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MSD Vacume advance ?

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4.7K views 56 replies 8 participants last post by  bigmike69ss  
#1 ·
Anyone know how much vacume advance a MSD pro billet distributer has ?
Say if I am at 36 total how much will be added with vacume advance, given that everything is working properly to get the most out of the VA?
Thanks in "advance" LOL
T.C.
 
#2 ·
The billet HEI I had from MSD added 20* vac advance at idle - this was w/it hooked up to manifold vacuum. So, it your initial timing was 18*, you hooked up the vac can, your timing light would show 38* at idle.

Remember, at WOT, vacuum dissappears so your vac can won't add anything. Reving in neutral will show that its adding, but under actual operating conditions (wot) you'll just have your 38*.
 
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#3 ·
If it provides that much... you will probably need to tweak it to provide less. You do not really want to see more than about 50* at cruise. If you have 20* VA, you would have to be at no more than 30* on the mechanical advance rather than the 36-38* that the Chevy motors often like.

You can find out what it does by disconnecting the VA and find out what your total timing is, both RPM and maximum advance. Then reconnect the VA and run the test again and the difference will be the amount of advance in the VA.
 
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#4 ·
Thanks for the replys
I was looking at this article on a Nova site about VA operation and VA cans, seems to be a prety good article?
The guy said he was helpfull in the evelopment of VA, if I read it correctly?

This is a quote from that posting:

Most GM V8 engines (not including “fast-burn” style heads), and specifically Chevys, will produce peak torque and power at wide open throttle with a total timing advance of 36 degrees (some will take 38). Also, a GM V8 engine, under light load and steady-state cruise, will accept a maximum timing advance of about 52 degrees. Some will take up to 54 degrees advance under these conditions. Once you advance the timing beyond this, the engine/car will start to “chug” or “jerk” at cruise due to the over-advanced timing condition. Anything less than 52 degrees produces less than optimum fuel economy at cruise speed.

It can be found here: http://stevesnovasite.com/forums/showthread.php?t=11689

Anyway I figured if I knew what a MSD was from the factory I could then find a vacume can to get the VA can I need to make up the 52 degree advance.
I know I have 36 total and 18 intial, juess I was just being to lazy to get out the timing lite or pull the distributor and read the part # on the can.
Hoping someone knew the answer,and it sounds like 20* may be the answer.
T.C.
 
D
#6 ·
I just bought a new MSD hei type distributor with vac. advance, part # 8365 the instructions said " the vacuum advance will advance the timng up to 10 degrees during partial throttle driving. (with 15 lbs of vacuum) the vacuum line should be routed to a ported vacuum outlet above the throttle plate." I'm sure lbs was a misprint and they mean 15 inches of vacuum. Dave
 
O
#8 ·
Performance aftermarket parts sold in the
U.S. are subject to laws that govern which
parts can be legally sold, distributed and installed
on street driven vehicles with emission
controls. It is our responsibility at MSD Ignition, to comply
with these laws and to notify you of which MSD products
have received California Air Resources Board (CARB) Executive
Order numbers. At this writing, the EPA accepts a
CARB exemption as meeting the requirements of its Memorandum
1A and the Clean Air Act Amendments
of 1990; consequently, MSD Ignition
parts with E.O. Numbers are legal for sale,
distribution and installation in all states. Below
is a brief description of the types of products
we offer and how to identify which parts
are legal for emission controlled vehicles.

If you want to sell products with CARB approval, you are going to instruct the customer to install it in the most smog-legal way.
 
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#9 ·
The part I found most interesting was the 52* timing, I was allways afraid to hook the VA up, because I didnt want to burn a piston or valve.
I guess it's ok though?
T.C.
 
#10 ·
TC said:
The part I found most interesting was the 52* timing, I was allways afraid to hook the VA up, because I didnt want to burn a piston or valve.
I guess it's ok though?
T.C.
Click to expand...
As I understand it, your not under load at cruise, so no piston burning.
 
