Hi all.
Been looking for an inexpensive (but not cheap) boost gauge.
I found a product called the ScanGauge and I asked them if it shows boost.
The guy e-mailed me and said:

"It provides Manifold-Absolute-Pressure (MAP) which is what the waste gate is set to control and is more important to the engine and power development than boost. MAP is the amount of pressure above a vacuum (0 PSI pressure). Boost is the amount of pressure above the current atmospheric pressure. Current atmospheric pressure changes with altitude, temperature and weather."

Do "normal" boost gauges show MAP or boost. I know some show vacuum, too as does my AccessPort. Is the AccessPort showing MAP? (it does go negative for vacuum)


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It's a cool device (search the 'net) and it does these things:
Fuel Economy
Fuel Rate
Battery Voltage
Coolant Temperature
Intake Air Temperature
Engine Speed (RPM)
Vehicle speed (MPH)
Manifold Pressure (not available on some vehicles)
Engine Load
Throttle Position
Ignition Timing
Open/Closed Loop

Maximum Speed
Average Speed
Maximum Coolant Temperature
Maximum RPM
Driving Time
Driving Distance
Fuel Used
Trip Fuel Economy

I'd have to look up this product, but it sounds like a generic OBDII reader with a few extras. Not sure how they can calculate actual fuel economy because there is no fuel flow rate data available in the ECU.
As for boost vs. MAP, any so-called boost or turbo gauge is actually a "relative manifold pressure" gauge. It measures the pressure in the manifold relative to atmospheric so it will read zero with the engine off, vacuum at idle and when coasting, crusing or decelerating and positive pressure/boost when under load.
The MAP sensor, which is what the ECU reads and what a scanner will display, shows absolute pressure and will therefore provide readings that are offset from those on a gauge by an amount equal to the atmospheric pressure. The ECU may need MAP data but as the driver you want to see relative pressure, not absolute.
I don't know how the AP is calibrated.
If all you want is a decent boost gauge there are plenty of options, you don't need to buy this kind of gadget just to measure boost/vacuum.

Thanks for the info.

Most of the boost gauges (except the el cheapo ones) cost more than this device. I was just wondering.

I guess I'll look for a used factory boost gauge.

Well you get what you pay for. Stay away from cheap gauges because, well, they're cheap. While the factory gauge is ok if you can get one at a reasonable price, you can do better. The Omori 45mm gauges are a nice alternative to the factory one and fit in the same pod, also they read in psi which the factory gauge does not. The night illumination on the Omoris is also far superior to the OEM Lamco gauge.
Check out J-Spec Tuning for the Omori gauges, you can find them over in the Vendor Classified section.
Also using an electronic display to measure boost is a bad idea. A conventional analog gauge is far better for this due to the rapidly changing values which are hard to read on a digital display, there is also a limit to how fast the display can update.