Submitted: Thursday, Apr 01, 2010 at 16:41
ThreadID: 77373 Views:2273 Replies:2 FollowUps:3
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hello all,
just a quick question, can anyone explain to me, the difference between turbo boost pressure and map(manifold absolute pressure) where are they mesured and if there any direct relationship between the two. watching on scangauge but caun,t figure

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Reply By: fugwurgin - Thursday, Apr 01, 2010 at 18:00

Thursday, Apr 01, 2010 at 18:00
hi Bill
thanks for asking that question, im interested in knowing the answer too.
I have a 03 RA Rodeo. My MAP (on scanguage) reads a static 14.8. I have the turbo xgauge programmed in, offset the variable for altitude (im at sea level and it just happens to be 14.8) i get a reading of 0.0psi for my boost? they seem to cancel each other out. the problem is my MAP reading doesnt change, just stays on 14.8. The aussie distributor told me it may just be my rodeos ecu is not compatible for the BST xguage. out of interest what car do u have and what readings are you getting? Does your MAP change as you drive?

AnswerID: 411315

Follow Up By: bill - Friday, Apr 02, 2010 at 17:25

Friday, Apr 02, 2010 at 17:25
scangauge fitted to 2009 nissan 3ltr patrol common rail.. MAP varies constantly whilst driving from memory up to 20 30 psi turbo boost up to 15psi seems to be little relationship between two regards bill
FollowupID: 681506

Reply By: Member - Oldplodder (QLD) - Thursday, Apr 01, 2010 at 18:03

Thursday, Apr 01, 2010 at 18:03
Guessing a bit here, since I don't know which vehicle you are talking about.

But at a guess, turbo boost pressure may be measured just after the turbo, before the intercooler, and is in relation to air pressure in the pipe after the turbo.
That is where I have my boost gauge connected.

air pressure is about 1 atmosphere or bar, or about 14 psi.
My boost gauge reads in relation to atmosphere, so 10 psi on the guage is 24 psi absolute.

manifold absolute pressure may be measured at the manifold (doh!) and sounds like it may be absolute.

Remember as air is cooled in the intercooler, it condenses, so pressure will drop in the manifold as you have the same mass of air in the same volume, and there will be pressure drop due to resistance through the intercooler. Also manifold pressure will vary as inlet valves open.

As simple as i can make it, and there is a few slightly inaccurate statements in there. :o)
AnswerID: 411317

Follow Up By: pop2jocem - Thursday, Apr 01, 2010 at 23:10

Thursday, Apr 01, 2010 at 23:10
Yeah my 2 bobs worth is you are on the money. Atmospheric pressure at sea level is 1 bar or 14.7 psi. I would think that absolute pressure would be 0 psi in a perfect vacuum. So as you said John most pressure gauges show 0 when actually they are disregarding atmospheric pressure of 14.7 psi. I guess for convenience they just show how much the turbo has boosted above this sea level reading. This is the reason turbo engines work better at higher altitudes than naturally aspirated ones and as you quite rightly pointed out if you boost 10 psi you are actually 24 psi absolute.

Cheers Pop
FollowupID: 681445

Follow Up By: fugwurgin - Friday, Apr 02, 2010 at 07:03

Friday, Apr 02, 2010 at 07:03
that makes perfect sense to me now, i know i didnt ask the original question but am following with interest. when i set the xguage code for the scanguage i needed to put in the psi for my altitude, because i am at sea level i put in 14.8. like you were saying the gauge obviously subtracts this pressure so to only show any pressure above ie the boost psi! i just wish my map reading would change so it would show boost. maybe my turbo isnt working properly and the 0.0psi reading is correct.
FollowupID: 681458

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