ignition timing.pdf

fast tech

/ TECH / IGNITION TIMING /

fast tech

MAKING ADVANCES

Think advancing your engine’s timing is a quick

and easy way to go faster? Think again. Here’s

what you should know before you have a go.

BEFORE

STARTING…

The ﬁ rst rule of working on

cars and using tools of any

kind is don’t ever skimp on

decent protection. Goggles,

gloves, ear defenders,

masks and a set of overalls

should be in your garage.

Use them.

When using power tools,

protective gear is essential

— grinders and welders can

make a real mess of your

soft skin and bone if you get

it wrong.

Never work under a car

without supporting it using

axle stands. A car falling on

you is not something you’ll be

laughing about down the pub.

INTAKE COMPRESSION IGNITION

EXHAUST

at Motorsport

Developments recently that was

pinking really badly. It transpired

that the owner had advanced the

timing as far as he could, as his

mate told him that would make it

as fast as it could be!

This got me thinking that maybe

an article on spark timing and its

purpose would be interesting, and

maybe save some of you from

destroying your engines. So here

we go...

feature, so as

always, let’s take

you back to basics

and I will try to

explain some of

the basic factors

affecting the

point at which we

require the spark

to happen. Once

you have a basic

understanding of

this, we can then

move on to the more

complex variables

that affect the timing of the spark.

Most petrol engines work on

what is known as the four-stroke

principle (see diag. above). The four

strokes are: induction (the sucking

in of the air); compression (the

compression of the air); ignition

(the ignition of the fuel and the

power created); and exhaust (the

release of the spent gas into the

exhaust system so we can start

again and reﬁ ll it).

The stroke we are interested in

here is the ignition/power stroke

and more speciﬁ cally, what actually

happens both before and after we

initiate the combustion with a spark

from our spark plug. Incidentally,

these four strokes are often easy

to remember as, ‘suck, squeeze,

bang, blow’. You can be sure that

most of the blokes reading this will

Piston heights are one way

of determining compression

ratio, which in turn affects

compression level (see text)

SPARK ADVANCE

Spark advance (or ignition timing

as it’s more commonly known)

is the term used to describe the

point in the engine cycle that the

spark is generated at the spark

plug in the engine’s cylinder. This

point is referenced against the

piston’s position in the cylinder at

the time of ignition. As an example,

if we were to ﬁ re the spark plug at

the exact time the piston is at 20

degrees before it reaches the top

of the cylinder, we would call this

ignition timing 20 degrees before

top dead centre (TDC).

This spark happens at an

almost inﬁ nitely variable point

in the engine’s cycle depending

on many different factors.

Understanding some of those

factors is the main reason for this

smile and remember it forever now.

I certainly did!

Words: Stewart Sanderson

COMPRESSION

So, our inlet valve opens and the

piston heads downwards, creating

a partial vacuum and drawing in air

— just like a syringe. The intake valve

then closes, sealing the cylinder

completely as the piston moves

back upwards and compresses the

mixture into a space much smaller

than it initially ﬁ lled.

This compression level is

determined by our compression

ratio — for example, an 8:1 engine

will compress the mixture into a

space eight times smaller than

it originally ﬁ lled. And a 500cc

cylinder will compress that

500cc of air/fuel mixture into a

space around 62cc in size. Once

0102 SEPTEMBER 2006 FAST FORD

FAST FORD SEPTEMBER 2006

0103

MAKING ADVANCES

WE had a car in

fast tech

/ TECH / IGNITION TIMING /

fast tech

INLET VALVE

CLOSED

SPARK

PLUG

than normal, carrying more latent

heat that was unused for power

creation on the power stroke.

PCP arriving a little too early

has the potential to be far more

damaging. If the peak pressure

wave arrives with the piston still

on its way up the bore or slightly

past TDC, it will try to force it to go

back down, wasting energy trying

to compress the piston and con

rod and naturally robbing us of

important torque.

