(Not So) Poor Man's Vacuum Gauge

I had the thought that you could use a MAP sensor off a GM vehicle as a sort of vacuum gauge. The type I'm talking about is like this:



To test this theory I stopped by the parts store and picked one up along with a length of hose. When I got home, I hooked it up to a 6 volt lantern battery and my digital volt meter and ran some tests. Everything seems to check out so far. What's nice about this set up is that I wasn't getting the "bounce" that you normally get with a standard vacuum gauge. I'm going to do more testing and compare my measurements with my carb sticks. When I am done I will report my findings here along with a write-up detailing the process.

The electrical hook up I'm using can be found on page three of this Wells Manufacturing brochure (.pdf). The pin connectors are labeled on the opposite side of the MAP sensor: A, B, and C.

Pin A -- Ground
Pin B -- Sensor output
Pin C -- +5 volts

The digital volt meter is hooked between Pin B and Pin A. The positive lead of the digital volt meter goes to Pin B.

UPDATE:
I did some additional testing with my vacuum pump and came up with some base readings. Here they are:



When I was testing this on the bike, I was getting a voltage reading of around 1.3 volts (if I recall correctly). According to the chart that value puts me somewhere in the 11 inch pounds of vacuum range. I tested cylinders 1, 2, and 4 and they all measured about the same. I then hooked my carb sticks to them and they all measured almost exactly the same on it as well. The acid test to see if this contraption will work for synching carbs will be to deliberately misadjust the synch screws then see if I can dial all four carbs back into about the same voltage values. Once that is done, I can then hook the carb sticks up to verify my results and accuracy.

That's about it for now...

Copyright and Intellectual Property notice:

The preceding information is the Intellectual Property of the Member which did write this Post.
Copyright: May 2, 2008
ALL Rights Reserved.
The material may NOT be used in whole or in part without express written permission of the Author.
(R. Massey for XJBikes.com)

 

(staring at Gamuru intently)

 

<BUMP>

New information add.

 

excellent, how much $?

 

Ummm... Forgot to look. I work for CARQUEST Auto Parts so I got it at cost +10%. I'm guessing the MAP sensor would retail for around $50 new. I was thinking a guy could go to his local auto salvage yard and get a used one plus its pigtail for about $15.

I'm not done with my testing so don't run out and get one just yet. It still may not work.

 

Don, you know so much that sometimes you're almost scary.

Just curious... Has anyone ever met this alleged "Gamuru" in person, just to confirm his identity?
Or are we corresponding with a massively parallel underground computer, near Ft. Meade, MD?

 

I think this is brilliant!
I hope it works and you can invent a digital device for syncing carbs.

Beware of the prying eyes on the visitors who are trolling the net just looking for a good idea to steal.

Good luck, Don.
I really have a high degree of confidence that you are on to something.
I hope you make some money from your ingenuity in devising this method.

 

Don, would you need 4 map sensors to sinc' all the carbs at once?

 

Give the man time to see if it works first.

Outside of that, I imagine one could utilize Rick's patented One Gauge Sync Method.

 

yeh, i hear ya. i was just trying to get my head around how user freindly it would be.
just as a caveat to rick's "old school method" of syncing carbs, i read somewhere about having 1 or 2 house fans blowing on the engine while all the tinkering is going on, i guess you guys would do that automatically, it's just not in the method statement.

 

We figured that if anyone had enough smarts to get far enough into tuning that they were ready to attempt syncing their carbs Old School ... would be able to figure that one out.

Maybe not!

I'll go back in and change it to include the caveat for not overheating the engine.
Because ... you're right!

Without it being spelled-out; someone could overheat their bike and blow a Head Gasket and it would be my fault.
And, ... that would suck big-time.

Imagine all that work getting to where you are ready to sync carbs and not having fans on to cool the engine while its running as you do the syncing.

Boom!
Oil all over the place.
Curses.
Having to take a Head off and put-on a gasket.
Not good.

I'll straighten it out!
Good call!

