Anyone have any information on how accurate different boost gauges are? I've always had pretty bad experiences with Autometer being upwards of 2 to 5 PSI off at the limit. Needless to say, that much inaccuracy in an FD could be the difference between a happy apex seal and a dead one. I'm looking at these Stewart Warner gauges as a possibility, they look nice and have the 30-0-15 readout with 270 degree sweep I'm looking for, as well as a matching water temp gauge:

http://tinyurl.com/k5c56

Comments? Suggestions?

 

That boost gauge doesn't look bad at all, and if ever in the future I'll only be running 15psi, so that would be a perfect gauge for me also....I'd like to know more about it also, as I have an autometer too and I've also heard they are sometimes off.

bump for the answer to this man's question! =O)

 

I have found that my analog boost gauge from PFS (pricey) is a bit off, when I tested it with a Mityvac. It could be that my Mityvac is a bit off, but I found that it was ~.75in/hg off.

 

For about $40, VDO used to sell a nice looking boost gauge with the same 30 inHg to 0 to 15 PSI range, but I think it has been discontinued. (I think they still have one that goes to 30 PSI.) I compared mine to the MityVac and it was perfect on boost, but a little low on vacuum.

 

You guys crack me the hell up! Is my boost gauge accurate? I don't know. Let's see how it compares to the Mighty Vac. Oh no! It reads different than the Mighty Vac! The boost gauge is wrong!

How the heck do you know the Mighty Vac gauge is right???? You haven't proven either to be right or wrong, you've merely proven they don't agree. The correct outcome of that test is "inconclusive". It proves nothing.

Here's how I verify my boost gauge's accuracy. FWIW the tire pressure gauge reads 0-20 psi and is caliberated by its manufacturer. I have since installed a VDO gauge instead for aesthetic reasons. Both the Autometer and the VDO were perfectly accurate.

 

I agree with him. However there is one instance were you can be sure your gauge is off. If the guage dosen't sit at 0 when the car is off you can be sure it is inaccurate.

 

And your $75 tire guage is calibrated only to 20 psi?

 

That is not true either. Any gauge that doesn't have a peg to stop the needle at zero psi is going to show slightly different readings at rest from day to day due to atmospheric pressure not being the same everyday. And even so you don't care wtf about the gauge being accurate around zero, you care about around 10 psi (assuming stock car).

If you want to check your gauge's accuracy you have to check it against a known good source. Relying on brand name, dollars spent or anything else is fooling yourself.

 

Yep; but it only reads to 20 psi so it's caliberated over its entire scale. That particular gauge belongs to a buddy with a mod car that runs 15 inch wide tires on a 1400 pound car. It only needs about 14 psi in the tires

People running karts, super light cars with huge tires and some dragsters use such gauges because their tire pressures are set very low. To be most accurate it makes sense to use a gauge that measures over that short range rather than my personal one for instance which reads to 60 psi IIRC.

 

Not reading zero with car off also only tells you (something about) how accurate it is at 0 psi, which is a point that doesn't have much significance for a boost gauge. If it is off by 2 psi at atmospheric pressure, that DOES NOT mean that it will be off by 2 psi at 10 psi (actual pressure). With gauges that read both boost and vacuum, the error is likely to be greatest at 0 psi. And it doesn't tell you much about it's accuracy under boost or vacuum.

-Max

 

I've said it before, and I'll keep sayin' it .

Japanese gauges are the way to go. Install one, and you'll understand. Quality hardware (which is a HUGE plus), good quality, and some like the Greddy are reasonably priced.

Here's the comparison I did a while back -

http://www.clubrx.org/default.asp?id...ntent=40&mnu=3

Dale

 

^ and there you have it. Dale blunders right in and insists the Greddy is more accurate because it's more expensive, he likes the hardware better and the Autometer gauge doesn't read zero at rest. He never even bothered to compare either to a caliberated source. That article tells you zip about the accuracy of either gauge and to come to the conclusion the the Greddy is more accurate is completely unfounded.

You can like the Greddy better and find it prettier but without testing it you can't claim it better. Hell, the Autometer wasn't tested either but he's already decided it's inaccurate. With that line of thinking I can "prove" that red cars are always fastest. Why? I like red cars.

 

Damon, you hit it right on the head!

The key is calibration against a KNOWN ACCURATE GAUGE. You would not believe (actually YOU would...) the number of people who some "trusty" gauge to be accurate, even if it hasn't been calibrated for 20 years. That leads to more screwed up stuff than anything else!

 

Thanks for the info. I was wondering about my Autometer boost guage. It doesn't sit at Zero when the car is off. It's usually at .5 psi.

 

My guage sits at 1 lb. when the car is off, and it never passes 10 (what I set the profec to) while running.

So for me, as long as it stays at these numbers, I don't care how accurate it is. I figure it hasn't poped in 6 years with these numbers, so I should be ok.

 

Wow, where to start. First let me say that I am an Instrument and Control technician by profession and have worked in a Metrology lab with high accuracy pressure. Most of the comments in this thread are wrong.

