Timing Question

Discussion in 'Electrical and Ignition: Repairs and Modifications' started by wildjyoung, Dec 16, 2009.

  1. wildjyoung

    wildjyoung Member

    I have built a 350 bored .040 with a high flow intake TPI intake, moderate cam, 10.25:1 compression, Different cranks/rods/pistons, AFR heads, and some other small performance modifications. I set the timing at 6 degress BTDC per factory spec. However, there is almost nothing factory about my engine anymore. Should my base timing be different than factory spec?

    What brought up my question was a Horsepower TV episode and recommendation by a mechanic. Now granted, I do not think Horsepower TV is the trusted source for car advice nor is every mechanic. However, on Horsepower TV they advanced timing past factory set point and gained 22 HP. The mechanic told me to advance timing until you get detonation and then twist distributor back just enough to stop it. Is there any truth to either of these methods?
     
  2. grumpyvette

    grumpyvette Administrator Staff Member

    now thats a rather interesting question, that comes up frequently so lets go into how and why, you set ignition advance a bit.

    OK the BASICS

    "The mechanic told me to advance timing until you get detonation and then twist distributor back just enough to stop it. Is there any truth to either of these methods?"
    short answer, yes thats a rather crude but fairly effective route to finding the usable average ignition advance , but its not nearly ideal, because you can,t always hear detonation, and even detonation your can,t hear can rapidly damage engines

    your ignition system is used to ignite the compressed fuel air mix in the combustion chamber, the object is to build PRESSURE over the piston, that can be used to force the piston and rod assembly rapidly down the cylinder bore and spin the crank, to provide the force, thats converted to rotary motion thru the mechanical leverage the crank and rods provide. now that cylinder pressure builds up over a short but measurable time, and its caused by the rapid increase in heat , and expanding gasses as a flame front burns from the ignition point to the far reaches of the combustion chamber, the fuel air ratio, the combustion chamber and piston shapes and dimensions , the combustion chamber heat already present , the fuels octane,and the location of the spark plug, etc. are all factors in the flame fronts speed and the pressure curve, and the speed of the burn, increases as turbulence and swirl in increased, and as the rpms increase.
    your engines quench design and clearances also effect the speed of the flame front and burn speed as will the compression levels and the volume of the compressed fuel/air mix to be burnt.

    now lets say as an example it takes 30-40 thousands of a second at idle to build to max pressure,after the compressed fuel/air mix is ignited. as your piston compresses the fuel /air mix, but that same engine may only take 15-20 thousands of a second to burn the compressed mix under other conditions or at a higher rpm level. the burning mix builds heat and pressure, but its likely, to fall in the 150-240 psi range before its ignited.
    obviously you don,t want the piston trying to compress a great deal of fast expanding gasses before TDC is reached and your goal is to maximize the pressure over the piston after tdc, but keep in mind that the cylinder pressure drops rapidly after the first 40 degrees past TDC and as the piston moves rapidly away from TDC the volume in the cylinder increases rapidly making maintaining that pressure far more difficult.
    "BTW one reason NITRO,METHANE base fuels are popular in dragsters is that they provide some of their own oxygen and burn far longer ,maintaining pressure over the piston far longer"
    After its ignited it rapidly but not instantly spikes to 600-1000psi, as the fuel/air mix burns.
    [​IMG]
    the problem is balancing the pressure rise and matching it to both the crank rotation and the time available, remember at 700rpm, the cylinder fires about 6 times per second, at 6500rpm, its about 54 times a second so theres a huge reduction in the time available for the pressure to build, forcing you to ignite the compressed mix earlier in the rotation to get the useful pressure over the piston to build up significantly to produce torque.
    as the time decreases youll reach a point where the cylinders don,t fill and empty as efficiently and the power or pressure per power stroke falls off fairly rapidly as the rpms continue to increase, but since the number of power strokes still increases the power produced may still increase for a while higher in the rpm band.
    [​IMG]
    now, it should be obvious that any pressure that build as the piston rotates from before top dead center, up to TDC, tends to slow rotation and reduce the potential power produced and all the pressure over the piston after top dead center as the piston and rod assembly swings past tdc can be used to spin the crank and make torque, but its a compromise, as the pressure builds at a different rate as the engines rpms increase or decrease.

