I am confused on this issue. That is nothing new. On the fuel map in either tuner studioor MEgatune, vacuum and KPA have an inverse relationship. 20 inches of HG on my gauge sets the pointer into the lowest cell on the fuel map. This is somewhere around 30 KPA. As the vacuum reading approaches 0, the KPA scale goes up, and reaches 100. Which I take to be one atmosphere.

However, I went on line trying to find a conversion chart for "of HG to KPA, I could not find one that listed KPA as such. So, I made a guess, and maybe a bad one, that KPA was Kilo Pascals. I think I must be wrong here. When I fill in the hg column it yields values for KPA that rise with the hg numbers, not the inverse as the fuel map does. So what does KPA stand for, and does anybody have a link to a conversion chart for this?

I wasted an hour on this tonight. It is far easier to give one of you smart guys an opening to straighten me out once again.Thanks.

Sam

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yep, kpa is kilopascals..........

http://convert-to.com/conversion/pressu ... of-hg.html

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Thanks Ems. That is the same info the conversion table I used provided. But it does not agree with the fuel map in Megatune. There the pointer is in the 100KPA cello when my gauge reads is 0" of hg. And it idles around 30 KPA in the fuel map, while my gauge shows 18". What gives? It makes no sense to me at this point. I am trying to see the correlation between my vacuum gauge and the fuel map. Any MS guys understand this?

Sam

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Vacuum is a negative number. Kpa is a positive number. 0 Kpa is a perfect vacuum or about -30" Hg. 100 Kpa is atmospheric pressure or 0" Hg gauge pressure which is really 30" Hg absolute. Put both the vacuum and Kpa scales on a number line and you'll see how they correlate.

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Joshua

Obtain another vacuum gauge with kPa dial. Don't play with gauge in Hg and Megasquirt in kPa, this only end up in sheer frustration.

FYI: 101.325 kPa = 29.92 inHg at sea level atomsphere pressure at 20 C/68 F temp.

Vaccum the value drops. Pressure numbers rises.

* 0% vacuum = 760 torr = 14.7 psia = 29.92 inc mercury abs = 101.4 kPa abs
* 50% vacuum = 380 torr = 7.3 psia = 15 inc mercury abs = 50.8 kPa abs
* 99.9% vacuum = 1 torr = 0.01934 psia = 0.03937 inc mercury abs = 1.3 kPa abs

The automotive vacuum/PSI gauge is 0 at sea level pressure at 68F and sounds bit confusing but remember this is zero between PSI and Hg.
This is why I'm slightly puzzled with this convention.

Cheers, Wizard

Gauge is negative, add 30", then convert


So 18" on gauge is really -18" or 12" Hg, or 40.6 kpa

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Simple enough. I was certain you smart guys would get this figured out for me. I knew it had to be something like that. I thought the conversion formula would be more complicated, kind of like going from Celsius to Fahrenheit.

Check out this chart I put together using the link you provided. Does it look right? It seems right. Only I was surprised that 50KPA was not 15 exactly. I suspect it depends upon elevation above sea level to some extent.

80 KPA= 6.3
70 KPA=9.3
60 KPA=12.5
50 KPA=15.2
40 KPA=19.2
30 KPA= 21.1

There are no more vacuum cells in the fuel map available in MSI. The upper two are pressure, or boost. when MSII happens, and it will, it has 16 cells and a much finer resolution of tuning cells.

Sam

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Using 29.53 is slightly more accurate than using 30...........

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Close enough. How come you are up so early? Thanks.
Sam

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Glad this got settled nicely.

I was confused when I put my boost/vacuum gauge in because it reads in Hg on the vacuum side, and psi on the boost side! No units marked on either side on this cheap gauge. Had me goofed up for a little bit... The conversion there is quite close to 1 psi = 2 in Hg (and again, negative scale for in Hg).

These units: in Hg, psi, kPa, bar, atm... are all just pressure units (from different ways of measuring or defining pressure). Vacuum is defined as negative pressure relative to 1 atm of atmospheric (ambient) pressure. Of course, they leave out the negative sign, which can be confusing...

Lou

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Home of Slant6-powered fun machines since 1988

early................


I was up late...

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Most engine control modules (ecm) use MAP as their point of reference- Manifold Absolute Pressure. As engine vacuum increases, MAP decreases.
A simple way to put it: MAP + vacuum = Barometric pressure. So if baro is about 30" Hg, & you idle at 20"vac then MAP is 10".

All these units of pressure mesurement have their own assumptions ( doesn't EVERYONE know the textbook def of vac?)
I work on automotive EVAP systems, lots of fun playing with inches of WATER as my pressure units ; 2 "Hg = about 28 "H2O.

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63 Valiant Wagon
225 - 4 bbl

Too bad we can't settle on one simple metric unit for everything.

Lou

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Home of Slant6-powered fun machines since 1988

A bit off topic, but do they use inches of "something else" to measure...something?

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I have no idea what I am talking about.

inches of water (" H2O) and inches of mercury (" Hg) are two common measurement units for pressure.

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Home of Slant6-powered fun machines since 1988