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Timing for Dummies

Discussion in 'T-Bucket Engine and Driveline Articles' started by PotvinGuy, Dec 23, 2010.

  1. PotvinGuy

    PotvinGuy
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    TIMING FOR DUMMIES

    Ignition timing has to be the least understood, most poorly explained, and undeservedly mystical aspect of engine technology. But it doesn’t have to be this way. Timing is based on scientific facts, not black arts or folklore. This article will 1) give you the scientific facts, and 2) explain why ported vacuum is to be avoided, regardless of what any salesman, manufacturer or grease monkey says. I know, you’re thinking “uh, what’s ported vacuum?” Patience, my pet, all will be revealed.

    First a definition: timing is about deciding when to fire the spark plug to initiate the combustion process. That’s it. Now here is the main scientific fact that drives that decision: peak cylinder pressure should occur about 15 crankshaft degrees after TDC for optimum power, economy, and a bunch of other reasons. Gotta trust me here, many scientists and engineers have done the math, and lots of dyno testing has confirmed this. A disclaimer: we’re discussing street cars, not Top Fuel dragsters or NASCAR weenies.

    Since the combustion process takes time, we have to start it in advance of the 15 degree ATDC point, and so we talk about timing advance, and we define it in crankshaft degrees before TDC. So “20 degrees of advance” means 20 degrees before TDC.

    OK, how do we ensure that this happens for all circumstances of engine speed, engine load, phase of the moon, etc? Well, as it turns out, engine speed and load are conveniently the most important parts of the equation (phase of the moon is only useful if your car is named Christine). The equation:

    Optimum advance = f (engine speed, engine load, some other stuff)

    …Optimum advance is a function of engine speed, engine load, and some other stuff, most of which we can’t easily measure or compensate for, anyway (compression ratio, temperature, fuel composition, air/fuel ratio, combustion chamber design, etc).

    Why engine speed? Engine speed is critical because it determines how much crankshaft rotation we have available to reach our goal at 15 degrees ATDC. The faster the engine turns, the earlier in the rotation we need to fire the plug; this is advancing the timing, and it’s accomplished by the springs and weights inside the distributor, unless you have a fancy new electronic distributor, where it’s all done by magic. Speed advance is often called centrifugal advance, but that is, strictly speaking, incorrect. Centrifugal force is just the mechanism (remember the springs and weights?) used to accomplish speed advance. And since most new cars and many aftermarket distributors no longer use centrifugal advance, the term will probably fade into history.

    Why engine load? Loading the engine increases the amount of air/fuel mixture in the cylinder, and therefore its density as it is compressed. A denser mixture burns faster, so we start it later to hit the 15-degree goal. Conversely, a lighter mixture burns slower and so needs to be started sooner. OK, how do we measure load? Turns out that manifold pressure is proportional to engine load (more pressure pushes more mixture into the cylinder), so we use it to adjust timing for load. Since normally aspirated (no blower) engines always have manifold pressure below ambient air pressure, we commonly talk about manifold vacuum, and hence the term vacuum advance is used. The canister on the side of the distributor does the trick. Now pay close attention: the hose from the distributor canister should always connect to a source of full manifold vacuum. And this is where we confront the misguided notion of ported vacuum, which is a vacuum source derived in the carburetor. Ported vacuum was an attempt by the carmakers to meet government emission specs in the 1960’s. It gives no vacuum, and therefore no advance, at idle, which does indeed reduce emissions, but unfortunately makes your engine idle rough, run hot and have a generally unpleasant disposition. Fast forward 50 years and we have figured out how to control emissions without screwing up our engines, but old ways die hard, and some folks still think ported vacuum is the real deal. Smile, give them an 8-track of country music, and they will wander off.

    Note: drag racers don’t bother with vacuum advance because their only concern is full-throttle full-load operation. But you need it in your street machine. Vacuum advance makes your engine run cooler, get better mileage, make more power and have better throttle response. That’s a win-win-win-win.

    And that is the essence of ignition timing. The timing at any instant is the sum of three parts: 1) initial timing, which is set by twisting the distributor, plus 2) engine speed advance, plus 3) vacuum advance. At idle, your timing will be the sum of the initial timing plus the full amount of vacuum advance (assuming you are using manifold vacuum, as you should). As engine speed increases, the distributor will add (advance) timing. As engine load increases, the distributor will take out (retard) timing.

    Now before I wander off, two obscure facts to impress your friends: as engine speed increases, the air/fuel charge gets better mixed and hotter and burns a little faster, offsetting the need for more advance, so at some point, usually around 3000 RPM, we stop adding speed advance. And for those lucky gearheads with blower motors, manifold pressure can exceed ambient air pressure, and we call this condition BOOST (please stop drooling). We often retard timing with increasing boost, under the exact same logic mentioned in the section on engine loading: more manifold pressure, less advance. Vacuum advance and boost retard are two sides of the same coin. Hakuna matata.

    I wish I could give you magic numbers to make your engine sing, but each case will vary. How to pick the numbers is a whole other article.

    BUT always be alert for knocking, AKA pinging. This usually means you have too much advance. Knocking can be confused with pre-ignition, where the mixture spontaneously ignites due to heat and pressure even before the plug fires. Either condition can turn your pretty motor into a pile of scrap pronto. If you tune your car on a dyno, make sure the operator has a knock sensor and knows how to use it. I didn’t…once.

    I’ll be delighted to hear from you; email me at dr.kerrysmith@gmail.com. I don’t have all the answers, but will pretend I do.
     
