Quote: Originally Posted by SynthRocker
Great explanation.
So if your at WOT in a F/I car, then vaccum should read -? If 0 vaccum is atmospheric pressure and a turbo buts more pressure in.
Or am I thinking/going about this all wrong? lol
If you're asking what a true vacuum gauge would read while under boost then the answer is "0", but only because a "vacuum only" gauge can't read pressure, it only reads from 30in hg to 0in hg of vacuum. If you did have a vac gauge that read "pressure" as a negative number then the answer would be yes. However, when looking at a combined vac/boost gauge (like ours) think of a vac reading as a negative number (in relation to atmospheric pressure), and boost as a positive number. The only problem is, vac and boost are on two differnt scales... 10in hg vac is not -10psi (more like -5psi).
What ever you do though, don't confuse all vac readings for meaning there is an absence of air pressure. Only a reading of 30in hg. vac could be said to be void of air since normal atmospheric pressure is roughly 30in hg (therefore 30in hg of vac = 0 pressure). For example, a reading of say... 20in hg vacuum (our car at idle) is actually 10in hg. of
pressure on an absolute pressure gauge. Our gauge simply tells us the difference in pressure in the manifold compared to the pressure outside (assuming at sea level). A reading on the vac side means the pressure is lower in the manifold, a reading on the boost side means the pressure is higher. This all then relates to how much air is entering the engine.
If you want to compare the vac readings to boost readings, think in absolute pressure.
Here's a typical boost guage:
Here's what it would say in absolute pressure (I rounded it off):
