MSD Vacume advance ?

vrooom3440 · Feb 21, 2006

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.

TC · Feb 21, 2006

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.

TC · Feb 21, 2006

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.

69-CHVL · Feb 21, 2006

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.

vrooom3440 · Feb 21, 2006

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?

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,...

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?

69-CHVL · Feb 22, 2006

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.

TC · Feb 23, 2006

OK I checked this today:
My best idle vacume was 10" so if I get a vacume can like this VC1808 B26 5-7 8 @ 11-13, it should do what I am looking to do ?
The part number means it will start to move at 5-7" of vac.
It will have a total of 8 degrees that equals 16 degrees at crank that will be all in by 11-13" vacume.
So if I add the 16 degrees to my 36 total advance I will have 52 degrees at cruze where I have 15" of vacume.
Sound OK?
T.C.

vrooom3440 · Feb 23, 2006

The advance amount sounds reasonable, I have read 52* as the maximum cruise but quote 50* to be a little conservative

The vacuum levels are not quite right though. If you are idling at 10" you want a vacuum can that is all in by 9" at the most. Otherwise your idle may fluctuate as the vacuum rises/falls and tweaks the advance in/out. Actually from my own experience it usually only fluctuates one way---> down

I cannot lookup the can specs right now... but I can check the list tomorrow and see what number would work better. Was the list previously posted? Don't remember but it is floating around the net.

TC · Feb 23, 2006

Now your confusing me..?
I got my best vac. reading by advancing the dist. to about 45+ degrees at idle, the "test" you guy suggested.
My normal idle vac. reading is 4-5", I dont think they make a can that will bee all in by 3" ?
Also I thought you wanted the advance to go up/down with the rpm of the motor?
This part has the smallest vac./timing #'s I found, VC1808 B26 5-7 8 @ 11-13 unless there are more part numbers somewhere?
T.C.
Anyway here's the vac. can list I found :

Non-HEI Distributors:

P/N ID# Application Starts @ “Hg Max Adv
(Distr. Degrees @ “Hg.)

VC680 B1 1959 – 63 All Chevrolet 8-11 8 @ 16-18
1964 Corvette exc. FI
1964 Impala, Chevy II
1965 396 High Perf.
1965-67 283, 409
1966-68 327 exc. Powerglide
1967-68 All 396
1969 Corvette 427 High Perf.
1969 396 Exc. High Perf.
1969 Corvette 350 TI
1969-70 302 Camaro
1970 400 4-bbl
1970 396 High Perf.
1970 Corvette 350 High Perf.
1973-74 454 Exc. HEI

VC1810 B28 1965 409 High Perf. 3-5 8 @ 5.75-8
1965 327 High Perf.
1966 327 High Perf.
1964-67 Corvette High Perf. FI

--------------------------------------------------------------------------------------------------------------------------------------------

VC1605 B9 1965 impala 396 Exc. High Perf. 7-9 10.3 @ 16-18
1965 327 All Exc. FI
1969 327 Camaro, Chevelle, Impala
1969-70 Corvette 350 Exc. High Perf.
1969-70 350 4-bbl Premium Fuel
1970 350 Camaro, Chevelle, Impala High Perf.
1971-72 350 2-bbl AT
1971-72 307 All

VC1675 B13 1968 327 Camaro Powerglide 9-11 8 @ 16-18
1968 327 Impala AT
1968 307 AT
1968 302, 307, 327, 350 Camaro, Chevy II
1970 350 Camaro, Chevelle Exc. High Perf.

VC1760 B19 1969 350 Camaro, Chevelle, Impala 4-bbl 5.5-8 12 @ 14-18
1969-70 350 2-bbl

VC1765 B20 1965 396 Impala High Perf 5-7 8 @ 11-13
1966-67 Corvette Exc. High Perf.
1966-67 Impala 427 Exc. High Perf.
1966-68 327 Powerglide Exc. High Perf.
1969 307 All
1969-70 396, 427 Camaro, Chevelle High Perf.
1970 400 2-bbl
1970 307 MT
1973 Camaro 350 High Perf.

VC1801 B21 1971 350 2-bbl 7-9 10 @ 16-18
1971-72 400, 402
1971-72 307 AT

VC1802 B22 1971-72 350 4-bbl 7-9 8 @ 14-16

Other Part Numbers & Specs:

VC700 B3 8-10 11.5 @ 19-21
VC1415 M1 6-8 10 @ 13-15
VC1420 M2 5-7 11 @ 16-17
VC1650 B12 8-10 10 @ 15-17
VC1725 B18 8-10 12 @ 13-16
VC1740 A5 6-8 12 @ 15-17.5
VC1755 A7 8-10 12.5 @ 18-20.5
VC1804 B24 6.5-8.5 10 @ 12-14
VC1805 M13 6-8 12 @ 14.5-15.5
VC1807 B25 5-7 8 @ 13-15
VC1808 B26 5-7 8 @ 11-13
VC1809 B27 5-7 9 @ 10-12
VC1812 B30 5-7 12 @ 11.75-14

69-CHVL · Feb 23, 2006

TC, is it 45* w/the vac can hooked-up? What is your initial w/o the vac can hooked up?

TC · Feb 23, 2006

No my inital w/o can is 18*, total is 36*, I havent checked inital with the can hooked up.
The 45+* is what I turned the distributor to, to get my best idle vacume, witch was 10" w/o the va can hooked up.
confused? I am.........................
T.C.

69-CHVL · Feb 23, 2006

I'm a little confused too....I'm surprised you were able to turn your distributor that far w/o hitting something.

