timing and vacuum advance 101

8/7/2019 Timing and Vacuum Advance 101

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TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such asat idle and steady highway cruise, take longer to burn than rich mixtures;idle in particular, as idle mixture is affected by exhaust gas dilution. Thisrequires that lean mixtures have "the fire lit" earlier in the compressioncycle (spark timing advanced), allowing more burn time so that peakcylinder pressure is reached just after TDC for peak efficiency and reducedexhaust gas temperature (wasted combustion energy). Rich mixtures, onthe other hand, burn faster than lean mixtures, so they need to have "thefire lit" later in the compression cycle (spark timing retarded slightly) somaximum cylinder pressure is still achieved at the same point after TDCas with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timingpurely as a function of engine rpm (irrespective of engine load oroperating conditions), with the amount of advance and the rate at which it

comes in determined by the weights and springs on top of the autocammechanism. The amount of advance added by the distributor, combinedwith initial static timing, is "total timing" (i.e., the 34-36 degrees at highrpm that most SBC's like). Vacuum advance has absolutely nothing to dowith total timing or performance, as when the throttle is opened, manifoldvacuum drops essentially to zero, and the vacuum advance drops outentirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire thatlean, diluted mixture earlier in order to develop maximum cylinder

pressure at the proper point, so the vacuum advance can (connected tomanifold vacuum, not "ported" vacuum - more on that aberration later) isactivated by the high manifold vacuum, and adds about 15 degrees ofspark advance, on top of the initial static timing setting (i.e., if your statictiming is at 10 degrees, at idle it's actually around 25 degrees with thevacuum advance connected). The same thing occurs at steady-statehighway cruise; the mixture is lean, takes longer to burn, the load on theengine is low, the manifold vacuum is high, so the vacuum advance isagain deployed, and if you had a timing light set up so you could see thebalancer as you were going down the highway, you'd see about 50degrees advance (10 degrees initial, 20-25 degrees from the centrifugaladvance, and 15 degrees from the vacuum advance) at steady-statecruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the acceleratorpump, power valve, etc.), burns faster, doesn't need the additional sparkadvance, and when the throttle plates open, manifold vacuum drops, andthe vacuum advance can returns to zero, retarding the spark timing backto what is provided by the initial static timing plus the centrifugal advance

provided by the distributor at that engine rpm; the vacuum advancedoesn't come back into play until you back off the gas and manifoldvacuum increases again as you return to steady-state cruise, when themixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam

via weights and springs) is purely rpm-sensitive; nothing changes itexcept changes in rpm. Vacuum advance, on the other hand, responds to


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engine load and rapidly-changing operating conditions, providing thecorrect degree of spark advance at any point in time based on engineload, 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 ofoptimizing engine efficiency, throttle response, fuel economy, and idlecooling, with absolutely ZERO effect on wide-open throttle performance,as vacuum advance is inoperative under wide-open throttle conditions. Inmodern cars with computerized engine controllers, all those sensors andthe controller change both mixture and spark timing 50 to 100 times persecond, 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 fullmanifold vacuum, along came emissions requirements, yearsbefore catalytic converter technology had been developed, and allmanner of crude band-aid systems were developed to try andreduce hydrocarbons and oxides of nitrogen in the exhaust

stream. One of these band-aids was "ported spark", which movedthe vacuum pickup orifice in the carburetor venturi from belowthe throttle plate (where it was exposed to full manifold vacuumat idle) to above the throttle plate, where it saw no manifoldvacuum at all at idle. This meant the vacuum advance wasinoperative at idle (retarding spark timing from its optimumvalue), and these applications also had VERY low initial static timing(usually 4 degrees or less, and some actually were set at 2 degrees AFTERTDC). This was done in order to increase exhaust gas temperature (due to

