Converting Manifold Vacuum Gauge to KPA Help

[RevTheory]

Fellas, I've found myself stuck in what should probably be pretty easy. I've purchased a Holley Terminator X EFI system and can't seem to get my brain to make the switch from manifold vacuum gauge readings to kpa to correctly set up the scaling on my fuel table.

The conversion charts I've found seem upside down, meaning 100 kpa, which as I understand it, should be on the verge of boost, are showing 29.53inhg instead of zero. I'm anticipating this engine to idle maybe at 15" of manifold vacuum on a gauge and I'd like to have my WOT settings pretty much in by 5 inches, again, read from a vacuum gauge.

I'm sure it's simple but my brain is hung up and all of these conversion charts seem upside down or backwards. How the heck do I get this thing scaled correctly at my anticipated 15" idle and WOT tuning beginning at 5" converted to kpa? From where to where in kpa?

Screenshot 2022-09-22 072321.png

Screenshot 2022-09-25 053549.png

Base Fuel kpa.png

[chimpvalet]

I'm gonna offer a WAG here that the kPa scale is constructed around a sea level average atmospheric pressure of 100, near enough. 15" of vacuum versus near 30 standard atmospheric reads quite like a 50% pressure reduction to my way of thinking, so I'd expect the kPa value to be near 50. It can be a challenge to flip one's view between pressure and lack of pressure, which is what "vacuum" actually is.

Steve

[RevTheory]

Thanks, Steve. I'm really struggling to understand why this seems like such a crazy question in EFI groups. "If I think my engine is going to idle around 15 inches on a gauge, about where is that in kpa so I can make sure I've got it scaled right?"

It's like I'm asking the dumbest, non-answerable question ever. I'm guessing it's me and hoping to get it figured out so I can move forward.

[RevTheory]

Using 50 kpa as an estimated idle pressure, I was able to drop it down on the scale and gained quite a bit of resolution in the part throttle/acceleration range. Hopefully that'll be good enough to get it fired up and run in on the dyno.

Learning as I go here and that's about half dangerous

Does this scaling look decent for a 10.5:1, n/a 406 SBC with a cam that specs 227/233 at .050 on a 106?

Base Fuel kpa with more resolution.jpg

[Stan Weiss]

Shawn,
I get 7.252 PSI for 50 kpa

I get 7.329 PSU for 15" of vaccum

Stan

[RevTheory]

Thanks, Stan

[chimpvalet]

What I've read indicates it's wise to go for finer resolution in the idle to light load conditions, as far as fueling goes in particular. FWIW

[gruntguru]

The charts you posted will look backwards if you are used to seeing vacuum in "Hg. That's because the charts are showing "absolute pressure" not "vacuum". The "vacuum" scale starting with zero at atmospheric pressure and increasing as pressure reduces below atmospheric, is one of the worst things ever invented.

Engine tuners would be well served to always think in absolute pressure units - with zero representing a perfect vacuum - the lowest pressure possible.

In this chart https://connecteddiscourse.wordpress.co ... and-below/ notice Hg(v) and Hg(a) go in opposite directions. (v=vacuum and a=absolute)

[RevTheory]

Grunt, that chart provided exactly the frame of reference I needed. Thank you!