cylinder heads and manifolds.pdf

fast tech

/ TECH / CYLINDER HEADS /

fast tech

EVERYTH ING FLOWS

Stu’s words of wisdom. This

month: exhaust tuning, theory

and practise.

about the cylinder

head: how best to get your fuel and

air in and then back out of the head

for both ultimate performance and

best engine efﬁ ciency.

So it seems natural that this

month we deal with what happens

after the gas leaves the head via

the exhaust ports. The majority of

you will already know that it enters

the exhaust manifold from here,

and is then routed to atmosphere

via the exhaust system and the

exhaust resonators and silencers.

So, is that it? Is the exhaust just a

series of pipes that direct the gas

out of the engine and into the

waiting atmosphere or is there

more to it than that? Let’s see...

Before we get into the theory

of exhaust tuning and the

various phenomena

surrounding this

subject, let’s go

back to basics

and look a little

more closely

at the life of

an exhaust

gas charge.

headers, creating a high-pressure

wave as it does so.

From the exhaust manifold

primary header pipes it travels down

to the point where all (or some)

of the cylinders’ primary headers

converge with each other. This point

of convergence is known as the

collector. Once in the collector, the

exhaust gases expand quickly as

the pressure wave propagates into

all of the available space and the

other lower pressure

primary header

pipes. As

you

all of the available space and the

other lower pressure

primary header

people lag their exhausts manifolds

and systems with heat-proof tape,

or even ceramic coatings. To keep

the heat in and the gas

speed high.

Pressure waves

on the other hand

base their speed on

the speed of sound.

While the pressure

wave’s speed also

decreases as it

travels down the

pipe due to gas

cooling, the speed will

increase again as the wave

is reﬂ ected back up the pipe

towards the cylinder when it

comes across any restriction. At

all times though, the speed of the

speed high.

on the other hand

base their speed on

the speed of sound.

pipe due to gas

cooling, the speed will

increase again as the wave

is reﬂ ected back up the pipe

towards the cylinder when it

comes across any restriction. At

all times though, the speed of the

IN THE MIX

As the piston rocks

over from TDC on the compression

stroke and heads back down the

bore on the power stroke, the

mixture burn has already been

initiated, and thus the rapidly-

burning mixture releases masses of

energy. This is then transferred to

the crankshaft via positive pressure

on the piston crown.

Once the burn is over and

expansion ceases, we are left with

a certain amount of red-hot waste

gas and particle matter. This waste

is commonly known and referred

to as exhaust gas, and although

nothing like the pressure it had

during the power stroke, it is still at

very high pressure.

Just before bottom dead centre,

the exhaust valve opens and the gas

naturally starts to exit the cylinder

due to the pressure differential

created, so it ﬂ ows to the lower

pressure exhaust port and out

into the awaiting exhaust manifold

would

expect,

the hot

exhaust gases

and part of the

pressure wave

energy make their way out of the

collector via its outlet and into the

main exhaust system, heading

for the tail pipe via any resonator

boxes and ultimately escaping to

atmosphere.

Based on the above visualization,

it should be quite clear that two

basic, yet very separate entities

are at work in the exhaust system:

exhaust gas particle speed and the

resulting pressure waves.

would

expect,

the hot

exhaust gases

and part of the

pressure wave

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.

NEED FOR SPEED

The pressure differential between

the cylinder and the atmosphere

determines the exhaust gas particle

speed — that much is relatively

straightforward. As the gases travel

down the pipe, rapidly expanding

and cooling, the speed decreases.

This is one of the reasons that

Above: exhaust gasses exit

the cylinder at high pressure

0130 APRIL 2007 FAST FORD

FAST FORD APRIL 2007

0131

LAST month we talked

or even ceramic coatings. To keep

the heat in and the gas

speed high.

Pressure waves

on the other hand

base their speed on

the speed of sound.

While the pressure

wave’s speed also

decreases as it

travels down the

pipe due to gas

cooling, the speed will

increase again as the wave

is reﬂ ected back up the pipe

towards the cylinder when it

comes across any restriction. At

all times though, the speed of the

primary header

pipes. As

you

fast tech

/ TECH / CYLINDER HEADS /

fast tech

system may be too restrictive and

this will hurt your top end power.

Too little gas speed tends to make

the power curve excessively peaky

and thus hurt your low-end torque.

Having nice big diameter tubes in

your exhaust will allow the gases to

expand, but this cools the gasses,

slowing them down along with their

pressure waves.