"lighting the fire late&quot to improve the effectiveness of the"afterburning" of hydrocarbons by the air injected into the exhaustmanifolds by the A.I.R. system; as a result, these engines ran like crap,and an enormous amount of wasted heat energy was transferred throughthe exhaust port walls into the coolant, causing them to run hot at idle -cylinder pressure fell off, engine temperatures went up, combustionefficiency 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 ofadvance, they had up to 34 degrees of advance in the distributor, in orderto get back to the 34-36 degrees "total timing" at high rpm wide-openthrottle to get some of the performance back. The vacuum advance stillworked at steady-state highway cruise (lean mixture = low emissions), but

it was inoperative at idle, which caused all manner of problems - "portedvacuum" 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 becausethey'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 rpmall the time, so they don't need a system (vacuum advance) to deal withthe 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


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advance is, how it works, and what it's for - there are lots of long-timeexperienced "mechanics" who don't understand the principles andoperation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines andmodified engines, especially if you have a lot of cam and have relativelylow manifold vacuum at idle. Most stock vacuum advance cans arentfully-deployed until they see about 15 Hg. Manifold vacuum, so thosecans dont work very well on a modified engine; with less than 15 Hg. ata rough idle, the stock can will dither in and out in response to therapidly-changing manifold vacuum, constantly varying the amount ofvacuum advance, which creates an unstable idle. Modified engines withmore cam that generate less than 15 Hg. of vacuum at idle need avacuum advance can thats fully-deployed at least 1, preferably 2 ofvacuum less than idle vacuum level so idle advance is solid and stable;the Echlin #VC-1810 advance can (about $10 at NAPA) provides the sameamount 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 stoutcam.

For peak engine performance, driveability, idle cooling and efficiency in astreet-driven car, you need vacuum advance, connected to full manifoldvacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it theydont understand it, they're on commission, and they want to sell "racecar" parts


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Every engine is a bit different, but imo, even a bone stocker will like muchmore than 10-12* initial, esp if ported vac source is used for the vacadvance [which is what I recommend]. I would want at least 15* at idle,and it will probably like more, even 20*. The problem is, most caddyadvance curves will have too much mechanical advance to allow muchinitial without over-advancing at higher rpm.

See how much mechanical advance is in it first. Make sure the weights

move freely. Put light spring [or no springs] on and set total timing [lessvac] to 36* or so, then see what it is at idle with normal springs. This willtell you how much is in the dist. Rev it up to 4000 or so, to be sure you seeall it'll give.

With total mechanical set to 36* ish, if the initial is too low, you will haveto swap weights/cam or modify wieghts to allow for less advance, in orderto have initial where you want it.

Work on the mechanical curve, then add the vac advance. With a propermechanical curve, it will probably want only 5-8* more from the can. Youwill probably have to fashion a stop or limiter of some sort, as most cans

have way too much travel. Without a limiter, it will over advance at cruise.As far as springs, some guys like it to come in all the way right above idle.I'd rather see a little slower curve, full in at 2500, maybe 2800 or so.

Dave


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reading plugs

Lets analyze this plug, sorry the numbered points aren't clear but it's 1-4you can figure it out.

#1 Is a timing indicator, you'll see a definite color change on the groundstrap, it doesn't show well here but you can still see it right about at thearrow. Too much timing and the color change will be very close to thethreaded body of the plug, too little and it'll be closer to the tip. Ideally wewant it right in the apex or center of the 90 bend on the ground strap. Thisplug shows too much timing for the combustion chamber efficiency oroctane level.

#4 arrow shows another indicator of timing, you'll usually see a brown ring

right at the tip of the porcelain area it should be a sharp and defined ringabout .020 wide. Wider indicates not enough timing and any smaller , or


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only 1/2 way around or nonexistent as in this image is the secondindication of too much timing in the motor.

#2 The tip of the ground strap is loaded with OIL deposits, fuel depositsare usually flat black in color and almost like a fine powdery deposit, thismotor is leaking oil into the combustion chamber, bad valve guides,leaking valve covers allowing oil to seep through the plug threads,whatever it needs to be fixed.

#3 The threaded portion of the plug gives you the heat range, look at thethreads you'll see that a few toward the tip are a dull burnt looking colorthe rest are black and shiny. You want about 2 threads showing the heaton the end of the plug and the rest of the threads to be shiny, this plug isimpossible to read because of the oil mess. If you using a longer reachplug than this one 2.5 to 3 threads is optimum.

