2001VHS AUTO1 IGNITION TIMING AUTO 2 2001VHS AUTO2 Ignition Timing Timing wasn’t adjustable on your Briggs & Stratton's Engines were designed to run.

2001 VHS AUTO 1

IGNITION TIMING

AUTO 2

2001 VHS 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

• 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 3

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

But it will

take some

Time to burn!

2001 VHS AUTO 4

Ignition Timing

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

2001 VHS AUTO 5

Overadvanced Timing (too soon)

• Causes• Detonation

• Second explosion of A/F after plug lights

• Hard cranking (balking)

• More of one type of emission

2001 VHS AUTO 6

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 7

Ignition Timing

Lets say this engine takes4 mS to burn the A/F to getgood pressure and we wantthe big push at 23o ATDC

Lets say this engine is running at 1650 RPM(write it down by the picture)

Look at this pictureIn your notebook(top left picture)

23o

2001 VHS AUTO 8

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

2001 VHS AUTO 9

Ignition Timing

40o

Write in 23o

ATDCCalculate how manydegrees before TDCthe spark will need startto get the big push at23o ATDC

Write

in answ

er here

2001 VHS AUTO 10

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 11

Ignition Timing

2500 divided by 60 = 41.66 RPS

41.66 times 360 = 15,000 degreesper second

15,000 divided by 1000 = 15 degreesper mS

15 times 4 mS = 60 degrees

17o

BTDC

2,500 RPM

Calculate when the big push willHappen and fill in on drawing

43o

ATDC

TOOLATE

2001 VHS AUTO 12

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

2001 VHS AUTO 13

Ignition Timing

23o

ATDCAt 1650 RPM17o

BTDC40o

2001 VHS AUTO 14

Ignition Timing

If we are going tomaintain the big pushat 23o ATDC at 2500RPM, Calculate where weare going to have to advance the timing too?

23o

2001 VHS AUTO 15

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 16

Vacuum Advance

Vacuum advance rotates points on breaker plate aroundThe distributor cam to advance when the points open and close

2001 VHS AUTO 17

VACUUM APPLIED

2001 VHS AUTO 18

Vacuum Sources

#1 Venturi

600-800 RPM 1200-1500 RPM

1400-2200 RPM

Venturi not strongEnough to use forVacuum advance

Find page innotebook

1

#2 Ported2

#3 Manifold3 16-21”

O-1”

15-20”

15-20”

1-3”

13-20”

13-20”

3-5”

0-2”

0-2”

0”

0”

19-26”

2001 VHS AUTO 19

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

2001 VHS AUTO 20

Mechanical Advance

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

2001 VHS AUTO 21

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

2001 VHS AUTO 22

Manual Scale Meter

2001 VHS AUTO 23

MILLIONS SIX WHOLE NUMBER

THOUSAND THREE WHOLE NUMBER

THOUSANDTHS (mili) THREE DECIMAL

MILLIONTHS (micro) SIX DECIMAL

2001 VHS AUTO 24

No “M”, “K”, or “m”.251 amps

674 ohms

6,740 ohms

6,740,000 ohms

.109 volts

.816 volts900 ohms

970 ohms

972 ohms

Infinite or immeasurable

.001173 amps

1,173 ohms

2001 VHS AUTO 25

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 26

Resistor By-PassRun

Start

Chrysler’s

2001 VHS AUTO 27

Resistor By-PassRun

Start

Ford’s

S I

StarterRelay

2001 VHS AUTO 28

Resistor By-PassRun

Start

GM’s

R S

Starter

2001 VHS AUTO 29

Resistor By-Pass Questions

2001 VHS AUTO 30

Ignition Timing

2001 VHS AUTO 31

Ignition Timing

2001 VHS AUTO 32

Ignition Timing