dynos.pdf

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ABOUT DYNOS

So you want to know what power your Ford makes? This month Stu

explains the differences between the various types of dynos and

why you shouldn’t always believe the ﬁgures...

discussing cars we

are going to look at dynos. There are

quite a few different types and it is

good to know which is which, how

they differ and what beneﬁts one

has over another, so let’s crack on.

Having worked as a tuner

for 17 years, Stewart ‘Stu’

Sanderson is one of the

most-respected names in

the business.

A Level 5-trained fuel-

injection technician, in

the past Stu has worked

for a Ford Rallye Sport

dealer, a well-known fuel-

injection specialist and

various tuning companies.

Then seven years ago he

joined forces with Kenny

Walker and opened up

Motorsport Developments

near Blackpool (01253

508400, www.remapping.

co.uk ), 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 jointly

responsible with

Webmaster, Petrucci for

www.passionford.com .

Started in 2003, it’s grown

rapidly from a few friends

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 every month

he’s just the man to

explain how and why

things work, and most

importantly how they can

be improved.

WHAT IS A DYNO?

In its simplest form it is a means of

measuring the force generated by a

rotating item. In an automotive case,

we would hope to get accurate

ﬂywheel power ﬁgures from it, but

that’s not always possible as you

will see when you read on. We also

expect that a good dyno will provide

us with a printout detailing useful

things such as rpm, engine torque,

engine horsepower and ideally

boost and Air Fuel Ratio (AFR), too.

Chassis dyno or rolling road

(below left) and hub dyno (above)

drive the whole car onto the

platform, strap it down and drive

it almost as you would on the real

road. This tends to be commonly

referred to as a rolling road.

HUB DYNO

This type of dyno measures

power at the hubs — this

removes any margin for

error with wheel slip and

PREPARING YOUR

CAR FOR THE DYNO

WHAT DIFFERENT TYPES OF

DYNO ARE THERE?

In essence there are three types

commonly available:

It’s worth a little mention

here of a few things you

must always do to ensure

your car is fully prepared

for its dyno run. First of all

make sure it’s running prop-

erly. There is no point taking

a faulty car for a power run.

No point at all, so don’t.

Secondly make sure you

have enough fuel and that it

is of the correct grade.

Make sure your tyres are

well inﬂated, ideally to the

fully-loaded high-speed set-

ting in the manufacturer’s

handbook. And please, get

there on time...

Words: Stewart Sanderson

CHASSIS DYNO

This type of dyno measures

power at the road wheels, or more

accurately, the tyres. This one is

the easiest to use as you just

Make sure your Ford’s running

right before you hit the rollers

FAST FORD OCTOBER 2007

0121

THE TRUTH

THIS month, instead of

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/ TECH / DYNOS /

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Engine dynos are the

only set-ups that can

give accurate ﬂywheel

power ﬁgures

static chassis dyno in a small room?

I wouldn’t like to be stood near it,

would you?

The fact is, all of these things

are far more likely to happen while

driving on the road than when under

controlled and monitored conditions

on the dyno during testing. This is

providing that the operator knows

what he is doing, and that the dyno’s

cooling fans are large enough.

When you are stood next to a car

at full power and 100-plus mph it

sounds very painful on the ears, but

you have to be aware that it is no

different to it doing it on the road, it

just sounds louder now because the

wind isn’t carrying the sound away.

should always receive 100 bhp from

this engine on this dyno. But there

is another small problem that needs

correction, too!

Most days there is also a

difference in atmospheric pressure,

so when the pressure is high your

engine will make more power and

conversely, when it’s low your

engine will make less... So we

now need another formula that

corrects the measured power for

atmospheric pressure variations.

We now have correction for both

temperature and pressure in the

dyno cell. Excellent, but remember

that the pressure and temperature

data has to be measured accurately

and entered into the system one

way or another before each run is

made, if this is not done, the run

cannot be accurate.

Just to add a curve ball, there is

more than one correction standard,

and they all give different results

yet technically all are correct. The

two most common are SAE-J1349

and DIN 70020. So as usual, nothing

is simple. No wonder some people

aren’t going to be happy unless

they can be measured on a TüV-

approved dyno!

to do is ensure that the reading

you’re looking at is quoting power

measured at the same place as the

reading you are comparing it to, such

as ﬂywheel or wheels. Secondly, are

both readings corrected using the

same correction standard?