So there you have it folks,

its not just a load of old

bent tubes after all,

well, it is, but

the sizes and

shapes they became bent into took

many hours of dyno time for some

very clever people to come up with,

so don’t go changing the shape of

yours every time you come to a

bump in the road will you?

This is a 4-2-1 mani-

fold: four primaries

lead into two second-

aries which then ﬂ ow

to the single pipe

With exhaust tuning, bigger

isn’t always better

Bends should be as

smooth as possible

pressure wave is greater than the

speed of the gas particles.

You need to understand that

pressure waves behave very

differently to gas particles when

a junction is encountered in the

exhaust pipe. When two or more

pipes come together, as they

would in any collector for

example, the waves travel

into all of the available

pipes — backwards as

well as forwards. Waves

are also reﬂ ected back

up the original pipe,

but with a negative

pressure. The strength

of the wave reﬂ ection

is based on the actual

size of the collector, or at

least the area size change when

compared to the area of the pipe it

travelled down there in.

pressure wave is greater than the

You need to understand that

pressure waves behave very

differently to gas particles when

a junction is encountered in the

exhaust pipe. When two or more

pipes come together, as they

An engine dyno is the only real place

to test manifold effectiveness

EXHAUST COMPONENTS

What makes up your

average system?

Exhaust system design is a

balancing act between all of these

complex events and their timing.

Even with the best compromise of

exhaust pipe diameter and length,

the collector outlet sizing can and

often will still make or break the

best design. Let’s take a look at an

exhaust system broken down into

its many different components.

too small will lose a little top

end power, but slightly too big

will normally lose quite a lot

of torque.

Header length has to my

mind been over advertised and

its importance blown out of all

proportion for many years. As

an example, how many times

have you heard the term ‘totally

equal-length headers’? The ﬁ rst

question you have to ask is, ‘Did

the engine actually need equal-

length headers? If so, why did it

not already have them?’

There are many engines out

there that not only will not

beneﬁ t from them, but may well

positively suffer from the use of

one. Many engine exhausts

are tuned to offer maximum

scavenging effect across a 3000-

4000 rpm engine speed, so the fact

that one exhaust header is tuned a

few hundred rpm short of another

is not of any great consequence

and can in fact extend the torque

range somewhat, even if it has

dropped the peak efﬁ ciency

down a few per cent.

Also worth considering is

that the extra length from

non-equal headers is

normally used to best

effect producing nice

radiused curves,

harnessing the available

space in the engine bay.

All engine requirements

differ and it has to be said

that the common in-line,

four-cylinder engine is

proven to be most

susceptible to header

length and tends to like

Exhaust gasses expand

quickly in the collector

pressure. The strength

of the wave reﬂ ection

is based on the actual

size of the collector, or at

least the area size change when

compared to the area of the pipe it

travelled down there in.

least the area size change when

compared to the area of the pipe it

Exhaust manifold design

is crucial to good power

RIDE THE WAVES

The basis of exhaust pressure

wave tuning is essentially to

harness these waves and make

them do good things for us. The

most common aim is to design

and assemble the pipes so that

the negative wave pulse reﬂ ection

coincides with the period of intake

and exhaust valve overlap.

This low pressure helps to pull in

a fresh intake charge as the intake

valve is opening and also helps to

remove the residual exhaust gases

before the exhaust valve closes.

Typically this phenomenon and its

effect is controlled by the length of

the primary header pipe. Due to the

extremely critical timing aspect of

this tuning there will most probably

be parts of the power curve where

more harm than good is done.

This type of tuning can not be

guessed and requires endless

dyno hours to perfect. Haphazard

guessing will normally end up with

nothing more than a seriously

compromised power curve.

Gas speed is not as simple to

achieve and harness as you may at

ﬁ rst think either. Having a gas speed

that is too high suggests that the

HEADERS/PRIMARIES

We must remember at all times that

bigger is not always better with

exhaust tuning. In fact, one of the

greatest fears when designing an

exhaust is that you will select a

tube too big and lose some of the

vitally important scavenging effect.

However, selecting a tube too

small will add a ﬂ ow restriction and

rob us of bhp. It is still worth noting

though that slightly too small is

actually better overall than slightly

too big due to the fact that slightly

them as near to equal length as

possible, but certainly not at the

expense of nice radiused curves,

free from nasty bends and harsh

angles. So inspect those manifolds

carefully before you hand over your

hard earned. Equal lengths with

nasty tight bends are not usually as

good as unequal lengths but lovely

smooth radiused bends.

