I am finishing a frame off on my 73 BB. I have a 454 that is bored .040. It has a 9.5 CR with a Lunati .575/.575 221/229 @.050 cam, stock manifolds and a 1902 Edelbrock carburetor. It is a 4 speed car with a 3.08 rear. It has an MSD distributor (8572?) with a vacuum advance marked B26.

I set the timing to 8 BTC initial and I have about 34 degrees BTC total at just before 3000 RPMs. The engine idles at 700 RPMs, and produces 14 “ Hg vacuum at idle. I cannot get cruise vacuum since the car is not road worthy yet. When I connect the vacuum advance to manifold vacuum the car stalls.

At idle, I connected an external source to the vacuum advance and I get 28 degrees total (8 initial and 20 vacuum) when the car stalls. It looks like the advance is not finished when the car stalls.

My questions:

1. Is the B26 advance the right advance for this engine?

2. If not which advance should I be using?

3. Should I be connected to Port or manifold vacuum? If I use manifold vacuum, I think I would be at 24 BTC (8 initial and 16 vacuum) at idle. Is this too much?

4. Is there supposed to be a stop for the vacuum advance? LARs paper suggests that I should get 16 crank degrees advance from the B26 and I believe I can get at least 25.

Thanks for any help you can provide.

 

Something is not right with that vacuum advance.
Below are the instructions for an MSD 8572 and it states vacuum is limited to 10 degrees.
https://documents.holley.com/8572.pdf
Did someone change the vacuum unit?
Why would it have a B26?

 

Thanks Mel. I missed the 10 degrees.

Not sure why it has a B26 vacuum advance, but that is clearly stamped on it. I think the car sat since 2006. I bought the car 4-1/2 years ago with the wiring harness removed and found a wiped cam before I tried to start it. My guess is that the PO had problems with the engine and kept throwing parts at it. The edelbrock is dated 2006 and the MSD distributor barely looks used.

 

I would contact MSD and try to get the correct vacuum unit, 10 degrees sounds perfect.

 

Specs on a B26 can are 16 degrees of CRANKSHAFT advance that starts at 6 inches of vacuum and should be all in (16 degrees timing) at around 12 inches of vacuum.
This might vary a degree either way on totals and also on vacuum inches. If someone stated it is limited to 10 degrees it might have a limiter of some sort installed on the vacuum advance can where the actuator rod goes through the slot in the hold down plate.
On my 69 427 I run 18 degrees initial and 17 degrees mechanical with a B26 can that produces (on the low side) 15 degrees advance. I am thinking of going back to the original stock MS360-12 can from 69 that produces 12 degrees vacuum advance. I can then run 20 initial and the 17 mechanical for a total of 37 and do it on 91 octane.

 

I think the MSD document is a bit deceiving. The statement indicates up to 10 degs advance when hooked to ported vacuum at part throttle. It does not say what maximum advance is. Is there a max advance spec I missed?

"Vacuum Advance (PN 8572 Only): The vacuum advance will advance the timing up to 10° during partial throttle driving (with 15 lbs. of vacuum). The vacuum line should be routed to a ported vacuum outlet above the throttle plates."

I have an 8572 distributor on my SB and it pulled over 10deg advanced as shipped. My engine pulls about 14-15" of vac at idle. I use a B28 can with a 10 deg limiter from Lars. I also use manifold vacuum. I run 16 deg initial, 10 vac and 20 mech. Works well with my mildly built SB.


Tom

 

The MSD distributors come with the B26. That's the right unit. Vacuum advance, as listed on the distributor spec sheet, is listed in distributor degrees - not crankshaft degrees. The B26 will pull in about 18-20 crankshaft degrees of timing, which is 9-10 distributor degrees, as advertised. It needs to be limited to 10 - 12 crankshaft degrees as outlined in my papers, but the full 20-degree crankshaft advance should not cause the engine to stall - it should idle great with 30-ish degrees of actual timing at idle. You have other tuning issues.

Lars

 

Thanks Guys. some more background information:
I did contact MSD (Holley) three times .
1. I was told it was a bad pickup. I Ohmed the pickup while shaking the wires and the reading stayed solid. I replaced the pickup anyway.
2. I was told it was my timing light. I have about a 45 year old Penske (craftsman 244.2115) light. The reading using this light is very stable and consistent so I did not replace the light.
3. I was told to replace the vacuum advance which is where I am at now.

I just tested the advance and it begins moving at 6" Hg and is all in by 14" Hg.

When I first started the car, it was idling at about 1000 and it kept running when I attached the vacuum advance to manifold vacuum,. It ran badly and the timing was fluctuating that badly that I could see the 270 degree mark on the damper coming in and out of view.

 

Thanks Lars. I tried to follow your papers in tuning. Where did I go wrong? I have 8 BTC initial and 34 total but I intend to bump that to 10 or 12 initial when I get the engine sorted out. Where do I look next? This is more critical to me than the advance issue. If it is set wrong or I did something wrong I want to fix that.

Based on what I read, it seems like I want to use manifold vacuum but then I need the vacuum advance all in by 12" Hg. Is that correct?

It seems like you have a way of limiting the vacuum advance. Can you explain?

I really do appreciate all the papers you published and the information you provide.

Thanks

 

You can e-mail me for info on my vacuum advance limiter. The B26, with a limiter, will work perfectly in your application.
Lars
V8FastCars@msn.com