turbochargers.pdf

fast tech

/ TECH / TURBOCHARGERS /

fast tech

SUPERSIZE ME

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.

Turbochargers: big or small? How does a bigger one make the power

difference without a boost pressure increase?

WHO IS STU?

A Level 5-trained fuel-injection

technician, in the past Ford nut Stu’s

worked for a Ford RS dealer, a well-

known fuel-injection specialist and

various tuning companies. Then six

years ago, he joined forces with Kenny

Walker and opened up Motorsport

Developments near Blackpool,

specialising in engine management

live remapping, as well as developing

a range of Evolution chips which are

now sold all over the world.

He’s also the brains behind www.

passionford.com. Started in 2003, it’s

grown rapidly from a few friends and

customers contributing, to one

of the biggest Ford communities on

the web.

Stu’s enviable knowledge of the

workings of modern-day Ford

performance engines means

that he’s just the man to explain

how and why things work, and

most importantly, how they can

be improved!

Having worked as a tuner for over

16 years, Stewart ‘Stu’ Sanderson

is one of the most respected

names in the business.

to-day

work as an engine tuner, I’ve learnt

that there are various concepts that

most car enthusiasts (and some

professionals!) can’t get their heads

around. One of these is turbo sizing.

When I recommend it’s time a

customer increases the size of the

turbo on their engine as part of a

power upgrade package, it’s almost

guaranteed the very ﬁ rst question

asked is, “Does this mean we can

run more boost, Stu?”.

The answer is usually: “While we

could if we wanted to, we aren’t

going to — we’re going to run the

same boost or less, and get much

more power!”

This reply normally leads — via

confused looks — to a discussion

about how a large turbo (Garrett

T4, for example) can produce more

power with a 20 psi intake plenum

pressure, than a much smaller

turbo like a Garrett T3 does at

exactly the same 20 psi intake

plenum pressure.

The facts are quite simple, if not a

little tricky to explain, so let’s look at

a few things shall we?

To make this 170 bhp we are

utilising the air pumping ability

of our 2-litre engine, via its four

500cc cylinders.

These cylinders are drawing in,

through the inlet valves, enough air

and fuel at the correct ratio to burn

safely and, importantly, expelling it

once burnt and processed via the

exhaust valves, to produce a power

at the crankshaft of 170 bhp.

The air processed and power

produced is related to the engine’s

volumetric efﬁ ciency (VE). (See

boxout for how this is calculated.)

So the engine in question is

making its 170 bhp with its nice,

well-designed standard 4-2-1

exhaust system. So, now let’s

redesign this engine and make

it turbocharged.

Starting at the exhaust system,

we’ll remove that 4-2-1 manifold

and stick a Garrett T3 turbo with a

relatively small turbine housing on

it. We’ll then jam close the exhaust

turbine bypass gate (also known as

a wastegate), and weld tight the

compressor wheel so it can’t spin

and pressurise our intake system —

just to see what actually bolting the

turbo on does to our engine.

In essence, we now have the

same, proven 170 bhp engine, but

with a far more restrictive exhaust

due to the turbo’s turbine housing.

Hands up anyone there who

thinks this engine will still make

170 bhp?

I’m sure we are all agreed that by

restricting our exhaust this way we

are now going to be lucky to see

100 bhp! But why does the power

Fitting a turbo actually costs

the engine some power

VOLUMETRIC

EFFICIENCY?

VE is the amount a cylinder ﬁ lls

itself with mixture on the

induction stroke. If a 500cc

cylinder draws in 500cc of air/

fuel on the induction stroke, it

achieves 100 per cent VE.

Words: Stewart Sanderson

FORCE THE ISSUE

The ﬁ rst question I have to ask you

is this: “Are you aware that a

turbocharger is universally

recognised as a form of forced-

induction that initially costs the

engine some power?”

Let’s look at this carefully with

a little tuning scenario. For the sake

of discussion, we’ll take a 1994

RS2000 I4 unit that’s had the

breathing improved by the ﬁ tment

of some sensible camshaft proﬁ les

and a nice porting job on the

cylinder head to produce a healthy

170 bhp.

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FAST FORD JUNE 2006

0117

DURING my day-

fast tech

/ TECH / TURBOCHARGERS /

fast tech

this costs us power, is a great

proportion of the energy produced

from the power stroke of each

cylinder burning the charge of fuel

and air is now wasted trying to push

the spent gas out of the previously

active cylinder’s exhaust valve, and

through the tiny turbine housing

into the exhaust before it can draw

in another fresh charge.

We also have some detrimental

knock-on effects from this back

pressure: the friction on components

caused by this pumping loss will

now add heat to our engine, too. This

heat was part of our power stroke’s

energy so is wasted power.

This pumping loss has also

caused a problem with a

phenomenon known as cylinder

scavenging. Scavenging is used

extensively with normally-aspirated

engines, but is reduced to virtually

zero on almost all turbocharged

engines due to the back pressure in

the relatively tiny turbine housing.

