Determining kPa values

tristanlee85 · Post by **tristanlee85** » Sat Jul 15, 2006 1:07 pm

I'm trying to determine a range of kPa values for the VE table. At idle, it's roughly 35kPa, so I made my lowest value of 30. I'm planning on running 10psi so I'd like the VE table to scale from complete vacuum to a litter higher than my maximum goal due to possible boost spikes.

So, I found the equation of PSI x 6.8947 = kPa. In my case, 10PSI would be 68.85kPa. Obviously I can't use that value as my maximum in the VE table since I also need to include atmospheric pressure. In this case, how do I determine atmospheric pressure for where I live? Would my maximum value for the VE table be atmospheric pressure + 68.85 = kPa value for VE table?

I'm using the MS1 v3.0, MegaTune v2.25.

Bernard Fife · Post by **Bernard Fife** » Sat Jul 15, 2006 1:17 pm

Would my maximum value for the VE table be atmospheric pressure + 68.85 = kPa value for VE table?

tristanlee85,

Yes, on the absolute kPa scale, vacuum is from 0 to about 101 kPa, and boost is above that. So 10 psi of boost is 101.3 + 68.8 = 170.1, if you live at sea level.

I would make the table from 20 to 180, if it were mine, allowing a little room on each end of the scale. The cruise vacuum might be lower than the idle vacuum (and decel vacuum certianly will), and you might want to turn up the boost slightly at some point.

Lance.

tristanlee85 · Post by **tristanlee85** » Sat Jul 15, 2006 1:33 pm

Thanks for the reply. So, I've tried google and other sources, but how do I find my closest value to sea level in Columbus, OH, or does it go by regions of say east, mid-east, etc.?

woh · Post by **woh** » Sat Jul 15, 2006 1:42 pm

If you know your elevation, you can use the table in the assembly instructions. It will tell you the kPa for your elevation.

Step 56
http://www.megamanual.com/v22manual/mass.htm#serial

Bernard Fife · Post by **Bernard Fife** » Sat Jul 15, 2006 1:45 pm

tristanlee85,

I would even try, if this were mine. The boost the engine sees is largely irrelevant, it's the MAP (in kPa) that matters, that's what determines how much air gets into the cylinders, and how much power you build.

So 170 kPa is always the same to the engine (at the same IAT). But 10 psi of boost can be anywhere from 126 kPa (in La Paz) to 170 kPa (in New York), meaning very different things to the engine.

However, for reference, there a average ambient pressure versus elevation chart in step #56 here: http://www.megamanual.com/v22manual/mass.htm#input and some sample cities here: http://www.megamanual.com/v22manual/mtune.htm#mt (scroll dow a bit)

Lance.

keithmac · Post by **keithmac** » Sat Jul 15, 2006 3:22 pm

Can`t you just turn the megasquirt on and read the kpa off Megatune with the engine not running to get true ambient pressure? Work your table out from that value..

tristanlee85 · Post by **tristanlee85** » Sat Jul 15, 2006 9:53 pm

Yeh, lol. It was that simple. I'm not sure why I wasn't paying attention to that in the first place. Wow, do I feel retarded now.