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adjustable vac advance?

Discussion in 'The Garage' started by dubl_t, Aug 26, 2002.

  1. dubl_t

    dubl_t 1/2 ton status

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    Is the "kit" to make the HEI vacumm advance adjustable worth the trouble to install it? And what about the mecahanical adv. springs?
     
  2. kennyw

    kennyw N9PHW Premium Member

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    I have heard good things about the adjustable dizzy's (vac advance). Have not yet tried one but have driven the result of a poor advance curve /forums/images/icons/grin.gif
     
  3. Blazer_Boy

    Blazer_Boy 1/2 ton status

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    Get one! I was lucky enough to find a brand new one in a pile of junk. Oh, and don't believe crane cams. There's comes with a little stop and the allen wrench like every one elses does. There clam is that the stop adjust amount while the allen wrench is for adjusting the rate. I noticed this because i was trying to see who made mine. They ALL have AS-20 written on them. Must be made by a single source and everyone else slaps their name on them. When you turn the allen wrench to the left a few turns and put some vacuum on the can, you can see the pull rod in their just stop shy of all the way. Turn it another turn and it'll be stopped even more. Springs are real important too. You can usually find what you need in old junk distributor. Usually you only have to change one spring to get things a little quicker. Crane has springs in their kit.
     
  4. dubl_t

    dubl_t 1/2 ton status

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    Thanks for the input, but now, what's the "real" method for setting up a proper advance curve? I've heard I NEED a dial-back timing light, true or false? Thanks again.
     
  5. Blazer_Boy

    Blazer_Boy 1/2 ton status

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    I was considering putting this in for a tech article, but I don't know. I need some pics and to read it over and over a bizillion times until i get it just right.

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    Many of us upgrade our ignition systems by adding aftermarket control units, "hotter" coils, or other such parts in an attempt to increase output. These modifications are fine in their own right, but often a more important aspect is over looked, timing. Ignition timing is one of the key factors in controlling throttle response, performance, and fuel economy. Yet it is often over looked in diagnosing problems or after engine upgrades.

    In most cases in a four-stroke engine, ignition occurs a few degrees before top-dead-center (BTDC) of the compression stroke. The mixture begins to burn while the piston continues to move a small amount further the top of its travel. This explosion begins to force the piston down and becomes the power stroke. The spent mixture is then cleared out on the exhaust stroke and a new fuel mixture is introduced on the intake stroke. If ignition occurs too late (slow), power will be reduced and under hood heat will increase because the mixture has not had adequate time to burn as it exits. If ignition occurs to early (fast) excessive turbulence can occur as the burning mixture is being compressed by the moving piston and will result in "pinging". To a point, the amount of advance an engine can tolerate is proportional to the speed that it is turning. Flame speed increase as engine speed does and a higher operating speed will not warrant additional ignition advance after 3,000 RPM. It's fortunate that flame speed does increase because it would difficult for efficient combustion to occur at higher speeds because of less time a spark plug can fire.

    Timing in the coil-in-cap, non-computerized, HEI distributor is controlled by three factors, initial timing, mechanical advance, and vacuum advance. Initial timing is seen when the distributor is adjusted at idle. It is a stationary setting that is controlled by loosening the hold down clamp and slowly rotating the distributor to adjust it. Moving the distributor counter-clockwise causes an increase, or "speeding up" of initial timing, such going from 8 degrees BTDC to 12 degrees BTDC. Moving it clockwise will cause a decrease, or "slowing down" of initial timing, as in going from 8 degrees BTDC to 4 degrees BTDC. Initial timing effects idle quality, emissions, and low RPM power/response. Typical factory settings for initial timing range from 4 degrees BTDC to 12 degrees BTDC. Street performance applications are usually set around 12-16 degrees BTDC.

    Mechanical advance is controlled by two small weights, a center plate, and springs under the rotor. As the distributor begins to rotate faster, centrifugal force causes the weights to move out and changes the positioning of the rotor. The springs in the mechanical advance control the rate that advance comes in. Lighter springs will bring in the total mechanical advance quicker and heavier springs will require an increase of engine speed to achieve full advance. A slow advance can cause poor throttle response and reduced mileage. If the advance comes in too soon "pinging" and poor throttle response again could occur. Stock HEIs will deliver a total 20 degrees of mechanical advance, with +/- 1 degree from manufacturing tolerances. A good performance mechanical curve will begin to come in just off-idle and smoothly increase to around 2,500-3,000 RPM. Additional advance beyond this point is not necessary due to combustion characteristics.

    The last factor controlling ignition timing is vacuum advance. Contrary to some beliefs, this is NOT a part used to achieve increased performance. The vacuum advance contains a spring loaded diaphragm that controls an arm which moves the magnetic trigger assembly in the distributor. The vacuum advance is connected to an engine vacuum source, hence the name. While the engine is cruising or under light load, vacuum is high, and the vacuum advance increases the timing. This helps to give slightly better throttle response, improved mileage, and cleaner spark plugs. If the throttle is opened wider or the engine is heavily loaded, vacuum drops along with the extra advance. This helps to reduce the chance of detonation because of the load sensitivity of the unit. When used with a proper carburetor, such as the Rochester Quadrajet, it can be connected to a ported vacuum source.

