A friend of mine has just replaced his old Mallory distributor that had no vac advance with an MSD one. The MSD book says specifically not to run manifold vac with this unit only ported vac. I run an original dist and don't know much about the MSD units so said I would ask if anyone here knew if manifold vac was okay to run with these as I subscribe to the Lars school of thought on manifold v's ported vac and run manifold vac successfully on my SBC.

 

Factory cam or smaller performance with high vacuum at idle, use manifold vac even on the MSD.
Lumpy cam with low vacuum at idle and auto trans, use ported.

 

The MSD distributors come with the B26 vacuum advance control unit. This pulls in full vacuum at 13 inches of manifold vacuum. If your engine is pulling 15 inches or better (numerically higher) in neutral (or "drive", if auto), you're fine running manifold vacuum. In fact, it's recommended.

MSD knows that a lot of their stuff will be installed on engines pulling numerically less than 15 inches of vacuum, so they recommend running ported vacuum in order to keep the timing stable. Rather than running ported vacuum, run a B28 vacuum advance on manifold vacuum if you have engine vacuum in the 8-13 range.

This assumes you are running the ProBillet tach drive distributor, since you did not state which distributor you're using nor what type of car or year you're working on...

Lars

 

Where can I get a B28? I have an MSD Tach drive on my 72 LT-1.

 

I have an MSD 8572 non-HEI distributor and ordered my B28 can through Advance Auto Parts. The box shows the part number as Airtex 4V1053. "B28" is stamped on the mounting arm. Hope that helps.

 

Thanks John, Lars, I hated to say it but its a 351 Windsor motor in a '56 F100 that has been rodded, its fitted with a mild cam and pulls about 16" vac at idle.

 

Then the vacuum advance unit installed by MSD will work just fine running off manifold vacuum.

 

The B28 is readily available. NAPA discontinued their part number VC1810, but it crosses over to AC/Delco part number D1312C, and VC177 in Standard Hygrade – both of these numbers are available at this time. Wells still sells it under their part number DV1810, and Borg Warner has it under part number V329. GM part number is 88924985, and is still available. Another good source for the vacuum advance control units (under the Wells part numbers) is from “My Parts Garage.com” so you can easily get them on-line through that source.

Lars

 

Hi Lars,

I enjoyed reading your posts, i am not working on a vette but i am trying to understand MSD distributor ported vs manifold. I was not able to send you a PM for some reason. I am running an all in one MSD distributor 8387 for a 70 charger with a 440. This distributor gives vacuum advance and the directions say use port, as you said if less than 15 vacuum, i will check my vacuum. If i use manifold and iam pulling more than 15, does the vacuum from the manifold add to the vacuum pulling from the distributor? I know little about this topic.
thanks

 

That's awesome...I am going to put a B28 on my 406! You learn something everyday!

Jebby

 

I hope you do not mind if I answer this. The vacuum can pulls what it is rated for (it is adjustable for actual advanced pulled) when the hose is hooked up and running of course. Your timing is checked with the hose pulled and whatever the timing is at idle is exactly that. When you put the hose back on it adds to the initial timing. So if your initial is 15 and the can is adjusted to pull 12.....then you have 27 at idle. But the can only works under vacuum....which is only under light or zero load (Like cruise or idle)...so when the vacuum comes in, it will pull advance with it as the engine will run smoother and more efficient this way......it is a great system once it is dialed in......when you accelerate...the vacuum goes away and so does the vacuum advance (The 12 degrees mentioned earlier).....and then the mechanical weights in the distributor start to advance the timing as the RPM's rise.......they work together. And having the correct mechanical advance curve and the proper vacuum pull and pull-off is critical for a properly running engine.
Ok....did you get all of that? Now....the reason it is suggested to go ported with a low vacuum/large cam is that generally the overlap of the cam makes for sporadic vacuum at the manifold and if you put a gauge on it, it flutters a bit....this would cause a vacuum can to oscillate wildly....and the timing can be unstable using a light.
It is a lot to understand at first but I have taught this to dozens of guys and when they get it....really get it, they can tune damn near anything....from Vette's to a Rambler.....
Lastly, ported vacuum was used by GM and others to help NOx emissions at idle along with their pathetic initial "timing" specs. Millions of GM cars (and others, including your Charger) were about 10 degrees retarded from optimal from the factory. Street racers caught on quick and found out that proper timing and a set of headers on most of these old musclecars was worth about a second in the 1/4 mile (average).
My little brother has a 383 in his 69' Charger and I put an MSD ready to run in it 12 years ago......it was a slug when he bought it. I did the curve and put on a 750 Mighty Demon on the C3 Eddy intake......the first time we took it out.....we went to get beer up the road.....he wanted me to drive it back.....at night....so I hopped on it leaving the Party Store and it blew both airshocks and shot the damn packing out of the glasspacks LOL No lie!
Have fun.....440 Chargers are a rip....

Jebby

 

Thanks for that! i dont know why its so hard for this to sink in, your explanation is helping! iam getting close! So since i have a stock engine, no big cam, not sure of the idle vacuum yet - rebuilding my carb right now. So unplug the hose from the dist, cap off all ports, if my vacuum is close to 15 then connect to manifold vacuum? If it is a lot less then use port. If i remember it is around 15. I am kind of an idiot when it comes to using a timing light, i hear it is a little hard to find TDC and not to rely on the marks already there. I can vacuum time? Just turn dist until i get max vac at idle,, probably not the best way to time.
Thanks a great story! I hope to see that kind of power one day!

Thanks!
Frank

 

Lets start a new thread rather than tack on to a 7 year old unrelated post because you cant send PMs yet.