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The fun never stops.

Good point.

Been out of town for a couple of days. Missed some of the fun. My whole point is: That if your carbs are sensitive to fluctuations in fuel pressure....what range of pressure fluctuations cause a problem. If its more than say....2 psi.....then ANY gauge...even a gauge with nothing but tick marks and no numbers will do you fine. In this manner....if teh engine stumbles and you see the gauge needle flicker...you know what the problem is. I "think" it is this diagnostic function that Scott Novak is referring too....when he says that you don't need a very accurate gauge for just driving. Use it as an "indicator"....not a test tool. Fine with me.

But...if your carbs floats have a sensitivity to pressure fluctuations of 1 psi or less.....and you hope to find out when that happens...or even if it is happening....a 5 psi gauge of only 2% accuracy in the middle of the scale will not help you very much.
So why do I say this? I mean jeez...2% of 5 psi is like....microscopic....right? Actually thats +/- 2%....which is a 4% spread....that could be a problem with some applications.

The problem with cheap guages ...is that many are class "B" (2% mid scale and 3% rest of scale) and most are actually class "D" (5% whole scale).
That rating is not JUST a reflection on their scale accuracy.....but is a reflection on heat and vibration proofing, dial tolerance, heat and cold tolerance....and above all....repeatability. They are generally just crappy build quality.

The gauge may come to you at a class A or B rating but even in general use they do not stay that way very long. Typically a class "B" gauge.....tested against a reference gauge is out of spec within just a short period of time. Typical readings are +/- 10% full scale. Thats a 20% spread! Still want to trust a crappy class "B" gauge for more than just an indicator? In extreme envrionments....which is under heat and vibration in an engine compartment.....these gauges can become innacurate in days....not months or weeks.

Also...since most of these gauges are"bourdon tube" type gauges....they have two seperate ratings.
(1) ambient temperature range (the temperature surrounding the case)
(2) Process temperature range (the temperature of the liquid or gas acting on the bourdon tube)

You would be hard pressed to find ANY...ANY ...bourdon tube style pressure gauges that accpetably operate above 160F ambient temp. They can operate elevated process temperatures (liquid)...only when an isolating guard or buffer is installed to keep the process liquid from elevating the case temperature. the only exception to this rule are whats called "high accuracy" or lab tst gauges of class 2A made for extreme environments. they can use ambient temps of up to 250F. The yare also generally wall mount style...about 4.5" in diameter and start at about $90 each.

Because of this heat issue...there is no such thing as a reasonably accurate bourdon tube style gauge that operates in the engine compartment of an ACVW. Its just too hot. They will be Waaaaaay below a class "D" gauge as soon as the case gets hot.

Next problem.....I don't know the term for....but its just like the "burn in" you get from leaving your computer monitor on with no screen saver. Having a pressure gauge operating in an environment that is too hot for it....and staying in a constant pressure range for extended periods....de-tempers the phospher bronze, brass or stainless that its bourdon tube may be made from. It has atendency to then be wildly innacurate....and not even to be able to contract the bourdon tube and return to "0".

The problem with running a line to the dash is not that the fuel being a dead end stand pipe will not give an accurate signal.....its just that with a long tube.....there is too much operation for temperature expansion of both the tube and liquid within since it does not circulate. It can be way off compared to what is actually in the circulating fuel line. A rubber line may damp pulses and would be ok at low pressures.

The problem with heat on cheap bourdon tube style gauges is that most cheap ones are gear driven dials. When the case expands...the mounting of the gear drive changes position relative to the dial.....highly innacurate.

Most good quality gauges are direct drive....attached directly to the bourdon pressure tube. But...the trade off for bettter accuracy is that the tube can take a heat set as described earlier.

Yes...you can get some gauges that have higher ambient temp ratings. Most stainless case gauges are a little better than thermoplastic ones. All glycerin filled gauges cannot go higher than 160 because of liquid expansion issues on teh bordon tube and possible case leakage.
The only exception are gauges that have inconel tubes and stainless cases.
Most of those gauges exist only in the class 3A category with full scale accuracy of .25% of scale. They start at about $250 each.

