How Your Boost Gauge Lies To You. Constantly.
May 31, 2025Maybe lying is too strong and word but it’s more than likely that your boost gauge is misleading you, when it comes to power. Two cars, similar setups, both running the same boost level, but one consistently sends the other to Gapistan. Here’s why it happens.
Boost is not the whole story
Boost gauges read the pressure in the intake manifold. But what we are really interested in, is how much air is entering the engine and more accurately, what mass of air is entering the engine. More mass equals more power, always. More boost can me less power, some of the time.
What we really want to know is how much oxygen is entering the engine. The denser the air, the more oxygen.
Two engines can be running the same boost level, but if the air entering the engine is hotter in one car, all else being equal, it will make less power. Hot air is less dense than cold. Less dense means less oxygen. Less oxygen means less power.
Example
Two identical engines bar the turbo size. Both are making 15 PSI of boost. Engine one makes 15 PSI and intake air temps are 20C, while engine two makes 15 PSI but the intake temps are 40C.
This would mean engine one is ingesting air with a density of 2.43kg/m3 while engine two is breathing air with a density of 2.28kg/m3. This is a 6.2% difference meaning engine one will be making around 6.2% more power at the same boost pressure.
And this is how boost gauges mislead us. They give us a partial picture. 15 PSI of boost in winter is not the same as 15 PSI of boost in summer. To get a true picture we need to know the air density. With air density we can compare power on different engine setups. Turbo X makes less boost than turbo Y, but are we making less power with turbo X? Air density will tell us. Our boost gauge cannot.
I should have known – Boost Gauge vs Density
When researching this article I wanted to see if any companies were making a dedicated air density gauge.
While I couldn’t find a dedicated gauge, Gale Banks does (I should have known!) have density monitoring on his iDash product (when the appropriate external sensors are also used with it). And to illustrate the benefits of measuring air density as opposed to pressure, he has also done a comparison video.
Summary – Boost Gauge
A boost gauge can tell us if our wastegate/boost controller can maintain the boost we want. It can also tell us how much boost we are making.
What it can’t do is tell us if the 15 PSI boost today, is making more or less power than the 15 PSI yesterday.
A boost gauge also can’t tell us if we are making more power with our new upgraded intercooler.
Only an air density measurement can tell us if the intercooler upgrade increased power or if our new intake routing makes more power, or if our new intercooler ducting makes more power, or if our intercooler water sprayer is making more power and so on and so forth.
