cylinder heads.pdf

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/ TECH / CYLINDER HEADS /

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HEAD

MASTER

Pay attention as Stu offers up a masterclass in

what you need to know about cylinder heads.

months I intend

to cover some of the more old-

school yet fundamental tuning

processes that some modern

tuners and magazines seem to

have forgotten all about, since the

advent of electronic tuning trickery.

I’d like to start this series with

the good old-fashioned cylinder

head and the various modiﬁ cations

we can make to it. Any good tuner

will tell you that the cylinder head

itself is the heart of your engine

and without a good head on your

block you will never get good

power no matter what else you do

to it, and conversely you will also

never get good driveability or good

fuel economy either, as these are

all aspects mainly dictated by the

cylinder head. The cylinder head

and its associated items really do

make or break the torque curve

and dictate the potential power

output of your engine, so let’s

go back to basics as always and

examine why exactly the cylinder

head is so important to our engine.

related to its ability to pump air

in great volumes, and the limiting

factor to how much air can be

pumped is related to the pathway

into and out of the pump.

In the case of an engine this is

everything from the air ﬁ lter to

the inlet valve, and conversely

everything from the exhaust valve

to the exhaust back box, but today

we are dealing with the cylinder

head and its associated items.

So, if we increase that air volume

pumping ability then we increase

our potential power output. Simple

isn’t it?

Actually no, sadly it isn’t.

Although you would no doubt

instantly think that this means

bigger ports and

almost everywhere and yet retains

sensible fuel economy, driveability

and idle characteristics?

I would say that over 90 per cent

of us, myself included, want the

latter, so before you take a Dremel

to your cylinder head I suggest you

grab a coffee and absorb a little

more information on what does

and does not work in the world of

cylinder head porting and design.

Let’s look at the fundamentals.

will develop its peak power (see

Fast Ford issues 245 and 246

for more in-depth camshaft and

timing information). Don’t forget

that the airﬂ ow capability of the

head and the velocity of the

airﬂ ow itself are all factors that

are taken into account when

designing camshafts, so be aware

that modifying the head too much

can also mean the camshaft is no

longer suitable.

PORT FLOW VS

MIXTURE VELOCITY

We established earlier that an

engine’s ability to ﬁ ll its

AIRFLOW

REQUIREMENTS

Naturally the ﬂ ow requirement

of an engine increases

as its revs increase. For

example, an engine

isn’t it?

Actually no, sadly it isn’t.

Although you would no doubt

instantly think that this means

bigger ports and

never get good driveability or good

digesting 30

lb of air at 3000

rpm will try to digest 60 lb of air

at 6000 rpm, all things being equal.

One thing many people forget is

that the amount of time available

to actually ﬁ ll the cylinders with

this air/fuel mixture decreases

proportionately to rpm. It is quite

staggering when you think about

it — at 5000 rpm we only have

an average of six thousandths of

a second to get the air into the

cylinder before the valve closes

again. Once the engine’s airﬂ ow

demand outstrips the ability of the

head to deliver increased airﬂ ow,

VE tails off, as does peak power.

As a quick example, if a head can

ﬂ ow 30 lb of air per minute and the

engine at 3000 rpm requires 30 lb

of air per minute it will make 100

per cent volumetric efﬁ ciency here

and possibly its peak torque, but be

fuel economy either, as these are

all aspects mainly dictated by the

cylinder head. The cylinder head

and its associated items really do

make or break the torque curve

bigger cams

must be better, in

the real world that

isn’t the case because

various dynamics start to

take their part in the proceedings

and as always, things are not as

simple as they ﬁ rst appear to be.

cylinders is the

limiting factor

dictating what its ultimate

power output will be. In

normally-aspirated (not turbo

or supercharged) form, the

cylinders are ﬁ lled by the pressure

differential that is created when

the piston moves down the

bore against an open inlet valve

(barring any ram or shock wave

tuning effects). The actual cylinder

ﬁ lling efﬁ ciency is measured and

expressed in terms of volumetric

efﬁ ciency (VE).

The timing events of the valves

opening and closing is the primary

factor determining the rev range

in which the engine will achieve

its peak VE and where the engine

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.