    as a start point most chevy v8 engines will run reasonably well with an 8-12 degree advance at idle speed and a smooth ignition advance with the rpms up till about 3200rpm and at about 34-36 degrees of total advance,where the increase in ignition advance can be stopped as the burn, speed starts to catch up or advance, can be stopped, once you set your ignition advance curve, to that basic curve you'll usually be close enough that only minor adjustments are required, and careful reading of the plugs, will let yopu know the combustion chamber burn conditions

    read these threads, as each has info you need to understand the process, reading your spark plugs will give a good indication of the burn conditions in the combustion chamber
    YES IM FULLY AWARE thats a good deal of info to wade thru, but if you want to understand a good deal its worth the required time/effort

    http://auto.howstuffworks.com/ignition- ... /printable

    http://yarchive.net/car/advance_curve.html

    viewtopic.php?f=70&t=202

    http://www.rodandcustommagazine.com/tec ... index.html

    http://www.hotrod.com/techarticles/sett ... index.html

    viewtopic.php?f=44&t=937&p=6449&hilit=detonation#p6449

    viewtopic.php?f=38&t=1099&p=2152&hilit=volumetric#p2152

    viewtopic.php?f=53&t=726&p=5640&hilit=quench#p5640

    viewtopic.php?f=70&t=1809&p=4671&hilit=+pressure+curve#p4671

    http://www.hotrod.com/techarticles/sett ... index.html

    viewtopic.php?f=70&t=1135&p=2289&hilit=+curve#p2289

    http://www.gofastforless.com/ignition/advance.htm

    http://www.pontiacstreetperformance.com ... curve.html

    http://www.hotrod.com/techarticles/sett ... index.html

    http://www.73-87.com/7387garage/drivetrain/hei.htm

    http://www.centuryperformance.com/ignit ... g-219.html

    viewtopic.php?f=52&t=727

    viewtopic.php?f=70&t=875

    viewtopic.php?f=70&t=1015

    http://www.corvettefever.com/techarticl ... index.html

    without a decent timing light and a well marked damper, on which youve carefully verified TDC, your basically guessing at timing
    viewtopic.php?f=70&t=875&p=6241&hilit=+timing+light#p6241
    [​IMG]
    stock timing marks are very limited in the extent of timing changes that can be indicated
    [​IMG]
    [​IMG]
     
  3. grumpyvette

    grumpyvette Administrator Staff Member

    An HEI Ignition & Timing Primer

    by Lars Grimsrud

    The following two short articles are excerpts of some posts I did on the CorvetteForum.com regarding the timing curve on the non-ECM GM HEI Ignition Systems. Although not organized as “how-to” technical articles, I think you may find some interesting and useful information here….

    First, be sure to get a copy of my technical article, “How to Set Your Timing for Best Performance.” This contains a lot of the background information regarding ignition timing, but is applicable specifically for the point-style distributors. If you do not have a copy of this article, be sure to drop me an e-mail request for it. Also complimenting this article, and essential reading to understand the vacuum advance system is my article titled “Distributor Vacuum Advance Control Units Specs and Facts.” Drop me a note for a copy if you don’t have one available.

    Okay, here we go with some basics…

    Ignition timing has a marked effect on both engine performance/power output as well as on vehicle emissions. Unfortunately, the HEI-equipped C3 Vettes built in the ‘70s were more tailored for emissions than for performance, so there are significant performance gains to be made by “tweaking” the calibration of the ignition system.

    At idle, most engines produce the smoothest running operation with about 12-16 degrees of initial timing. The more retarded the timing is, the lower the Hydrocarbon emissions will be, so most HEI-equipped cars from the ‘70s have an initial timing spec in the 4-8 degree range. However, the more retarded the timing is, the more power you lose…

    As rpm comes up, the engine demands more timing advance in order to produce peak power. To control emissions, the HEI distributors are designed to bring the additional timing in very slowly, and to limit the total timing in such a way that emissions are minimized. This is not conducive to producing peak power…

    Typically, GM V8s will produce peak torque when a total timing of 36 degrees is brought in at the engine’s torque peak rpm – about 2500 rpm. If you have your initial timing set at 10 degrees BTDC, this means that you need the centrifugal advance to bring in an additional 26 degrees of timing by 2500 rpm in order to get best power. What we see, however, is that the stock HEI setup usually produces about 15 – 18 degrees of maximum centrifugal advance, limiting the total maximum advance (initial timing plus centrifugal advance) to about 25 – 28 degrees. This total advance will come in close to redline rpm – not at the torque peak. This is 10 degrees short of producing peak power!

    So to properly tune the car for best performance, we do a couple of things: First, the advance curve length must be corrected and/or verified. If the curve is too short to produce acceptable timing results, it must be lengthened. Second, we must change the rpm at which the total advance is allowed to occur. Since the engine’s torque peak occurs at 2500 rpm, we must change the advance curve to bring the total timing all in by 2500 – 3000 rpm. Finally, we must set and adjust the timing to produce a total timing setting of 36 degrees. Once these parameters are properly set, the car will perform its best.

    Keep in mind that these parameters will not produce best emissions. In order to produce low emissions, the timing must be retarded.