45* was the best # huh. With an initial of 18*, you would need a vac can to give you an additional 27* of vac advance at idle...I don't know if that can be done. My can gives me 20* and I thought that was alot.

Was there a big diff in vacuum between 35-40*?

TC · Feb 23, 2006

I would have to check it again as I was more interested in the vacume reading, I'LL do that in the morning and be more alert as to whats going on.
Yea the vac. can was pointing towards the carb, in fact thats why I stoped, the tubing was hitting the manifold.
I thought the reason for this test was to see what the best vacume I could get at idle was, w/o vac. advance?
So I just turned the dist. untill I seen the best vac. on the guage.
I did kinda check the timing, but didnt pay that much attention, it was 45-50 degrees though. (at idle)
T.C.

69-CHVL · Feb 24, 2006

Its just pretty obvious that your motor wants alot of initial timing (comfirmed by your testing). I forget but did you have an MSD? Just use their can w/your 18* initial and see how you like it, should give you around 38* initial hooked up to manifold. You just have to make sure your not pinging at cruise. If you do ping, you'll have to change it out. I suggest on of those Crane or Accell adjustable units for ~$22.00 so you don't have to buy another can if something doesn't work out. A good thing about alot of initiall is that you will be able to lower your idle screw alittle which should help your car not stink so much at idle.

I think we making this a little more complicated then it has to be.

TC · Feb 24, 2006

I agree it's really not that hard to figure out, I think I will recheck my vac/timing readings tomorrow.
Then order a can to where I am getting my va at a lower vacume number.
I am not shure what can MSD uses, but I do know they say it's 20* at crank, thats to much with 36 inital, I would like to keep it down around 50-52*.
I think I can get it.
T.C.

vrooom3440 · Feb 24, 2006

Sorry to make this confusing TC, there is just a lot going on here that all has to work together at the same time.

You have confirmed that your motor likes/wants a lot of advance at idle. Did you keep the same idle speed or did it increase as you added advance? If it increased perhaps we got a little bigger number than we really need. But we have a ball park figure to aim at, now the challenge is coming up with a setup that will get you there.

I am a bit surprised that your vacuum did not come up further, that is going to make things a bit more challenging.

One misconception to clear up is that the vacuum advance is not about advancing the ignition with RPM. It is about retarding the ignition with load.

You may have to limit your mechanical advance to get where you need to go. Purely as an example, we are still shooting for the 36* total, but rather than 20* advance it might be better with 10* advance. This would move your initial up to 28* instead of 18*. Then add 12* from vacuum advance and you idle at 40*.

Of course there are two gotchas hiding here... first 40* is still less than your motor liked, but it is closer and may be enough. The second is bumping the initial up that far may make life tough on your starter. I have read that bumping initial up too far can cause hot start issues but do not remember what the threshold was (what timing starts to cause the problem). You probably want to do a search here and see what initial you think you can get away with.

I am presuming your dist takes an HEI type vacuum advance can? You did quote a non-HEI part number previously. If HEI, so here are some candidates I see in the list:
VC1862 AR31 2-4 8 @ 6-8
VC1825 AR1 3-5 9 @ 6-8
VC1868 AR35 2-4 10 @ 6-9

These all provide more than the 7* you need to go from 36* to 50*. This can be a good thing actually. You can make a travel limiter to prevent them from extending out as far and this will trim the bottom of the vacuum range. So using that last AR35 as an example, limited to 7* that 2-4 changes to a 3-5 or maybe just a bit higher. I made a travel limiter using .035" K&S stainless steel strip I picked up at an Ace hardware store. It will be more commonly available from a hobby shop though. It is a pretty simple L shape drilled to bolt on with one of the VA mounting screws.

You will probably end up with a bit of a compromise on idle timing when you are done. But it should be better than where you are now.

TC · Feb 25, 2006

Thanks again for all the replys:
Here is what I did today, warmed motor.
Checked the timing w/vac. hooked up @600rpm= 22*
checked the timing w/vac. unhooked @600rpm =22*

This tells me I have no vac. advance at idle, but I only have 7" of vac. so I am not suprized.
I have a MSD point style distributor that has a B26 vac. can on it, that equals 5-7 8 @11-13.
So it's not moving at idle, correct.
The next thing I checked was my timing at the best idle setting without going overboard.
The results are 45* making 12.5" of vacume, I could keep raising the timing to get better /faster idle but the vacume didnt go up so I quit at 45*.
I then hooked my vac. pump to the dist.and cranked the motor, let idle and then pumped to 15" of vacume.
The idle sounded much better with the extra 16* of timing.
I then checked to see what I had at idle w/15" vac. on the can.
It was 36*, so that tells me I have 18* inital and 18* vac. advance.
So if I take my 36 total and 16* from the can. below, I will have 52*.
I may not be perfect but I bet it will be better.
Seeing as my distributor has a "B-26" on it now tells me I need a "non HEI" type vac. can.
The best I can see in the chart I have it the VC1810 "B28" 3-5 8 @ 5.75-8 , this should be almost all in at my idle.
I am still open to suggestions.
T.C.

vrooom3440 · Feb 25, 2006

It sounds like you have gone from a bit confused to having a very good idea where you are and where you are going.

The B28 sounds quite workable. You may even be able to jump up to an all-in by 10" can, but I do not see one in the points-version list. It seems like there is a much wider variety of VA cans for the HEI distributors.

Let us know how it works out with the B28 can.

bigmike69ss · Feb 25, 2006

Vroom, You mentioned earier you had lousy brakes, are they power? low vacuum would explain lousy brakes. Are you losing vac at the brake booster?

bigmike69ss · Feb 25, 2006

Meant TC with bad brakes

MSD Vacume advance ?

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