AIR WAYS

To describe what exactly an

engine is I always ﬁ nd it best to

compare it to an air pump. In

reality, all complexities aside, that

is exactly what it is — it draws air

in, develops power with it after the

addition of fuel, and then expels it

again. This pumping cycle happens

very quickly indeed, but the fact

remains that the power-producing

capability of the engine is directly

POWER OR

DRIVEABILTY?

Big brash ports and cams, while

often allowing the best top-end

power ﬁ gure, will invariable

damage our low-end driveability,

fuel economy and torque. Is that

the result you wanted? A dog of an

engine that goes OK up near the

limiter but nowhere else, or did you

actually want a good overall engine

that goes better than standard

Words: Stewart Sanderson

0130 MARCH 2007 FAST FORD

FAST FORD MARCH 2007

0131

everything from the air ﬁ lter to

the inlet valve, and conversely

everything from the exhaust valve

to the exhaust back box, but today

we are dealing with the cylinder

head and its associated items.

So, if we increase that air volume

pumping ability then we increase

our potential power output. Simple

OVER the next few

and does not work in the world of

cylinder head porting and design.

Let’s look at the fundamentals.

PORT FLOW VS

MIXTURE VELOC

XTURE VELOCTY

We established earlier that an

engine’s ability to ﬁ ll its

IXTURE VELOC

XTURE VELOCI

REQU

Naturally the ﬂ ow requirement

of an engine increases

as its revs increase. For

example, an engine

digesting 30

lb of air at 3000

rpm will try to digest 60 lb of air

at 6000 rpm, all things being equal.

One thing many people forget is

that the amount of time available

to actually ﬁ ll the cylinders with

this air/fuel mixture decreases

proportionately to rpm. It is quite

staggering when you think about

it — at 5000 rpm we only have

an average of six thousandths of

a second to get the air into the

cylinder before the valve closes

again. Once the engine’s airﬂ ow

demand outstrips the ability of the

head to deliver increased airﬂ ow,

VE tails off, as does peak power.

As a quick example, if a head can

ﬂ ow 30 lb of air per minute and the

bigger cams

must be better, in

the real world that

isn’t the case because

various dynamics start to

take their part in the proceedings

and as always, things are not as

simple as they ﬁ rst appear to be.

cylinders is the

limiting factor

dictating what its ultimate

power output will be. In

normally-aspirated (not turbo

or supercharged) form, the

cylinders are ﬁ lled by the pressure

differential that is created when

the piston moves down the

bore against an open inlet valve

(barring any ram or shock wave

tuning effects). The actual cylinder

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/ TECH / CYLINDER HEADS /

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Having massive inlet ports can actually lose you

power, as it slows down speed of the airﬂ ow

to continue ﬁ lling the cylinder,

cramming in the air, sometimes

above 100 per cent VE as we

actually compress some air due to

the piston moving upwards and the

gas still travelling downwards when

the intake valve closes.

A good example of inertia is

when we spin a heavy wheel,

maybe on a car or push bike. Once

we have let go after our spinning

movement, the wheel takes no

more energy to continue spinning,

so it should have just stopped

when we let go, but it didn’t, did it?

In fact, if we try to stop it spinning

now it resists us. This is inertia.

EXHAUST GAS

VELOCITY & INERTIA

In the exhaust port, many of the

same considerations apply as

with the inlet ports. In the exhaust

ports, instead of drawing in air

and fuel to ﬁ ll the cylinder we are

actually pushing it out with a piston.

Maintaining a good gas velocity

here helps to scavenge the cylinder

of spent gases to promote good

cylinder ﬁ lling of fresh air/fuel charge,

because don’t forget we cannot ﬁ ll a

cylinder with 100 per cent fresh fuel

and air, if 15 per cent of the old burnt

garbage is still in there, can we?

If we look at an exhaust port with

poor ﬂ ow velocity due to excessive

port size, we will see that we will

usually have a large, slow-moving

pocket of burnt gas literally slowed

down to almost stationary awaiting

the next push of gas to shove it into

the manifold. Why? As the piston

reached the top of the exhaust

stroke and then began to slow

massively ready for its direction

change, the lack of inertia in our

gas (due to the large port) meant

the mixture also stopped and in

some cases, it can actually reverse

direction and ﬂ ow backwards as

the inlet valve is open at this point

allowing sometimes less pressure

in the cylinder than the exhaust

port (a gas will always ﬂ ow to the

point of least resistance). This is

not at all helpful as it dilutes our

incoming fresh mix. This effect is

usually far worse at low rpm where

ﬂ ow rates and velocity are at their

lowest due to obvious factors.