Equal-length primaries

don’t suit all engines

COLLECTORS/SECONDARIES

Let’s now take a closer look at the

collector and secondary pipe

conﬁ gurations. It is not easy to

discuss collectors/secondaries in

general due to the various different

conﬁ gurations in existence, but the

collector itself is the point where

the primary headers join each other

to ﬂ ow into the secondary pipes.

In a 4-into-1 system we have a

large all-in-one collector and a

secondary pipe. In a 4-2-1 system

we have a collector for each pair of

headers, then a pair of secondary

pipes ﬂ ow the gas to one

bigger collector that collects

the gas from both secondaries

0132 APRIL 2007 FAST FORD

FAST FORD APRIL 2007

0133

tech

system may be too restrictive and

this will hurt your top end power.

Too little gas speed tends to make

the power curve excessively peaky

and thus hurt your low-end torque.

Having nice big diameter tubes in

your exhaust will allow the gases to

expand, but this cools the gasses,

slowing them down along with their

pressure waves.

So there you have it folks,

its not just a load of old

bent tubes after all,

well, it is, but

the sizes and

system may be too restrictive and

shapes they became bent into took

many hours of dyno time for some

very clever people to come up with,

so don’t go changing the shape of

yours every time you come to a

bump in the road will you?

This is a 4-2-1 mani-

fold: four primaries

lead into two second-

aries which then ﬂ ow

to the single pipe

Bends should be as

smooth as possible

pressure wave is greater than the

speed of the gas particles.

You need to understand that

pressure waves behave very

differently to gas particles when

a junction is encountered in the

exhaust pipe. When two or more

pipes come together, as they

would in any collector for

example, the waves travel

into all of the available

pipes — backwards as

well as forwards. Waves

are also reﬂ ected back

An engine dyno is the only real place

to test manifold effectiveness

Exhaust manifold design

is crucial to good power

them as near to equal length as

fast tech

EXHAUST COMPONENTS

Correctly-designed silencers

are vital for good ﬂ ow

complex parts of an exhaust to get

correct. The two factors we spoke

about earlier need dealing with

separately at the silencer if you are

to make the exhaust work properly,

and of course, legally.

The Pressure wave needs to be

deadened by a suitable material to

keep the system quiet for the road,

yet the ﬂ ow capacity of this sound

absorbing material must still allow

us to get the gas through with no

major power robbing restrictions.

Bear in mind we can’t just make it

huge and have lots of it, as the

bigger the bore of the back box, the

more sound we tend to generate.

The sound absorption is normally

dealt with in one of two ways: The

cheapest and most efﬁ cient way

is to pack the rear box

with a large

format is the reason that years ago

one of the ﬁ rst mods we ever made

to a car was a shiny and loud back

box. And it was louder because...

The second most common type is

the absorption type. This utilises a

far better ﬂ owing design. Essentially

the exhaust pipe runs directly

through the back box, and in this

pipe are drilled hundreds of uniform

holes, almost like a sieve. Packed

around this exhaust tube is a similar

type of glass-ﬁ bre

sound

absorption

these then the possibility of you

losing power via your exhaust

system goes up dramatically, but all

is not lost.

The ﬁ rst rule when dealing with

catalysts is that we use the highest

ﬂ owing components we can. A

catalyst by its very nature is

seriously restricting to both gasﬂ ow

and pressure wave progress as all

your exhaust gas must pass through

a monolith (almost like a ceramic

sponge) in order for it to

do its job chemically and

remove all the poisons

from the gas.

Nowadays there does

exist such a thing as a high-ﬂ ow

performance catalyst, available

from all good exhaust suppliers.

These catalysts have far coarser

monoliths inside and ﬂ ow gas far

better than a production unit ever

would. But they are currently still

very expensive

and directs it into our main

exhaust system.

The ﬁ rst point to make

about performance here is that the

secondary pipe diameter is just as

critical as the primary pipe. When

choosing a diameter for the

secondary pipe or the collector it is

always good to start by multiplying

the primary pipe diameter by 1.75.

As for the secondary length, you are

likely to ﬁ nd that experimenting

here gives the most surprising

changes on the dyno than any

other part of the exhaust.

Getting secondary

lengths optimised will

be worth power on

almost any engine.

us to get the gas through with no

major power robbing restrictions.