    Finally, we have the issue of vacuum advance. Vacuum advance is added on to the mechanical advance curve discussed above, and is a method for improving fuel economy and throttle response at light, part-throttle settings (at cruise speed). A GM V8 at light throttle cruise performs best when timing is at 52 degrees BTDC. If timing goes much beyond 53 degrees at cruise, the car will start to buck and jerk due to too much timing advance. But 52 degrees is just about right.

    Since the factory designs the total mechanical part of the advance curve with about a 26 – 28 degree limit, they install a vacuum advance control unit that adds on about 24 degrees of vacuum advance at cruise. This puts the timing right up at 52 degrees max, and is a perfect cruise setting. But, if we now go in and change our advance curve for peak Wide Open Throttle performance (36 degrees all in at 2500 rpm), we see that the factory vacuum advance unit will now add another 24 degrees on top of our 36 degree setting, giving us a total ignition advance of 60 degrees. This is too much advance for any car to handle, and will result in the car jerking and bucking severely at cruise speed. Not a good thing.

    So to compliment the performance curve of our distributor, we must now replace the stock vacuum advance unit with a unit that will give us a total vacuum advance of not more than 16 degrees. By carefully researching the specifications of the vacuum advance units, it is possible to produce a perfect setup.

    This method of tweaking and tuning will result in a significantly improved level of performance and throttle response, from off-idle through redline rpm. Mind you, it’s not like bolting on a supercharger or anything like that, but it will produce a noticeable, seat-of-the-pants difference in performance and “feel” on the throttle. My past tuning and performance measurements show that in order to produce a seat-of-the-pants difference in performance, you have to change horsepower by at least 10hp. Getting 10 horses by tweaking your timing curve is a pretty neat mod to do.

    For assistance on comments on this subject, drop me a note to:

    V8FastCars@msn.com


    The following CorvetteForum post discussed my development of a curve kit for the HEI distributors. It contains some interesting information about the HEI curve and how to change it. I am not currently actively marketing or selling these kits, but I can assist you in parts selection if you need help.

    Development of the HEI Ignition Curve Kit is complete, and I’ll have the first kits ready to go by the weekend. Parts are trickling in slowly from the various suppliers, and I’m hand-fabricating the special advance weights that are required to make some of the HEI systems work right. I can build one kit every evening, so I will fill orders as they come in just as quickly as I can build kits. For more information, drop me an e-mail. The kits are $45 including postage, and I trustingly accept non-rubber personal checks. Please keep in mind that I do this for the passion and love of the hobby, so I’m not putting a worthwhile profit margin on this – if you bounce a check on me it’ll hurt… so please don’t…. Payable and mail to:

    Lars Grimsrud
    1285 Cressida Court
    Lafayette, CO 80026

    V8FastCars@msn.com

    I highly recommend that you buy yourself an adjustable timing light – it makes this job much easier and more precise. Use this as an excuse to go visit your friendly Sears Craftsman store…

    The development of this kit was very interesting, and revealed several things…

    First, the stock HEI setup leaves HUGE room for improvement. It responds very well to the correct tuning technique, but this is the key: As important as the parts you use to modify your car is the TECHNIQUE you use to set it up.

    The HEI systems are primarily tailored to provide low vehicle emissions. In most cases, this means that the ignition curve is slow and retarded. Retarding the timing reduces Hydrocarbons, but it also retards your performance…. Typically, we find that the HEI systems are set up for an initial timing of about 8 degrees. The “length” of the advance curve varies greatly from one distributor to the next, but typically the centrifugal advance will allow an additional 15 – 18 degrees of advance, with the maximum advance coming in at about redline rpm. This gives you a total mechanical advance of about 26 degrees on your HEI ‘Vette.

    26 degrees. You gotta’ be kidding. This is 10 degrees short of best power, and the rpm curve is off by 2500 rpm. Pretty significant.

    The advance curve on the HEI systems is limited by the design of the centrifugal weights. The weights have a unique geometry which causes the weights to “bind up” and not advance any further once they hit a certain advance point. You can advance your timing to achieve 36 degrees total advance, but on many cars, this will result in an initial timing setting of about 18 – 22 degrees. This works well for radical cams, but does not work well for most stock or near-stock setups.

    What we find, then, is that the total length of the advance curve must be increased for most applications, and the curve must be brought in at a much lower rpm: 2500-3500 rpm is the target range for full 36-degree advance. To do this, your tuning technique must include a verification of your stock advance curve length, and then a thoughtful modification and correction to this curve length. For best throttle response and performance to stock and near-stock cars, the initial timing should be around 12 – 14 degrees (as measured with vac advance disconnected), centrifugal curve length should be about 22-24 degrees, with total mechanical timing at about 36. If you can get close to this, you’ll get a noticeable performance improvement – anything is better than the stock 25 degree max setup…

    In doing the tuning we also found that none of the aftermarket kits provide good instructions, and none of the kits produce the correct curve length. Some of the kits improve things slightly, while others actually make your car run worse than stock. No kidding.
     