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#11 ·
I took my car for a ride today to see what I had for vacume at idol, cruze, WOT and part throddle.
I have 4-5" at idol,13.5-14" at a 1000 rpm cruze, 4" at wot, and 15" at cruze above 1200 rpm.
The max I had was about 23" when I let off the gas.
So what I am going to do is look at for a VA can that will start to come on at about 4-5" and give me a total of 8 degrees and be all in by about 13-15".
When I times the 8 degrees by 2 that will give me 16 degrees vacume advance, plus the 36 total I have will equal 52 degrees at cruze.
Will it work..........?
T.C..
 
V
#12 ·
Your setup is messed up...

I have a 402 with a HR296, i.e. less motor CI to feed and more cam trying to feed it. I run around 10-11" of vacuum at idle. I would never classify 1000 RPM as any kind of cruise, that is lugging. Cruise is more like 2000 RPM and I see around 15" vacuum at cruise. On deceleration I can see 25+".

What is your timing at idle? Since you have not hooked up VA I would bet good $$$ that you have insufficient ignition timing advance at idle for your combination. Thus you probably have the carb opened up too far to get it to idle leading to excessively low vacuum.

Your motor will be much happier with VA. But like anything else you have to make sure you do it right or you can make your motor very unhappy.

Every motor can vary in exactly what it wants, but this should make for a very good starting point:

Setup your mechanical advance to provide 36* all in by/at 3000 RPM. You do not need/want further advance at higher RPMs as intake flow turbulence at higher RPMs will assist in mixture burn making the burn occur faster. Note that we give the most attention to the most critical parameter here: total mechanical advance. Much less attention to initial ignition timing. But many mechanical advances will provice around 20* advance, so given a 36* total you end up with 18* initial.

Now for the vacuum advance you want to max out at 50* at cruise. This is the combination of mechanical and vacuum advance. The simple approach is to take the 3000 RPM mechanical advance timing of 36* and say that vacuum needs to be 12-14* maximum. You may be able to get away with more, IF you cruise at a lower RPM with less mechanical advance. That total vacuum advance should be all in by 1-2" less vacuum than you have at idle. The vacuum advance should be connected to manifold vacuum, not ported vacuum.

So at idle you will have a lot more ignition advance because you will have initial plus vacuum advance. My motor idles with about 30* for example. But because you get a significant portion of this advance from vacuum, your starter will not have to fight excess advance either. Double bonus.

I would not recommend using a VA can with advance changing at your idle vacuum levels. This can cause a wandering idle. I sometimes have issues in my setup where the engine falls down. The engine slows down a bit, idle vacuum drops, ignition retards, the engine slows down some more and so on. If I bring it back up with some gas pedal it all gets better.

Also have to make sure you talk about the same stuff in these discussions. The distributor folks talk about a different degrees than the motor folks. It makes a big difference if you are talking about distributor degrees or crank degrees.
 
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#13 ·
Thanks for te reply Steve:
I do have 36 total and 18 at idle without the va hooked up.
I realize that 1000 is lugging but as I was out in the car writing things down it was just another figure to add to the mix.
Is I mentioned I also have 25" of vacume when I let off the gas or deceleration as you call it.
When you say you have 10-11" at idle, is that in gear?
My figures were all taken with the car in gear,I have more when it's in park/nutral. (my car idles at 500-700 rpm)
I would have to check that and will do so today, I bet it's between 7 & 10".
I also understand the difference between distributor degrees and crank shaft degrees, thats why I doubbled the vacume can degree figure in my post.
T.C.
 
#14 ·
TC,

If your vac guage is accurate, your running very low vacuum at idle. 9"-10" should be about right for all the XE284 I've read about in BB's. Is your mixture screws adjusted correctly? Is your valves adjusted too tight? You may need to raise the idle a bit - the vac will probably shoot up.Car must not idle so good w/that little vac.
 