Bear in mind we can’t just make it

huge and have lots of it, as the

bigger the bore of the back box, the

more sound we tend to generate.

The sound absorption is normally

dealt with in one of two ways: The

cheapest and most efﬁ cient way

is to pack the rear box

with a large

around this exhaust tube is a similar

type of glass-ﬁ bre

sound

absorption

sponge) in order for it to

do its job chemically and

The ﬁ rst point to make

about performance here is that the

secondary pipe diameter is just as

critical as the primary pipe. When

choosing a diameter for the

secondary pipe or the collector it is

always good to start by multiplying

the primary pipe diameter by 1.75.

As for the secondary length, you are

likely to ﬁ nd that experimenting

here gives the most surprising

changes on the dyno than any

remove all the poisons

from the gas.

material,

but this time

it is packed

extremely densely around

the exhaust tube and the pressure

wave is attenuated as it ﬂ ows

through, what is essentially, a sound

attenuation chamber. This type of

system is harder to design and

manufacture and normally offers

less silencing ability but far more

ﬂ ow.

exist such a thing as a high-ﬂ ow

performance catalyst, available

from all good exhaust suppliers.

These catalysts have far coarser

monoliths inside and ﬂ ow gas far

better than a production unit ever

would. But they are currently still

very expensive

NEXT MONTH

Performance cats

usually ﬂ ow better,

but they’re expensive

As part of our A-Z Of Tuning

special, Stu gives the low-

down on Ford’s top engines

As part of our A-Z Of Tuning

special, Stu gives the low-

down on Ford’s top engines

SILENCERS: BIGGEST

POWER LOSS?

Inappropriate silencer selection will,

in a very effective manner, destroy

almost any advantages you gained

by correct manifold, primary,

collector and secondary pipe tuning.

The problem with silencers is that

they are actually one of the most

amount of glass-ﬁ bre wadding and

zig-zag our pipe through it, so by the

time the exhaust gas and pressure

wave exits the rear box, much of its

energy is lost.

This system, whilst efﬁ cient at

silencing, plays havoc with the ﬂ ow

and causes a power restriction. In

its cheapest and most restrictive

CONTACT

zig-zag our pipe through it, so by the

time the exhaust gas and pressure

wave exits the rear box, much of its

ﬂ ow.

CATALYSTS

Finally, those law-abiding citizens

amongst you with cars built from

1991 will know that their exhaust

system utilises a catalytic converter.

Sadly, if you have to have one of

Stewart Sanderson co-owns

Motorsport Developments

in Blackpool:

01253 508400

www.remapping.co.uk

seriously restricting to both gasﬂ ow

and pressure wave progress as all

your exhaust gas must pass through

a monolith (almost like a ceramic

sponge) in order for it to

do its job chemically and

remove all the poisons

from the gas.

Nowadays there does

exist such a thing as a high-ﬂ ow

performance catalyst, available

from all good exhaust suppliers.

These catalysts have far coarser

monoliths inside and ﬂ ow gas far

better than a production unit ever

would. But they are currently still

very expensive

the exhaust pipe runs directly

through the back box, and in this

pipe are drilled hundreds of uniform

holes, almost like a sieve. Packed

around this exhaust tube is a similar

keep the system quiet for the road,

yet the ﬂ ow capacity of this sound

absorbing material must still allow

us to get the gas through with no

Correctly-designed silencers

are vital for good ﬂ ow

and directs it into our main

exhaust system.

The ﬁ rst point to make

about performance here is that the

secondary pipe diameter is just as

critical as the primary pipe. When

choosing a diameter for the

secondary pipe or the collector it is

always good to start by multiplying

the primary pipe diameter by 1.75.

As for the secondary length, you are

likely to ﬁ nd that experimenting

here gives the most surprising

changes on the dyno than any

other part of the exhaust.

Getting secondary

lengths optimised will

be worth power on

almost any engine.

and directs it into our main

material,

but this time

it is packed

extremely densely around

the exhaust tube and the pressure

wave is attenuated as it ﬂ ows

through, what is essentially, a sound

attenuation chamber. This type of

system is harder to design and

manufacture and normally offers

less silencing ability but far more

ﬂ ow.

As part of our A-Z Of Tuning

special, Stu gives the low-

down on Ford’s top engines

SILENCERS: BIGGEST

POWER LOSS?

Inappropriate silencer selection will,

in a very effective manner, destroy

amount of glass-ﬁ bre wadding and

zig-zag our pipe through it, so by the

CONTACT

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