All the boost pressure gauges that you have, from the most expensive to the least, are not very accurate. I have emailed Defi repeatedly and they have never told me what their accuracy specification is (they have told me that they do not understand what I am asking, and I have tried to re-phrase it). Autometers Nexus (the best they have) line is rated at +/- 2% of full scale. The Nexus boost gauge scale is 30"Hg to 30 psi. Since 30"Hg is equal to approx. 14.7 psi, we will say the full range is 30 + 15 or 45psi. 2% of that is .9psi. So the gauge is accurate to +/- .9 psi, not bad for a 52mm gauge.

I have tested a Defi D-Gauge and Autometer (from Pettit, I assume it is AM) on a Cosa T3500 dead weight tester. The AM read applied pressure to within 1 minor division, or 1 psi. The Defi read to within 2 minor divisions, or 2.5 psi. One explanation for the poor performance was that the "zero" indication had shifted when applied pressure was vented off - not good. I emailed Defi with the results, and I asked for the gauge zero to be reset or a new gauge, as I found the results completely unacceptable. They replied that these results (+-2.5 psi) were acceptable to them!

I would expect that on average, a mechanical gauge will wil be about +/- 5% full scale, and an electronic one (Link, Nexus, etc.) will be about +/-2% full scale. In comparison, the least accurate gauge I use at work is +/- .5%FS, and the most accurate is +/- .05%FS. The dead weight testers used to calibrate the guages are usually 10 to 20X better.

A change in atmospheric pressure will not affect your boost gauge. All presssure measuring instruments actually measure differential pressure. In the case of boost gauges, they measure the difference between atmospheric and applied pressure. The actual units are not psi, but psig. The "g" stands for gage (gauge and gage are the same). It means that the gauge is measuring applied pressure referenced to atmosphere. If you are walking around with a gauge that reads 0 psig, it doesn't matter whether you are standing on Everest or at Death Valley, the gauge will always read zero. 10 psig of boost is always 10 psig because it is always 10 psi above atmosphere, the actual value of the atmospheric pressure being irrelevent.

There are specialty gauges that measure psia, the "a" being absolute. These gauges measure applied pressure referenced to a vacuum. Since this gauge would actually measure atmospheric pressure (approx 14.7 psia), a change of atmospheric pressure would affect the gauge.

If you purchase a gauge, make sure it has a good zero. This is the only point you can be sure of without having other calibrated equipment to check the gauge against. Comparing one gauge against another is meaningless unless one of the gauges is periodically calibrated to a known standard. If it has a bad zero, best to just return it.

Apologies for being long winded!

 

I can perform a calibration on your gauge for you.... =)

I do it for a living... avation/electronics cal tech... a gauge is simple. I'll use my 3688 Pressure Standard and run it up to 15psi and even do some inHG tests for you... I'll even give you a certificate of calibration so you can say it's accurate =)

Oh... as stated by the member above (ReadyKW)... I also work in a Metrology Lab.. I've been doing it for roughly 4 years.

 

I have an autometer boost guage and I havent had one prolbem with it. It has always shown rocksolid boost, not once have I seen a suspecious reading from the guage. I dont know anyone with the same guage, so I can only speek for myself, and I am very happy with it.

 

Excellent post.

 

ReadyKW: Class is in session

 

That's a little harsh, Damon.

Few points to consider from my experience -

- EVERY Japanese gauge I've ever seen, installed, etc. has been right at the zero point at rest. Autometer gauges are all over the zero "box". Even a lineup of brand-new gauges right next to each other in the display case are off. They're even off if you go through pics on Autometer's web site. Until someone can come up with a good explanation for that , I'm still not comfortable with that.

- Autometer's lighting and hardware are horrible. That's plain and simple.

I've never understood why people spend so much effort defending Autometer gauges. THEY ARE THE CHEAPEST GAUGES YOU CAN BUY. You get what you pay for.

Dale

 

Ready KW: excellent post nice to have a clear and concise post spelled out without biased opinions based on loyalties or ignorance. I've been looking at gauges myself and came across SPA brand. Don't know a thing about them except that they readout digitally ( dual readouts) and are mostly used in aircraft technology/usage. Do you have any words of wisdom on this particular brand.

v

 

Hmm...I tested the accuracy of my boost gauge by comparing it to the ECU (PFC readout), which I would HOPE is the most accurate? Any thoughts?

Oh and FYI, I've found (as many others have) that Defi boost gauges (the D-gauge, aka mechanical) are a bit off...which is prob the reason why they discontinued the guage and will prob be introducing an electronic version.

~Ramy

 

Haha, maybe I'll take you up on that. That way, I can have the most accurate PSF guage around!

 

ReadyKW: I knew there had to be at least somewhere here with the logical and scientific information regarding this matter. Good post!

dhahlen: Just, curious....how much would a service like that cost? Not that it'd be very viable or worth it for such gauges....but it'd be good to know.

FWIW, I have a $40 Intercomp tire pressure gauge that reads 0-60. It's by no means 100% accurate. Of course, it's not the high end of the deal either. The zero rest is a range of about 2psi. I've also discussed it with Intercomp and they mention that I need to keep the gauge upright too, or my reading might be off by 1psi. I should probably get my gauge tested, so I can note the difference.