I've seen many discussion about the variances between LCR meters and their affect on the accuracy of a MPS adjustment. But I have not seen any discussion about the accuracy of the other measuring device involved in the process - the vacuum gauge.

I'm preparing to adjust my MPS. I have a Wavetek LCR55, a relatively cheap vacuum gauge and I have a new diaphragm in transit. My vacuum gauge is attached to the tool I use to check for leaks in the MPS and also to bleed brakes.

I had no idea how accurate it was so I went to my local shop that buys nothing but the best tools. He had two identical vacuum gauges and when I attached them to mine to compare readings with mine at 15 inHG, one read 10 and the other 20. God knows if any of them are correct.

So I bought a liquid-filled vacuum gauge on Amazon that had about 1,500 reviews and a rating of 4.8. It arrived today and when mine read 15, the new one read 18.

Any thoughts, suggestions?

Fortunately, I have access to a brand new, in the box MPS that was purchased from a Porsche dealer a few years ago and was never used. I plan to use that as my standard so the accuracy of my measuring equipment will not be an issue.

But I would still like to know how we can be confident when adjusting to the specs published by Anders?

Be sure your inductance values are factored for your location they are ambient air pressure dependent. Anders gives the process to calculate the correction factor.

Anders values are also known to be lean for some reason...not sure why. He based them on measured, NOS, and known good MPS values back then. The Wavetek will get you close, but a wide band and O2 sensor or dyno will be more accurate to the specific needs of your engine.

But again, what good does it do to be so meticulous and adjust for elevation if you don't know if you have an accurate vacuum gauge. I've checked 5 gauges and got 5 different readings.

You make an excellent point. Consider sending your best gauge out to a calibration service. Google for it.

But then, there is a question of what Anders’ vacuum gauge was calibrated to. Touche, this could explain a lot of variability.

There are gauges available whose calibration is traceable to a recognized standard for about $100, but it doesn't do much good because we don't know whether our reference (Ander's gauge) was accurate.

For my purposes it doesn't matter since I have access to a brand new MPS that I can use as a reference. It will be interesting to see how my readings compare to Ander's. I'll be sure to share them.

I view using a wavetek and a vacuum gauge as a starting point. I too had nos mps’s the use to calibrate to but they were to fitted to brand new factory motors. Adjust your mps then check your work using a O2 set up to get your AFR at cruise to 13.7 to 1.

I agree with Geoff
The original calibration was for 1974 gas which is not the same as 2021 fuel

This the original calibration numbers are too lean

Your motor is also not the same as it was from the show room

A wide band (real time) has much more useful data

I received the new, in the box, 043 MPS today and recorded reference readings using a Wavetec.

Note that I showed my absolute reading as well as readings adjusted for Ander's altitude. After the adjustment my readings at 0 and 15 match almost exacting, but the reading at 4 is a little off.

I thought I'd share for what it's worth!

Click to view attachment

That appears to be within a minor tolerance. The question is, will this produce good A:F ratio for your car. In my 50K mile engine they are too lean

It's strange that 0 and 15 match almost exactly while 4 is off.

When I was comparing gauges I found that differences were greater at low vacuum and started to converge at higher vacuums. I wonder if this is a symptom of a difference between my gauge and Ander's? If so, which is correct - if either is?

I understand that these settings might result in a lean mixture, but it will only be temporary. I have a stock 2.0 right now but am wrapping up a 2056 build which will have a AFR meter to help calibrate.

jeff, when you rebuilt my MPS, did you run it in your car for real world testing after setting the initial tune? I ask because you said your car will be lean if using initial settings. I am not running the MPS you rebuilt and tuned for me because the AFR i have showed it very lean compared to the one i am running right now. it was about 15 when my other one in same condition was about 13.5 this is in 3rd and 4th gear at about 3000 rpm part load flat ground. same at other settings and conditions, including WOT under load. SO i am interested in now getting this fine tuned but it would be interesting to compare them with a Wavetec LCR before and after making any changes.

@JeffBowlsby

@adolimpio if you are ever down my way or come to HHI or Savannah let me know!

Hi Phil, I build/calibrate many MPSs, but only 'field test' them on my car when there is some reason to do so. I use the same Wavetek LCR as Brad A and know what the numbers need to be. I own and have possed multiple NOS and excellent condition MPSs over the years and you might be surprised at the variations in calibrations I have encountered. Brand new NOS in the BOSCH box MPSs with no leakage - and they are calibrated differently. I am happy to recalibrate this for you at no charge (including return shipping) if you send it to me and can tell me what LCR numbers it needs to be at for your engine. One customer many years ago mentioned the same trouble he was having he and had another MPS that he said ran very well on his car. He sent me both MPSs and I recalibrated the one I had rebuilt to match his other one. His other one was so completely far off any reasonable spec that I don't know how it ran at all, but it did.

Therein is the reality. Each engine is different - different wear, different calibrations on the TPS, TS2, ECU knob, injector flows, fuel pressure, timing, dwell, valves, trigger points, a heat-damaged, corroded terminals, worn-out harness, etc. I usually recommend that folks check all these other variables as much as possible, to factory specs before further adjustments to the MPS. If yours is too lean, you might try increasing the fuel pressure a little bit to compensate (up to about 3 psi) to see if that improves things.

thanks jeff that’s a great offer. let me see if i can either test this one or i could send both and you could calibrate them new rebuilt one to sam’s values ans the other one.
i agree and i think at the factory i have heard that every car got its MPS tested and calibrated before being sealed sent off . it makes sense because of all the variables.

You can easily calibrate your vacuum gauge with a tape measure, some clear plastic tubing and water, if your gauge is in inches of mercury, 1 inch of mercury equals 13.6 inches of water,
so for 15" of mercury you'll get 203.9 inches of water

Interesting?

Distilled water? At what temp?

Sort of like this?

BTW I add a bit of food colouring to see it better and a drop of Dawn dish soap to break the surface tension.

And the 996 crank pulley makes it sporty.

Guess that's why the mouth suck test doesn't work eh?

Nice, now I have a project for the weekend.

You may need a ladder as well 203" is 16.9 feet it you want to go that high.
However if you have a source of mercury, much shorter ladder

Yeah I only needed to measure between 1-10"WC.

I built it as a math/science lesson for my kids, but the odd time I need this to measure 5"WC. I tested it against my friends $300 pro unit, that came off the Snap-on truck and it's surprisingly accurate.