S4 Up High... Altitude Issues...

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Jun 22, 2001 | 06:56 AM

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S4 Up High... Altitude Issues...

I noticed that there is considerably more lag down low but almost the same power after a few seconds of full throttle and waiting. Also my trip computer got more milage.. i dont know if i understand that. This car is WONDERFUL up high. I had a 2.8 Q before and it was choking over vail pass previously. Any stuff i should do to make it better? I know it is hard to prefect perfection but oh well.

Jun 22, 2001 | 07:08 AM

CobaltJacket

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Does the Bosch/Siemens system compensate for altitude/air pressure like the Saab and Volvo systems?

Jun 22, 2001 | 07:10 AM

Shredster

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Obviously... some higher revs when disengaging the the clutch should help... but you know this

Jun 22, 2001 | 07:24 AM

Nick Gustas

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It backs off the requested boost so the turbos don't overspin at altitude

at 14k+ feet the absolute pressure was around 590-600 millibar and the car requested hardly any boost, less than 1000mbar absolute if I recall, but I wasn't able to create much load. It's hard to pull boost going downhill on pikes peak, too dangerous

1st and 2nd don't have enough resistance, and I am NOT going to try to build boost in 3rd, on a narrow dirt road on the side of a mountain.

sooo, limited testing

anyone in CO wanna verify?

Jun 22, 2001 | 07:29 AM

Nick Gustas

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Boost takes longer to build because the engine is "smaller" at altitude, the turbos need flow to

build boost, once they do start to build boost, the engine puts out more exhaust and the turbos spin even faster, and then you are back to normal, to a point, the ecu will reduce boost to prevent overspinning the turbos if the air pressure is low enough.

Your gas mileage would improve because the reduced oxygen levels would require less fuel for a given RPM, similar to a smaller engine.

I had the same observations when I went to Colorado, power was down, especially off-boost, but compared to the other cars I encountered the S4 was a rocket.

Jun 22, 2001 | 07:34 AM

Shredster

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yeah... what he said.

what do I know, my *** is practically at sea level... as a matter of fact, we have a swell that's hitting today... 4-5 foot. I can't wait to paddle out tomorrow morn. I don't surf, but have been bodyboarding for more years than the ages of most posters. I really love 10 foot plus waves! Why am I typing all this here?

Jun 22, 2001 | 07:39 AM

Jimmy Pribble

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Old School altitude info...

This applies to my UrS4, which uses an older Bosch Motronic system, but I'm guessing the parameters are similar:

Altitude Compensation

From: Motronic Engine Management System for the Audi S4 and the 4.2 Liter Audi V8 --- Service Training Self Study Program 218 - 3/92

Barometric Pressure Sensor F96

"The barometric pressure sensor informs the Motronic engine control module of the current barometric pressure. At altitudes above 3200 feet (low air pressure), the charge air pressure is reduced to avoid overreving of the turbocharger."

"At altitudes above 10,000, the charge pressure control system is switched off." ( - you have boost still, but it's totally based on baseline WG spring pressure)

----

Anyway, even if we don't know the exact profile, it is a fact that Motronic (all versions) compensates for altitude.

HTH,