Determining top dead center

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by: 41willys, Alittle1, Cobalt327, Crosley, Jon, Pepi, Techinspector1
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Contents

[edit] Before you begin

One of the first things that you will need is a degree wheel, this will tell you EXACTLY where you are at when you turn the engine over. You can take this picture of the degree wheel and have it blown up to a reasonable working size by your local photo copy center. A couple of ways to use it is to have it laminated or glue it to a piece of sheet metal or aluminum.

Degreewheel1.jpg

Degree wheel download

Another option is to make a timing tape as described below.

[edit] Verifying top dead center at the damper line/pointer with the engine assembled

[edit] The damper

First off, the large round hub attached to the front of the crankshaft is called a harmonic damper by some and a harmonic balancer or simply "balancer" or "damper" by others.

If it is bolted to the crankshaft of an engine that is internally balanced, then it serves only the function of being a harmonic damper, dampening vibrations set up in the crankshaft as a result of the rod journal springing back and forth from the forces applied to it during operation.

If it is bolted to the crankshaft of an engine that is externally balanced, then it serves the dual purpose of damper and balancer.

Unless the engine has been unaltered and is "as built" by the manufacturer, you have no idea if the TDC notch on the ring matches the timing pointer attached to the block or front cover, even if the outer ring has not slipped at all.

There are a multitude of different dampers and timing pointer locations on a small block Chevy. Refer to Timing Tabs and Damper TDC Lines SBC for more on them. Other engines can refer to a service manual.

The whole reason for doing this operation in the first place is to be able to time the engine with a timing light and know absolutely that the timing is correct. The elastomeric material that connects the outer inertia ring of the harmonic damper/balancer to the inner hub of the damper/balancer which presses onto the snout of the crankshaft begins to break down over time due to ozone in the atmosphere and oil and fuel or other foreign materials which may find their way onto the material. When this happens, the outer ring may slip circumferentially in relation to the inner hub, rendering any attempt to time the engine with a timing light futile. Even though this operation you are about to do will bring the timing marks back to correct for the time being, there is no guarantee that the ring will not slip further after a while. You also have no idea if the timing pointer matched the inertia ring in the first place if the engine has been disassembled and reassembled by someone else in its lifetime. If you want to bulletproof the operation, then start with a new or rebuilt damper and use the correct timing pointer for that damper.

[edit] Checking the outer damper ring for movement

Damper line.jpg
Draw a sharpie line as shown in the image below. When the timing light is pointed at the TDC line, the sharpie line will also be seen. By revving the engine and running it at different speeds, if the outer ring is loose, the line on the outer ring will be seen to move independently of the line on the inner hub.


[edit] Rebuilt dampers

One noted place to buy a rebuilt damper/balancer is Damper Doctor. They disassemble stock, OEM production dampers, clock the hub to the inertia ring and reassemble the unit with new elastomeric material pressed together under tremendous hydraulic pressure. An 8" damper for a 350 Chevy can be had for a mere $32.95 (ca. 2012), deal of the century!!!!

The option is a used damper/balancer that may be clocked worse than the one you have or an aftermarket damper/balancer that will cost more money and may not have been correctly machined on the inner hub diameter. Some of these offshore (Chinese) dampers being sold are bored either oversize or undersize for the production crank snout diameter. The damper/balancer hub MUST BE A SNUG PRESS-FIT on the crank in order to properly transfer harmonics from the crankshaft to the damper/balancer hub and on to the inertia ring, where harmonics are dissipated.

On a street engine or a drag race engine down to 11.00 E.T. in the quarter mile, an OEM-type damper/balancer may be used legally. At 10.99 E.T. and quicker, an aftermarket SFI-18.1 damper/balancer is required. Blower motors normally do not use a balancer/damper, but instead, use an aluminum toothed hub on the crank snout to drive the blower. On these blown motors, the large Gilmer drive belt functions as a dampener to dissipate crankshaft harmonics.

[edit] Damper suppliers

[edit] Damper fasteners

[edit] Piston stop

If the engine is a short block on the stand, you can determine TDC with a simple homemade piston stop made from a strap of metal bolted across two head bolt holes, with the strap drilled and tapped for an adjustable bolt/nut assembly. A dial indicator can also be used on a fixture that bridges the bore or on a magnetic base. This would be an ideal time to note the piston-to-deck clearance for use in computing the static compression ratio and quench distance.

If the engine has the heads on, use a spark plug-type piston stop tool. If there is a timing tab present, use it to mark the position with. If no tab, use a length of stiff wire that's attached to the engine to use to show the positions. This may be made easier by using a degree wheel or a timing tape on the outer ring of the damper.

  1. Set the plug hole-mounted piston stop to contact the piston close to TDC
  2. Rotate engine until the stop just contacts the piston- mark the location
  3. Then rotate in the opposite direction until the piston is stopped
  4. Half way between the two marks is approximately TDC
Strap-type and spark plug-type piston stop tools


[edit] A word on using the damper retaining bolt to turn the engine over

If the damper/balancer retaining bolt is used to turn the crank over, the bolt can loosen if turned CCW. If this happens the bolt will need to be retorqued (60 ft/lbs on a SBC, 85 ft/lbs on a BBC).

