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IGNITION TIMING. AUTO 2. 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

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ignition timing

IGNITION TIMING

AUTO 2

VHS AUTO

ignition timing1
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

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ignition timing2
But it will take some

Time to burn!

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 timing3
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
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
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 timing4
Look at this picture

In your notebook

(top left picture)

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 timing5
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 timing6
Write in

23o

ATDC

Write in answer here

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 timing7
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 timing8
TOO

LATE

17o

BTDC

43o

ATDC

2,500 RPM

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 timing9
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 timing10
23o

ATDC

Ignition Timing

17o

BTDC

40o

At 1650 RPM

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ignition timing11
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 timing12
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

Vacuum advance rotates points on breaker plate around

The distributor cam to advance when the points open and close

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vacuum sources
1

2

3

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”

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19-26”

vacuum advance notes
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 Advance

Mechanical (centrifugal) advance advances the distributor

cam to the distributor shaft to open points sooner

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mechanical advance notes
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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slide23
MILLIONS SIX WHOLE NUMBER

THOUSAND THREE WHOLE NUMBER

THOUSANDTHS (mili) THREE DECIMAL

MILLIONTHS (micro) SIX DECIMAL

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slide24
No “M”, “K”, or “m”

.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
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

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resistor by pass1
Resistor By-Pass

Run

Chrysler’s

Start

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resistor by pass2
Run

Start

S

I

Starter

Relay

Resistor By-Pass

Ford’s

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resistor by pass3
R

S

Starter

Resistor By-Pass

Run

GM’s

Start

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