Distributors, vacuum versus mechanical
Race Director
Joined: Mar 2003
Posts: 11,608
Likes: 780
From: Eustis ( Area 51 Bat Cave ) Fl
Personally I wouldn't drive a street car without V/M advance,
I have done just as much air cooled vw stuff as corvette stuff over that last 30 plus years and in the ACVW world as far as mods go one of the big ones was to swap in the 009 mech advance distributor,
Then mixed with the solex carb 99.9% would have a bad flat spot when you would get on it, but they were cheap and easy, I ran them many times and fought with carb adjustment to help the flat spot.
Then I needed a new distributor and on ebay was a V/M ( vacuum and mech advance ) not unlike what is ran on c3 corvettes,
I installed it set the timing and tweaked the carb, I was shocked the difference it made, in performance and drivability, this was on my current VW trike and it made it a different creature,
I then started doing some research and no one in the vw circles were running the 009 any longer,
Seems that the mech only distributors are not bad for some things like drag cars, but on street cars where all kinds of things vary while driving the best is the V/M advance distributors,
There will be guys who will explain it far deeper, techier and better than I did but the end result is the same....
I have done just as much air cooled vw stuff as corvette stuff over that last 30 plus years and in the ACVW world as far as mods go one of the big ones was to swap in the 009 mech advance distributor,
Then mixed with the solex carb 99.9% would have a bad flat spot when you would get on it, but they were cheap and easy, I ran them many times and fought with carb adjustment to help the flat spot.
Then I needed a new distributor and on ebay was a V/M ( vacuum and mech advance ) not unlike what is ran on c3 corvettes,
I installed it set the timing and tweaked the carb, I was shocked the difference it made, in performance and drivability, this was on my current VW trike and it made it a different creature,
I then started doing some research and no one in the vw circles were running the 009 any longer,
Seems that the mech only distributors are not bad for some things like drag cars, but on street cars where all kinds of things vary while driving the best is the V/M advance distributors,
There will be guys who will explain it far deeper, techier and better than I did but the end result is the same....
Race Director
Joined: Jan 2000
Posts: 12,399
Likes: 247
From: San Jose CA
If you're asking if I prefer a "vacuum advance" distributor to a "mechanical advance" distributor, I think you're misunderstanding a mechanical ignition system and how it determines timing.
In low load situations, such as steady state cruise, more advance can be used to improve fuel burn. Load and vacuum actually have an inverse relationship, and as such, vacuum can be used to gauge load pretty well. So when you're cruising down the road at a steady throttle state, load is low and vacuum is high. Vacuum advance kicks in to add more ignition advance.
The mechanical portion applies the concept of centrifugal acceleration to ignition timing by adjusting the relationship of the advance plate to the ignition, bringing the spark in sooner. The amount of centrifugal acceleration is directly related to engine RPM (engine spins faster, weights go out further and move the advance plate more "near" the trigger).
For a street car, both are required.
Fully computer controlled ignition is better because it covers situations that the simple system of centrifugal acceleration and vacuum can not accommodate, but it's also best coupled with fuel metering as well.
To understand the concept of vacuum, if you don't alreadym I highly recommend temporarily connecting a vacuum gauge to your car and driving around (carefully) to watch the relationship between acceleration, cruising, coasting, heavy throttle, etc.
Good luck!
In low load situations, such as steady state cruise, more advance can be used to improve fuel burn. Load and vacuum actually have an inverse relationship, and as such, vacuum can be used to gauge load pretty well. So when you're cruising down the road at a steady throttle state, load is low and vacuum is high. Vacuum advance kicks in to add more ignition advance.
The mechanical portion applies the concept of centrifugal acceleration to ignition timing by adjusting the relationship of the advance plate to the ignition, bringing the spark in sooner. The amount of centrifugal acceleration is directly related to engine RPM (engine spins faster, weights go out further and move the advance plate more "near" the trigger).
For a street car, both are required.
Fully computer controlled ignition is better because it covers situations that the simple system of centrifugal acceleration and vacuum can not accommodate, but it's also best coupled with fuel metering as well.