If PCP happens much too early

it can be really bad and destroy

things. Just imagine this scenario:

we have the force of the road

wheels, the transmission, the

other cylinders’ power strokes

and the crankshaft and ﬂ ywheel

inertia all forcing the con rod and

piston up the bore in a particular

cylinder, and a huge pressure wave

of combusting fuel and oxygen

forcing the piston back down!

The results, as you can probably

imagine can be catastrophic and

terminal for the engine. Holed and

molten pistons are the normal

outcome of this scenario as the

massive pressures lead onto

artiﬁ cial ignition of the fuel known

as detonation or knock; this knock

can act as a thermal lance and

literally melt the alloy piston away...

start the fuel burning earlier

than that.

So let’s take an educated guess

and ﬁ re the spark at around 20

degrees BTDC (before top dead

centre). This gives us 35 degrees of

crankshaft rotation for our mixture

to ignite, start propagating and

ultimately reach the point of peak

expansion rate while at our desired

crankshaft position, which we will

take as 15 degrees ATDC.

It’s worth bearing in mind the

kind of speeds we are talking

about here: 3000 rpm is 50 full

engine rotations per second. And

we are dealing here with only 35

degrees of one solitary turn. It is all

happening incredibly fast.

After initial initiation of the

spark, it normally takes between 2

and 15 degrees for our mixture to

react with an exothermal process

and start to raise the cylinder

pressure above the compression

line at which it would be with no

combustion. This time delay is

known as the delay period.

From here the ﬂ ame front

propagates rapidly outwards,

ultimately reaching the PCP and

then ending a few degrees after this.

Let’s presume that this ignition

point was perfect for our particular

scenario and we reached PCP at 15

degrees. Why can’t we just set the

spark timing ﬁ gure to 20 degrees

BTDC and have done with it? Well,

that’s where it gets complicated

and we have to start looking at the

variables that affect just how fast

the mixture burns, and of course

how much time we have to deliver

our burning fuel to achieve PCP in

the correct place.

Let’s look at the various factors

that inﬂ uence our spark timing.

3000, as the piston is travelling at

half the speed. We can also assume

twice as much at 6000 rpm as the

piston is travelling at twice the

speed. So we now have 10 degrees

at 1500 rpm, 20 degrees at 3000

rpm and 40 degrees at 6000 rpm.

If you are following me so far,

then congratulations, you are

starting to understand and plot your

very ﬁ rst advance curve for a four-

stroke petrol engine!

set-up and mapped for 98 RON will

normally result in molten pistons

and open wallet surgery.

EXHAUST

VALVE

CLOSED

PETROL\AIR

MIXTURE

BURNS

COMBUSTION

CHAMBER

CHARGE DENSITY

This is largely related to the load the

engine is under, but basically, the

more air we cram into the cylinder, the

denser the air/fuel mixture becomes

and the less advance it requires due to

its increasing burn speed.

As a very simple example, the

density of the air fuel mixture in

the cylinder at idle will be far less

than it would be at, say, 25 psi and

4000 rpm. The charge density in

the cylinder normally relates very

well to the torque graph, with peak

torque also being peak air/fuel

density ignited in the cylinder,

and thus requiring the least spark

advance of all due to its immense

burn speed.

PISTON IS

FORCED DOWN

BY EXPANDING

GASES

CONNECTING

ROD

FUEL MIXTURE

The petrol mixture and composition

itself will affect the spark timing

quite considerably. Petrol will burn

quite easily when mixed with air

at ratios between 10:1 and 20:1.

In most engines we typically use

between 11 and 16:1. All these

mixtures have a slightly different

burn rate and thus a different

advance requirement.

Most petrols burn fastest in the

12-13:1 range, which is of course

the reason why peak power is found

at this particular air/fuel ratio. It is

also worth noting that the higher

octane the fuel, generally speaking

the slower it actually burns, so

adding that ultra-high octane fuel

to your engine that is not setup

for it will normally cost you a few

horsepower, and in reverse, running

95 RON fuel in your engine that’s

Colder air/fuel mixture burns

faster and requires less advance

CRANK

SHAFT

DISTRIBUTION

It is very important that the engine

is designed in such a way as the air

fuel molecules in the compressed

charge end up in a uniform state.

