From a 2016 thread. The figures are what individual owners logged, not limits. For why boost can flatten at high rpm see Supercharged boost that goes flat above about 5,000 rpm. For the mapping side see Supercharged NA ECU mapping.
One owner (2016) had just bought a stage 2 car and wanted to check boost before taking it to Courtenays for final mapping. His first calculation was wrong because he mixed absolute and gauge readings. What owners said:
- Absolute versus gauge. One owner said the MAP sensor reads absolute pressure from the ECU's point of view, so 100 kPa is atmospheric, or 1 bar, and idle vacuum is around 30 kPa. A boost gauge treats atmosphere as zero, so you read about minus 0.7 bar at idle. A max reading of 15 kPa would mean either 0.15 bar of boost or 0.85 bar of vacuum, which makes no sense at full throttle. For a gauge reading, subtract the atmospheric pressure from the absolute reading (our arithmetic: 180 kPa absolute is about 80 kPa on a gauge, or about 11.6 psi).
- What to expect. Engine off, the MAP should log about 100 kPa (1 bar, atmospheric), so check this. At idle about 30 to 40 kPa (0.6 to 0.7 bar of vacuum), depending on idle rpm and cams. For a stage 2 supercharged car at full boost, about 180 kPa, which is +0.8 bar or about 12 psi. The maximum depends on the exhaust, the head and the pulley, and a low reading at higher revs could mean belt slip or a boost leak (though you would usually also get a vacuum leak).
- The Courtenay stage 2 doubles it. One owner pointed out that the Courtenay stage 2 uses a "faked" 2 bar MAP sensor in place of the OEM 1 bar one, so the logging software, which translates the sensor voltage using the 1 bar specification, shows half the real pressure. Multiply what you log by 2.
- The case. Stage 2, 3.35 inch pulley, standard cams, standard exhaust with the pre-cat removed, 68 mm throttle body. Logged with OBD Doctor Free on a tablet: engine off 54 kPa, idle 22 kPa (940 rpm), maximum 15 kPa. Doubled that is 108, 44 and 30. The first two fit, but one owner said 44 kPa at idle is too little vacuum (the ideal is about 30) and suspected a leak. Another owner said that at 940 rpm about 40 kPa is normal and at 800 rpm you should see 30 to 35, and that a full-throttle reading with more vacuum than idle must be a wrong value (perhaps logged on deceleration). When he logged again in April the maximum was 81 kPa, which doubled is 162 kPa or about 8.9 psi, matching Courtenays' expected 9 to 10 psi. Another owner's earlier range for full throttle was 80 to 90 kPa logged, or 160 to 180 doubled. The owner asked whether the extra revs of the final map would bring it nearer 180 kPa, which wasn't answered.
- Barometric pressure. One owner asked whether people record outside barometric pressure when comparing logs, and said to subtract the standard value from the MAP reading to compare. He wrote it as 1013 kPa, which we take to be 1013 millibar or 101.3 kPa.
- A simple check. A third owner suggested a temporary gauge for test purposes.
Help wanted
We don't have the ECU or logging settings that give true MAP on a Courtenay car, or a log from a standard supercharger kit with the matching pulley size.