why is my map sensor showing kpa (pressure) instead of vacuum?

[blazed350]

i just noticed that was i am datalogging and i step into the gas my pcm retards the timing even though the ks is not going off, the knock counts do not climb just retards timing instead of adding. i think my map is showing inaccurate data according to tunerpro. as i gas the car the map climbs.... i thought it was susposed to go down due to a vacuume in the manifold? any thoughts?

[RobertISaar]

MAP = manifold absolute pressure. it's signal can be interpretted as vacuum, but it measures from 10.35 kPa absolute to 104.45 kPa absolute.

10.35kPa = max vacuum, 104.45 = total lack of vacuum. in terms of vacuum, 104.45kPa is 0" of vacuum, 10.35kPa is ~30" of vacuum.

retarding timing with rising MAP is normal... can't run the same amount of advance at 50kPa than at 100. if you ran the advance from 50kPa in the 100kPa range, you'd have knock that would be absolutely ridiculous. if you ran the 100kPa advance in the 50kPa column, you'd have bad power and even worse fuel economy.

[PJG1173]

yep, this is a concept I struggled with when I first started tuning since I was used to tuning a carb with a Vac gauge. just takes some getting used to.

[blazed350]

so i guess i must go back to the drawing board. any suggestions on what a max advance should be (more or Less). do i just advance until knock sensor goes off and retard a couple degreees. i will include a bin file when i learn how to post files here.

[PJG1173]

max advance is all dependant on your setup (quench, dynamic compression, head design, etc) I just advance mine 2* at a time until I get knock or power loss then back off 2*. I'm at 33* on mine so far with no knock and not wanting to push if further.

have you tested your knock sensor by tapping on the head with a hammer yet?

[EagleMark]

Yes make sure your knock circuit is working before you start trying to push limits of spark advance! You can add say 6 degrees to like 2000 RPM 75 to 100 MAP so when you do a half throttle from start you'll see instant knock counts or spark retard when you hit that RPM.

Easy way for us old guys who are used to vacuum and now have to deal with MAP kpa is it's opposite!

What motor and ECM mask ID are you running so we have something to work with. I've been wanting to do a writeup of building a correct spark table from a stock bin file to work with, it works just the same as what GM did with out one numbskull in the end needing to increase or decrease timing in a certain area to meet emissions and he just grabbed and area and pulled it up, yeah it worked but man does it look bad and hard to add a little timing smoothly because of the disaster they came with.