The charge present between

the spark plugs electrodes at the

time of ignition must be of an

easily combustible composition,

meaning somewhere between

10 and 20:1. Ideally the charge

present throughout the combustion

chamber should be a similar

composition and evenly distributed,

so as to allow smooth propagation

of the ﬂ ame front at a controlled

rate once the burn is initiated.

Sudden rich or lean pockets of

mixture can have dramatically

negative effects on power output

and engine health.

The ignition/power stroke

CHARGE TEMP

The temp of the mixture in the

cylinder has a dramatic effect on

burn speed and spark requirement

of that particular combustion event.

A cold and dense mixture normally

burns at a very different rate to a

hotter and potentially less dense

one. Sometimes, hotter mixtures can

burn even faster than cold, requiring

more retard. It’s a complex subject

that cannot be covered fully here.

compressed to this immense

level, our mixture of fuel and air

becomes extremely volatile and

a single spark from the spark

plug will initiate the combustion

of it and release the energy from

the mixture, forcing our piston

smoothly back down the bore and

producing our next stroke — the

power stroke.

created when the crankshaft is

at an advantageous angle. That

angle is normally between 10

and 20 degrees after the piston

has reached TDC. This gives the

maximum leverage effect possible

at the crankshaft and generates the

maximum torque. The pressure we

are talking about here is known as

the peak cylinder pressure (PCP).

Getting PCP point wrong will

result in one of two things: if we

achieve Peak Cylinder Pressure

too late then we have missed

the point of peak leverage on the

crankshaft and lose a little torque

as a result. Also, we will transfer

less combustion heat into the

water jacket around the cylinder

head, thus potentially running the

engine hotter.

Another by-product of late PCP

is our exhaust gas is somewhat

hotter due to the burn cycle

continuing further down the bore

CHARGE DILUTION

Any exhaust gasses left over from

the last exhaust stroke will dilute the

fresh charge and slow down burn

rates. The main culprit of this waste

gas being left over is the camshaft

timing. Excessive overlap can result

in large amounts of exhaust gas

TIMING’S RIGHT

OK, so we now know that we want

to have our PCP between 10-20

degrees after TDC and we know

that too early or too late is bad for

our engine and/or performance. So

all we need to know now, is what

affects the burn and how we get it

to peak at the required place with

our well-timed spark.

Before you can understand spark

advance requirements, you must

understand a little about the fuel

mixture and what happens when

we initiate combustion of it. The

ﬁ rst point to understand is

that our fuel and air mixture

will take a particular time to

actually ignite and burn after

the spark has been initiated

(this time is almost inﬁ nitely

variable, but that comes

later). The second point to

understand is that our engine

is also travelling at a certain

speed (revolutions per

minute) and that this speed

must be accounted for when

planning our spark.

For now, let’s imagine

our engine is running at

3000 rpm. It should be fairly

obvious to the majority of

you that to achieve our

cylinder pressure peak

somewhere between 10 and

20 degrees after the piston

hits TDC, we must actually

CYLINDER PRESSURE

Simple enough to understand so

far, I hope? OK, let’s proceed.

One of the most common

misunderstandings is that the fuel,

once ignited, simply explodes and

forces the piston down! Nothing

could be further from the truth and

if that were the case your engine

would be destroyed in seconds.

To generate maximum pressure

on the piston crown, we need

to ensure that maximum point

of expansion of the hot gases is

Skimming the cylinder head raises the compression and makes

the mixture burn faster, requiring more retarded ignition

ENGINE SPEED

The ﬁ rst and indeed main reason we

need to adjust our point of spark is

the fact our engine doesn’t rotate at

a set speed. In the example above I

have used 3000 rpm and shown that

we needed 20 degrees of advance

to get the PCP in the correct place.

