boost gauge question, I'm going nuts...

[ScottD]

I'm going nuts, I can't find a boost gauge that isn't WAY off up here...

I've had a VDO and Defi gauge in the past, both mechanical... I *think* they gave accurate numbers.... I'm beginning to wonder if electrical gauges are the problem...

I do know that sealevel atmosheric pressure is around 14.73psi, in Denver we average around 12.7psi. Given that 1psi = 2.036inHg the gauge *could* show 4inHg with the key on, engine off (or do I have that wrong?)

First I had a ProSport electrical... key on, engine off it was reading 8-12 inHg, idling I was pegging at 30 inHg vacuum, boost I was hitting 12psi when the ECU was showing 14.5psi

Then I got an STRI electrical, it reads 5inHg with the key on, engine off, vacuum about 22inHg, and peak boost of 12.5psi (ecu shows 14.5psi).

The STRI numbers may be accurate with the altitude thrown in to consideration, but I was under the understanding that a gauge reads gauge pressure and not absolute pressure. 14psi at sealevel should be 14psi here... after all, the ecu is reading it correctly.

The old VDO and Defi mechanical gauges I had showed barely under zero with the key on and engine off... they were on a different car, so I can't compare them to ecu readings... am I going nuts? I just want a nice looking gauge that works at this altitude. I'm beginning to think a mechanical gauge is the way to go again....

[thiazole]

They can definitely make an electronic gauge that reads they way you want. My HKS EVC shows the same boost pressure as my mechanical boost gauge shows.

[Kibo]

I think it has to do with the specific brands, Scott. Think about how you're datalogging the boost from your ECU--an electrical sensor, right?

Even though I agree that it sounds like your gauges are off based on the vacuum readings with the engine off, I urge you to consider that the reading from your ECU might not be accurate either. I don't know the Subie platform at all; but based on what you described to me, it sounds like you're logging the equivalent of the MDP emissions sensor that's used to control the EGR valve on a DSM. It's possible that this reading is only good enough for its intended purpose, so take it with a grain of salt.

My SPA boost gauge allows me to zero out any offset due to atmospheric pressure, voltage, temperature, wiring resistance, whatever. This is necessary because the sensor reports psia, which the gauge head translates into psig. It baffles me that other electronic gauges don't allow for this calibration, unless they're using a sensor that reports psig by referencing the measurement to a secondary (atmospheric) port. It's also possible for the gauge to read the sensor prior to boost/vacuum being applied (i.e. immediately upon power-up), as this would represent atmospheric pressure. Regardless, it sounds like the two gauge brands you've chosen don't compensate for atmospheric pressure correctly.

[ScottD]

Thanks guys, that's what I thought, probably a problem with the gauge manufacturers.... just hard finding a gauge that matches up to the stock cluster well... my Talon had a Defi non-controller gauge that would have been great, all they have is those blue racers ones now...

So, psig should be what the gauge is reading right? So shouldn't 14psi be 14psi regardless of where you are?

[Kibo]

Wouldn't the Defi Link Meter gauges match your car well? Pretty pricey, but they seem like nice gauges.

Yes, 14psig will be 14psig anywhere. However, the sensor is likely measuring psia. Check out this article:

MAP sensors (http://www.asashop.org/autoinc/feb97/techtotech.htm)

[ScottD]

Wouldn't the Defi Link Meter gauges match your car well? Pretty pricey, but they seem like nice gauges.

Yes, 14psig will be 14psig anywhere. However, the sensor is likely measuring psia. Check out this article:

MAP sensors (http://www.asashop.org/autoinc/feb97/techtotech.htm)

They would, but then I need to buy the Link unit and everything... The old Defi D-gauges were the exact look of those, but didn't need the link unit... they were retired for the ugly (IMHO) Blue Racer line.... anyone have one to sell?

[ScottD]

I checked this morning and the number I'm getting back from the ECU is relative pressure. It seems to be more accurate as I can feel and hear the turbo right when the readings make a transtion to boost from vacuum.

I'm assuming that rather than reading psig, the gauges I've tried are reading psia and then are calibrated against sealevel atmospheric pressure (14.7).

Would that mean that every mechanical gauge I've used that reads accurately has a barometric sensor in it?

[thiazole]

I don't think it is that complicated. I've never torn apart a mechanical boost gauge, but I somehow think it has something more simple like spring tension on the vacuum and the boost side, so that no matter where you are, if there is no vacuum or boost relative to atmospheric pressure, the spring force pulls it to 0. If there is boost, it must fight the boost spring to climb, and if there is vacuum, it must fight the vacuum spring to drop. I'm not saying this is exactly how they work, but I think in principle it is similar.

[Kibo]

I agree with Rob, a diaphragm referenced to atmospheric on one side and manifold pressure on the other side seems most likely.

[D Walker]

The basic problem is a MAP sensor measures ABSOLUTE pressure and a boost gauge measures differential pressure, which is not nec the same thing. For example, my AEM MAP sensor reads -2.XX typically while the boost gauge is showing 0. When the boost gauge is showing 10psi, the MAP sensor will show 7.XXpsi.

[ScottD]

The basic problem is a MAP sensor measures ABSOLUTE pressure and a boost gauge measures differential pressure, which is not nec the same thing. For example, my AEM MAP sensor reads -2.XX typically while the boost gauge is showing 0. When the boost gauge is showing 10psi, the MAP sensor will show 7.XXpsi.

Thanks Don, as I was reading up on it more it seems that's exactly what I found... so an electrical boost gauge is always going to read off (in my definition) up here unless it either has a way to calibrate for atmospheric pressure or resets to atmospheric pressure on power on.

I don't think I'm ok with that, I'll probably go back to a mechanical boost gauge.... True, the load on the engine is different even at the same 15 psi, but I don't want to see 15psi at sea level, 12psi here, and 10psi up in Copper because the MAP sensor is making a measurement against absolute pressure.

[Kibo]

Uhh...that's what I said. *shrug*

[ScottD]

Sorry Erik, your post was super informative and I appreciate it, it didn't quite sink in as to what you were saying until I did some more research, then it all made sense.

[Mirage]

One issue that I had with an Autometer elec. boost gauge was that it would calibrate wrong upon startup.

This particular gauge calibrated itself on every power up. The guy who installed it put it to a switched hot that gets killed while cranking. When he released the key from starting it, the gauge calibrated itself to a vacuum state. If this was wired to a key on hot that stayed hot while cranking, it would have fixed it.

It ended up showing about 6 in/hg shy of a normal reading and was off by about 5 psi.

Marcus

[ScottD]

That's interesting Marcus... I'm hooked up to a switched power as well, maybe I'm also calibrating to a vacuum state. Should I have the the MAP sensor connected to a full-time hot connection? The instructions never mentioned anything like that....

[ScottD]

That's interesting Marcus... I'm hooked up to a switched power as well, maybe I'm also calibrating to a vacuum state. Should I have the the MAP sensor connected to a full-time hot connection? The instructions never mentioned anything like that....

Wait, I guess I answered my own question... key on, engine off it reads 6 inHg... so it just never calibrates to atmospheric pressure...

[magikgsx]

They can definitely make an electronic gauge that reads they way you want. My HKS EVC shows the same boost pressure as my mechanical boost gauge shows.

i had a electronic boost gauge that i was having trouble with for a while, and gave up on it. i personally prefer the mechanical gauge, in my opinion they are more reliable and the most accurate.