Another risk is in stripping the threads or breaking the bolt off if you use the damper bolt to turn the engine over. If you feel resistance, or if the bolt tightens more as you attempt to turn the crankshaft- STOP! You will need to use a remote starter switch, or turn the damper by hand (plugs out), or turn the flexplate/flywheel from beneath the vehicle.

The safest/easiest way to turn the crank is to use a tool that allows a large ratchet or breaker bar to be used.


[edit] Getting started

Attach the degree wheel or refer to How To Make A Timing Tape or buy a timing tape that matches the diameter of the damper. Also buy or make a piston stop tool. Using a tool that has a hole drilled through the center of the probe will allow pressure or vacuum to escape through the hole from the piston moving up and down in the bore with the rocker arms disabled (valves on their seats). This makes turning the engine over w/the piston stop installed easier.

You'll need access to the harmonic damper outer ring. If necessary, remove the fan, belts, shroud and water pump pulley. You may have to remove the water pump to gain full access to the damper ring and do the measuring needed for this operation.

If this is necessary (removing the water pump), this would be a good time to replace the pump if there is any question as to its condition. Pumps and gaskets are not that pricey. Whether or not you replace it, remove the pump and check for impeller slip on the pump driveshaft by holding the impeller securely with one hand and the drive hub of the pump with the other hand and twist in opposite directions. If there is any movement, replace the water pump before it fails completely. Even if the pump is good, you may want to replace it with a high-flow unit. FlowKooler, Stewart and Edelbrock are names that come to mind, there may be others who produce a quality high-flow pump. A good high-flow pump is nearly MANDATORY on a 400 SBC or any other engine which uses siamesed cylinders.

[edit] Water pump suppliers

[edit] The procedure

Thoroughly clean the harmonic damper and timing pointer.

[edit] Procedure done with valves closed

Remove the valve cover for #1 cylinder and back off the rocker arms for both valves for that cylinder. COUNTING THE NUMBER OF TURNS YOU LOOSEN THE ROCKER NUTS WILL MAKE IT A SNAP TO GET NEAR TO THE CORRECT LASH WHEN YOU TIGHTEN THEM BACK AFTER THIS OPERATION.

Disabling the valves by backing off the rocker arms will prevent interference between the timing tool probe and the valves while turning the crank. Turning the engine over by hand will be easier if you remove ALL the spark plugs.

With a socket on the damper retaining bolt or using the tool described and a long socket handle, rotate the crankshaft clockwise while your buddy uses a strong flashlight to look into the #1 spark plug hole.

Bring the piston up to top dead center, then continue rotating the crank very slowly so that the piston comes down in the bore slightly. You want the piston slightly past top dead center down in the bore, but not so far down in the bore that the probe of the top dead center tool will not contact the piston crown.

At this point, we are not concerned with where the crank/piston is in the total 720 cycle of operation. In other words, we have the valves disabled, so it doesn't matter whether you are on the exhaust cycle or the compression cycle as the piston comes up to TDC. All we are working with at this point is the 360 degrees of the damper, regardless of the cam and valves.

[edit] Procedure without removing valve cover

If checking an assembled/long block engine, you will install a piston stop tool into #1 spark plug hole (domestic V8 firing orders). Screw the top dead center housing into the spark plug hole and snug it down. Insert the probe of the tool into the tool housing and screw it in until you feel resistance of the tool probe against the piston crown. Snug it down slightly against the piston crown and start from there.

Attach a degree wheel or affix a 4 to 5 inch length of masking tape to the damper ring with the left end of the masking tape about 1 inch to the left of the timing pointer on the timing cover, positioning the masking tape toward the block-side edge of the damper ring, leaving room at the front edge of the ring to affix your timing tape later.

With a ball-point pen, make a thin mark front to rear on the masking tape right at the point of the timing pointer. Rotate the crankshaft clockwise (here's where you'll be glad you purchased a piston stop tool with a hole drilled through the center of the tool probe) until the piston comes up against the piston stop tool probe again. Easy does it here, you don't want to bring the piston up against the tool so hard that it will dent the piston crown.

Make another mark on the masking tape with your ball-point pen to coincide with the tip of the timing pointer. With your 6 inch caliper, measure the distance between the two marks you made with the pen. Divide this distance in half.

Move the jaws of the caliper to show this half distance. With one caliper jaw on one of the marks you made with the pen, the other jaw of the caliper will be at true top dead center. Make another mark on the masking tape at the caliper jaw to show this center (or middle) position on the masking tape and affix your degree tape onto the ring, aligning TDC on the timing tape with the center mark you made on the masking tape. If the timing tab and damper line agree with this, they're accurate. If not, the tab can be readjusted or another line made on the damper to coincide with true TDC.

