25 PSI manifold pressure

[SACTX]

So I have a Gm Stage 2 tune, stock charger, and a pulley that's labeled 77.9. Attached is my second long run of my car (it's long) and there are a couple of times that the pressure has hit 25+ psi. I didn't think that was possible. Can someone shed some light on it for me? Is there a setting wrong somewhere?

I used the KPA graph to find the high spots. Look at the peaks at 16:33:24-16:33:30.

Thanks!

 

[Ross]

I saw at 04:33:28 (or 16:33:28 in military time) that the MAP sensor reads 182 kPa.
1.82*14.7 = 26.7 PSI... okay we are seeing the same thing, just different units.

I looked at an old log file of mine, I see 167 kPa, which is 24.5 psi.
Your 77.9 mm (3.05") pulley is a bit smaller than my stock 3.35" pulley, so that is probably why you are making more pressure.

 

[SACTX]

I was thinking that the stock supercharger should be pushing 12-13 psi and the Stage two kit was 18 or so. Seeing it peak at 25 to 26 psi was surprising. Is it because I am not at full throttle at that RPM?

 

[ctuinstra]

If I'm not mistaken, you have to substract 14.7 from the 25+ psi. Think about it this way - 100kPA = 14.7PSI or atmosphere pressure. At atmosphere at the intake is neither vacuum nor boost so it is effectively 0 boost (and 0 vacuum). so 100kPa is = 0 boost. 182kPA is around 12 psi BOOST above atmospheric pressure.

There is no way that stock supercharger with a 3.05" is pushing 26PSI above atmosphere pressure.

 

[ctuinstra]

kPa Vac/Boost
0 29.5
5 28.1
10 26.6
15 25.1
20 23.6
25 22.1
30 20.7
35 19.2
40 17.7
45 16.2
50 14.8
55 13.3
60 11.8
65 10.3
70 8.9
75 7.4
80 5.9
85 4.4
90 3.0
95 1.5
100 0.0
105 0.7
110 1.5
115 2.2
120 2.9
125 3.6
130 4.4
135 5.1
140 5.8
145 6.5
150 7.3
155 8.0
160 8.7
165 9.4
170 10.2
175 10.9
180 11.6
185 12.3
190 13.1
195 13.8
200 14.5
205 15.2
210 16.0
215 16.7
220 17.4
225 18.1
230 18.9
235 19.6
240 20.3
245 21.0
250 21.8

 

[Ross]

I don't see your throttle position sensor in your log file (but I do see the RPM going up), but in my log file I reached those pressures while at full throttle.

 

[ctuinstra]

I will say one thing, you have a very smooth LTFT graph, however it's adding fuel all the time. I would rather see one removing fuel. The base tables need to be richen up and MAF tune.

 

[Ross]

If I'm not mistaken, you have to substract 14.7 from the 25+ psi. Think about it this way - 100kPA = 14.7PSI or atmosphere pressure. At atmosphere at the intake is neither vacuum nor boost so it is effectively 0 boost (and 0 vacuum). so 100kPa is = 0 boost. 182kPA is around 12 psi BOOST above atmospheric pressure.

There is no way that stock supercharger with a 3.05" is pushing 26PSI above atmosphere pressure.

Good point Chad.
Although, a stock (Naturally Aspirated) engine never really has 1 atmosphere in the intake when the engine is running.

 

[SACTX]

Wouldn’t a manifold pressure measurement be based on 14.7 atmospheric? + is boost and - is vacuum.

That’s why I’m wondering if I have something set up wrong. But, looking at my boost gauge on my dash, it reads negative psi a lot. That’s coming through the OBDII port just like the scanner is for HP tuners.

What you’re saying and those numbers make sense but I don’t know why HP tuners would set up that way from the factory.

 

[SACTX]

I will say one thing, you have a very smooth LTFT graph, however it's adding fuel all the time. I would rather see one removing fuel. The base tables need to be richen up and MAF tune.

I agree with you on that. That was why I started scanning. I thought it was running lean.

 

[ctuinstra]

Good point Chad.
Although, a stock engine never really has 1 atmosphere in the intake when the engine is running.

Not at the intake (before the supercharger) but it does at the MAP inside the manifold (after the supercharger). It will see from vacuum to full boost at the MAP.

 

[ctuinstra]

Wouldn’t a manifold pressure measurement be based on 14.7 atmospheric? + is boost and - is vacuum.

See the chart I posted. Anything less than 100kPa is less then 14.7PSI (atmosphere) so that is what you think of as vacuum. So any number on the chart that is less than 100kPa is vacuum and anything over is boost. I totally understand your confusion, it confused me for the longest time and then it finally clicked.

 

[Ross]

Wouldn’t a manifold pressure measurement be based on 14.7 atmospheric? + is boost and - is vacuum.

That’s why I’m wondering if I have something set up wrong. But, looking at my boost gauge on my dash, it reads negative psi a lot. That’s coming through the OBDII port just like the scanner is for HP tuners.

What you’re saying and those numbers make sense but I don’t know why HP tuners would set up that way from the factory.

Yes. The engine has a manifold pressure sensor, and an atmosphere pressure sensor, and it does the math to give positive and negative boost pressures. It reads negative a lot because we mostly are not under boost.
HP tuners is just telling you what the MAP sensor is reading. It doesn't claim it is boost pressure.

 

[SACTX]

Got it. Thank you guys! That makes perfect sense now and is a lot more reasonable.

 

[SACTX]

That’s why it says absolute manifold pressure…

 

[ctuinstra]

That’s why it says absolute manifold pressure…

BINGO!

 

[jayrolls]

Yes. The engine has a manifold pressure sensor, and an atmosphere pressure sensor, and it does the math to give positive and negative boost pressures. It reads negative a lot because we mostly are not under boost.
HP tuners is just telling you what the MAP sensor is reading. It doesn't claim it is boost pressure.

Question, so I think I am following this. It is normal at idle then to see negative or vacuum pressure versus boost pressure?

 

[Ross]

Yes, that is normal. Congrats on a first start!

 

[jayrolls]

Yes, that is normal. Congrats on a first start!

Awesome. That’s what I thought I was reading. Ty. Making good progress.

 

[ctuinstra]

Question, so I think I am following this. It is normal at idle then to see negative or vacuum pressure versus boost pressure?

Think of it this way, when the throttle butterfly is closed, that's the same and holding your hand over the end of the hose on a vacuum cleaner, you'll get max vacuum (as measured inside the hose).