MAP sensor reading vs. engine vacuum

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AbandonedBronco
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MAP sensor reading vs. engine vacuum

Post #1 by AbandonedBronco » Tue Oct 30, 2018 11:07 am

Hi all,
Still getting used to the nuances of the Holley Sniper I have installed (love it so far).

However, one thing I have some curiosities about.
Holley documentation states the conversion between the pressure that the MAP sensor reads (kPa) and the vacuum gauge reading (hg).

100 kPa = 0 hg
75 kPa = 7.5hg
50 kPa = 15hg
25 kPa = 22.5hg
0 kPa = 30hg

Basically, this comes out to

Engine vacuum (hg) = 30 - kPa * 0.3

The issue I'm having is that there's quite a disparity between what the MAP sensor is reading, and what my vacuum gauge is reading (which is on direct manifold vacuum).

At idle, my MAP sensor reads 40kPa, which mathematically comes to 18hg. However, my vacuum gauge reads 14.5hg (which would be about 52 kPa). There's quite a difference between 14.5hg at idle and 18hg. I checked the vacuum reading with 3 different vacuum gauges, and they all read similar.

Shouldn't these be reading roughly the same? They're both intake manifold pressure (or lack thereof).

Thoughts?
1985 Ford Bronco Eddie Bauer. 300 I6. 9.1:1 CR, roller rockers, EFI manifolds and 2.5" exhaust. DUI ignition. 3.55 final drive, 5 speed ZF5. Holley Sniper EFI w/Offenhauser C Intake. 32" BFG KO2 A/Ts.

pmuller9
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Re: MAP sensor reading vs. engine vacuum

Post #2 by pmuller9 » Tue Oct 30, 2018 10:46 pm

A MAP sensor by definition reads absolute pressure while a Gauge reads relative pressure.
They are not the same.
Boise at an elevation about 2500 ft has an absolute pressure of 92 kPa.
92 kPa - 40 kPa = 52 kPa pressure difference.

What does the Map read when the engine is not running?

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Re: MAP sensor reading vs. engine vacuum

Post #3 by AbandonedBronco » Wed Oct 31, 2018 11:18 am

Thanks for the info pmuller. I figured both were just intake pressure (or lack thereof) and should directly correlate.

When I first turn my key to "on", the MAP reads 94kPa, which is very close to what you listed?
Where did you get that info? While I was looking around online yesterday, all I could find for Boise was around 103kPa. The weather map says Boise is 30.56hg, which converts to about 103.

92 would make a lot more sense since that's about an 11 - 13kPa difference than "sea level" and, the difference I was getting with 40 (my reading) vs. 52 (what 14.5hg vacuum equates to) is about a 12kPa difference.
1985 Ford Bronco Eddie Bauer. 300 I6. 9.1:1 CR, roller rockers, EFI manifolds and 2.5" exhaust. DUI ignition. 3.55 final drive, 5 speed ZF5. Holley Sniper EFI w/Offenhauser C Intake. 32" BFG KO2 A/Ts.

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Re: MAP sensor reading vs. engine vacuum

Post #4 by pmuller9 » Wed Oct 31, 2018 12:14 pm

Here is one of the references.
https://www.engineeringtoolbox.com/air- ... d_462.html

Boise Firebird racetrack was one of our favorite tracks. The track elevation is 2595 ft.
At this moment the uncorrected barometer is 27.53 inHg which is 93 kPa
The team I worked with rented the track for test and tune. It was fun having the entire track to ourselves.
Instead of going back to the pits the car just went back to the starting line for adjustments.

Here is one of our best runs at Boise during a regular race. 6.82@210 mph
https://www.youtube.com/watch?v=cGX-3u_huvc

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Re: MAP sensor reading vs. engine vacuum

Post #5 by AbandonedBronco » Wed Oct 31, 2018 12:36 pm

Hey, that landscape looks familiar!
Yes, driven by Firebird many times on our way out to Emmett for car shows. My dad has some 50's classics he likes to share.

That 49 scoots!!! Wow. Is that one of yours?