To increase the number of burnt threads increase the heat range of theplug, if you have 4-5-6 threads burnt you need to get a colder plug.

Looking at the color of the porcelain I'd give this carb a passing grade atthe mid range and not to bad on the idle circuits although that dam oilleak makes it tough to really get a good read. I'll get some better shots forlesson 2.

I'm going to keep working on this page and try and get some real goodshots of various plugs, we'll study each one and find the good and bad telltales of each.

I'll also start pulling some plugs out of my race car at various settings andget pictures so we can see how jetting, timing and heat range affect the

plugs in the same engine under the same conditions.

The plug is showing me by the deposits on the tip of the electrode andalso the deposits right on the edge of the threaded body.


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that it's slightly fat at idle.

The white porcelain is showing a lean condition at WOT, it's not too faradvanced as the total timing mark or color change is right in the apex ofthe ground strap curve. The Idle timing is shown by the triangular hazingup on the flat of the ground strap and without even looking at thedistributor specs I can tell you that the timing on this SB Mopar is about18-20 initial and 34 total.

I would need a better picture of the threads to determine the heat range.

Changes:I would try and lean it out just a touch at idle and up the jets by 2 points tofatten up the WOT circuit.

That slightly lighter color at the tip of the ground strap indicates too muchgap, nothing serious but next time you change plugs I'd go to about a .036

gap from the current .040. Too much resistance caused by too wide of aplug gap can cause excessive heat on the tip which will shorten the life ofthe plug and really give you no benefits. I believe excessive plug gaps arenot required on most Muscle and bracket cars, once you get into real bigcompression and major power you would open up the gap and replaceplugs 2-3-4 times a year.


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Let's look at Image 1.......Note the change of color on the ground strap and it's position, this showsa timing issue. Unfortunately the reader who sent this image in didn't saywhat engine he was working on but by the condition of the plug I wouldguess a Small Block Chevy, 3 years old with 38* total timing and anEdelbrock Carb......I'm sure he will respond :-)

In the second image I used a green arrow to show the area where we wantthe timing mark to be on this engine and of course you can see that the

motor has too much timing.


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Image 3 shows only one thread heated on the plug and rest are oil soakedand wet this normally indicates that the plug is too cold, the oil indicates avalve cover leak and a New set of Cometic Aramid fiber gaskets wouldcure it.

The color on the plug indicates a decent idle A/F ratio and a good ignitionsystem probably an MSD or some other type of aftermarket ignition. Thehot spark is keeping the porcelain clean on the top area anyhow. Now ifwe look down into the bottom of the plug as seen in the above imagewhere the porcelain reaches below the steel body we see black depositsindicating a rich condition at mid range to WOT. I would guess this engineis a little lazy in mid range and could get better mileage.

Changes:Back off the total timing but keep whatever the initial is as it appearspretty good, I would shorten the total by 4* and do another check.Fix the oil leakGo up one heat range and after about 50 miles check to see if you have 2-3 threads heated up and discolored for correct heat range. Heating up theplug will help reduce the carbon build up and further tuning with the A/Fratios can be done

Before you can really tell what to do with the carb the ignition must be

right as well as the correct heat range on the plug to get true readings.

...and check your Idler arm it appears to have some slack in it and yourleft rear tire needs 3# of air.

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This is what you don't want...This is Bob Riggles Hurst Hemi Under Glass motor after a pass...a little toomuch timing and or not enough fuel. Note the spark plugs completelymelted. The piston detonated itself into fragments and beat the

combustion chamber to a pulp...his day was done...


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The hole in the piston in the second picture is not a window to inspect thewrist pin, although it seems to work well....


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wheres the ground strap!?!?!?---------------------------------------------------------

WOT Race Tuning

Installing a thermo coupler sensor (Available on our Stewart Warner Page)in the combustion chamber is about the only real way to know what ishappening inside your engine the alternative is to learn how to accuratelyread your spark plugs. Ignition timing and A/F ratio's are critical toachieving optimum performance with going to far and falling off the edgeinto detonation while still not being so rich as to not reach maximumpower potential.