It’s worth bearing in mind that

this particular issue is normally

made far worse with increased-

overlap, high-performance cams.

The result is an engine that is slow

to get on the cam as engine speed

picks up some velocity in the

exhaust port, and very poor low-

speed engine operation and bad,

sometimes non-existent idle.

Ultimately, both the intake

and exhaust ports must balance

ﬂ ow velocity and ﬂ ow capacity to

achieve ﬂ exible performance over

a broad range of engine speeds.

This is a necessity in a road-based

performance engine. We should

always maximize the efﬁ cient use

of the port to achieve the required

gasﬂ ow, instead of simply making

bigger ports to give ultimate high-

end bhp.

SQUISH EFFECT

Something I have seen badly

modiﬁ ed many times over the

years is the squish area of the

cylinder head. It has often been

destroyed by someone who quite

possibly doesn’t even know that it

exists, let alone what its purpose

was. Squish is the term used to

describe the area of the cylinder

head that comes into very close

proximity to the piston at top dead

centre (TDC).

It’s a known fact that in over 90

per cent of engines, a faster burn

and a more turbulent mixture are

obtained when the compression

height of the piston is set to take

the maximum advantage of the ﬂ at

quench face of the cylinder head.

In other words, if the area

between the ﬂ at face of the head

and the piston top is sufﬁ ciently

close, as the piston reaches

TDC on the compression stroke,

the mixture that occupied the

area over the piston is forced at

high speed into the direction of

the advancing ﬂ ame front. The

beneﬁ ts of this are faster burn

and greater turbulence at the time

of combustion. This is called the

squish effect. In a nutshell, the fuel/

air mixture at the outer edges of

A large, inert volume of air in the inlet is slow to provide a signal

to the MAP sensor/airﬂ ow meter, giving lazy throttle response

VELOC

same considerations apply as

with the inlet ports. In the exhaust

aware that the same head will only

supply 30 lb of air per minute to the

engine, which at 6000 rpm requires

twice as much to make the same

torque as it did at 3000 rpm. All of

these airﬂ ow issues are dealt with

by the cylinder head (obviously

ignoring any inlet hardware

restrictions that may exist such

as throttle body, airbox etc).

So we know that large amounts

of airﬂ ow are paramount to high

engine speed performance and

thus high bhp output. However, if

drainpipe-sized inlet and exhaust

tracts and ﬁ t massive combustion

chamber-ﬁ lling valves.

This would create an engine that

was useless, it’s unlikely it would

even start and idle. Why? Because

velocity is as important as volume.

velocity means that the air column

is moving faster and this means

information will be relayed more

accurately to the management. Far

more important than this is a totally

different dimension that has now

opened up to us — gas inertia.

the combustion chamber is literally

squished into the centre of the

chamber by the rising piston.

A second beneﬁ t of this

squish phenomenon is that the

engine’s mechanical resistance to

detonation is improved when close

piston-to-head clearance is utilised.

Detonation almost always begins

in an area other than that where

the main ﬂ ame front is initiated.

With a ﬂ at-topped piston, the area

opposite the spark plug in the

quench area is prime detonation

territory. If this area is sufﬁ ciently

tight, the ability for it to detonate

reduces since the combustion

at the desired ﬂ ame front occurs

quicker due to the squish effect

and lessens the time for heat rise

in the mixture at the far end of the

chamber. Furthermore, the thin

section at the squish area exposes

only the small volume of mixture

to an equally high surface area,

diminishing the heat rise in this

part of the mixture.

INLET VELOCITY

It’s a sad fact that as port volume

increases, velocity decreases. A

huge port between the throttle

body and the inlet valve will create

a proportionally large volume of air.

The actual motion of air through the

inlet tract to the cylinder is dictated

by the volume of the cylinder made

available by the piston moving down

the bore. So the nearer we get to

matching the two volumes up, the

slower our gas speed will be on the

cylinder ﬁ lling event.

As an example, imagine the

same scenario using a pint glass

and a straw. You have 3 seconds

to ﬁ ll the pint glass from a tap

using a straw. The speed the water

travelled through the straw to the

glass would be very high. However,

if you perform the same test but

use a hose pipe to ﬁ ll the glass, the

actual speed of the water would be

a lot lower, due to shifting far more

volume per second. It’s the same

principle in an engine, but we are

talking thousandths of a second, so

the speeds are far higher.

