IGNITION TIMING

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IGNITION TIMING

• AUTO 2

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Ignition Timing

• Timing wasnt adjustable on your Briggs
  Stratton's
• Engines were designed to run at a steady governed
  speed so timing was set at factory to work best
  at that speed
• An Automobile needs good power and fuel
  efficiency at all driving speeds so timing needs
  to be changed to meet all these demands

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Ignition Timing

• Time it takes to burn an A/F mixture depends on
• Compression ratio
• Mixture (ratio mixed up)
• Combustion chamber shape and size
• Placement of spark plug in chamber
• Throttle opening RPM
• Other small factors

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Ignition Timing

• Engineers generally agree that we need the
  biggest push down on the average piston around
  23o ATDC

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Overadvanced Timing (too soon)

• Causes
• Detonation
• Second explosion of A/F after plug lights
• Hard cranking (balking)
• More of one type of emission

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Retarded Timing (too late)

• Causes
• Low Power
• Less efficiency (mpg)
• More emissions
• Backfiring out the throttle plate
• Lower engine vacuum (manifold)
• Higher cylinder temperature
• Hotter running engine

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Ignition Timing
Lets say this engine is running at 1650
RPM (write it down by the picture)
23o
Lets say this engine takes 4 mS to burn the A/F
to get good pressure and we want the big push at
23o ATDC
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Ignition Timing

• At 1650 RPM, how far is the crankshaft going to
  move during 4 mS (.004 seconds)? Think think
  think thinkkkkkk, how am I going to do this?
• Lets get RPM to RPS
• 1650 divided by 60 27.5 RPS
• There are 360o in a circle so
• 360 times 27.5 about 10,000o per second of
  crank
• There are 1000 mS per second so
• 10,000 divided by 1000 10o per mS
• 10o per mS times 4 mS 40o

I got it, At 1650 RPM the crank will move about
40o in 4 mS
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Ignition Timing
40o
Calculate how many degrees before TDC the spark
will need start to get the big push at 23o ATDC
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Ignition Timing

• Now take the same engine and rev it up to 2500
  RPM and say it still takes 4 mS to get the good
  burn and push on the piston.
• If we leave the starting point at 17o BTDC the
  BIG PUSH will happen too late
• Lets calculate

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Ignition Timing
2500 divided by 60 41.66 RPS
41.66 times 360 15,000 degrees per second
15,000 divided by 1000 15 degrees per mS
15 times 4 mS 60 degrees
Calculate when the big push will Happen and fill
in on drawing
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Ignition Timing

• So what we need to do is to advance the starting
  point when we rev up the engine
• Copy the numbers from your top engine over to the
  top engine on the next page in your notebook

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Ignition Timing
17o BTDC
40o
At 1650 RPM
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Ignition Timing
If we are going to maintain the big push at 23o
ATDC at 2500 RPM, Calculate where we are going to
have to advance the timing too?
23o
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Ignition Timing

• So when we speed the engine up, we will need to
  have the spark take place sooner
• We are not going to change the dwell
• Specifications needed to maintain proper
  saturation time and point opening
• So how are we going to change the timing
  according to engine load and speed?

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Vacuum Advance
Vacuum advance rotates points on breaker plate
around The distributor cam to advance when the
points open and close
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VACUUM APPLIED
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Vacuum Sources
Find page in notebook
600-800 RPM
1200-1500 RPM
O-1
1 Venturi
15-20
2 Ported
16-21
15-20
1400-2200 RPM
3 Manifold
3-5
1-3
Venturi not strong Enough to use for Vacuum
advance
13-20
0-2
13-20
0-2
0
0
19-26
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Vacuum Advance Notes

• Vacuum comes from either
• Manifold
• Ported
• Dwell variation is the amount the dwell changes
  as the vacuum advance arks around the distributor
  cam or as the distributor cam wobbles in its
  bushing.
• Maximum acceptable variation is 3o of dwell

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Mechanical Advance
Mechanical (centrifugal) advance advances the
distributor cam to the distributor shaft to open
points sooner
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Mechanical Advance Notes

• Weights over-come springs to turn the distributor
  cam as engine speed increases
• As engine speed decreases, springs pull back
  weights and retard distributor cam back to where
  we started
• Advance affected by
• RPM
• Spring tension
• Weight of weights

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MILLIONS SIX
WHOLE NUMBER
THOUSAND THREE
WHOLE NUMBER
THOUSANDTHS (mili) THREE DECIMAL
MILLIONTHS (micro) SIX
DECIMAL
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.251 amps
674 ohms
6,740 ohms
6,740,000 ohms
.109 volts
.816 volts
900 ohms
970 ohms
972 ohms
Infinite or immeasurable
.001173 amps
1,173 ohms
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Resistor By-Pass

• Ballast resistor is used to cut voltage and
  amperage
• Point type ignition first was used on 6 volt
  systems
• When switched to 12 v systems points couldnt
  handle added amperage
• Resistor was added to prevent burning points
• During cranking, battery voltage may drop to 10v
  leaving around 4 v pushing at of coil
• Makes spark weaker when we need it the strongest
• Resistor by-pass by-passes resistor while
  cranking to give full battery voltage to of
  coil

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Resistor By-Pass
Run
Chryslers
Start
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Resistor By-Pass
Fords
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Resistor By-Pass
Run
GMs
Start
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Resistor By-Pass Questions
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Ignition Timing
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Ignition Timing
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Ignition Timing