The best way to tune for WOT is by doing a shut down at the end of thetrack and doing a proper WOT reading on the plugs, use caution, coast to


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a safe spot away from return lane traffic. Or tow the car back to the pitswhere you can read all 8 plugs for combustion chamber efficiency.

Accurate measurement of the combustion chamber temperature is theonly way to achieve the perfect tune up. A combination of a O2 Sensorwith a data logging feature combined with accurate and rapid respondingEGT Gauges will achieve the quickest and most accurate track tuning.Dyno's are OK for a baseline setting, NO DYNO will ever replace a goodtrack tuner with the right tools.

I look at it this way....you spend $400-$600 at the dyno shop all conditionsare determined by the dyno operator and climatic conditions of theambient air of the particular day of the test. You now take your engine andbolt it into a car with a transmission, engine compartment heat, differentfuel, fuel pressure variances, G forces, tire slip, wheel stands, elevation

etc. All your money did was set a baseline which can easily be done inyour own shop with a screwdriver....$1.99.

Spend that money on a WEGO 5 wire, wide band, data logging O2 sensor,it'll last for years and give you accurate combustion chamber efficiencyfrom the burn-out box to the freeway. Team that up with a Stewart WarnerEGT (Exhaust Gas temp gauge) and it won't matter if it's 105* or 55*outside, 300' or 5,000' elevation you'll be able to make decisive changeswith no guess work or risk to your investment.

In My Opinion ...Dyno's are a waste of money and fall into the category of"worthless information".

If your interested in a WEGO or EGT system call I'd be happy to discusshow they work with you.

The Air/Fuel MapLook at the porcelain and divide it into 3 areas, top, middle and bottom in

about 1/3rds.

Top Area: This area is your idle circuit and for the racer real has very littlemeaning. However if your car is a street driven vehicle then we need toattempt to get this portion to a dark gray color by tuning the AFR, Idle AirBleeds and the four corner Idle adjustments until we get a clean crisp idleand total combustion of the fuel at idle. Keep in mind that the idle circuitsare totally isolated from the rest of the carburetor and can be tuned withvery minimal affect on the WOT position A/F Ratio.

Center Area: This is the area that will show the combustion chambercondition at part throttle or cruise with the power valve and secondary

venturi's closed. Usually about 30-50 MPH depending on the gear andweight of the car.


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Bottom Area: This is where the racer needs to concentrate all his efforts,using a 5x or 10x scope you need to look right down to the bottom of theporcelain where it meets the metal housing of the plug. This is the areathat will leave the color that you need to study and tune to. The other2/3rds of the porcelain will be burned clean by the high heat generated byWOT and high RPM, this is not the time to read for cruise or Idlecharacteristics.

We're looking for a colored ring right at the base of that porcelain which Irefer to as a Fuel Ring. The color will vary depending on the type of fuelyour burning, Unleaded Pump gas will leave very little color because of allthe additives such as injector cleaners and methanol. Most of us run a 108or 110 in our bracket cars and these fuels will leave a tan color while theC17 and higher octane fuels will leave an almost gray fuel ring. If youstudy the ring you will see that it starts to show color just below the baseof the ground strap, this is caused by the shielding affect of the strap

leaving that portion cooler thus showing the most color. The ring willdevelop from each side and form a full circle of correct color as you getcloser and closer to the correct tune up. If you look down into the bottomof the porcelain and you detect a thin oily deposit you are seeing oil fromthe combustion chamber caused by either worn or improperly seatedrings, you may also detect this with a new engine that hasn't fully seatedthe rings yet, just don't confuse this with the "Fuel Ring".

Plug Characteristics

Shiny or Glazed Porcelain: This occurs for two reasons, excessive idling orrich idle condition leaving heavy soot deposits. These deposits will burn

and create excessive porcelain temps causing the glass to melt usuallyresulting in a down track popping through the exhaust..

You will usually see splotchy deposits on the plugs after the run, eithergreen or yellow in color and of course the tell tale shiny glaze. Theporcelain should have a chalky or dull finish.