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#15 ·
Once again this was in gear, I will check it again.
Accually the car idles really well, dont have good brakes but it does idle well.
My vacume guage is a Sears vacume pump type, so I couldnt tell ya how accurate it is? (will try to find a better one)
Not to argue but it seems like all the post I have seen on the xe284 and vacume, people were complaining about low vacume and brake problems?
Like around 7" at idle?
I may be wrong though, my wife keeps telling me I usually am? LOL
T.C.
 
V
#16 ·
TC,

18* is what I have before VA and my idle numbers are in neutral at 850 RPM. The vacuum falls off pretty quickly at idle speeds less than that. It also does not idle so well stopped in gear with the Muncie :)

If you can get yours to idle all the way down to 500 you are way ahead of me on that aspect. This also further supports the fact you have less cam than I do. And that you should have a lot more vacuum. To give you a better idea of where I am coming from, here is the process I went through on my setup:

I used to have mine setup on ported vacuum and a 5-15" 20* vacuum can. To get it to idle required opening up the throttle too far. So I set the throttle to where the ported vacuum was just closed at idle. Then to get enough for idle I added a valve controlled vacuum leak to set idle speed. I saw around 7" of vacuum at idle with this setup. I have always set the basic timing using the total timing of 36* without VA hooked up.

I did some research and learned a bit more about how vacuum advance is supposed to work. I borrowed a hand vacuum pump so I could measure what my vacuum can actually did. I tried to tweak the adjustment on it and found it to be worthless, it only provided about a 3" change.

My old/prior distributor had a fixed vacuum can marked "UNI-9-11" but I never had any idea what that meant. With the hand pump I figured out this was a 3-9" vacuum 11* advance. Cost was free... so I installed it along with a movement limiter. So I basically created a 6-9" 6* vacuum advance can. Changed to manifold vacuum rather than ported.

Absolute night and day difference in idle quality and vacuum. I was able to eliminate the mickey mouse controlled vacuum leak idle kludge. My vacuum increased to 11" rather than 7". All from going from 18* at idle to 30* at idle on the ignition timing.

Note that part of the theory on this stuff is the detail that as cylinder charge pressure/density goes down, the mixture takes longer to burn. As you get up into the more aggressive cams the charge density at idle really drops. So they start needing more ignition advance and becoming much more critical of having the ignition advanced. I would suspect that a lot of people who step from a 270 or less cam to the xe284 probably apply their same tune setup even though it is no longer applicable. Lord only knows I had no idea what I was doing as I initially tuned my motor with the HR296. I was trying to go by the book and use factory specs. But the cam was part of a pretty good package deal... so I have gotten educated and no longer try and use factory specs ;)

BTW my vacuum boosted brakes work just fine without any pumps of reserve cans or anything.
 
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#17 ·
An article I read, either carcraft or Hotrod tested he XE series in a 454. The 284 made big power, but they said that the cam was almost unsuitable for the street.

Looks like your timing is spot on. I'm just surprised you can get that bad boy to idle that low, especially in gear!
 
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#19 ·
Thanks for the replys guy's
I got a professional vacume guage today and checked the idle vacume again.
This is what I have
in gear @600 rpm=6"
in park @600 rpm= 7.5"
in park @ 1000 rpm =13.5"
So the idle speed does have alot to do with vacume, but I dont really want to idle my car at 800-1000 rpm, remember it's an automatic.
I was thinking I am off the idle curcut on the carb, but it idles down so good I dont see how?
I am going to check to be shure I am on man. vacume and not ported? (99.99% shure I am)
As for this cam producing low vacume, I am not that much of a cam guy to tell ya why?
I think the difference we are seeing is between an auto. and 4 speed, it's easy for your 4 spd to handle the 800-1000 idle, where my auto would want to pull me through stop lites.

