You have to distinguish between total WOT (wide open throttle) advance and total cruise advance. Vacuum advance is there because lean, low density, exhaust gas diluted mixtures slow combustion propagation, so more advance is required to achieve best thermal efficiency/minimum EGT, and this additional advance is provided by the vacuum advance, which adds advance with decreasing load. A more detailed explanation is in my tuning seminar, which is included in the "restoration documents" thread near the end of the sticky section.
I'll use my 340 HP SWC as an example of what a small block needs with a high overlap camshaft. I modified the spark advance map back in 1965 to the '64-'65 365/375 HP specs by swapping in those weights and springs to bring all 24 degrees centrifugal in by 2350 engine RPM (OE was 4600), and the VAC starts at 4" and adds a maximum of 16 crank degrees at 8" and above.
With the 3.08 axle the engine is turning about 2350 at 60 MPH in top gear, so full centrifugal and full vacuum advance are in on a level road at 60 MPH. I run about 38 total WOT advance, and the VAC adds 16 for a total of 54 degrees above 2350 and relatively light load, which would be any legal cruising speed above 60 MPH on a level road up to where manifold vacuum drops below 8", which is probably at least 100 MPH. The engine operates very smoothly and gets about 22 MPH at legal freeway speeds and mid-teens around town.
My experience with early Corvette big blocks with closed chamber heads is that they don't like as much low speed/low load advance as small blocks, which is probably why GM equipped these engines with 28-32 degrees centrifugal and specified less initial than medium to high overlap cam small blocks. I believe the reason is that the "dual quench" zones of the chamber design effectively increase the rate of combustion propagation compared to open chamber designs like small blocks and later big blocks.
Your OE distributor offers a total of 28 degrees centrifugal so if you have, with the VAC deactivated, 14 at 1000, 39-14 = 25, so that tells me that the centrifugal starts below 1000 and is adding about 3 degrees at 1000, but that's not necessarily a bad thing. My SWC's modified spark advance map does the same thing since centrifugal starts at about 700 so idling at 900 centrifugal adds about 2 degrees and total idle advance is about 32, of which 16 is from the VAC and the the rest is the sum of 14 initial and 2 centrifugal . I don't even bother trying to measure initial since the engine will not idle below 700 long enough to even get a reading before it stalls, but I don't care. I set the total WOT advance in the 38-39 range at a little over 2500, then blip the throttle to about 6000 to be sure it doesn't over advance, and I'm good to go.
The most efficient idle setup on a small block that minimizes fuel flow and EGT is low 20s to low 30s advance and this is provided by the sum of initial plus full vacuum advance. High overlap cams generate a lot of exhaust gas dilution at idle, maybe as much as 30 percent, which slows combustion propagation so they like the upper range. Low overlap cam small blocks like base engines have only about 10 percent exhaust gas dilution at idle, so they like the lower end of the range and medium overlap cam small blocks like L-79 are happiest somewhere in the middle. So on a base small block the best setup is about 30 centrifugal and 6-10 initial, while high overlap cams like 24 centrifugal and 12-16 initial.
It's very important that idle advance remain fixed, Otherwise the engine may not idle stably and tend to stall. This is the reason for the Two-Inch Rule. If the engine doesn't generate enough idle vacuum to keep the VAC locked at full advance, vacuum advance will back off, which will cause a loss of idle speed, resulting in more advance and idle speed loss until the engine stalls. This was a problem when my SWC was new. The Duntov cam didn't generate enough vacuum at a reasonable idle speed, like 800-900 to keep the 15.5" 201 15 VAC locked at idle. This was the first year that Chevrolet added vacuum advance to Duntov cam engines. It was a good idea, but just not properly executed. They had not yet figured out the "Two-Inch Rule".
Since early closed chamber head big blocks, both the mechanical lifter versions and 390/400 HP versions that have fairly aggressive hydraulic cams (closer to a L-79 than base small block cam), don't like as much idle and low speed/load advance, this is likely why their spark advance maps look more like base small blocks than mechanical lifter small blocks.
If an engine is over-advanced at light load you will likely feel some roughness or maybe "trailer-hitching". If it runs smoothly and responds instantly to increased throttle input it's likely not over-advanced.
The '66 Corvette big blocks had 12 degree VACs. Recently a L-36 needed the OE VAC vacuum advance replaced, but only 16 degrees VACs are available today. We also decided to quicken the lazy centrifugal and since I was concerned that these changes would cause low speed/load driveability issues due to excess advance in this regime we replaced the 30 degree cam assembly with a 32 degree unit that was OE in '67 on l-36/68, and the max we could run initial for detonation free operation on California 91 PON fuel was 4 degrees. So we got 36 degrees total WOT advance, which is in the 36-40 ideal range, and the engine was very smooth and responsive under all speed/load conditions.