  4. grumpyvette

    grumpyvette Administrator Staff Member

    http://www.corvette-restoration.com/res ... Timing.pdf
    How to Set Your Timing for Peak Performance
    by Lars Grimsrud
    SVE Automotive Restoration
    Musclecar, Collector & Exotic Auto Repair & Restoration
    Broomfield, CO Rev. A 2-7-01
    This tech paper will discuss setting the timing on a Chevy V8. This procedure also applies to other GM
    V8s.
    The procedure outlined here differs from the Service Manual, and is based on my years of experience doing
    this work in the quickest, least painful, most economical way while keeping the level of quality high. It is
    recognized that other people will have different methods of doing things, and may disagree with specific
    methods and procedures that I use.
    How to Set the Timing
    When you think about it, setting the timing at idle speed makes no sense at all: You donÂ’t operate your car
    at idle, and timing changes as the rpm changes. Fact is, the timing spec at idle speed is provided as a simple
    way for most people to set the timing, and is not a good procedure for optimum performance.
    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. If you have an adjustable timing light, this is very easy to check.
    If you donÂ’t, you need to scribe a 36-degree mark on your harmonic balancer. HereÂ’s how:
    Measure the circumference of your harmonic balancer using a sewing tape measure (or other flexible tape
    measure). Get it as accurate as you can. Take this measurement and divide by 10. The number you get is
    the distance to 36 degrees. Measure this distance CLOCKWISE from your existing harmonic balancer
    timing mark and place a clear mark on the balancer.
    Remove your distributor cap and rotor. Remove the 2 centrifugal advance springs. Install the rotor and the
    cap (without the springs). Disconnect the vacuum advance.
    Start the engine. It may kick back a little due to the advance coming in immediately without the
    springs. If youÂ’re using an adjustable timing light, set the light to 36 degrees advanced. Now rev the
    engine just a little while observing the timing marks with the light. It shouldnÂ’t take much rpm to peg out
    the advance without the springs installed. With an adjustable light set at 36 degrees, align the stock timing
    marks with “0” when the timing is “pegged out.” With the non-adjustable light, align your new 36-degree
    mark with “0.” Rev the engine a little to make sure the timing will not advance any further. Shut it down.
    Pop the cap and rotor and re-install the springs. Put everything back together, but leave the vacuum
    disconnected. Start it up. For future reference, make a note of the timing setting at idle. This is your new
    curb idle timing spec. Now give the engine a few quick revÂ’s past 3,000 rpm and verify that the full timing
    (36 degrees) is coming in. If itÂ’s not, you need to change to a softer set of springs until you get full 36-
    degree advance before 3000 rpm. (NOTE: A stock set of springs will usually not allow full centrifugal
    advance to come in before redline rpm. If you have stock springs installed, donÂ’t rev the engine beyond its
    limits to try to force full advance in.)
    Shut it down and hook up the vacuum. Now do a road test.
    The 36-degree 2500 rpm advance curve is optimum for performance, but may require premium fuel. Lug
    the car around, and punch the throttle at low rpm while listening for detonation (“engine knock”). If you’re
    getting any audible knock, you MUST retard the timing. Retard the timing in 2-degree increments until
    engine knock stops. Engine knock will seriously damage engine components if not corrected. If you get
    no knock, you may see slightly improved performance at 38 degrees total timing. This is particularly true
    if youÂ’re running at high altitude.
    If you have no engine knock under acceleration, but the car “chugs” or “jerks” at cruising speed (light
    throttle application), you are getting too much vacuum advance on top of the mechanical advance. You
    may need to change out the vacuum advance diaphragm with an adjustable unit available from aftermarket
    sources. Adjust these units so that you get the most vacuum advance possible without any “chugging” or
    “jerking” at cruise speed.
    Your timing is now set for best possible performance. Make note of the new setting, and use this for your
    future tune-up work.