This back pressure has now also

decreased the amount of air the

exhaust pulse drew through the

inlet valve at overlap (the point at

which both inlet and exhaust valves

are slightly open), so the maximum

cylinder ﬁ ll (VE) has reduced. Not

The much larger T4 turbo will produce a lot more power at the

same boost, but as it has larger housings, it’s very laggy

A T3 turbo will produce good power at a given boost limit with

relatively low lag, giving good response and driveability

Our turbo’s pressurised air has to

travel through hoses, intercoolers,

throttle bodies and then ultimately

the plenum, before it has fuel added

to it and it travels through the inlet

valve into the awaiting cylinder.

Once combusted and the energy

from this mixture is converted into

crankshaft energy as best it can

be, the piston travels back

upwards with the exhaust valve

open, and the air is expelled into

the turbine housing and exhaust

awaiting a fresh charge through

the inlet valve...

So, how do we make our engine

shift more air and thus create even

more bhp?

The engine will only process

more air if we do one of the

following things:

What did bolting a T4 onto our

engine do? The head hasn’t been

ported or cams ﬁ tted so there’s no

route improvement.

We are running the same boost

pressure as on the smaller turbo, so

there’s no harder push.

So, have we changed the engine’s

VE? We must have!

only that, but the back pressure

created means that some of the

exhaust gas no longer escapes at

all, thus diluting down our next

cylinder of air/fuel with dead (and

extremely hot) gases.

Down goes the engine’s VE

again... things are looking really bad

for our power curve now!

So conversely, as we now have

less airﬂ ow on overlap, we are going

to start dumping heat through our

exhaust seat and port and are

heating our soft alloy head up. Why?

Simple: designers use scavenging

on overlap as a very simple and

effective way of cooling valve seats,

guides and ports.

How does that work? Well, it’s

simply because when we reach

overlap in our cam timing event,

we have both a cold inlet and

hot exhaust valve open. This gives

the exhaust valve and relative

components time to cool down

from their grievous job only

moments after shifting a mass

of immensely hot air through its

system. So it’s a great relief for

them to sit in some nice cold-

ﬂ owing air for a second and

transfer a bit of excess heat away!

Our fancy new engine design

isn’t looking too hot now is it? Well

actually, it’s getting very hot!

So, hopefully you can now

see why the turbo costs an engine

horsepower just by its very

existence, and if you understand

that, you are now well on your way

to understanding how a bigger

turbo will make more power than

a smaller one with the same

inlet pressure.

T3 V T4

The necessary exhaust back

pressure caused by the turbine

housing assembly is the key

element between the T3 and T4 —

the T4 ﬂ ows far more exhaust gas

than the T3 so pumping losses are

far reduced.

But let’s deal with the delivery of

our air into the engine now, and for

that let’s use the Cosworth four-

cylinder YB power unit.

A T4 produces far more volume of

air at a given pressure from its

compressor housing than a T3 —

that’s universally agreed. And we

agree both turbos have the same job

— to pressurise our engine’s intake

system and keep the air ﬂ owing

through it, mixed with fuel at the

correct ratio, to generate power.

So let’s look at the route of the air

a little closer:

Below. Unlike the manifold on

the left, all the gas on a turbo’d

car has to be forced through that

little square opening you can see

in the turbine housing. It is then

spun so that when it reaches the

turbine wheel, it has a higher ve-

locity than when it left the engine,

creating turbine inlet pressure

Above. In a perfect world,

exhaust gasses would have

no restrictions to cause losses,

like when a 4-2-1 manifold is

ﬁ tted to a NA engine

TECH NOTE

To explain how, I am afraid we have

to get a little bit technical. First, the

Compressor Stage:

On a standard YB, a Garrett T3

equipped with a 50-trim compressor

(2wd Cosworth), running our desired

boost pressure of 20 psi may be

spinning at around 120,000 rpm with

a compressor efﬁ ciency of 70 per

cent (depending on air consumption

at the time of measurement).

A Garrett T4 equipped with a 60-

trim compressor (RS500), running

our desired boost pressure of 20 psi

may be spinning at only 90,000 rpm

with a compressor efﬁ ciency of

drop? What has actually changed

that’s cost us 70 bhp?

LOSE OUT

The answer is that we have

dramatically increased pumping

losses. Pumping losses are the

amount of power used by the

power stroke of a cylinder, to pump

out the exhaust gas from another

cylinder. The harder it is to get the

gas out, the more power is lost in

doing so.

This is the biggest issue with

turbocharged engines. The reason

1. Improve the air’s route into

the cylinders

2. Increase the pressure we push it

in with

3. Improve the volumetric efﬁ ciency

Here you can clearly see the difference in size of the T3

(left) and T4 compressor wheels. The T4 will move a far

greater volume of air thus improving the engine’s VE

CYLINDER

SCAVENGING

This is the extra amount of

waste gas drawn out of the

exhaust valve at top dead

centre (TDC), due to the

presence of a slight depression

that was created by the

evacuation of gas through the

exhaust valve.

0118 JUNE 2006 FAST FORD

FAST FORD JUNE 2006

0119

fast tech

T4 can supply it all in its stride, due

to its nice, large compressor.