In short...decide what you are trying to measure....what your level of tolerance will be before you have problems...shop a gauge accordingly....keeping in mind where you plan to mount it.

For real accurate tuning and setting of floats....it really would pay to buy a really good gauge.Ray

Ray, Scott, David, et al,
Ray's last post gives the best real world advice on this subject. Using an economical guage as a diagnostic aid is, in most cases, more than adequate. Consider this; Weber recommends 2.5-3.0 psi for their IDF, IDA, and ICTs. From past (back when dual 40's w/manifolds, air cleaners, and linkage cost less than 200 bucks) experience, I know that duals will run, without trouble, at 4-4.5 psi. I set them at 2.5-3.0 with my old Sears pressure/vacuum guage at 3-3.5k rpms. I don't fudge. That gives me all of the leeway I need to compensate for pressure rise that could occur. This is not the accuracy that Ray describes, but, it seems to be within the tolerance that Webers can live with. Making a fuel pressure check part of your regular tune-up might seem unusual, but it would surely prevent a lot of frustration when it comes to tuning and jetting. This applies only to carbs. FI, no doubt, requires a great deal more precision.

The sad part is...that most of the people (not all) over on the fuel injection forum....do not pay nearly as much attention as the yshould to fuel pressure. The current crop of fuel injection people these days spend too much time believing that any innaccuracies will just be corrected by software of their programming.
Of course they forget that the ECU has no idea what your fuel pressure is. It just assumes you set it at a pre-determined level. With a fuel injection system and a fuel pressure setting at idle of say....32 psi.....every 1 psi you are off is 3.125% ....so yeah....it can make a big difference.
In carbs...its just supply. It doesn't affect the metering as much unless you run dry. Ray

say I'm running a holley 12-804,it has two outlets.
Can I get a pressure reading from one,and use the
other to feed the carbs?
I'd rather have a dedicated diagnostic port and
a gauge that I can screw into it than a gauge
permanently trapped in my engine compartment

Just a thought... you can get diaphragm mounts for most gauges. Could you set one up on the line and run some tubing to the gauge up front from the dry side of the diaphragm? May not be super accurate but installed correctly should give you the base line data you seek without the danger of actual fuel in tubing issues.

An industrial spec sensor (Honeywell 0-5v etc) is the way to go, and you can calibrate them yourself vs. the software if you have a good reference gauge.

SMC pressure (0-150 PSI) or vacuum/pressure (-30/+30 PSI) programmable (ZSE or ISE series) are also excellent and will run just fine on 12-24v. Make for neat little readouts mounted in the engine compartment/on the engine.
(These make for a reliable idiot light device BTW)

Almost any mechanical gauge will suffer a fast, miserable death if hard connected to the engine, and a much longer slower death in the engine compartment on a flex hose.

But as a temporary install for troubleshooting/indication almost anything is better than nothing.
Usually you are looking for excursions, not absolute values.

Even home Depots finest are OK for that.

Diaphragm mounts are overkill for fuel.

Gentlemen,

We use an engine compartment-mounted fuel pressure gauge just to set psi at idle, with a few RPM runups to see if the gauge is responding.

Most, if not all, road racing organizations forbid any fuel pressure gauge in the driving compartment except those operated by electric senders. Obviously they don't want any liquid fuel piped to the dash.

Our concern is that the driver should see some kind of info on the performance of the fuel pump, typically a dead electrical pump, or an erattic, failing pump. We found a simple fuel pressure warning light kit from JEGS, and set to 0-5 psi. No light means no pressure. A flickering light means erattic pressure.

Now, if we were trying to diagnose some on & off fuel problem, the best thing in the world is to have a gauge in front of the mechanics face as the engine runs under load. Just idling won't help much, even RPM runups won't tell you much. It has to be under load.

To that end, you put temporary T-connectors ahead and behind of your fuel filters, run good hose-clamped lines to them, and mount two cheap fuel pressure-slash-vac gauges to them and drive.

That way you'll see pressure failures from the pumps (we use two ganged pumps) to the filters, then from the filters to the carbs.

FJC