To understand the concept of vacuum, if you don't alreadym I highly recommend temporarily connecting a vacuum gauge to your car and driving around (carefully) to watch the relationship between acceleration, cruising, coasting, heavy throttle, etc.
Good luck!
Zen Vet Master Level VII
Joined: Mar 2016
Posts: 5,218
Likes: 1,174
From: Southlake, TX
I used to mess with old Mopars. A trick we would do with our Chargers and Roadrunners was to plug the vacuum advance and advance the timing as far as it would go. We based it on what the vacuum gauge said, not worry about a timing light or until the pinging from detonation was louder than our Styx 8 track.
Burning Brakes
Joined: Apr 2008
Posts: 1,222
Likes: 93
From: shelbyville West Michigan
Personally I wouldn't drive a street car without V/M advance,
I have done just as much air cooled vw stuff as corvette stuff over that last 30 plus years and in the ACVW world as far as mods go one of the big ones was to swap in the 009 mech advance distributor,
Then mixed with the solex carb 99.9% would have a bad flat spot when you would get on it, but they were cheap and easy, I ran them many times and fought with carb adjustment to help the flat spot.
Then I needed a new distributor and on ebay was a V/M ( vacuum and mech advance ) not unlike what is ran on c3 corvettes,
I installed it set the timing and tweaked the carb, I was shocked the difference it made, in performance and drivability, this was on my current VW trike and it made it a different creature,
I then started doing some research and no one in the vw circles were running the 009 any longer,
Seems that the mech only distributors are not bad for some things like drag cars, but on street cars where all kinds of things vary while driving the best is the V/M advance distributors,
There will be guys who will explain it far deeper, techier and better than I did but the end result is the same....
I have done just as much air cooled vw stuff as corvette stuff over that last 30 plus years and in the ACVW world as far as mods go one of the big ones was to swap in the 009 mech advance distributor,
Then mixed with the solex carb 99.9% would have a bad flat spot when you would get on it, but they were cheap and easy, I ran them many times and fought with carb adjustment to help the flat spot.
Then I needed a new distributor and on ebay was a V/M ( vacuum and mech advance ) not unlike what is ran on c3 corvettes,
I installed it set the timing and tweaked the carb, I was shocked the difference it made, in performance and drivability, this was on my current VW trike and it made it a different creature,
I then started doing some research and no one in the vw circles were running the 009 any longer,
Seems that the mech only distributors are not bad for some things like drag cars, but on street cars where all kinds of things vary while driving the best is the V/M advance distributors,
There will be guys who will explain it far deeper, techier and better than I did but the end result is the same....
read this, I think it came from this site:
TIMING AND VACUUM ADVANCE 101
Written by John Hinckley! Thank you John for sharing this with everyone! You are an asset to the Corvette Community.
The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.
The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.
At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).
When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.
Tech Contributor
Joined: Jun 2004
Posts: 20,934
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
I used to mess with old Mopars. A trick we would do with our Chargers and Roadrunners was to plug the vacuum advance and advance the timing as far as it would go. We based it on what the vacuum gauge said, not worry about a timing light or until the pinging from detonation was louder than our Styx 8 track.
Tech Contributor
Joined: Jun 2004
Posts: 20,934
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
LOL I had the same experience when I had my dunebuggy...
read this, I think it came from this site:
TIMING AND VACUUM ADVANCE 101
Written by John Hinckley! Thank you John for sharing this with everyone! You are an asset to the Corvette Community.
The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.
The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.
At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).
When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.
read this, I think it came from this site:
TIMING AND VACUUM ADVANCE 101
Written by John Hinckley! Thank you John for sharing this with everyone! You are an asset to the Corvette Community.
The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.
The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.
At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).
When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.
The extra vacuum advance at idle is necessary due to the additional residual exhaust gas in the cylinder, which slows the burn rate down. Anyone who has ever has an EGR valve go wacky and introduce an excessive amount of exhaust gas into the cylinder has felt the engine fall on its face (due to a sudden reduction in torque), due to the slowdown in combustion burn rate. The vacuum advance canister compensates for the slower burn rate at idle caused by the low cylinder pressure and the dilution from exhaust gas trapped in the cylinder at TDC exhaust stroke due to exhaust system backpressure (compared to the intake manifold pressure/vacuum).