But we aren’t making more

power simply because the T4

pumped more air at 20 psi, are we?

I have now proven that we are

making more power because this

turbo improved the volumetric

efﬁ ciency of our engine. The

improvements are mainly through

exhaust back pressure reductions,

and an improvement in outlet

temperatures at the compressor

itself. This temperature improvement

is due to a factor known as Adiabatic

Efﬁ ciency (which we’ll discuss in a

future issue).

This is achieved by narrowing

the turbine housing down prior

to it terminating at the turbine

wheel, increasing the compression

of the gas and ultimately releasing

maximum heat — but creating

the maximum back pressure.

A small turbine housing and

wheel will spool the turbo up

quickly, but leave you with lots of

exhaust back pressure and a

torque curve that at high rpm will

drop off very quickly due to the

pumping losses generated at high

rpm when we try and move

maximum volume of air in the least

possible amount of time.

The size of the turbine housing

is expressed in an A/R ﬁ gure, and

in most cases, the smaller the

number, the smaller the housing,

and therefore, the faster the spool

will be and the higher the ultimate

back pressure.

When we increase the turbine

housing A/R, we drop the back

pressure, and in doing so, also drop

energy and velocity at the turbine

wheel, thus slowing the turbine’s

response, which ultimately harms

the compressor’s response and

making the engine’s ‘time to

boost’ worse. Not to mention the

possibility of engine and turbo-

damaging surge.

This lower TIP and corresponding

compressor response has the

effect of moving the engine’s

power band higher, but damaging

low-end torque at the same time.

Turbo choice is a science that

requires an educated amount of

give and take. You cannot yet have

your cake and eat it — at least,

not until variable geometry

turbochargers are ready for us to

start reliably bolting to your petrol

engines. But that’s another story...

The exhaust, or turbine housing as its known will make all the

difference to how a car drives — too big and it will be laggy, too

small and it won’t make big power but will have instant response

BIGGER = BETTER?

Therefore everything about the

bigger turbo is good, isn’t it? So why

don’t we always ﬁ t a huge, great

turbo and beneﬁ t from the greatly

ﬂ owing turbine stages?

Well, that brings me to the

downsides of ﬁ tting a larger turbo...

Where do we lose out with a larger

turbo? And why?

Turbochargers require high

back pressures prior to the turbine

wheel to drive them correctly. This

pressure is referred to as the Turbine

Inlet Pressure (TIP), and

is the ﬁ rst part you must match

when designing a turbocharged

installation on an existing engine.

A normal ratio here will be in the

order of 2:1 (for example, 40 psi

turbine inlet pressure and 20 psi inlet

valve boost pressure). The

way the turbine housing works is

to accelerate the gasﬂ ow and

concentrate the heat energy, so that

it meets the turbine wheel with more

velocity and heat energy to drive it

hard, thus spinning our compressor

hard and generating the maximum

boost pressure in minimum time.

82 per cent (depending on the

air consumption at the time of

the measurement).

another exhaust back pressure

improvement there at all times,

wastegate open or closed.

Let’s look now at the boost

pressure seen at our intake valves.

Since our exhaust back pressure

is now largely reduced, our cylinder’s

demand for and ability to process

air has increased. We have overlap

efﬁ ciency gains during valve open

events, we have thermal efﬁ ciency

gains in the compressor stage

meaning our air is cooler coming

out of the bigger turbo and thus

denser, and we can suck more air

into the cylinder, mix it with more

fuel and generate more bhp due to

increased cylinder evacuation on

the exhaust event. Result: we are

now processing more air and this

Turbine Stage:

The T4 P trim turbine wheel ﬂ ows a

lot more air than the standard T3

trim rear wheel. But it conversely

takes more energy to spin it to

speed, so the ﬁ rst thing to note is

we have an exhaust gasﬂ ow

improvement due to a better

ﬂ owing rear wheel.

Secondly, we now have a

wastegate that will open much

sooner and much wider than it

would on the T3 turbo, as less

exhaust volume is required to

spin the turbine and compressor

to generate our required inlet

pressure, due to improvements

made to the efﬁ ciency of both the

turbine and compressor.

At the turbine we now have a

30,000 rpm improvement in

efﬁ ciency at our rated 20 psi inlet

pressure — hey, and that’s another

exhaust back pressure improvement

there, isn’t it?

A nice, wide-open wastegate

is also a by-product of a more

efﬁ cient turbocharger — the

wastegate bypasses the restrictive

turbine housing as a means to

stabilise and regulate boost

pressure. This also makes the

piston and crank’s job of pumping

the exhaust gas from the engine a

little easier again, so the wider the

better please.

Since our T4 is actually using an

altogether bigger turbine housing

area radius (A/R) as well, we have

Even on Cossies, big turbos don’t always make for great road-car

installations, simply because of the increased ‘time to boost’

NEXT MONTH

What’s a compressor map?

Plus how it helps you to

make the correct turbo

choice and avoid surge

FAST FORD JUNE 2006 0121

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