 

after looking at the Wells article, i wonder where i get the +5v from?

 

:lol: Yep, I'm real. Jdpesz (Jeff) can confirm my state of being for you.

Thanks, Rick! That would be nice, but there already exists digital manometers. However, they cost big $$$ and are cost prohibitive for the backyard mechanic. This, on the other hand, is not... depending on where you source your parts from.

You could if you were certain that you have four MAP sensors that read the same. I don't have that much faith in auto parts store electronics. I had imagined, as Schmuckaholic surmised, that you would use the one unit to set all four carbs. The voltage value produced by vacuum isn't nearly as important as just setting all four carbs to the same value. Synchronization is found in symmetry.

At the moment I'm using a 6 volt lantern battery. It seems to work just fine. I was thinking that a "AAA" battery holder capable of holding three batteries from Radio Shack could be purchased, too. This would put you at about 4.5 volts (a little closer to spec. voltage). I tried 12 volts from the battery on the bike, but the measurements became so "tight" that it was almost indistinguishable when you changed pressures. Obviously running at or near the designed voltage would produce the best results.

Thanks for all your comments!

 

You could use a computer power supply. The four wires going to the drives are +/- 12v and +/- 5v. Just find an old AT power supply (the older ones where the power switch was not momentary). These are almost useless as computer parts today.

Even better yet--find a external computer drive power supply. It will be compact and have 5v.

Just a thought.
Stephen

 

"Synchronization is found in symmetry."
not only are you a photography and how to wizard, your a gad dang philosopher poet too

 

My state of being and whatever reality is associated with it may be brought into question, but Don is definately for real. He is a knowlegeable mentor, a willing servant, and is fast becoming one of my best friends.




Still up for that brake job, Don? :lol:

 

Try reading his blogs. He's a great read!

 

Noting that Don works for CarQuest prompts a comment:

I belong to a classic car club in my area. The local Carquest affiliate auto parts store gives 20% off on club members' purchases there, which IMHO is very classy. Sure builds customer appreciation and loyalty.

XJ Bikes is a much larger group but so widespread the analogy's not perfect. And we saw how hard Rick worked trying to get group benefits from vendors who don't seem to give a flip. Yet I wonder if other Carquest stores would offer any discounts, or if that's strictly one local store's generosity.

[and if 'formal membership' is an issue, maybe XJB could offer downloadable member cards for everyone who donates a certain amount to help support the resources SnoSheriff, the Moderators and other experts provide here.]

 

I wanted to give you all an update, but first a couple of responses to comments recently made.

Jeff, yep, let me know when you're ready for that brake job.

Jeff and Polock, thanks for the kind words.

gcrick, unfortunately I am not in a position to extend to all members of XJBikes.com a discount all across the U.S. and Canada. I wish I could, but I can't. However, if you come visit me at my store and let me know you're a fellow XJ rider then you bet I'll take care of you. I won't leave you empty-handed, either. When you go into a CARQUEST store, ask for their "Prefered Customer Discount". The account number should be PC1000 (At least that's what it is here in Washington). That'll get you about a 10% discount off the retail price on many items. Hey, it's better than a sharp stick in the eye. :wink:

Now, back to our project. I've some good news and some bad news for you.

The bad news:
I was mistaken about the vacuum fluctuation. It was pretty pronounced today while trying to set the carbs. So bad, in fact, that I had to finish the synching with the carb sticks. Why? I believe it is because the barometric pressure changed overnight when a storm rolled in. I noticed my base reading was way different today than yesterday. I'm speculating here, but I believe that since the base pressure was so different, it changed the sensitivity of the sensor. Whatever the cause, it made it virtually unreadable... and that's not good.

The good news:
I discovered that by pinching off the hose, I could "control" the sensitivity of the MAP sensor. This is promising news because it means I can add an inline vacuum restrictor which will dampen the pulsations from the engine. On Monday, I'm going to purchase and install an Edelmann Vacuum Restrictor:


PN 715300

This should quiet down the erratic readings I'm getting (I hope).

