Sorry, I don't. It would be dependent on the sensor and electronics. I can tell you however, that a digital display is inherently more accurate than an analog display. This is because an analog display will suffer from hysteresis (error caused by a difference in reading the identical applied pressure in the upscale and downscale directions), and parallax (error caused by not reading the display head on). A digital display does not have this.

The problem with digital displays is that they really don't give you a sense of scale. A quick glance at an analog gauge and you know if you are OK, without knowing the exact value. With a digital number, you have to put it into perspective yourself.

The ECU and sensor are not calibrated on a periodic basis against a known standard, so on a technical level it doesn't do you any good. However, two gauges reading exactly the same increases the probability that they are good.

P.S. Can't remember how to get the "Originally Posted by" in there. Any hints?

 

Thanks for the info ReadyKW.

Haha. Sure. [QUOTE=ReadyKW] then type... followed by endquotes

 

I'm asking myself just how critical it is that the guage is exact. At least on my car with a stock ECU. Since that stock ECU tends to be on the rich side anyway, and since relatively speaking, my boost is held at a given psi, and if the pattern is as Mazda intended it be (a relative 10-8-10) .....do I need to be all that concerned?
Regardless, it was an interesting read ReadyKW. Thanks.

 

[QUOTE=FDNewbie]Haha. Sure. Thanks, I'm on a roll now

 

Well that's the whole reason I started this post. My original autometer 30-0-25 gauge always read about 2 psi too low. I confirmed this when it was installed on my friend's WRX that was bone stock and it maxxed out at 2 psi under the factory boost levels at sea level, so he was all worried his car was underboosting. He later installed a VDO and that read exactly factory boost. So with the autometer, we're talking the difference between seeing 12 psi and hitting 14 in a stock-ECU'd FD with intake and exhaust mods and I'm not willing to take that risk, which is why I asked for opinions initially My autometer 30-0-30 that I use now on my galant displays up through about 15 PSI more or less accurately, but anything higher (according to my digital peak hold gauge that's built into the turbo-timer) usually pegs the autometer at 30, no matter what it is. Seems pretty useless to me, which is why I won't be making that mistake a 3rd time.

 

So now I'm confused, If I read correctly any gauge can be off. So an autometer or greddy or defi are on the same level?

 

EDIT

 

Just what I like to see a start to good debate...

Wahoo...just made 1000 posts--with minimal post whoring!!!

 

Hell no, just because you cant tell the difference doesnt mean its not there. Were just yet to find a proactive, properly-equipped member to prove what we all assume.

 

Something else to consider is the responce time of analog versus digital gauges.
I'm using a VDO 0 - 15 PSI boost gauge that has a 270 degree sweep. This makes reading it very easy and gives it really nice "resolution". I use the term resolution losely since anything analog/mechanical has pretty much infinite reolution. This is more in reference to the human perception.
I also use a Greddy Profec B to control boost, which has a built in gauge and peak/hold feature.

The two compared to each other at a static pressure are equal (yes, I've tested it with a Mityvac). Yet during operation the peak shown by the Greddy BC will always show a small spike of about 0.7 to 1.0 PSI (driving me nuts, cause I can't seem to get rid of it...). While the mechanical nature of the VDO gauge lags behind just enough to make any spike nearly imperceptible, at most registering .2 - .3 if anything at all.

This makes using an analog gauge to tune the boost pressure without the backup of a good peak/hold feature rather dangerous. Especially if one insists of running near the very ragged edge of the fuel systm.

Just my $0.02.

 

I wasn't defending Autometer, I was pointing out that the article you linked to on your site proves absolutely nothing about either gauge being accurate. Your own comments show why you prefer the Greddy and I tend to agree with them, but you did absolutely nothing to prove either gauge accurate.

You felt the Greddy was more expensive, was prettier, had better lighting, smaller sensors, nicer hardware etc. With that info to then make the jump that it is more accurate is completely unfounded by the facts given in the write up.

