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kPa to PSI conversion - Confused! 

 

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 Post subject: kPa to PSI conversion - Confused!
Posted: Wed Apr 29, 2009 9:54 pm 
Tyre Shredder
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Hi All,

I'm playing around with with the boost/rpm levels that I introduce water injection into my engine. Its controlled by a motec M800. I'm trying to beat a pinging issue.

This is the thing. I can set the water to begin injecting at these increments of 'Effcy kPa' - eg. 80, 100, 110, 120, 140, 170 kPa, etc. Rpm points are also adjustable but not related to my question. Sounds fine....but when I google the kPa to PSI conversion I find this....

kPa --> PSI
* kPa x 0.145 = psi

or

PSI --> kPa
* PSI x 6.98 = kPa

I know that atmospheric pressure is 101.325 kPa....and I guess that atmospheric pressure should be the same as '0' PSI. But if you do the math.... atmospheric pressure is 101kpa x 0.145 = 14.6 PSI! Obviously atmospheric is not 14.6 psi.

If this were correct and I set my water injection to deliver water at 'Effcy kPa' of 100, then it wouldn't inject until I'd reached 14psi. I know this isn't correct because the water does activate at quite low PSI, and my engine doesn't even reach 14 psi!

Can someone shed some light on the kPa to PSI conversion, or even how this works on my motec management?! Would certainly help to know what I'm doing.....!

Cheers,
Joe

 

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 Post subject: Re: kPa to PSI conversion - Confused!
Posted: Wed Apr 29, 2009 10:02 pm 
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the conversion you got there is right...
atmospheric pressure is 101.325 kpa at see level??

we are above see level by how much i dont know?

 

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Last edited by TROYMAN on Wed Apr 29, 2009 10:07 pm, edited 1 time in total.

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 Post subject: Re: kPa to PSI conversion - Confused!
Posted: Wed Apr 29, 2009 10:06 pm 
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1 pascal = 1Nm^2 (one newton-meter squared), so one 1kPa = 1000NM^2. One newton is a kilogram times 9.81ms/s (the force of gravity), so 1 newton meter squared is the force of 0.10kg acting on an area of 1 square meter. Now you have identical units which can be compared, 0.10kg/m^2 and PSI, pounds per square inch.

Kilograms * 2.2 = pounds
Meters squared * 1550 = inches squared

So if you have 100kPa (remember that kilo is 1000, so that's 100,000Pa), that's the same as 100,000Nm^2.

100,000 * 2.2 = 220, 000 pounds per meter^2.

220, 000/1550 = 141.93 pounds per square inch.

Hope you got all that... I think I got it right? Someone feel free to correct me if I'm wrong.

 

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 Post subject: Re: kPa to PSI conversion - Confused!
Posted: Wed Apr 29, 2009 10:17 pm 
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here is something i found on wickapedia...
14.7 PSI is the atmospheric pressure at sea level this drops as we gain altitude, for instance, 1 mile above sea level has an atmospheric pressure of approximately 12.2 PSI since 986.9 millibar equals 14.7 PSI 12.2 PSI equals 819.1 millibar and as Low or high pressure cells move into any area, the atmospheric pressure will vary in kind.

now i guess you need to work out how far above see level you are.

then you should get a better reading..

 

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 Post subject: Re: kPa to PSI conversion - Confused!
Posted: Wed Apr 29, 2009 10:40 pm 
Tyre Shredder
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Brisbane is on a river, so I guess I'm at about sea level! So, being at sea level, my atmospheric pressure is 14.7 PSI .... if this is the case it starts to make sense with what the motec is telling me.

 

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 Post subject: Re: kPa to PSI conversion - Confused!
Posted: Thu Apr 30, 2009 10:25 am 
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Your motec is thinking in 'absolute' pressure, whereby 0 in any pressure measurement is a perfect vacuum.
So ~100kpa absolute would read as 0 on your boost gauge (unless you have an absolute pressure boost gauge :lol:)
You need to take the amount of boost you are wanting to start injection (lets say 5psi) and then add that to 14.7psi to get your 'absolute' boost pressure.
Convert 19.7 psi back to kpa, and there you have it.

 

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 Post subject: Re: kPa to PSI conversion - Confused!
Posted: Thu Apr 30, 2009 12:42 pm 
Tyre Shredder
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SteadyED, that's exactly what I was looking for! That makes sense now.

So, if I want to inject water at 4psi, I convert 14.7psi+4psi (18.7psi) to kPa. Which would be (psi x 6.98 = kPa) 18.7psi x 6.98 = 130.5 kPa. Awesome - that is very clear now and makes sense.

Thanks for clearing that up Steady, Troyman and fnp. I'm keen to start getting my head around using this motec management myself....water injection adjustment is a safeish first step!

Cheers,
Joe

 

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 Post subject: Re: kPa to PSI conversion - Confused!
Posted: Thu Apr 30, 2009 1:00 pm 
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Yep thats it!
No worries mate, good luck!

 

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