It’s also important to know that

since the large column of air in

the inlet tract is the only means by

which the cylinder communicates

with an engine’s MAP sensor or

airﬂ ow meter, a large, lazy volume

of air in the port is slow to provide

a signal to your management about

what exactly is happening and

what our fuel requirements are. The

end result of this is a lazy throttle

response and an engine that

feels ﬂ at. Conversely, a high port

INLET GAS INERTIA

Basic physics tells us that a body in

motion (in this case, the air column)

will build momentum and will

want to keep moving in the same

direction, even as the force acting

on it is removed. It’s called inertia.

In the case of the air within the

port, the force acting on the air

column is the pressure differential

in the cylinder as the piston moves

down and the pressure acting

upon it from the atmosphere.

When the piston slows as it

reaches the bottom of the cylinder

at the end of the induction stroke,

good port velocity is helpful

SWIRL & TUMBLE

After dealing with the port’s ﬂ ow

capacity and of course velocity, the

next logical step is to recognise

the presence of externally-induced

mixture motion. The important part

here is to recognise the fact that the

mixture will ﬂ ow through the port

and into the cylinder in a very un-

uniform fashion, and will normally

enter in a rotational fashion.

This is best compared to a plug

and drain in a sink. When you pull the

plug, the water in the sink doesn’t

just pour down the plug hole and out

of the drain like it was poured from a

pan does it? No, because it also has

to ﬂ ow down a port of sorts, it exits

in a rotational motion following the

perimeter of the bore.

Tumble is a little harder to put

into words, but imagine pouring

a liquid quickly into a wine glass

down one side, and the liquid

quickly rises up the opposite wall

of the wine glass then almost free

falls into the centre of the glass,

this is what I call port tumble.

Many factors can inﬂ uence

tumble but the most dominant

factor is the intake valve’s position

in the combustion chamber when

looked at in relation to the intake

port’s centreline. Many older

combustion chamber designs that

placed valves around the perimeter

of the chamber induced mixture

tumble. Modern inline valves tend

to promote far more swirl, which,

overall, is far better.

THEORY TEST

So, there you have it, I hope that

has given you a small insight into

what goes into the modifying of a

cylinder head, or at least the theory

behind the modiﬁ cations and why

you cannot simply take a Dremel

to a cylinder head and make

everything bigger as many home

tuners do.

Correct valve size is crucial to good driveability

as well as outright performance

WHAT IS VE?

An engine’s ability to ﬁ ll

its cylinders dictates what

power it will produce

This is the ratio between the

mechanical volume available

within the cylinder, and the

volume of mixture the cylinder

actually ingests. For example, if

the cylinder is completely ﬁ lled

with fuel/air, the volumetric

efﬁ ciency is 100 per cent. If it is

only 70 pe cent ﬁ lled with fuel/

air we say the engine at that

point has a volumetric

efﬁ ciency of 70 per cent.

achieving large ﬂ ow ﬁ gures was all

that we needed to create a good-

quality cylinder head, port design

would not represent a signiﬁ cant

engineering challenge at all. We

would simply bore in some huge

Squish effect is where the head face and piston top are

very close at TDC giving a faster, better mixture burn

0132 MARCH 2007 FAST FORD

FAST FORD MARCH 2007

0133

aware that the same head will only

fast tech

/ TECH / CYLINDER HEADS /

fast tech

CAN WE DIY?

Removing casting marks from

the ports is well worth doing

The previous pages cover the majority

of the key elements that need careful

consideration before you start boring

out your cylinder head and ﬁ tting the

biggest valves known to man.

So what can you do on a DIY level

that will improve things before you have

to give the head to a professional? Well,

basic cylinder head porting will improve

the performance of any production

cylinder head by simply removing the

ﬂ aws that are present due to the

procedures used to manufacture these

large castings in a mass-production

environment. Most of the work in a

basic porting project is focused on

reducing the restrictions and

compromises have to be made with:

Removing sharp edges in the

valve guide boss and seat

areas speeds up airﬂ ow

the valve seat areas. Sharp

edges create large amounts of

turbulence as they work just like a

ramp to the high-speed air, forcing it

to tumble and crash into walls,

slowing its progress massively. All

areas should be nicely radiused for

maximum smooth progress of the

high-speed gas.

but a good smooth ﬁ nish on the

exhaust ports can be beneﬁ cial in

resisting carbon build up which does

of course slowly cut down our

available port ﬂ ow.

