IGNITION TIMING
AUTO 2
2001
VHS AUTO
1
Ignition Timing
• Timing wasn’t 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
2001
VHS AUTO
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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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VHS AUTO
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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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VHS AUTO
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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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VHS AUTO
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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
2001
VHS AUTO
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Ignition Timing
Lets say this engine is running at 1650 RPM
(write it down by the picture)
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
Look at this picture
In your notebook
(top left picture)
2001
VHS AUTO
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23o
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 = it,
27.5
I got
AtRPS
1650
the crank
• There are 360oRPM
in a circle
so
o per second of crank
• 360 times 27.5will
= about
10,000
move
about
• There are 1000 mS
40oper
in second
4 mS so
• 10,000 divided by 1000 = 10o per mS
• 10o per mS times 4 mS = 40o
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VHS AUTO
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Ignition Timing
Calculate how many
degrees before TDC
the spark will need start
to get the big push at
23o ATDC
2001
VHS AUTO
40o
23o
ATDC
9
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
2001
VHS AUTO
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Ignition Timing
17o
BTDC
TOO
LATE
43o
ATDC
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
2,500 RPM
Calculate when the big push will
Happen and fill in on drawing
2001
VHS AUTO
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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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VHS AUTO
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Ignition Timing
17o
At 1650 RPM
2001
VHS AUTO
40o
23o
BTDC
ATDC
13
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?
2001
VHS AUTO
23o
14
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?
2001
VHS AUTO
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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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VHS AUTO
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VACUUM APPLIED
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VHS AUTO
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Vacuum Sources
Find page in
notebook
#1 Venturi
#2 Ported
#3 Manifold
600-800 RPM
1200-1500 RPM
1
O-1”
2
3
15-20”
16-21”
1400-2200 RPM
15-20”
3-5”
1-3”
Venturi not strong
Enough to use for
Vacuum advance
13-20”
0-2”
13-20”
0-2”
0”
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VHS AUTO
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0”
19-26”
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 it’s 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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Manual Scale Meter
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MILLIONS
THOUSAND
SIX
WHOLE NUMBER
THREE
WHOLE NUMBER
THOUSANDTHS (mili) THREE
DECIMAL
MILLIONTHS (micro)
DECIMAL
2001
SIX
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No “M”,
.251
“K”,
amps
or “m”
674 ohms
6,740 ohms
6,740,000 ohms
.109 volts
.816 volts
900 ohms
970 ohms
972 ohms
Infinite or immeasurable
.001173 amps
2001
VHS AUTO
1,173 ohms
24
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 couldn’t
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
2001
VHS AUTO
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Resistor By-Pass
Run
Chrysler’s
Start
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VHS AUTO
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Resistor By-Pass
Run
Ford’s
Start
S
I
Starter
Relay
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VHS AUTO
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Resistor By-Pass
Run
GM’s
Start
R S
Starter
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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
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