Total Advance---Timing
Thread Starter
Instructor
Joined: Jul 2004
Posts: 106
Likes: 0
From: Brandon Mississippi
Total Advance---Timing
---NEED to know if this is RIGHT!!! 
1980 Stock--L48---NO smog(no AIR pump--no Cat.--no EGR valve)stock exhaust with turbo mufflers.
1. recurve kit-(weights and silver and blue springs)
2.Crane:adjustable vacuum advance--(screwed clockwise 9 turns)
" :vacuum timing limiter--(second notch from end)
NOW
Timing readings(timing tape on balancer)---with OLD style Timing light(NO dial)
1. curb idle--700 rpm-------8 deg.--------vacuum unplugged
2. curb idle--700rpm-------8 deg.--------vacuum connected
3. 1000 rpm--------------12 deg.--------vacuum connected
4. 1500 rpm--------------33 deg.--------vacuum connected
5. 2500 rpm--------------45 deg.--------vacuum connected
6. 3000 rpm--------------48 deg.--------vacuum connected
DOES this sound RIGHT!!!!!
THANKS for the help
MERRY CHRISTMAS and HAPPY NEW YEAR!!!!!!
Melting Slicks
Joined: Jul 2002
Posts: 3,128
Likes: 3
From: Deltona (son of Deland and DAYTONA) FL
That looks about right. Try it with the vac advace disconnected so you can check mechanical advance. Should be 36-38 degrees at or before 3000 rpm.
From what I've read, our SBC's will run well with up to 52 degrees on vac advance at cruise.
From what I've read, our SBC's will run well with up to 52 degrees on vac advance at cruise.
Instructor
Joined: Aug 2004
Posts: 139
Likes: 0
From: Austin Tx
agteacher, this is a good question. I would like to see an answer regarding your vacuum advance vs mine. I have a 1976 L48 stock engine.
At 600 rpm and no vacuum I get 8º advance.
At 700 rpm with vacuum pluged in I get the full vacuum advance of 18º for a total advance of 26º.
It looks like agteacher has no vacuum advance at idle and I have the full vacuum advance.
So let the discution begin. (I would like to see a spell checker available for people like me)
At 600 rpm and no vacuum I get 8º advance.
At 700 rpm with vacuum pluged in I get the full vacuum advance of 18º for a total advance of 26º.
It looks like agteacher has no vacuum advance at idle and I have the full vacuum advance.
So let the discution begin. (I would like to see a spell checker available for people like me)
Burning Brakes
Joined: Nov 2004
Posts: 1,028
Likes: 0
From: San Antonio Texas
my L82 (79) is set at 12 degrees BTDC at idle. i've got 20 degrees of mechanical advance for 32 and another 20 with vaccuum for 52 total. i tried going to about 15 degrees at idle because i too have heard that 56 total is "optimal" for small blocks but i was getting some detonation on hard acceleration so went back to 12 initial per specs.
Melting Slicks
Joined: Jul 2002
Posts: 3,128
Likes: 3
From: Deltona (son of Deland and DAYTONA) FL
this is from Lars' paper that can be found here
http://www.corvettefaq.com/engine.asp under "how to set timing"
"Small block Chevys (and most other GM performance V8 engines) perform best when the total timing (full centrifugal advance plus the initial timing setting with vacuum advance disconnected) is all in by 2,500 – 2,800 rpm and is set to 36 – 38 degrees. "
Note that while setting the mechanical advance, the vacuum advance is unplugged. Plug that hose so you don't create a vacuum leak. The procedure in that paper is designed to increase acceleration. During that time, vacuum is minimal. At idle and cruise vacuum builds. I guess that's the beauty of vacuum advance. It increases engine efficiency when it can. When it cause detonation under acceleration, there is little or no vacuum, so it doesn't advance the timing.
http://www.corvettefaq.com/engine.asp under "how to set timing"
"Small block Chevys (and most other GM performance V8 engines) perform best when the total timing (full centrifugal advance plus the initial timing setting with vacuum advance disconnected) is all in by 2,500 – 2,800 rpm and is set to 36 – 38 degrees. "
Note that while setting the mechanical advance, the vacuum advance is unplugged. Plug that hose so you don't create a vacuum leak. The procedure in that paper is designed to increase acceleration. During that time, vacuum is minimal. At idle and cruise vacuum builds. I guess that's the beauty of vacuum advance. It increases engine efficiency when it can. When it cause detonation under acceleration, there is little or no vacuum, so it doesn't advance the timing.
Tech Contributor
Joined: Aug 1999
Posts: 14,356
Likes: 6,328
From: At my Bar drinking and wrenching in Lafayette Colorado
The readings with the vacuum hooked up are not meaningful. You need to check and set up your timing with vacuum advance disconnected. With the hose disconnected, set total max timing to 34-36 degrees. You can then use a vacuum advance control unit that adds up to an additional 16 degrees of timing on top of the 36 total mechanical. From your readings, you're pretty short on timing. For 36 total, intial will usually be up around 12-18 degrees, 36 total mechanical, and 52 total combined mechanical + vacuum.