Please remember, a basic porting

job does not attempt to correct any

design or engineering deﬁ ciencies

present in the casting itself, and is

intended purely to improve upon the

existing design without really

changing anything dramatic.

As soon as you try to change the

way things work in a large way, such

as changing port angles or changing

valve sizes, you really are beyond the

scope of a DIY basic head port and

should be enlisting the help of

professionals who do this for a living.

BASIC PORTING

Whilst very time consuming, basic

porting isn’t actually hard work. I

would allow around 8 hours for an

average DIY mechanic to do a decent

job on the average four-cylinder, 16-

valve head and thus produce a decent

head with no nasty casting marks,

burrs lumps or sharp edges to ruin our

gas ﬂ ow.

CASTING

ROUGHNESS

The castings on an OEM head will be

quite rough, and its worth cleaning

them up so they become smoother. A

mirror ﬁ nish is pointless and can in

fact be detrimental on the inlet side,

mirror ﬁ nish is pointless and can in

fact be detrimental on the inlet side,

gas ﬂ ow.

You can do basic head mods

yourself. But stuff like ﬁ tting

larger valves, leave to the pros

CASTING MARKS

AND RIDGES

These are present on port ﬂ oors, roofs

and walls and are caused by the

casting process. Such items ruin the

smooth ﬂ ow of gas and create

turbulence and swirl as well as taking

up precious room in the tract. A simple

ﬂ ap wheel can often remove these

issues to a suitable standard.

out or break a waterway or oil gallery.

If you do, the head is scrap.

SHARP EDGES

These can be found in the valve guide

boss area and in most cases the top of

STEPS FROM

MANIFOLD TO TRACT

These steps are simply there due to a

mismatch in port size and manifold

size or position, and it’s well worth

matching the two up so the ﬂ ow from

manifold to head (see pic on the right),

or indeed head to manifold on the

exhaust side, is restriction and

obstacle free for your high-speed gas.

A Dremel with a decent cutter is

needed here, and of course you need

to be very careful not to cut too much

boss area and in most cases the top of

TESTING

Dyno testing is the only sure-ﬁ re way to

ﬁ nd out if your head mods have worked

If you don’t

test on a dyno

you will never really know what

gains you have made, or indeed

where you have lost out. A ﬂ ow

bench is ideal to study and test what

you have done to the airﬂ ow

characteristics, but any professional

with ﬂ ow bench and dyno

experience will tell you that once

you bolt it onto an engine it all

means very little anyway as ﬂ ow

bench data often doesn’t relate to

engine performance in any way

shape or form.

Beyond frictional ﬂ ow losses,

other factors affect airﬂ ow in an

engine and cannot be duplicated by

ﬂ ow bench testing. These are air

density, thermal transfer of heat to

the incoming charge from the

engine, the mass of the fuel in the

air, your piston crown conﬁ guration,

the rod/stroke ratio, the piston

velocity, engine rpm and intake

manifold tuning characteristics, to

name only a few.

This does not discount the validity

of ﬂ ow testing and with some simple

mathematical equations, an

accurate prediction of engine

performance can be derived from it,

but never assume the ﬂ ow bench

has proved your head will work,

often what a perfectly educated

man thinks is right will prove to be

totally and utterly wrong.

Airﬂ ow still hides itself very well

behind a mask of invisibility and

even the motor manufacturing

giants’ own scientists still struggle

to get their head ports right. Real-

world experience works best in

this arena and that’s why I suggest

beyond a basic port clean-up you

entrust your head to a pro with a

lot of experience with your

particular head.

NEXT MONTH

Vital knowledge about exhaust

manifolds and systems

CONTACT

Stewart Sanderson co-owns

Motorsport Developments

in Blackpool:

01253 508400

www.remapping.co.uk

0134 MARCH 2007 FAST FORD

If you don’t

test on a dyno

engine, the mass of the fuel in the

air, your piston crown conﬁ guration,

world experience works best in

this arena and that’s why I suggest

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