  2. AZCOWBO

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    You have brought up an interesting question for me. I am running a Wieand 2 x 4 ram and running ports for vacuum. Front carb port to MSD all-in-one distributor with MSD Blaster 2 coil. Rear carb port is connected to the tranny. No ports are a available on the intake. The carbs are new Demon Jr. 525s which I am changing to Holley 390s this week, because they are too much for my cam and having a stall issue upon acceleration, which also might be enhanced because of vacuum advance problems.
     
  3. PotvinGuy

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    The distributor needs manifold vacuum for a good idle. Some folks get away with ported carb vacuum by cranking the initial advance way up, like 30 degrees, but that is hard on the starter and a dumb thing to do. The tranny can use either manifold or ported carb vacuum, since the ported vacuum is equal to manifold vacuum once you are moving. But I wouldn't do it.

    Lemme know how the 390s work out. And I'd like to know your timing numbers.
     
  4. BOB S

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    Setting your initial timing perfectly without starting the engine and using a timing light
    1. bring your engine up to top dead center first
    2. now move the harmonic balancer to whatever initial timing you would like
    3. Remove the number one sparkplug attach the wire and lay it up on the intake manifold
    4. turn the ignition switch to on (not start ) just on
    5. retard the distributor then slowly advance the distributor when the commutator connects with the rotor it will spark the plug snug down the distributed
    6. put number one sparkplug back into the head and hook up the plug wire
    7. start the engine it is a foolproof way to start the engine first time around

    bob
     
  5. old round fart

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    Why do y’all u need to remove plug?
     
  6. Gerry

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    Loading the engine increases the amount of air/fuel mixture in the cylinder,

    amount/volume? Can you explain this a little more please? I can't quite figure it out for myself. Really good info and well written:thumbsup:
    G
     
  7. PotvinGuy

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    When the load is increased (accelerating, going uphill, etc), the throttle butterflies are open further, causing the pressure in the manifold to increase. This increased pressure forces more of the air/fuel mix into the cylinders.
     
  8. fletcherson

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    Just because the port is on the carb doesn’t mean it’s “ported vacuum” it can be manifold vacuum. Ported vacuum is low at idle and increases with rpm, manifold vacuum is high at idle and drops during acceleration. With the engine idling, pull the vacuum line, do you have vacuum? Better yet, plug a gauge onto it. If it’s manifold vacuum, when you pull the line, the rpm should change. A because the a/f mixture changed and B the vacuum advance was eliminated. If it’s ported vacuum, little change will be noticed at idle. Basically, if the vacuum on the carb port increases with acceleration, it’s ported, if it’s higher at idle and drops off during acceleration, it’s manifold vacuum. Most carbs have a manifold vacuum port on them for vacuum advance. The manifold vacuum port is usually on the base plate of the carb and ported is up on the body. Eddlebrock afb carbs have ported and manifold vacuum ports on the front and are labeled.
     
  9. PotvinGuy

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    Yes, the ported vacuum source on the carb is just above the throttle blades, so it isn't active until you go off-idle. The full, or manifold, vacuum source on the carb is below the throttle blades and is always active. Of course you can get manifold vacuum at other places on the manifold. I drilled and tapped my manifold in 2 places to accommodate all the lines I have.

    I wish the carb makers would eliminate the ported vacuum, as it has no purpose and just confuses people. I'd put that on the SEMA bucket list, but they never look at it...
     
  10. Gerry

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    OK. Did wonder but as you say "butterflies open more". I didn't consider that, just that load somehow did it on its own, if you see what I mean.
     
  11. PotvinGuy

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    It can be a confusing business. Load, manifold pressure and throttle opening are related; change load or throttle and the manifold pressure changes. And to add to the confusion we usually talk about manifold vacuum, since the manifold pressure is always below ambient air pressure (unless you have a blower, where the manifold pressure can rise above ambient and we like to call that condition boost). Here's some info to make your head hurt: Manifold vacuum - Wikipedia
     
  12. Gerry

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    Think I'm OK with it all.
    Vac log I made for the kinsler as they only have one feed for the MAP sensor and i need one for the trans as well. Tried to make it look like a little V8.... I love to fiddle.:)

    vac logg (5) [1024x768] (2015_11_13 11_07_31 UTC).JPG
     
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  13. Neshkoro

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    Nice work!
     
  14. fletcherson

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    I have never found a use for ported vacuum on a non stock smog era engine. I’ve seen it utilized on choke pull offs, egr valves, and various smog related gizmos involving vacuum switches, amplifiers, etc... there was a short period where a few vehicles had vacuum advance canisters with two ports on them. I think one was timed, but that’s fuzzy to me, early smog era. I find one simple vacuum cap simplifies things immensely. Vacuum is basically a non issue on recent production vehicles. Everything is electronic... I’m not sure it’s all good, many failures of components causing major failures, transmissions, hvac, ie blend doors, etc... I’m ok with electronics, studied it, have a degree, but some simple mechanical things function just fine, why complicate them? Answer: control. IMO
     
  15. fletcherson

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    Flat head? Lol
     
  16. Gerry

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    Yep. Change a head gasket in 15 minutes. Bet you remember that as well as I do. Did a few on the side of the road, BITD.
    G
     
  17. fletcherson

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    Omg, the things that I’ve done on the side of the road or in a parking lot... on my way to or from work or so I could get there in the morning... the good days? At least the cars were serviceable on the side of the road back then...
     
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  18. mountainman

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    :):eek::rolleyes::sneaky:.............Oh, never mind! You were talking about working on cars but MY memory took a different direction.:speechless:
     
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  19. Neshkoro

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    Fortunately, I've spent more time trying to do that than actually fixing cars on the side of the road. But I've done plenty of car fixing, too.
    Mostly in my younger years with some beater I was driving. Actually, both were in my younger years!
     
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  20. fletcherson

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    Both are more hassle than they are worth now...
     






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