Conclusion:
I did attempt to synch the carbs. I started off with the following measurements on cylinders three and four (I'm excluding cylinders one and two because I couldn't obtain reliable readings from them):

Cyl #3 = 8" of Merc. or 1.902 volts
Cyl #4 = 11" of Merc. or 1.730 volts

After adjusting cylinder four to the same voltage as cylinder three, I reinstalled the carb sticks and found that the mercury for both cylinders was at the exact same level.

I am now confident that if I can get the vacuum "bounce" under control by adding the restrictor inline I should be able to synch all four carbs.

More to follow when I make the modifications...

 

I like the idea,,,but how long it holds up with unrestricted vacuum fluctuations would be a real ? imo....sounds like it maybe coming apart inside already,,,maybe lets hope not..
my hats off to Don for putting it out there sounds like your having fun...
I would love to see 4 of them with digital readouts on my dash for on the fly tuning...Dan

 

I don't think so. It's actually behaving the way a typical vacuum gauge should. The anomaly, in my opinion, was when it gave a reading without the rapid voltage fluctuation. I've still got my hopes up that the inline restrictor will solve the problem.

 

Those restrictors are really easy to fabricate. Just a handful of hose splices and the tubes off carb cleaner.

Run the tube all the way through the union.
Fill the union with epoxy.
When it hardens ... take a razor blade to both ends.
Bingo. Restrictors!

 

This is VERY interesting. Please keep the peanut gallery posted on your progress.

 

Here's a quick update. I picked up that inline vacuum restrictor I mentioned earlier. When I finally got home tonight I installed it and hooked up the MAP sensor to the bike. At first, I added it right up next to the MAP sensor but didn't get the results I desired. I then thought that maybe it should go up next to the vacuum port at the manifold and that quieted down the fluctuations substantially. I didn't have a chance to mess around with it a whole bunch as I've got other irons in the fire right now, but things are looking promising. It may be a few days before I can get back to this project. The boss is taking the rest of the week for vacation and I'm on for five more 11-hour days. That, of course, doesn't leave much time for tinkering.

 

I'm actually a bit disappointed that it turned out to be necessary to add a restrictor. I added one to the gauge I was using, but NOW I hear from Rick that those don't work so well with the mechanical dial gauges I use.

(sigh)

 

The restrictor is nessessary because the feedback from the sensor and sampling rate of the digital volt meter are so quick that the displayed number changes too rapidly to be useful. To slow it down to where us humans can use the number, we must dampen the oscillations. Besides, we're not interested in the full spectrum of vacuum measurements from the cylinder, we're just interested in it's maximum value, and that's what the restrictor helps us to see. As it turns out, this solution was recommended to me by an expert from BellJar.net. I wrote to him about this problem and this is his reply:

It can't be all that bad of an idea. It's recommended by an expert. :wink:

 

What you're looking for with the vacuum gauge is an average.

If you put an un-dampened it will wildly fluctuate as the intake valve opens and closes.

With mercury sticks the sheer mass of the mercury dampens the system.

Theoretically, if you put a very small orifice at the intake, and an infinite volume chamber on it in enough time (infinite!) the chamber will reach the average pressure of the intake.

In practice, putting an orifice (restrictor) near the vacuum nipple and a length of hose gives enough volume to dampen the reading, whether with a dial gauge or a MAP sensor - although I'll bet the MAP sensor has less moving mass than a dial indicator - making it extra responsive.

An alternative on the MAP sensor Don might to put a capacitor in parallel with your meter. Electrical damping.

 

Analog meter, anyone?

 

When I do them One Gauge ... I use restriction. It settles-down the wild fluctuation that could have the gauge bounce.

I also use a small filter as a baffle. Probably don't need the baffle but I think it also soften the response to the gauge.

The needle on the Gauge shows the amount of vacuum being pulled while its fluctuating between the cycles. It's accurate enough to get the right value to set-up the others.