Sanity check my boost math

[JohnJL]

I must be losing it. I am looking at a friend's datalogs and am seeing some weird readings. Along those are some boost spikes.

This is indeed a stupid question because I used to know the answer but please check me...


The logs show the car running under manifold pressures of 19kpa up to 165kpa. In PSI that's 2.75 psi to 23.5psi at WOT.

Average air pressure is 14.7, right. So when standardizing the language, he's really running 23.5-14.7 = 8.8 psi boost at WOT, no?

Thanks for the sanity check.

[1986 930]

I have not done the math to figure the differences, but remember that 14.7 lbs/square inch is for a standard day.

A standard day is defined as:

Sea Level
59 degrees F(15C)

Humidity factors in as well.

That might explain some of the variations. One of the magical(some would say black magic) qualities of our 930's is the variation in boost and HP given the weather conditions.

Just a thought.

Good Luck,

Mark

[WPOZZZ]

He is running 23.5 psi at WOT, but to put it in boost gauge terms, 23.5 psi/14.7 psi = 1.59 bar

[Speedy Squirrel]

Your readingsare absolute pressure chief.You have to subtract 101.3 kPa (14.7 psi) to get the gage readings that most people are used to.

[Tippy]

100kPa is 14.7. Subtract 100 from the 165 which = 65

Turn into a decimal and multiply by 14.7.

14.7 * 0.65 = 9.55lbs or 0.65 rounded 0.7 bar.

[908/930]

Hmmm, when the engine is off, what pressure is the data logger giving? if it is zero then you are developing 23 psi of boost. Is it possible that is between shifts.

[Tippy]

Engine off should read 100 kPa's.