This section of the site will be comprehensively improved and restructured in 1Q03, so apologies for the fairly flat and disorganised appearance until then.


Boost Pressure Measurement (A rough guide)

One of the most frequently asked questions by S2 owners is how much boost is normal for a healthy engine. This question does not have a simple answer because of the way the ECU controls boost levels in relation to load (air intake), engine speed, air temperature and atmospheric pressure. I have managed to locate the official factory workshop info for measuring boost pressure on the ABY engined S2 and have replicated it on this page along with my own notes that help understand it (hopefully). The measurement process for the 3B engine is identical, but the results will of course be different, and I will add those as soon as I get them.

The official workshop procedure calls for a snazzy digital pressure tester, known as the VAG1397/A - so you can show off at your dealership by quoting that to the service manager if you like :) This clever little machine displays ABSOLUTE pressure in the inlet manifold, whereas the regular analogue boost guages of the world measure RELATIVE pressure. The simple calculation that connects absolute and relative (boost or vacuum) pressure is as follows -

The traditional analogue boost guage ignores ambient or atmospheric pressure to give a reading which is relative to that i.e increased pressure in the form of 'boost', or decreased pressure in the form of a 'vacuum'. So if you ever wondered why your boost guage gives higher or lower readings on some days, then this is because your ECU is aware of ambient pressure and your boost guage is not !

Ambient pressure is nominally ~1bar at sea level or thereabouts. One counter-intuitive aspect of any turbocharged engine is how it reacts to reduced ambient pressure such as found at high altitudes. In such 'thin air' environments, the absolute pressure permitted in the system is actually reduced by the ECU so as to prevent mechanical damage to the turbo in a frenzied attempt of overspooling whilst trying to attain the same boost pressure as at sea level.

The table below depicts the official absolute pressures for the factory ABY engined S2 with the following pre-requisites for the test. Remember that if using an analogue boost guage to make this measurement, that ambient air pressure must be added to the result to give the desired result. All of the pressure values given here are in units of bar - The Audi S2 is almost 100% metric after all :) If you need help with unit conversions then click here .

• Have a second person on-board to read the boost guage - it is just about impossible to get accurate readings when driving the car at the same time

• All hose pipes and guage connections must be secure and airtight

• Ensure the engine is properly warmed up, but not suffering heat soak from traffic lights etc

• Take note of the intake charge temperature (if using VAG-COM) but ambient temperature should suffice if VAG-COM is not available for the test

• Take note of the ambient air pressure (also available in VAG-COM) if not using the VAG1397/A test equipment

• Accelerate the car in 4th gear at full throttle from a speed of ~60km/h and observe the rev counter

• At 3000 RPM take the measurement from the pressure guage. Repeat a few times to calculate an average reading

Absolute charge pressure at 3000RPM (4th gear from 60 km/h) - ABY Engine only
Ambient Temperature (Celsius)	-10˚ to 0˚	0˚ to 10˚	10˚ to 20˚	20˚ to 30˚	Above 30˚
Intake air temperature (Celsius)	10˚	25˚	35˚	45˚	55˚
Altitude above mean sea level (MSL)	Absolute pressure in bar. Overboost for maximum 15 seconds - initiated by rapid throttle opening.
0m	2.027	2.091	2.188	2.245	2.336
900m	1.945	2.006	2.099	2.154	2.241
1600m	1.919	1.967	2.028	2.066	2.128
2300m	1.808	1.913	1.947	1.983	1.983
Altitude above mean sea level (MSL)	Absolute pressure in bar at full load.
0 - 1400m	1.838	1.935	2.011	2.106	2.234
1600m	1.826	1.933	2.005	2.057	2.128
2300m	1.804	1.908	1.942	1.978	1.978

Note also that permitted boost pressure is increased at higher intake air temperatures - often thought to be an ass backwards approach, but Audi's intent was to have consistent engine performance all year round. The alternative is an engine which has less oomph on hot sunny days. The ECU allows higher boost at higher air temps in order to try and equalise performance.

This is why bigger intercoolers, and other measures to reduce intake air temperature - such as intercooler misters (aka garden sprays) and water injection systems can be a complete waste of money on the S2 unless the chips are reprogrammed accordingly.

The units quoted in Audi documentation are always 'BAR' and so it is important to know how to convert these values into or from other units that you may be more familiar with, or that appear on test equipment.