Base of threads sooty: This is usually a indication of the plug not beingtight enough allowing it allow air to pulled in to the chamber and fuelbeing pushed out, change the plug, tighten to spec. This area should havea nice dark to medium gray color without heavy deposits.

Black or brown specs: This is an indication of detonation usually caused bytoo hot of a plug, drop one or two heat ranges and it should go away. Theplug gets so hot that it will start to detonate the fuel before the ignitionfires, this cause a double flame front and reduces the efficiency of thecombustion process as these two flame fronts battle each other in thecombustion chamber. The black spots are the result of the fuel depositsbeing burned onto the porcelain by the double flame front. Once you cooldown the plug and remove the secondary ignition source you can re-setyour timing to produce the correct combustion chamber temp for optimumpower.

I've heard 100's of so called tuners and engine builders tell people thatthis is normal and nothing to worry about......WRONG. Do not


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underestimate the damage that this condition can do to your engine, dropthe plug heat range.

Shiny little diamond like deposits: It's too late you've already started toblow the pistons out of it. Start over from scratch, fatten the carb, back-upthe timing and cool down the plugs. Now start a proper tuning procedureand hopefully you caught it before too much damage was done.

No color mark on ground strap: If your showing good color on theporcelain but the ground strap fails to show an indicator then your plug istoo cold, go up 1 heat range.

If the ground strap is showing a good color line but the porcelain isshowing no color at the base then your plug is too hot, drop down a range.I will use maybe 3 different ranges on one bank, make a pass and fromthose three different plugs determine which one is correct for the engine.

Variable heat readings: On a small block motor most have adjoiningexhaust ports, these will always be your hottest cylinders and in somecases where there has been excessive port work and thinning of the metalbetween the ports it may be necessary to run a plug one step cooler inthose cylinders, use a plug indexing washer to back the timing down inthose cylinders slightly, or if you have an adjustable ignition system youmay want to electronically back the timing down in the hot cylinders.

Always tune to MPH and not ET: There are too many variables on the ETside of the time slip, tire slip, lane choice, driver error, tire pressure etc. Ifyou use the MPH most of these variables are eliminated, but the driver stillhas to do his job, once a baseline is established don't change anything

forget about your RT, do everything the same every pass and the endresult will be a very quick and extremely consistent car.

Tire Pressure: This is off the topic but I want you to think about this criticalaspect of Bracket racing and consistency.I always see guys adjusting their tire pressures in the staging lanes justbefore a pass ...why?

1st pass: Think about this.....your tire temp is 50* you set it at 8#, you doa big burnout and the tire temp goes to 135* the pressure picks up to9.5#.

2nd Pass: So you go back to the lanes and you wait for an hour, the suncomes out and your tire temp is now at 75*. Just before you make yourpass you check the tire and it's at 8.5#s so you drop it to 8#, into the boxand tire heats to 135* but because the tire was already considerablywarmer than the first pass the pressure goes only to 9# and the car runsquicker in the 60'.

3rd pass: Now it's noon and the ambient air is at 90*, you check your tireand its at 8.25 so you deflate it even more, but this time because of thestarting temp of the tire after your burnout the tire only grows to 8.5#,again the 60' improves, you break out and your looking for the trailer and

the beer stand.


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Try this, set the tire according to ambient air so if it's 50* in the morningyou know that the tire will grow pressure as soon as you get into theburnout box so set it low, maybe 7.5#. As soon as you get back to thestaging lanes set the tire to 8.5 Hot and leave it alone. If you check beforeyour next run you'll see that it's dropped pressure considerably but whenyou get through with your burnout and the tire is back to 135* yourpressure will be the same on every pass. Have your crew chief use aninfrared gun and determine how long your burnout needs to be to reachthe desired temp, count down your burnout's and do them exactly thesame. Adjust your tire pressure immediately after each run, you'll besurprised to see that once you get it down right you won't be doinganything to the tires after the second test and tune pass.

The theory being that your making your pass with a tire temp of 135*, sowhy set your tire pressure when it's at 70*?

Consistency is the name of this game, engine, tranny and tiretemperatures are critical to Consistency,find a way to make them the same every pass and you'll win rounds.

timing and vacuum advance 101

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