    lars.grimsrud@lmco.com
     
  5. grumpyvette

    grumpyvette Administrator Staff Member

    Distributor Vacuum Advance Control units
    Specs and facts for GM Distributors
    by Lars Grimsrud
    SVE Automotive Restoration
    Musclecar, Collector & Exotic Auto Repair & Restoration
    Broomfield, CO
    Rev. B 8-19-02
    I’ve been seeing a lot of discussion and questions regarding distributor vacuum advance control units; what do they
    do, which ones are best, what was used on what, etc., etc. To clarify some of this, I thought I’d summarize a few
    facts and definitions, and provide a complete part number and specification listing for all vacuum advance control
    units used by Chevrolet on the points-style distributors. I’m also providing a listing of the specs for all other GM
    (non-Chevrolet) control units, but without the specific application listed for each (it would take me a bit too much
    time to research each part number by application across each of the GM Motor Divisions – it took me long enough
    to compile just the Chevy stuff…!). This latest revision to this paper also includes the HEI listings (the HEI
    distributors use a longer control unit, so the non-HEI and HEI vacuum advance control units CANNOT be
    interchanged).
    As always, I’m going to include the disclaimer that many of these are my own comments and opinions based on my
    personal tuning experience. Others may have differing opinions & tuning techniques from those presented here. I
    have made every attempt to present factual, technically accurate data wherever possible. If you find factual errors in
    this information, please let me know so I can correct it.
    Background
    The vacuum advance control unit on the distributor is intended to advance the ignition timing above and beyond the
    limits of the mechanical advance (mechanical advance consists of the initial timing plus the centrifugal advance that
    the distributor adds as rpm comes up) under light to medium throttle settings. When the load on the engine is light
    or moderate, the timing can be advanced to improve fuel economy and throttle response. Once the engine load
    increases, this “over-advance” condition must be eliminated to produce peak power and to eliminate the possibility
    of detonation (“engine knock”). A control unit that responds to engine vacuum performs this job remarkably well.
    Most GM V8 engines (not including “fast-burn” style heads), and specifically Chevys, will produce peak torque and
    power at wide open throttle with a total timing advance of 36 degrees (some will take 38). Also, a GM V8 engine,
    under light load and steady-state cruise, will accept a maximum timing advance of about 52 degrees. Some will take
    up to 54 degrees advance under these conditions. Once you advance the timing beyond this, the engine/car will start
    to “chug” or “jerk” at cruise due to the over-advanced timing condition. Anything less than 52 degrees produces
    less than optimum fuel economy at cruise speed.
    The additional timing produced by the vacuum advance control unit must be tailored and matched to the engine and
    the distributor’s mechanical advance curve. The following considerations must be made when selecting a vacuum
    advance spec:
    How much engine vacuum is produced at cruise? If max vacuum at cruise, on a car with a radical cam, is only 15
    inches Hg, a vacuum advance control unit that needs 18 inches to peg out would be a poor selection.
    How much centrifugal advance (“total timing”) is in effect at cruise rpm? If the distributor has very stiff centrifugal
    advance springs in it that allow maximum timing to only come in near red-line rpm, the vacuum advance control
    unit can be allowed to pull in more advance without the risk of exceeding the 52-degree maximum limit. If the
    engine has an advance curve that allows a full 36-degree mechanical advance at cruise rpm, the vacuum advance
    unit can only be allowed to pull in 16 more degrees of advance.
    Are you using “ported” or “manifold” vacuum to the distributor? “Ported” vacuum allows little or no vacuum to the
    distributor at idle. “Manifold” vacuum allows actual manifold vacuum to the distributor at all times.
    Does your engine require additional timing advance at idle in order to idle properly? Radical cams will often require
    over 16 degrees of timing advance at idle in order to produce acceptable idle characteristics. If all of this initial
    advance is created by advancing the mechanical timing, the total mechanical advance may exceed the 36-degree