Duke
I'll use my 340 HP SWC as an example of what a small block needs with a high overlap camshaft. I modified the spark advance map back in 1965 to the '64-'65 365/375 HP specs by swapping in those weights and springs to bring all 24 degrees centrifugal in by 2350 engine RPM (OE was 4600), and the VAC starts at 4" and adds a maximum of 16 crank degrees at 8" and above.
With the 3.08 axle the engine is turning about 2350 at 60 MPH in top gear, so full centrifugal and full vacuum advance are in on a level road at 60 MPH. I run about 38 total WOT advance, and the VAC adds 16 for a total of 54 degrees above 2350 and relatively light load, which would be any legal cruising speed above 60 MPH on a level road up to where manifold vacuum drops below 8", which is probably at least 100 MPH. The engine operates very smoothly and gets about 22 MPH at legal freeway speeds and mid-teens around town.
My experience with early Corvette big blocks with closed chamber heads is that they don't like as much low speed/low load advance as small blocks, which is probably why GM equipped these engines with 28-32 degrees centrifugal and specified less initial than medium to high overlap cam small blocks. I believe the reason is that the "dual quench" zones of the chamber design effectively increase the rate of combustion propagation compared to open chamber designs like small blocks and later big blocks.
Your OE distributor offers a total of 28 degrees centrifugal so if you have, with the VAC deactivated, 14 at 1000, 39-14 = 25, so that tells me that the centrifugal starts below 1000 and is adding about 3 degrees at 1000, but that's not necessarily a bad thing. My SWC's modified spark advance map does the same thing since centrifugal starts at about 700 so idling at 900 centrifugal adds about 2 degrees and total idle advance is about 32, of which 16 is from the VAC and the the rest is the sum of 14 initial and 2 centrifugal . I don't even bother trying to measure initial since the engine will not idle below 700 long enough to even get a reading before it stalls, but I don't care. I set the total WOT advance in the 38-39 range at a little over 2500, then blip the throttle to about 6000 to be sure it doesn't over advance, and I'm good to go.
The most efficient idle setup on a small block that minimizes fuel flow and EGT is low 20s to low 30s advance and this is provided by the sum of initial plus full vacuum advance. High overlap cams generate a lot of exhaust gas dilution at idle, maybe as much as 30 percent, which slows combustion propagation so they like the upper range. Low overlap cam small blocks like base engines have only about 10 percent exhaust gas dilution at idle, so they like the lower end of the range and medium overlap cam small blocks like L-79 are happiest somewhere in the middle. So on a base small block the best setup is about 30 centrifugal and 6-10 initial, while high overlap cams like 24 centrifugal and 12-16 initial.
It's very important that idle advance remain fixed, Otherwise the engine may not idle stably and tend to stall. This is the reason for the Two-Inch Rule. If the engine doesn't generate enough idle vacuum to keep the VAC locked at full advance, vacuum advance will back off, which will cause a loss of idle speed, resulting in more advance and idle speed loss until the engine stalls. This was a problem when my SWC was new. The Duntov cam didn't generate enough vacuum at a reasonable idle speed, like 800-900 to keep the 15.5" 201 15 VAC locked at idle. This was the first year that Chevrolet added vacuum advance to Duntov cam engines. It was a good idea, but just not properly executed. They had not yet figured out the "Two-Inch Rule".
Since early closed chamber head big blocks, both the mechanical lifter versions and 390/400 HP versions that have fairly aggressive hydraulic cams (closer to a L-79 than base small block cam), don't like as much idle and low speed/load advance, this is likely why their spark advance maps look more like base small blocks than mechanical lifter small blocks.
If an engine is over-advanced at light load you will likely feel some roughness or maybe "trailer-hitching". If it runs smoothly and responds instantly to increased throttle input it's likely not over-advanced.
The '66 Corvette big blocks had 12 degree VACs. Recently a L-36 needed the OE VAC vacuum advance replaced, but only 16 degrees VACs are available today. We also decided to quicken the lazy centrifugal and since I was concerned that these changes would cause low speed/load driveability issues due to excess advance in this regime we replaced the 30 degree cam assembly with a 32 degree unit that was OE in '67 on l-36/68, and the max we could run initial for detonation free operation on California 91 PON fuel was 4 degrees. So we got 36 degrees total WOT advance, which is in the 36-40 ideal range, and the engine was very smooth and responsive under all speed/load conditions.
Duke
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