I dont know if I should put this in the DFI forum or here. I am building a twin turbo 355 and have been looking at different fuel management systems. My question is, MSD sells MAP sensors and they have 3 available. 1 is for N/A and it says it is a 1 bar. The next is for turbo up 20 psi and it is a 2 bar and the last is for 30 psi and it is a 3 bar. What I want to know is what do they mean x amount of bars. Any help will be greatly appreciated.

 

shot in the dark but, a bar refers to one atmospheric pressure. I think 1bar = 1atm = 14.7 PSI pressure. (easy to remember, stoich is also 14.7 )
If that's what they mean....

 

That is what I thought it would mean but when I thought some more I was not sure. Anything over 14.7 psi will be like entering a second atmosphere. Then 29.4 would be the third. It sounds good. I am not sure

 

Huh? if you go higher in the atmosphere there is less pressure. Higher PSI would be under water towards the center of the earth.

For all practical purposes, free air is 14.7 PSI at sea level, 2bar is about 2x atmosphere pressure or 2*14.7, and 2-bar is 3x atmosphere pressure or 3*14.7PSI = 44.1 PSI. An air pressure gauge reads 0 PSI at 14.7 PSI of atmosphere pressure. Boost is relative to fee air or 14.7 PSI of atm. pressure.
That is why people refer to psia and psig. psig = psia -14.7. The 2bar or 3bar is refering to psia.

So, relative to free air then 2bar is 29.4 - 14.7 = 14.7 PSI OVER atmosphere pressure (what an air pressure gauge would read). 3bar would read 3*14.7 - 14.7 = 29.4 PSI.
Those are generic rough calcs. Go to the Garrett turbo website in the Tech area for a more detailed and accurate summary.

Basically, use a 2bar for up to 15 PSI of boost and a 3bar for up to 30 PSI of boost.

 

You're confusing bar with atm (atmospheres) and PSIA with PSIG.

PSIA – Pounds per Square Inch ABSOLUTE – where 0 is 0, no atmosphere at all
PSIG – Pounds per Square Inch GAUGE – where 0 is normal atmospheric pressure, 14.7psi

PSIG = PSIA – 14.7psi

1atm = 14.7psi
1bar = 14.5psi (don’t ask why, I don’t know, it’s stupid, I know, probably invented by someone that was on a mountain and due to lower pressure was not getting enough O2 to his brain)

A 1 bar map _should_ read 5VDC at 0psig, a 2 bar map _should_ read 5VDC at 14.5psig and a 3 bar map _should_ read 5VDC at 29psig, but some are odd, like the Honda 1 bar maps can put out 5V at somewhere around 7psig.