 

This is not true of all gauges in my experience. This question came up one day about tire gauges and being the geeks we are we experimented. For instance some tire gauges have a relief vavle in the body to equalize the pressure inside the instrument to atmospheric. The reason being so that atmospheric pressure would always read "zero" on the gauge. We wanted to test if gauges without this valve would read differently.

We wanted to know if the gauges displayed differential pressure or absolute pressure so we put them into a little vacuum chamber rigged from a plastic box and an air compressor. If the gauge reads differential pressure its needle shouldn't move. If it reads absolute pressure the gauge should read the pressure inside the chamber. Results were mixed but perhaps we overlooked another variable?

 

What counts is that the instrument's reading follow the source that drives it. When the car is at rest there is no source driving the mechanism so the needle will tend to rest wherever the machanism is "happy" and balanced at rest. This is true of most gauges that are not lab instruments and do not have a positive stop for the needle. Why does the fuel level gauge with the key off read one thing and then another when you turn the key on? Same reason.

 

We use boost gauges to measure manifold pressure. What we really need to know IMO is what manifold pressure the ECU is reading because that is what fuel is based from. If people are going to gripe about tiny psi differences in gauges you sure as hell had better have your boost gauges tee'd into the MAP sensor line since this is the location from which the ecu reads boost. If you have the gauge tapped anywhere else you will see slight differences (less than a psi I'm sure) in manifold pressure because of flow differences throughout the manifold itself.

 

On a stock or nearly stock vehicle, aren't we really using the gauge to monitor the health of the turbos and the functioning of the sequential system? If the stock ECU is designed on the rich side, we have a decent BC and aren't pushing the envelope other ways... will +/- 1 psi error in the guage really be significant?
I guess I bought my gauge assuming it was accurate and responsive, but also with asthetics in mind. I would prefer to keep it unless it's really a liability.

 

Amen

 

The perfect zero is also a result of the electronic age greedy, defi (elec), Autometer (full sweep electric) all reset the zero at ignition turn on. The Autometer take manifold reading as the "key" sweeps between on and start. The world has many gauges, we could spend 5 grand on a boost gauge if we wished. As far as our application their are a number of factors, price, reliability, accuracy, readability, looks, interior match, lighting, features. To each his own I use big Autometer mech. ProComp Ultralight gauges on my race car; they are easy to read, I do not race at night, they stand up to the vibrations, they are well made (Autometer has a million product lines, and even with same name model x and y can be different ie, all aluminum case in 2 5/8 gauge vs plastic in 52 mm gauge) and mech. senders down tax electrics. On my FD I use full sweep elec. 52 mm, so as to get memory, easier installation blah blah blah. I guess to keep the car Japanese, I should have just taped 100,000 yen notes to the dash and asked my passengers: hey like my Japan gauge, the corners curl when the boost go's past 17 psi, but its ok, those six notes will get me a new engine.

 

While we're on the topic of autometer, and if you want I'll move this to it's own thread, I find that the gauge's tendancy to zero out at 1 psi boost is dependent on sunlight or lack thereof, striking my a-pilar cluster. On a cold day, no sun, it tends to zero at 1 psi boost. It also tends to under-report vacuum by 3-4 mm Hg. When it was new it zeroed out at 0 and all numbers were idle = 18 mm Hg and top boost was at 10 PSI, set by a Greddy Profec B. Today, it is cold out and the gauge zeros at 1 PSI, it reports idle vacuum at 15 mm Hg and top boost at 11 PSI. When it is warm out yesterday and the cluster is hot to the touch, the gauge zeros out and reports readings like it did when it was new. The tendancy to zero out at different PSI under different temperatures is definitely influencing top readings in boost and vacuum, consistently. Do a Google search of "Autometer boost gauge" and zero and the results are interesting if you have all night to read about the same problems on Mitsus, Mercs, Nissans and Toyotas. And, by the way, anyone know a way to calibrate an Autometer Carbon Fiber Ultralite? There's no set screws on it anywhere that I can find. I suspect I too will be looking for another gauge when I can afford it.