If indeed the readings are taken

from the same place and the

correction factors are the same then

one of the most common reasons for

the high or low ﬁgure is inadequate

dyno cooling. The ﬁrst clue to cooling

being inadequate is that the more

runs you do the worse the power

gets. This is due to the intake system,

transmission, hubs and brakes all

getting far hotter than they would

do on a motorway doing equivalent

speeds and generating equivalent

power due to the lack of cooling

effect from the high speed airﬂow.

Secondly, many of the twin-drum

roller systems generate signiﬁcant

heat and friction in the vehicle’s

tyres and as such they are largely

dependent on tyre pressures, tyre

temperature, tyre speed and even

vehicle weight (so all your mates in

the boot may actually lower your

power ﬁgure), and can certainly affect

the power reading in most cases.

where those gains or losses are

achieved. As long as your dyno

achieves this, then your dyno

is ﬁne, no matter what ﬁgures it

produces. It is after all a tuning tool

for making comparisons, and not

strictly speaking a tool for accurately

measuring power.

As mentioned earlier, an

inertia dyno has a head start on

repeatability because being a known

mass it will never change from one

day, month or year to the next, it

will always require the same energy

to accelerate and rotate it and

therefore should, by its very nature

be at least accurately repeatable as

far as the results go.

A braked dyno has the cards

stacked against it from the

start for repeatability due

to the electronics and load

ACCURACY AND REPEATABILITY

Let’s make one thing clear... It is

possible to have a dyno with 100

per cent repeatability regardless

of whether or not it is accurate.

The purpose of a dyno is to give a

repeatable power graph, so that

when you’re tuning and making

adjustment you can see for certain

whether you have made gains or

losses with your adjustments, and

CORRECTION FACTORS

COMMONLY USED

The very term ‘correction factor’

instantly makes people think

that this is a complicated subject

whereas in fact it is actually quite

simple. If you look at the dyno

printout you will ﬁnd the number

that relates to a correction factor

that has been used. The reason we

have a correction factor is because

whenever we use a dyno we wish

it to output the same result on the

same engine every time we test

it otherwise our ﬁgures from one

day to the next will be completely

incompatible, meaning the ﬁgures

are rather useless.

The problem we have is this: let’s

say that we run the car up on a cool

Friday evening with 10 degrees C

ambient temperatures and achieve

100 bhp. We then lock the unit up

and go home for our tea and come

back in the morning to ﬁnd it’s a nice

sunny Saturday and the ambient

temperature is now 30 degrees C.

Fabulous... We make ourselves a

nice cup of tea and proceed to run

the car up once again and get it

warm. On our power run we only

make 94 bhp... what has happened?

We didn’t change anything on the

engine! Well, the 6 bhp loss is very

simple to explain, the 20 degree

difference in air temperature has

resulted in an air density change

that has cost us horsepower.

Because of this, dynos normally

have a correction factor that will

correct all readings back to a known

standard air temperature. Using this

correction factor, no matter what

the ambient temperature is, we

HOW TO READ A DYNO GRAPH

other issues, such as having

more than one tyre contact

point. (Real Tarmac roads

have one contact point per tyre

— many dynos have two). This

one is the next easiest to use as all

you have to remove are your road

wheels to use it.

INERTIA

An inertia dyno uses a heavy solid

steel drum or drums, and no brake!

The drum is free to be accelerated

by the driving force. A computer

logs the rate of acceleration and

since the mass of the drum is a

known constant, uses simple maths

to plot the power and allows us to

plot the power against either road

speed, or engine rpm.

reason that anyone actually tuning

cars and mapping as well will need

to buy a braked dyno over an inertia

one if they require a dyno.

Inertia rollers are really for

dynamic testing only. This means

that when you open the throttle the

drum always accelerates. It is not

possible to hold a steady under-load

rpm. So, making changes to fuel and

ignition maps is much more difﬁcult.

No dyno feature would

be complete without a

section that explained

how to read the graph,

so let’s have a crack at

this before we get in

depth about dynos.

Most graphs will

have an X and a Y

axis. The vertical axis

is normally the power

and the horizontal is

normally engine speed.

You will then have a

squiggly line running

from one end to the

other representing

your power and anoth-

er one representing

torque. These lines

are simply the power

and torque numbers

for each engine rpm

speed point on the

horizontal chart con-

nected together so

you can see the

power curve.

If you look at the

graph on this page,

you’ll see that the

solid line representing

power shows a peak

reading of virtually

372 bhp at 6500 rpm.

The dotted line shows

the car made 350 lbf.

ft of torque at 4500

rpm. You can see how

much power or torque

was made at any point

in the rev range by

simply reading it

off at the correct

graph intersection.