Also, if a wideband A/F ratio meter is installed on any of our antiques, the idle mixture is almost always on the rich side of stoichiometric, not the lean side.
Total timing is initial + mechanical + vacuum. The total timing that the piston/cylinder "sees" (regardless of what the driver/"tuner" thinks) is that crank/piston position when the spark occurs. And that point is the sum total of initial plus instantaneous mechanical plus instantaneous vacuum.
Safety Car
Joined: Jan 2009
Posts: 3,736
Likes: 56
From: anchorage ak
I used to mess with old Mopars. A trick we would do with our Chargers and Roadrunners was to plug the vacuum advance and advance the timing as far as it would go. We based it on what the vacuum gauge said, not worry about a timing light or until the pinging from detonation was louder than our Styx 8 track.
To the op..as was stated, the mechanical is for performance and the vacuum is for cruise and idle, for both fuel efficeintcy and emissions, it also helps keep the idle temps down and if you have a lot of duration on a camshaft, it takes more timing at idle to burn all that extra fuel and the vacuum advance helps by adding timing when the caburator is on the idle circut with out adding so much timing that the engine pings under load.
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Michael S. Palmer
Pro
Joined: Feb 2014
Posts: 546
Likes: 7
From: Kansas
Never heard of such, we would time by ear when we were teenagers and to broke to aford a timing light or a dwell meter, take the wrench and go set the timing in an empty parking lot. Power brake to the point of almost spinning the tires, if it didn't ping advance it till it did,, back it off a slight bit and try again and repeat just until it didn't. They next step was to take it up pearl hill lugging it in the highest gear that you could. if it pinged back it off a little bit and repeat, but usually after the power brake setting, it didn't ping.
To the op..as was stated, the mechanical is for performance and the vacuum is for cruise and idle, for both fuel efficeintcy and emissions, it also helps keep the idle temps down and if you have a lot of duration on a camshaft, it takes more timing at idle to burn all that extra fuel and the vacuum advance helps by adding timing when the caburator is on the idle circut with out adding so much timing that the engine pings under load.
To the op..as was stated, the mechanical is for performance and the vacuum is for cruise and idle, for both fuel efficeintcy and emissions, it also helps keep the idle temps down and if you have a lot of duration on a camshaft, it takes more timing at idle to burn all that extra fuel and the vacuum advance helps by adding timing when the caburator is on the idle circut with out adding so much timing that the engine pings under load.
For some reason mine wouldn't accept vacuum advance and we couldn't get it to hold timing. So I'm at 36* total timing with no vacuum advance hooked up. So I guess that puts me at straight mechanical. Ideals and cruises great with zero issues, but it's more a street / strip car.
BTW it got even better when I mixed the race gas last weekend at half 92 and half trick 101. Which leads me to believe that I'm not really at 10:8:1 compression. I think it might be even higher than that maybe even 11:1 seeing how much it likes race gas.
With Mickey Thompson ET Street SS tires she had a best et of 11:89 in the 1/4 mile after 3 runs.
Safety Car
Joined: Jan 2009
Posts: 3,736
Likes: 56
From: anchorage ak
For some reason mine wouldn't accept vacuum advance and we couldn't get it to hold timing. So I'm at 36* total timing with no vacuum advance hooked up. So I guess that puts me at straight mechanical. Ideals and cruises great with zero issues, but it's more a street / strip car.
BTW it got even better when I mixed the race gas last weekend at half 92 and half trick 101. Which leads me to believe that I'm not really at 10:8:1 compression. I think it might be even higher than that maybe even 11:1 seeing how much it likes race gas.
With Mickey Thompson ET Street SS tires she had a best et of 11:89 in the 1/4 mile after 3 runs.
BTW it got even better when I mixed the race gas last weekend at half 92 and half trick 101. Which leads me to believe that I'm not really at 10:8:1 compression. I think it might be even higher than that maybe even 11:1 seeing how much it likes race gas.
With Mickey Thompson ET Street SS tires she had a best et of 11:89 in the 1/4 mile after 3 runs.
Last edited by bluedawg; Jul 22, 2016 at 02:41 AM. Reason: Paused to get a beer...