all the boost has been programmed out above about 5000rpm. I haven't verified this by logging the boost with vcds But the curve is not what a boosted engine should be doing.

Hence, reprogramming will allow full use if the boost available. Installing a smaller SC pulley (or larger drive pulley) increases the capability of maximum boost but without programming will not result in its use. The bypass valve will just open under the same psi as before.

Mike_L

Actually, its off the car right now. Gotta take it in for service tomorrow and since they're gonna be looking for excuses not to cover stuff, I'm not giving them any.

I'll probably hit the track at some point, if I keep the car. Thats always up in the air with me though.

forza1976

Shiv knows his stuff. Had his tune on my 335. The BMW programming definitely compensated for elevation as my car consistently required about 2.5psi more boost to hit the boost target versus sea level. It was interesting to see when I logged the car.

Mike_L

I can verify that. Thats why even the piggyback makes a ridiculous amount of power.

The 3.0T could easily make 390-400hp if they didn't limit boost. Why Audi chose to neuter the motor is beyond me. Should have just let it make its power, destroy the 550i and E550, and be done with it. They do the same thing with the S6s 4.0T. They give it 420hp, when the exact same motor can make 560hp if they'd let it.

tenspeed

Less power = less drive train warranty work.

feralc

The 3.0tfsi has around 340-350 hp from factory, the 310hp number is underrated, but still is "neutered" and why? In my opinion different factors:
1 Fuel consumption is a big factor in selling cars, people want "everything" they want a fast car but without paying that much money for fuel, even at performance expense.

2 Emissions; sometimes in order to meet the guidelines they need to sacrifice power.

3 How would you sell a RS7 if your potential buyer can get almost the same power with a properly tuned S7? Product differentiation, and you have different buyers for two similar products.

4 Reliability, less power=less stress=less warranty repairs and longer engine life


The A6/A7 target market is not performance oriented, it is a balance, if the target market was performance oriented there would not be diesel versions available (because diesel motors are NOT performance oriented, there are no S, RS or R cars with Diesel engines and no, the European SQ5 doesn't count because it is not a car)