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Believe mechanical gauge or ECU?
coyoteboy - 23/9/13 at 10:54 PM

Found an interesting connundrum. Got two mechanical boost gauges which read within 1.5psi of each other, but when reviewing EFI logs (MS) I see a fairly large difference. The mech gauges give me 0.95 of boost (13.8psi), the MS logs give me a peak of 176kpa (11psi).

I'm inclined to accept the MS logs more than the 10 year old mechanical gauges, what do folks think. Anyone else noticed such disparity?

owelly - 23/9/13 at 11:11 PM

Absolute or gauge pressure?

coyoteboy - 23/9/13 at 11:27 PM

Gauge (Ok the 176kpa is absolute but I've converted it to gauge psi for ya! )

Bare - 24/9/13 at 01:53 AM

Dunno.. EVU gets input from the MAP sensor These can be great OR crap.
Mech gauges ? Ditto I use a Warranteed 2% accurate boost gauge from Mc Master Carr ~$20 too Compared to one (most?) of the aftermarket Bodge Gauges.. this thing is like an Atomic clock for accuracy .
Before you can go pointing accusing fingers at the existing hardware.. you... Really! ...need to test with a genuinely Trustworthy gauge.

v8kid - 24/9/13 at 02:01 AM

Calibrate the gauge yourself. 10m head of water is 14.2 psi.
Set up gauge attached to Hosepipe filled with water, vertically that is, and drop height of top of hosepipe until reading on gauge matches previous ones. Pro rata to 10m head to get true value. Google for other stuff to ensure accuracy. Bingo!
Cheers!

cliftyhanger - 24/9/13 at 06:04 AM

Like others above I would question the sensor that is sending the signal to ms as much as a a mechanical gauge. You need to calibrate the mech one or you will never be sure.

balidey - 24/9/13 at 07:48 AM

As above, you have to check as you have no way of knowing.

Although it does depend how old you are. Most younger generation automatically believe anything digital or electronic must be better or more accurate.
The number of times I am told my speedo is out because the GPS in their phone says it is. How do they know?
Digital calipers are 'more accurate' then my dial vernier.
etc.

matt_gsxr - 24/9/13 at 08:30 AM

What does the MS gauge read in normal air? should be about 99kpa, but depends on your altitude and climatic conditions on the day.

I assume they get their pressure feeds from the same place (before and after intercooler can give you the kind of drop that you describe).


Personally I don't trust mechanical gauges any more. In turbo applications they can struggle because of the fluctuating pressure, are either of them liquid filled.

mookaloid - 24/9/13 at 09:50 AM

It is possible that they are both 'accurate' but the sampling rate of the digital one might reveal stuff that the hysteresis loop on the mechanical gauge might be masking.

just thought I'd chuck that one in

coyoteboy - 24/9/13 at 12:05 PM

quote:

Calibrate the gauge yourself. 10m head of water is 14.2 psi. Set up gauge attached to Hosepipe filled with water, vertically that is, and drop height of top of hosepipe until reading on gauge matches previous ones. Pro rata to 10m head to get true value. Google for other stuff to ensure accuracy. Bingo! Cheers!


V8 - Bit difficult as I can't check the linearity of the gauges as I can't get to 10m height

Yep, I'm at 99kpa at initial turn on on the MS, so I know that's approx correct, but again doesn't mean the rest of the range is.

Sample rate shouldn't be an issue unless I have significant boost fluctuation in the order of 10Hz, which I don't think I do! I've tried sustained boost periods of 3-4 seconds and only see peaks of 176kpa. You're right though mook - it could simply be spotting the peaks that the mech gauge averages out. I'll get more logs and check for this. Neither are liquid filled and both have some lovely oscillations at light throttle which seems to be a resonant point in my intake geometry!

I tend to trust digital items more because in my experience they're usually more accurate as there's usually much tighter tolerances on the manufacture and they can be linearized in hardware and temperature compensated when designed for a specific task.

I'm half-way between old and young so I float and look for peoples experience! I trust a GPS over a mechanical speedo because, at steady state with good signal, they are more accurate by definition of their method of operation (time compensated triangulation from 12 simultaneous sources with atomic clocks, versus a 10 toothed gear wheel in a gearbox with unknown tyre pressure)

Bah, I'm going to have to go down the route of trusting no-one and calibrating aren't I. Damnit.

scudderfish - 24/9/13 at 12:41 PM

quote:
Originally posted by coyoteboy
Bah, I'm going to have to go down the route of trusting no-one and calibrating aren't I. Damnit.


Do you trust the tools/methods you're going to use for calibration?

coyoteboy - 24/9/13 at 01:03 PM

No

Boost profile from a light throttle press showing 0.95bar on the mech gauge...
https://www.dropbox.com/s/t069p9psppsznzw/boost.png

beaver34 - 24/9/13 at 03:58 PM

i have the same with my map sensor on the ecu reading lower than the boost controller does, i take the map as correct as that what the engines fuels for

coyoteboy - 24/9/13 at 06:22 PM

It poses quite a risk though, if conventional wisdom suggests x psi is sensible max.
In my case I really am questioning the ms too because it's significantly more powerful than stock yet using only 60% duty on the n injectors too. A world of questions!