Vac. Advance unit
Thread Starter
Instructor
Joined: Jan 2007
Posts: 232
Likes: 41
From: Simpsonville SC
Vac. Advance unit
Going through my distributor tonight and correcting some sloppy end play. Took a close look at my vac. advance unit and cleared away some crud to see the number. It has 626 10 stamped on it. This doesn't seem to match anything on Lars vacuum spec document. I connected a vac. pump and it starts to move at 4Hg and is done by 9Hg. Does this seem correct? I have 78 L-82 with distributor #1103291.
Last edited by bytor; Aug 7, 2007 at 10:18 PM.
Racer
Joined: Mar 2006
Posts: 454
Likes: 0
From: SAN CLEMENTE CALIFORNIA
Goring through my distributor tonight and correcting some sloppy end play. Took a close look at my vac. advance unit and cleared away some crud to see the number. It has 626 10 stamped on it. This doesn't seem to match anything on Lars vacuum spec document. I connected a vac. pump and it starts to move at 4Hg and is done by 9Hg. Does this seem correct? I have 78 L-82 with distributor #1103291.
Tech Contributor
Joined: Aug 1999
Posts: 14,375
Likes: 6,375
From: At my Bar drinking and wrenching in Lafayette Colorado
The specs on the units are listed in distributor degrees - not crank degrees. That unit pulls in 20 degrees of crankshaft advance, and that's too much for a performance application where you have 36 degrees of total mechanical timing coming in fairly quickly. 36 mechanical plus 20 degrees of vacuum advance will produce a total maximum timing at high speed cruise of 56 degrees. This will likely cause the car to "buck" and jerk a little at cruise, and that's not good. Suggest you use a vacuum advance unit that has about the same activation points, but with a shorter "stroke," limiting vacuum advance to 16 degrees (8 distributor degrees). I like to use the VC1838 for cars with a stock cam that pull at least 14 inches of vacuum in "drive," and I use the VC1862 for cars with bigger cams and less vacuum signal.
Racer
Joined: Apr 2006
Posts: 477
Likes: 43
From: Millbrae California
the 10 on a delco-remy original vacuum advance of that vintage showed the degrees of advance at the crank. the # 1973626 vac advance as per the delco remy spec book (1.2) shows the advance starts at 3 to 6 inches of vacuum and has 5 distributor degrees of advance or 10 degrees at the crank at 7 to 9 inches of vacuum. to me that vac can is a keeper.
for the reformulated gas we have here in california, i limit the vac advance to 10 to 12 degrees of advance at the crank when we are using a performance mechanical advance curve (36 degrees @ 2800-3200 rpm)
henry @ oles
for the reformulated gas we have here in california, i limit the vac advance to 10 to 12 degrees of advance at the crank when we are using a performance mechanical advance curve (36 degrees @ 2800-3200 rpm)
henry @ oles
Thread Starter
Instructor
Joined: Jan 2007
Posts: 232
Likes: 41
From: Simpsonville SC
I'm still confused about the crank vs. distributor advance numbers. I seem to see ~10* with the timing light at the crank when it's fully engaged so that = 5 distributor degrees correct? What number do I use in my total timing calculation 10* or 5*? I have read so many articles on this stuff now it's kinda getting all mixed up 
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Thread Starter
Instructor
Joined: Jan 2007
Posts: 232
Likes: 41
From: Simpsonville SC
Thanks for the feedback. Got my distributor back in here's what I ended up with. I only get 18* mechanical advance max so I set my initial at 16*. This gives me 34* with an additional 10* of vacuum advance. The only thing I notice is a 'very slight' stumble when transition from idle while cruising. I haven't had time to investigate it yet. Any ideas?
Last edited by bytor; Aug 13, 2007 at 02:33 PM.
Racer
Joined: Dec 2004
Posts: 336
Likes: 0
From: crystal lake Il.
I have that can on my dist. It`s an msd dist. that came with a bad adjustable vac can, so I put #vc1703 on. I got a little pinging so I retarded it. All was well for a while and the pinging started again. Should I replace it with #1838 or run without vac advance, It seems to run well without it.