    limit by a significant margin. An appropriately selected vacuum advance unit, plugged into manifold vacuum, can
    provide the needed extra timing at idle to allow a fair idle, while maintaining maximum mechanical timing at 36. A
    tuning note on this: If you choose to run straight manifold vacuum to your vacuum advance in order to gain the
    additional timing advance at idle, you must select a vacuum advance control unit that pulls in all of the advance at a
    vacuum level 2” below (numerically less than) the manifold vacuum present at idle. If the vacuum advance control
    unit is not fully pulled in at idle, it will be somewhere in its mid-range, and it will fluctuate and vary the timing
    while the engine is idling. This will cause erratic timing with associated unstable idle rpm. A second tuning note
    on this: Advancing the timing at idle can assist in lowering engine temperatures. If you have an overheating
    problem at idle, and you have verified proper operation of your cooling system components, you can try running
    manifold vacuum to an appropriately selected vacuum advance unit as noted above. This will lower engine temps,
    but it will also increase hydrocarbon emissions on emission-controlled vehicles.
    Thus, we see that there are many variables in the selection of an appropriate control unit. Yet, we should keep in
    mind that the control unit is somewhat of a “finesse” or “final tuning” aid to obtain a final, refined state of tune; we
    use it to just “tweak” the car a little bit to provide that last little bit of optimization for drivability and economy. The
    vacuum advance unit is not used for primary tuning, nor does it have an effect on power or performance at wide
    open throttle.
    With these general (and a little bit vague, I know…) concepts in mind, let’s review a few concepts and terms. Then
    it’s on to the master listing of specs and parts…..:
    Part Number
    There are many different sources for these control units. Borg Warner, Echlin, Wells, and others all sell them in
    their own boxes and with their own part numbers. Actually, there are very few manufacturers of the actual units:
    Dana Engine Controls in Connecticut manufactures the units for all three of the brands just mentioned, so it doesn’t
    make much difference who you buy from: They’re made by the same manufacturer. The part numbers I have listed
    here are the NAPA/Echlin part numbers, simply because they are available in any part of the country.
    ID#
    Every vacuum advance control unit built by Dana, and sold under virtually any brand name (including GM), has a
    stamped ID number right on top of the mounting plate extension. This ID, cross referenced below, will give you all
    specifications for the unit. So now, when you’re shopping in a junkyard, you’ll be able to quickly identify the
    “good” vs. the “bad” control units.
    Starts @ “Hg
    Vacuum is measured in “inches of Mercury.” Mercury has the chemical symbol “Hg.” Thus, manifold vacuum is
    measured and referred to as “Hg. The “Start” spec for the control unit is a range of the minimum vacuum required
    to get the control unit to just barely start moving. When selecting this specification, consideration should be made to
    the amount of vacuum that a given engine produces, and what the load is on the engine at this specification. For
    example, an engine with a very radical cam may be under very light load at 7 inches Hg, and can tolerate a little
    vacuum advance at this load level. Your mom’s Caprice, on the other hand, has such a mild cam that you don’t
    want the vacuum to start coming in until 9 – 10 inches Hg. For most street driven vehicle performance applications,
    starting the vacuum advance at about 8” Hg produces good results.
    Max Advance
    Since the vacuum advance control unit is a part of the distributor, the number of degrees of vacuum advance is
    specified in DISTRIBUTOR degrees – NOT crankshaft degrees. When talking about these control units, it is
    important that you know whether the person you’re talking to is referring to the distributor degrees, or if he’s talking
    crankshaft degrees. All of the listings shown in the following chart, and in any shop manual & technical spec sheet,
    will refer to distributor degrees of vacuum advance. You must DOUBLE this number to obtain crankshaft degrees
    (which is what you “see” with your timing light). Thus, a vacuum advance control unit with 8 degrees of maximum
    advance produces 16 degrees of ignition advance in relationship to the crankshaft. When selecting a unit for max