Remove the piston stop tool probe and housing. If the rocker arms were loosened previously, tighten the rocker arms back the same number of turns you used when you loosened them. With your buddy holding his thumb over the #1 spark plug hole, rotate the crankshaft until he feels compression against his thumb.

Continue to rotate the crank slowly until the timing pointer aligns with about 10 degrees before top dead center on the timing tape. STOP. DO NOT ROTATE THE CRANKSHAFT ANY MORE.

Replace the spark plugs and wires. Remove the masking tape, reinstall water pump, pulley, fan, shroud and belts if they were removed previously. Replace any coolant/water you may have lost in the operation.

Remove the cap from the distributor and align the rotor with the cap terminal that coincides with #1 plug by rotating the distributor housing. Looking down on the motor from a birds-eye view, the rotor needs to be pointing toward 5:30 on an analog clock face (pointed toward #1 cylinder). The gears that drive the distributor off the camshaft are helical, so you need to drop the distributor into place so that it can spiral down and be in the correct position when the distributor housing is seated on the intake manifold. On a Chevy, clockwise rotation of the housing retards the ignition timing, counter-clockwise advances it.

On a Chevy V8, because of the pitch of the helical gears on the cam and distributor, the rotor will turn about 2 inches CW as the distributor is lowered into position. That means the rotor needs to start out about 2 inches CCW from where it needs to end up with the distributor fully seated.

From #1 plug position on the cap, the wires will be attached clockwise around the cap following the firing order (Chevy V8: 1-8-4-3-6-5-7-2). As was stated in the last paragraph, the rotor should be pointed to the 5:30 o'clock position to fire #1 plug. This will give maximum clearance between the vacuum canister and the runners of the intake manifold and firewall so that you have room to twist the distributor to fine tune the ignition timing when you get the engine running.

If the valve cover was removed earlier, set the valve cover on the head to keep oil from flying everywhere and start the engine and allow it to warm up. Do the final adjustment on the #1 cylinder rockers.

  • Chevy: back off the rocker until it audibly clatters, then tighten it down 1/2 to 3/4 turn past where it quiets down. If you want to adjust the valves on the rest of that side of the block, now would be a good time. Replace the valve cover.

[edit] Timing the engine

Hook up your timing light to #1 plug wire.

Remove the vacuum advance rubber hose from the vacuum canister at the distributor and plug the end of the hose with a golf tee or other suitable plug. Adjust initial timing at the crank to what you want by rotating the distributor housing. Some use the factory setting while others prefer to set it a little more advanced for good throttle response. If using a little more initial advance at the crank, make certain you don't have so much mechanical advance in the distributor that you exceed the total timing (initial and centrifugal) specified for the engine. Most small block Chevy's will run best with around 30-34 degrees (initial and centrifugal) with fast-burn heads and 35-36 degrees with conventional heads.

If using a camshaft with more duration, you may want to increase the ignition timing lead at the crank and limit the centrifugal advance in the distributor to achieve your total ignition timing. Some distributors, like those sold by MSD, use different bushings to control the amount of mechanical advance. The rate of mechanical advance is tailored by changing the springs and/or centrifugal weights. Usually just spring changes are all that is needed. If working on a GM HEI, the original weights are almost always a better choice than the weights sold in the various advance curve kits available from Mr. Gasket, Crane, Summit, Moroso, etc.

If you are using a radical cam and/or a converter that allows the engine RPM to come up past where you would normally limit centrifugal advance (about 2800 rpm), you may want to modify the distributor so that centrifugal advance is locked out and put your total amount of ignition advance in at the crank. Of course, the engine will not want to crank against this much ignition lead, so you will want to install a momentary switch in the wire going to the "+" terminal of the coil to disable the coil while you crank the engine. Once the engine is spinning, release the switch and the engine will fire normally.

[edit] Resources

Crankshaft Coalition Wiki articles:
  • FACT AND FICTION CONCERNING VACUUM ADVANCE....
  • The following two articles review the basics of distributor tuning quite well and have worked for years and years and are based on sound *engineering principals. I thought it would be helpful for some to review these prior to hacking up their distributors. Hacking up your *distributor to compensate for a poorly tuned, misapplied or defective carburetor is not very sound engineering, for a street application or *otherwise.
  • Here's an interesting article on vacuum advance written by a GM engineer:
  • As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those *systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and *as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further *from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to *understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and *performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's *also informative.
  • TIMING AND VACUUM ADVANCE 101
  • 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 key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes *it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing *the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By *today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel *economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open *throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark *timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.
  • Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold *vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid *systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported *spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold *vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative *at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees *or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting *the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the *A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust *port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion *efficiency went down the drain, and fuel economy went down with it.
  • If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 *degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at *high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean *mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-*converter crude emissions strategy, and nothing more.
  • What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? *They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - *they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum *advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without *manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they *don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't *understand the principles and operation of vacuum advance either, so they're not alone.
  • Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have *relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, *so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in *response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified *engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, *preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) *provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in *idle timing even with a stout cam.
  • For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full *manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want *to sell "race car" parts.
  • 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.
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