Presuming that our engine speed

has doubled to 6000 rpm. What will

happen if we initiate the combustion

at the same place as we did earlier,

20 degrees BTDC?

Yes, that’s right, our piston will

have gone way past our ideal 15

deg ATDC due to the fact the piston

itself is travelling at twice the speed

it was earlier but our propagating

mixture’s ﬂ ame front is travelling at

the same rate as it was at 3000 rpm

(this example assumes an identical

volumetric efﬁ ciency at the two

engine speeds).

Based on this, it’s reasonable to

assume that we need half as much

advance at 1500 rpm as we do at

Get Peak Cylinder Pressure wrong and

you could end up with a holed piston

0104 SEPTEMBER 2006 FAST FORD

FAST FORD SEPTEMBER 2006

0105

DISTRIBUTION

fast tech / TECH / IGNITION TIMING /

DETONATION

KNOCK

Fitting a larger turbo can

affect the spark advance

almost everywhere in the

load and rev range

Detonation or knock is deﬁ ned

as a form of combustion which

involves an energy release

which is far too fast. This

release gives rise to massive

temperatures and cylinder

pressures. Normally detonation

is an explosion of a part of the

fuel mixture not currently

involved in the normal

propagation process.

FLAME

PROPAGATION

Propagation is the correct term

for the way your air fuel mixture

actually burns. It is far more like

a wave than an explosion with

the mixture burning smoothly

from its point of ignition

outwards, rather like the ripple

that is produced if you throw a

pebble into a nice still pond.

being present when the exhaust

valve ﬁ nally closes. Exhaust back

pressure also has a huge effect,

the higher the back pressure is, the

harder it is to ﬂ ow the gas out of

the cylinder on the exhaust stroke.

This is one of the reasons a larger

turbocharger increases volumetric

efﬁ ciency so much — as explained

in the June 2006 issue.

So there you have it. Hopefully

you’ll now have some idea of not

only what spark advance actually

is, but also what sort of things

govern the ﬁ gure required, and

why it is almost inﬁ nitely variable.

One last subject worth covering is

the common things people do that

affect spark advance requirements

without them knowing it.

HEAD SKIMMING

The most common thing of all is

probably having the head skimmed.

Skimming the head, block or

both raises the compression and

instantly increases the amount that

the air/fuel charge is compressed,

generally making it burn faster,

thus requiring more retard. Go too

far and uncontrollable detonation/

knock will be your only reward as

the fuel is compressed to the point

it auto ignites.

ensure your chip is matched to

your new turbo or a meltdown

could be imminent!

band around, and thus

the spark requirements.

Advancing your cam will

often require a spark retard

at high engine revs, and vice versa.

Advancing cam

timing often

requires a spark

retard at high revs

CAM PROFILES

Again, the volumetric efﬁ ciency

of the engine is altered quite

dramatically with new cams just

like it is with a larger or smaller

turbocharger. A camshaft proﬁ le

that shifts more air and makes more

power will have a different spark

advance requirement both on and

off its power band.

NEXT MONTH

ENGINE CHECKS

The moral of the story is simple;

when you have added a new go

faster part or made an adjustment

to any of the mentioned settings

you must always check the engine

not only for correct fuelling, but

also for any engine damaging

detonation/knock.

Obviously I would recommend

you leave all setting and adjustment

work to an experienced professional

with the correct equipment to do so.

Take care out there.

CAMSHAFTS: What they do,

how to time them and what

different timing does.

ADDING A

LARGER TURBO

This has a large effect on the

volumetric efﬁ ciency of the engine,

especially if it has a larger turbine

housing, and thus can affect the

spark advance almost everywhere

in the load and rev range. Always

CONTACT

CAM TIMING

Simple cam timing adjustment is not

quite as bad as new cam proﬁ les

but remember that moving the cam

timing moves the engine’s power

Motorsport Developments

01253 508400

www.remapping.co.uk

0106 SEPTEMBER 2006 FAST FORD

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