    advance spec, the total centrifugal timing at cruise must be considered. Thus, a car set up to produce 36 degrees of
    total mechanical advance at 2500 rpm needs a vacuum advance control unit producing 16 degrees of crankshaft
    advance. This would be an 8-degree vacuum advance control unit.
    Max Advance @ “Hg
    This is the range of manifold vacuum at which the maximum vacuum advance is pegged out. In selecting this
    specification, you must consider the vacuum produced at cruise speed and light throttle application. If your engine
    never produces 20” Hg, you better not select a control unit requiring 21” Hg to work.
    The following listing (Non-HEI) is as follows: The first two part number listings are the two numbers that are
    most commonly used in a Chevrolet performance application. The “B1” can is the most versatile and user-friendly
    unit for a good performance street engine. As you can see, it was selected by GM for use in most high performance
    engines due to its ideal specs. The “B28” can was used on fuel injected engines and a few select engines that
    produced very poor vacuum at idle. The advance comes in very quick on this unit – too quick for many
    performance engines. Do not use this very quick unit unless you have a cam/engine combination that really needs an
    advance like this. It can be used as a tuning aid for problem engines that do not respond well to other timing
    combinations, and can be successfully used in applications where direct manifold vacuum is applied to the can (see
    paragraph and discussion on this above)
    After this, the listing is by Echlin part number. The Chevrolet applications are listed first by application, followed
    by a complete listing of all of the units used on any GM product (all GM units are interchangeable, so you can use a
    Cadillac or GMC Truck unit on your Vette, if that’s what you want to do).
    Non-HEI Distributors:
    P/N ID# Application Starts @ “Hg Max Adv
    (Distr. Degrees @ “Hg.)
    VC680 B1 1959 – 63 All Chevrolet 8-11 8 @ 16-18
    1964 Corvette exc. FI
    1964 Impala, Chevy II
    1965 396 High Perf.
    1965-67 283, 409
    1966-68 327 exc. Powerglide
    1967-68 All 396
    1969 Corvette 427 High Perf.
    1969 396 Exc. High Perf.
    1969 Corvette 350 TI
    1969-70 302 Camaro
    1970 400 4-bbl
    1970 396 High Perf.
    1970 Corvette 350 High Perf.
    1973-74 454 Exc. HEI
    VC1810 B28 1965 409 High Perf. 3-5 8 @ 5.75-8
    1965 327 High Perf.
    1966 327 High Perf.
    1964-67 Corvette High Perf. FI
    -------------------------------------------------------------------------------------------------------------------------------------------
    VC1605 B9 1965 impala 396 Exc. High Perf. 7-9 10.3 @ 16-18
    1965 327 All Exc. FI
    1969 327 Camaro, Chevelle, Impala
    1969-70 Corvette 350 Exc. High Perf.
    1969-70 350 4-bbl Premium Fuel
    1970 350 Camaro, Chevelle, Impala High Perf.
    1971-72 350 2-bbl AT
    1971-72 307 All
    VC1675 B13 1968 327 Camaro Powerglide 9-11 8 @ 16-18
    1968 327 Impala AT
    1968 307 AT
    1968 302, 307, 327, 350 Camaro, Chevy II
    1970 350 Camaro, Chevelle Exc. High Perf.
    VC1760 B19 1969 350 Camaro, Chevelle, Impala 4-bbl 5.5-8 12 @ 14-18
    1969-70 350 2-bbl
    VC1765 B20 1965 396 Impala High Perf 5-7 8 @ 11-13
    1966-67 Corvette Exc. High Perf.
    1966-67 Impala 427 Exc. High Perf.
    1966-68 327 Powerglide Exc. High Perf.
    1969 307 All
    1969-70 396, 427 Camaro, Chevelle High Perf.
    1970 400 2-bbl
    1970 307 MT
    1973 Camaro 350 High Perf.
    VC1801 B21 1971 350 2-bbl 7-9 10 @ 16-18
    1971-72 400, 402
    1971-72 307 AT
    VC1802 B22 1971-72 350 4-bbl 7-9 8 @ 14-16
    Other Part Numbers & Specs:
    VC700 B3 8-10 11.5 @ 19-21
    VC1415 M1 6-8 10 @ 13-15
    VC1420 M2 5-7 11 @ 16-17
    VC1650 B12 8-10 10 @ 15-17
    VC1725 B18 8-10 12 @ 13-16
    VC1740 A5 6-8 12 @ 15-17.5
    VC1755 A7 8-10 12.5 @ 18-20.5
    VC1804 B24 6.5-8.5 10 @ 12-14
    VC1805 M13 6-8 12 @ 14.5-15.5
    VC1807 B25 5-7 8 @ 13-15
    VC1808 B26 5-7 8 @ 11-13
    VC1809 B27 5-7 9 @ 10-12
    VC1812 B30 5-7 12 @ 11.75-14
    The following listing (HEI) is as follows: The first four part number listings are the 4 numbers that are most
    commonly used in a Chevrolet performance application. The “AR12” can is the most versatile and user-friendly
    unit for a good performance street engine. The AR 15 and AR23 are almost identical, with only slight variations in
    their “start-stop” specs. The “AR31” can is the HEI equivalent to the “B28” Hi-Perf can used on the early engines:
