I have my distributor off the car, and I'm checking things out. I've got to pass Colorado emissions *ONCE* to get plates for it in this state. I'll get the collectors' plates, though, and will never do emissions again!

Anyhow, I was looking at the vacuum advance function, as it never seemed to act the way it was supposed to. Connecting it to my Mityvac, I found that it functions, and doesn't seem to leak.

The pod's arm is marked "7R." So, I believe this is a seven degree unit, right?

Here is the real issue. The MP Electronic Ignition conversion kit directions indicate that turning the 5/32" allen wrench clockwise will reduce the vacuum advance. From my testing with the mityvac, I find that their directions are backwards. Turning the allen wrench clockwise allows full advance to occur at lower and lower vacuum pressures. With the screw lightly seated, I get full movement at 12 inches of mercury (in Hg). Turning the screw counterclockwise, I see an increasing need for vacuum pressure to achieve full movement. 1 turn outwards increases vacuum required by about 1 in Hg.

My two questions:
1. 12 in Hg seems like a lot of vacuum. Will such a vacuum value ever been seen at the advance nipple on the carb? Especially at Colorado Springs' 6200-foot elevation? (sea level ambient air pressure is 14.7 psi (29.9 in Hg), while here it is around 11.7 psi (23.8 in Hg))

2. Has anybody else noticed that the MP direction sheets are backwards? They say to turn the screw 1/2 turn clockwise if hesitation or knocking are heard in a part-throttle run--which seems to indicate that they've got it backwards.

Thanks for any help!

Mark W. Marasch

Hey Mark,

I cannot comment for certian on the MP directions as I just made similar tests to yours and used my best judgement to eliminate pinging while keeping the advance going at the largest throttle opening value. It does appear that they are backwards based on your description. You basically need to remove the advance when the vacuum reading drops below a certain value, like 10-15 in Hg or so as you open the throttle and the cylinder pressure increases. 7R indicates 7 degrees of distributor (cam) rotation, or 14 of crank rot. I am running, I think, a 9.5 and a 10.5 in my dists. I have seen as much as about 23 in Hg in my big cammed (250 @ 0.050") motor at part throttle and ~ 2500 rpm or higher, but at sea level. Engines pull a lot of manifold vacuum at off-idle speeds.

I believe your tests may be influenced by your measuring tools and the high altitude. First, if I remember right, vacuum gauges read "gauge pressure," which means you read zero with no pull applied and the gauge diaphragm is set up relative to sea level pressure. It may be that this will not affect readings of PSI, but I believe it does - I can't remember how those gauge sensors are built offhand - might be something to think about/check into. Also, the spring pressure in the MP pod is set up for low elevations, so it will need proportionally more pressure relative to ambient pressure to pull it in at high altitude.

Hope this helps,

Lou

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Looks to me that the MP instructions are backwards. Testing the unit (as you are doing) is the only way to know for sure.

Reading some other instructions, looks like full clockwise is full / fastest V. Advance and counter-clockwise reduces the amount.
Here is the link to that info.
http://www.dutra.org/doug/draft-webpage ... or/pg4.jpg
DD

Doctor Dodge,

Thanks for your help! The link you've given is to a set of directions that are printed correctly. Indeed, my bench test shows that full/fastest vacuum advance is achieved at full clockwise. I'm not sure why the hardcopy I've got is printed backwards. A quick websearch for this topic brought me to this site, where the directions are the same word-for-word as the ones I've got, except that they call an allen wrench an "alien" wrench. http://www.imperialclub.com/Repair/Elec ... lectronic/

In summary, I'm glad I'm not crazy! I wonder how many others have been misled by these instructions.

Lou,

Just about any pressure gauge (besides a barometer) references ambient pressure as zero. Also, the backside of the diaphragm on the vacuum advance pod is vented to the atmosphere, so the gauge reading on my mityvac is what the pod is seeing. However, I've noticed that the gauge on this abused mityvac is twisted, and 8 in Hg is providing full movement, which seems more reasonable.

Because of the above reasons, the same gauge-measured vacuum reading will provide exactly the same advance at any altitude (within reason). However, you're right that adjustments will have to be made for altitude:

In my case, my engine is bored-out .060 over, has a MP head, Clifford intake and headers, 2.5" exhaust, Holley 390cfm, Crane .440" lift cam, etc...

The combination of a free-flowing setup and stock compression leads me to dail-in 28 degrees of initial advance for the best idle and off-idle performance. Because of lower ambient pressure, I see less pressure in the combustion chamber, giving me the equivalent of only 7.2:1. Often, at altitude, people will run higher compression ratios simply because they can. Why leave it this way, though? Eventually, I'm going to do multiport injection and an Eaton-type supercharger, so the engine will eventually see sea-level conditions and then some!

Wow, your 23 in Hg is a lot of vacuum! Standard atmosphere is 29.9 in Hg at sea level, so you couldn't pull a whole lot more. Are you measuring this at the carb's vacuum advance port, or at the intake manifold? I've never seen anything close to this at the manifold (even considering my altitude). I could see where the good ole Bernoulli effect might give a higher vacuum at that port.

With that in mind, I'll do separate measurements at the advance port when I'm tuning. Thanks for that data point!

You're right, the 7R added 14 degrees of advance using the mityvac on my engine today.

Thanks for the help, guys!

Mark W. Marasch

That all sounds good, Mark. Glad I could be of some limited help.

Truthfully, I don't remember whether I measured vacuum at manifold or TBI timed advance port, but they should be the same at part throttle anyway. This measurement was at part throttle but decelerating, so it is sort of a "highest possible" reading, and it was at sea level (Santa Monica, CA). I have heard of people seeing up to 27-28 in Hg at the manifold under decel and higher RPMs (like 2500+).

Lou

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Home of Slant6-powered fun machines since 1988