Thanks for the article 69-chvl, I probably should have gone with the xe274, but mine is in and broken in so it's staying in.
Besides I like the real lumpy idle, it's the lack of brakes and gas bill that ar getting me.
If you guys can come up with something to try to get more vac. please let me know.
Thanks T.C.
 
#20 ·
I agree I don't like any higher idle than necessary, and its even more important w/an auto. It seems that more initial timing will give more vacuum. If you are running manifold vacuum, you will get the best idle IMOP. Just for test puropses, increase you intial timing while lowering your idle speed and see if you can get that # up. If you can, maybe try one of those Crance adjustable vac cans so you can run more initial and not mess w/the total timing.
 
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#21 ·
TC,

I think what Vince is suggesting is to try an expirement to see if your motor wants more advance, and how much, at idle. Might be a good expirement to see what it tells you. Disconnect the VA and move the timing around until you get the best idle. Then see what you have for actual timing and vacuum.

Then it should not be too hard to "engineer" a solution that gets you there by adding some vacuum advance on top of your normal initial. If you want to post your results back I can look up in an article I saved to see if there is a decent match with a Napa vacuum advance can.

BTW I would rather have a lower idle like yours... but my cam will not support it. The lower you can go and still be stable the better IMO.
 
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#22 ·
I am at 18 inital now, dont really want to go any more, I have total all in by 2500 (36 deg.) and if I get a vac. can that starts moving at 4-7 and is all in by 15, I should be good, dont you think?

I at one point I used a vacume guage to adjust the distributor to get the highest vac. reading.
It wound up being about 45 degrees inital advance, that would have been about 63 total.......Way to much total.
I have heard the Crane part doesnt fit a MSD?
The NAPA cans are only $8-$12, I think I can find one that will get me where I want to be.
I have a vacume pump on the way, so my brakes will be better soon.
The only other issue I have is the th400 shift points, but that all hinges on how much vacume I have as to where all that is set. (4 spd. would be much simpler).
T.C.
 
#23 ·
OK, I see what he is saying, I'll try that tomorrow.
Tonite it's off to the car show............ soon as I find my shorts, kinda warm out today 78-80.
Hope the A/C works? LOL
T.C.
 
#24 ·
I shoulda been more clear...

If your timing light is showing say 30* at idle (w/can hooked up) try turning the distributor to get 34/36/38* at idle and see how it responds. You will have to lower idle speed b/c the engine will pick up rpm as you advance the timing. I'm not saying drive the car - just an idle test. You may be able to pick up more vacuum w/more vac advance. If it does, make a note of where your timing is and get a vac can that will get you there w/your 18* initial.

Again just for the record, I run about 17-18* initial, and w/the can hooked up to manifold vacuum I get 38* at idle. Runs perfect, idles great, 16" of vaccum.
 
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#25 ·
TC said:
I am at 18 inital now, dont really want to go any more, I have total all in by 2500 (36 deg.) and if I get a vac. can that starts moving at 4-7 and is all in by 15, I should be good, dont you think?
Click to expand...
As noted, we were not suggesting driving it this way. Just information gathering.
tc said:
I at one point I used a vacume guage to adjust the distributor to get the highest vac. reading.
It wound up being about 45 degrees inital advance,...
Click to expand...
There you go, you are halfway there. Now you just need to know what the vacuum was at that advance point. Or maybe something a *little* more conservative, like 38*.

Then you can pick that NAPA can. Pick it to be all in at 1-2" less than the vacuum level you get above. I suspect it will need to be lower than 15", probably more like a 5-10" range.

Vince, with your setup you should be seeing 56* or so at highway cruise. That sounds like too much... but apparently it works. I presume you are running 36* total, what vac can parameters do you have? Might be that you get away with it with the reduced compression?
 
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#26 ·
Steve, I'm using a take-out ZZ502 distributor. My total timing is 38* actually, so that may push the vac advance # even higher. After reading this thread, I may try and back it off a little. But yea, no problems w/pinging that I can hear.
 
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