    The advance comes in very quick on this unit – too quick for many performance engines. Do not use this very quick
    unit unless you have a cam/engine combination that really needs an advance like this. It can be used as a tuning aid
    for problem engines that do not respond well to other timing combinations, and can be successfully used in
    applications where direct manifold vacuum is applied to the can (see paragraph and discussion on this above)
    After this, the listing is by Echlin part number. All GM HEI vacuum advance units are interchangeable, so you can
    use a Cadillac or GMC Truck unit on your Vette, if that’s what you want to do.
    HEI Distributors:
    P/N ID# Application Starts @ “Hg Max Adv
    (Distr. Degrees @ “Hg.)
    VC1838 AR12 1975 350 Buick 7-9 7 @ 10-12
    VC1843 AR15 1977 305 All Exc. Hi Alt, Exc, Calif. 3-5 7.5 @ 9-11
    1974 400 All w/2-bbl
    1977 305 El Camino
    1976 262 Monza Exc. Calif
    1976 350 Vette Hi Perf, Incl. Calif
    1975 350 Z-28
    1977 305 Buick Skylark
    VC1853 AR23 1976 350 All Calif. 5-7 7.5 @ 11-12.5
    1976 350 Vette Calif., Exc. Hi Perf
    1976 400 All, Exc. Calif
    1975 350 4-bbl
    1974 350 All w/1112528 Distr.
    1978 350/400 Heavy Duty Truck, Exc. Calif, Exc. Hi Alt.
    VC1862 AR31 2-4 8 @ 6-8
    -------------------------------------------------------------------------------------------------------------------------------------------
    VC1703 N/A 1978-79 Vette Special Hi Perf N/A N/A
    1979 305 El Camino Calif.
    1978-79 350 Blazer & Suburban
    1979 Buick 305/350
    VC1825 AR1 1976 454 Caprice, Impala 3-5 9 @ 6-8
    1975 454 Caprice, Chevelle, Monte, Suburban
    VC1826 AR2 5-7 12 @ 10-13
    VC1827 AR3 5-7 9 @ 9-11
    VC1828 AR4 1975-76 350 Buick & Olds 6-9 10 @ 12-14
    1976 350 Pontiac
    VC1831 AR7 6-8 12 @ 14-16
    VC1832 AR8 1975-76 455 Buick Electra 4-6 12 @ 12-14
    VC1833 AS1 1975-76 500 Cadillac Exc. Calif. 4-6 14 @ 15-16
    VC1834 AR9 4-6 13 @ 13-16
    VC1835 AS2 1975-76 350 Olds 5.5-7.5 12 @ 15-17
    VC1836 AR10 1977 305 All Hi Alt, Exc. Calif. 3-5 9 @ 11-13
    1977 350 All exc. Calif.
    1977 350 Vette Exc. Calif, Exc. Hi Perf
    1976 305 All Exc. Calif
    1976 350 All Exc. Vette, Exc. Calif
    1976 350 Vette Exc. Calif., Exc. Hi Perf
    1975 262, 350 All w/2-bbl carb
    1975 350 All 4-bbl w/ 1112880 & 1112888 Distr.
    1977 305 Chev Truck Light Duty
    1975-76 350 El Camino 2-bbl
    VC1837 AR11 1976 305 Blazer, Exc. Calif 6-8 12.5 @ 10.5-13.5
    1976 350/400/455 Pontiac 4-bbl
    VC1839 AR13 4-6 12 @ 11-13
    VC1840 AR14 1975-76 350/400/455 Pontiac Firebird 6-8 10 @ 9-12
    VC1841 AS3 1975-76 500 Cadillac Calif. 5-7 10 @ 13-14
    VC1842 AS4 1976 350 Olds Cutlass 5-7 12 @ 13-15
    VC1844 AR16 3-5 12 @ 13.5-15.5
    VC1845 AS5 1978-79 425 Cadillac w/F.I. 4-6 14 @ 14-16
    1977 425 Cadillac
    VC1846 AR17 1977 301 Buick Skylark 3-6 13 @ 10-13
    1977 301 Pontiac
    VC1847 AS6 1978 403 Motor Home 4-6 12 @ 12-14
    1977-79 350/403 Buick LeSabre Hi Alt, Riviera, Olds
    1977-79 350/403 Pontiac Hi Alt
    VC1848 AR18 4-6 12 @ 9-12
    VC1849 AR19 4-6 12 @ 7-10
    VC1850 AR20 1977 350/400 Pontiac 4-6 10 @ 8-11
    VC1851 AR21 1977-79 350 Buick LeSabre, Century 5-7 12 @ 11-13
    1978-79 350 Pontiac
    VC1852 AR22 77-78 305/350/400 Chev Truck, Heavy Duty7-9 5 @ 12-14
    1975-76 350/400 Chev Truck Heavy Duty
    VC1854 AR24 3-5 13 @ 10-13
    VC1855 AS7 1977-79 260 Olds Cutlass 3-5 15 @ 10-12
    VC1856 AR25 3-6 15 @ 10-14
    VC1857 AR26 3-6 12 @ 13-16
    VC1858 AR27 1978-79 305 All 3-6 9 @ 11-13
    1978 350 Camaro
    1978 305 Chev Truck, M/T, Light Duty
    1978 350 Chev Truck Hi Alt
    1978 305/350 Buick & Olds
    1978-79 305 Pontiac
    VC1859 AR28 1979 350 Vette Exc Hi Perf 3-6 10 @ 9-12
    1978-79 305 w/1103282 Distr., Incl. El Camino A/T
    1979 350 Camaro, Impala, Nova, Malibu, Monte
    1979 350 Suburban
    1979 350 Buick Century
    1978 305/350 Buick & Olds
    1978-79 305 Pontiac Hi Alt.
    VC1860 AR29 3-6 12 @ 10-13
    VC1861 AR30 1978-79 301Buick 3-5 13 @ 11-13
    1979 301 Olds
    1978-79 301 Pontiac
    VC1863 AR32 2-4 10 @ 11-13
    VC1864 AR33 1978 305 Chev Truck, A/T, Light Duty 4.5-6.5 13 @ 11-13
    VC1865 AR34 1973-74 350 Vette Special Hi Perf 3-5 15 @ 8.5-11.5
    VC1866 AS8 1978-79 425 Cadillac w/carb 3-5 14 @ 13-15
    VC1867 AS9 2-4 10 @ 8-10
    VC1868 AR35 1979 305 Chev Truck & El Camino 2-4 10 @ 6-9
    1979 305 Buick & Olds
    1979 305 Pontiac A/T
    VC1869 AS10 2-4 12 @ 8-11
     

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