    Ported vacuum on the Quadrajet is different than many other carburetors in that it behaves almost like full manifold vacuum when the throttle is open. In one of the front primaries, barely above the throttle blade is a slot that is covered at idle. When the throttle begins to open, this port is exposed to manifold vacuum and will advance the timing because the vacuum advance canister is now receiving engine vacuum. The common misconception of ported vacuum being incorrect for a performance application comes from certain carburetors that have a port the feeds from near the venturi. Without going into great detail, the venturi is the heart of the carburetor. It is a narrowed section of the carburetor that incoming air passes through. In order for air to pass through this bottle neck, it must speed up as it is constricted. This causes a lower pressure area, which creates vacuum. This is phenomenon helps the carburetor with the metering of fuel and will run a vacuum port on some models. Extra vacuum advance under WOT is detrimental to performance and is not desired.

    Hooking up to a source that has full vacuum at idle will increase idle speed, emissions, and may harm off-idle and low throttle performance. Stock truck HEIs typically added about 20 degrees of advance, which is too much for performance applications. Using an adjustable style unit and adding as much advance possible is all that is required. The nature of vacuum advance is not predictable and there is no set amount to "shoot" for. Too much can create part-throttle "pinging" and/or "chugging" under acceleration. To use an adjustable vacuum advance, simply insert the correct size allen wrench and turn it one full turn. Typically one turn to the left will remove about 2 degrees of advance. Many times, but not always, the amount ends up being an extra 8-12 degrees if the can were engaged.

    With the vacuum advance disconnected, most small block Chevys make their best power around 32-38 of total advance. 32 degrees would be ideal for a very efficient combustion chamber, such as GM's Fast Burn cylinder head. Most engines will work best with 34-36 degrees and rarely would 38 be needed. The HEI will add 20 degrees of mechanical advance to the initial timing. If an engine was set at 16 degrees BTDC, then it would end up with a total of 36 degrees of total advance. Adjusting the initial timing anywhere from 12-18 degrees BTDC would adjust the 32-38 degree total. The mechanical advance should be in fully by around 3,000 RPM. To adjust this rate, simply change the tension on the springs. They lie under the rotor and can be replaced by hand. GM has used a multitude of springs and weights over the years and it is impossible to give an exact combination. Aftermarket kits and other distributors are an excellent source of springs. The weights and center plate that come in some kits should be avoided because of the poor results that they give. Remove the stock weights and inspect the pivots. The holes in the weights or their bushings, and pins they ride on should be true to round. If not, then replace the worn parts to ensure consistent operation. Clean the area and apply a thin coat of light oil over the moving parts to ensure smooth operation.

    A well-equipped individual can easily check up on their mechanical advance. First, the hose leading to the vacuum advance should be disconnected and plugged off. The initial timing should then be set the desired setting (12-16 degrees). If using a dial-back timing light, then the light should be set 20 degrees past the initial setting (ex. 16 initial + 20 = set light for 36). Timing tape will be required if using a standard style light. More expensive models of timing lights may have a tachometer built into them and can aid the user. If not, then have an assistant observe the tachometer or temporarily install one under the hood. Snapping the throttle should make the timing mark move. Pulling the throttle harder and holding it at 3,000 RPM should provide the desired reading. A dial back timing light would show 0 degrees (because the light would be dialed back 32-36 degrees) and a standard light would 32-36 degrees with the timing tape. If not, then lighter springs must be installed until the reading is near this RPM range. Fine tuning beyond this point is up to driver. Experimenting with initial timing and spring weights can help to fine-tune response. Initial timing beyond 16 degrees or bringing in the total timing below 2,500 is not recommended and should not be necessary. The vacuum advance can then be reconnected and adjusted.

    If satisfactory results can not be achieved, then other adjustments may be in order. Starter drag can be cured by using higher torque starters. The center plate in the mechanical advance mechanism can also be reshaped to allow more advance. This would mean that less initial advance to achieve the desired total timing. There is an excellent article by Jim hand that covers some of this process.

    A lean mixture can also result in higher combustion chamber tempatures and lead to detonation. Improper heat range spark plugs could also create problems. Spark plugs radiate off heat and are made in different ranges that control the rate of this. A "cold" plug dissipates heat easier than a "hot" plug because of design characteristics. Signs of too hot of a plug can be specs on the porcelain, or a lack of color. The porcelain should be a light brown, tan, or grey. Lack of color indicates a lean condition or the wrong plug. A fouled plug could be too cold, too rich of a mixture, or a weak ignition system.

    Coolant temperature can also affect the occurrence of pre-ignition. Switching from a 195 degrees thermostat to a 180 degree unit can help keep the combustion chamber cooler. Lastly would be selecting better fuel with a higher octane rating. If the owner is dead set against the use of premium fuel, perhaps they should reassess what they intend to do with their performance motor. A quality motor is expensive and using better fuel to power it may be part of the expense. If $.10 a gallon more is too much to spend, then the Goodwrench 350 with stock settings is a great 87 octane power plant.

    Ideal timing is difficult to achieve, but can yield impressive results. These modified settings may alter emissions levels and could cause the owner to fail testing. Therefore they should only be performed on exempt or off-road only vehicles. Extended driving with improper timing can also engine damage or failure. However, proper timing can also lead to better throttle response and more power!
     
  6. Blazer_Boy

    Blazer_Boy 1/2 ton status

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    "I'm gonna ride around in stlye. I'm gonna drive everybody wild. 'Cause I'll have the only one there is in town...." /forums/images/icons/laugh.gif
     
  7. TopOff

    TopOff 1/2 ton status

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