And yeah, it's like the water park here on a weekday. Once you go down a slide, you just turn around, run back up, and go again. There's no hours of waiting in line.
Unfortunately, people are catching wind of how great Boise is, and it's starting to get a population influx. :evil:


"Corrected" pressure is most likely all I'm finding. The 27.53 sounds MUCH more like it. 93kPa and my gauge read 94kPa when off. Cool.

Again, thanks for the info. Just trying to figure out how it all works.
If I were to correct my idle kPa pressure back to a 100, that'd put it around 48, which is 15.5hg. Much closer to the 14.5 - 15hg I'm reading on my vacuum gauge. :thumbup:
1985 Ford Bronco Eddie Bauer. 300 I6. 9.1:1 CR, roller rockers, EFI manifolds and 2.5" exhaust. DUI ignition. 3.55 final drive, 5 speed ZF5. Holley Sniper EFI w/Offenhauser C Intake. 32" BFG KO2 A/Ts.

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Re: MAP sensor reading vs. engine vacuum

Post #6 by pmuller9 » Wed Oct 31, 2018 12:53 pm

It is one of the cars I helped put together.
430 cu in SBF with twin 88s, 24 injectors on methanol, 2400 hp.
You can't see the engine for all of the bolt-on stuff
https://www.dropbox.com/s/4rz3c5rqf70si ... 2.JPG?dl=0

Here is my control center sitting over the intercooler on the passenger side.
https://www.dropbox.com/s/7w0nhlw6bhm83 ... 3.JPG?dl=0

pmuller9
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Re: MAP sensor reading vs. engine vacuum

Post #7 by pmuller9 » Wed Oct 31, 2018 3:14 pm

AbandonedBronco wrote:If I were to correct my idle kPa pressure back to a 100, that'd put it around 48, which is 15.5hg. Much closer to the 14.5 - 15hg I'm reading on my vacuum gauge.


Now correct the reading on the vacuum guage to sea level remembering that the kPa at sea level is 101.
Tell me what you get.

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Re: MAP sensor reading vs. engine vacuum

Post #8 by AbandonedBronco » Mon Nov 05, 2018 11:05 am

pmuller9 wrote:It is one of the cars I helped put together.
430 cu in SBF with twin 88s, 24 injectors on methanol, 2400 hp.
You can't see the engine for all of the bolt-on stuff
https://www.dropbox.com/s/4rz3c5rqf70si ... 2.JPG?dl=0

Here is my control center sitting over the intercooler on the passenger side.
https://www.dropbox.com/s/7w0nhlw6bhm83 ... 3.JPG?dl=0


That is one very cool, and intricate looking setup. I love it. :D
Very high quality looking control center, too. Did you get to drive it?

pmuller9 wrote:Now correct the reading on the vacuum guage to sea level remembering that the kPa at sea level is 101.
Tell me what you get.


If ambient here is 94, and sea level is 101, that would make a difference of 7kPa.
Not sure if this is the proper way to do it, but 40 (my idle) + 7 = 47
47 kPa equates to around 16hg vacuum. A bit closer to my 14.5.
1985 Ford Bronco Eddie Bauer. 300 I6. 9.1:1 CR, roller rockers, EFI manifolds and 2.5" exhaust. DUI ignition. 3.55 final drive, 5 speed ZF5. Holley Sniper EFI w/Offenhauser C Intake. 32" BFG KO2 A/Ts.

pmuller9
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Re: MAP sensor reading vs. engine vacuum

Post #9 by pmuller9 » Mon Nov 05, 2018 12:00 pm

I didn't drive the car. I wasn't license for a car that fast and besides, it is a good policy not to drive a customers race car.

I data logged 52 channels of data on that car and set it up so neither myself or the engine tuner had to be at the track.
While Ken was in Vegas or Boise, I sat at home in Spokane WA where I could see every engine and suspension parameter and the engine tuner made his adjustments from New York while looking at the same data.
This was done by way of Verizon broadband wireless and after the car returned to the pits.

You corrected vacuum gauge reading is close enough and you can say that the corrected reading is 16 inHg. Much Better!

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