There may also be

other information plot-

ted on the graph as

mentioned above, but

we are just concen-

trating on the actual

power part of the

graph. Incidentally, it

is worth noting that

the lines representing

power and torque on

your graph will always

cross at 5252 rpm as

shown in our example

— as long as the graph

uses the same scale

for power and torque,

and uses bhp (brake

horsepower) and lbf.

ft (a 1 foot lever with

a pound of weight

attached) as measure-

ments. There is no

getting round this fact,

so if they don’t cross

there, the graph is

inaccurate and totally

useless to you. We

will go into exactly

why that is in the

next issue...

WHY DO SOME READ HIGH AND

OTHERS LOW?

There are many reasons why

some dynos will read higher and

indeed lower than others. Very

occasionally the dyno may actually

be calibrated incorrectly, either

by mistake or maybe on purpose.

The latter normally being because

the operator knows his customers

prefer higher ﬁgures and it results

in less unhappy customers for him

to have to console. You can see

the attraction there I am sure...

More often a high or low sounding

ﬁgure is a simple case of people

not comparing like with like. For

example, the ﬁrst thing you need

ENGINE DYNO

This type of dyno, as its name

suggests, bolts directly to the

engine. Normally measuring from

the ﬂywheel.

This is the hardest to use as

you have to remove your whole

engine and bolt it onto it in a special

room, but it is the only one that

can give you accurate ﬂywheel

power ﬁgures.

DIFFERENCES BETWEEN THE

TWO SYSTEMS

The main difference is that a braked

system allows the operator to vary

the load at the rollers, so that the

engine can be held at constant

speeds so that such things as

ignition and fuelling can be adjusted

to optimum values under differing

throttle openings and engine rpms.

The load system is actually akin to

a home exercise bike where you

can vary the load on the pedals to

make your job as a pedalling engine

easier or harder... it’s the same

thing, just on a bigger scale. This is

very useful indeed for mapping an

engine management system and

you can get most of it done quite

accurately indeed. This is the main

WHAT DAMAGE CAN BE DONE

TO THE CAR OR ENGINE?

A dyno cannot itself damage an

engine, let’s get that straight right

now. Over-revving, shock loadings,

incorrect fuelling or ignition advance

and lack of cooling are all engine

damaging things... and dynos don’t

cause any of those, but the operator

could do so, if he wasn’t careful.

So if your engine does sustain

damage on an adequately-cooled

dyno, you can be sure that had

you operated it at the same engine

rpm and load on the road, it would

have happened there too! That said,

running a chassis dyno with bald

or ﬂat tyres could have disastrous

results. Can you imagine what a

blowout at 150 mph looks like on a

WHAT MEASURING SYSTEMS

ARE USED?

There are two main types in

use with chassis dynos and

they are as follows:

BRAKED

A braked-type dynamometer

measures power by braking

the rotating part and slowing,

or even stopping its rotation or

acceleration, be that part the

ﬂywheel, the hub or the tyre

depending on the type of dyno you

are using. It then uses a load cell

or spring balance (a mechanical

device that measures turning force

on the rollers) to measure the

torque required to hold the engine’s

rpm steady. Torque x rpm = power,

so from this simple calculation we

have the power ﬁgure at the point

of measurement.

Correction factors in the dyno software should

allow consistent ﬁgures whatever the conditions

Extra bodies in the boot to aid

traction could actually lose

you power...

FAST FORD OCTOBER 2007

0123

fast tech

/ TECH / DYNOS /

fast tech

generators involved, but

accurate and repeatable

braked dynos certainly do

exist, although I am not going to

start naming names as that’s not

why we are here.

component damage. It is also worth

noting that unless the cooling is

excellent, your charge temperatures

will increase signiﬁcantly (force-

inducted especially), and this

means you will lose power at the

wheels and you won’t get it back

in the graph as the inlet manifold

temperature is not compensated for

in the dyno compensation scheme,

only the ambient temperature.

As the speed on the rollers

increases, so does friction

from the tyres, sapping power

The problem with the system is

that it only accurately measures

the resistance in your transmission

when it is not under load. You’ll

ﬁnd that any mechanical system

would generate more friction in

things such as gears and thrust

bearings when it is under load, yet

these systems regularly measure

completely unrealistic transmission

losses, thus giving you completely

useless ﬂywheel power graphs.

Some systems just leave it there

and others add a percentage loss as

well... That brings us neatly to...

tyres will increase too, thus sapping

more power. Please note that engine

rpm is irrelevant to this as the main

point of relevance is road and thus

tyre speed, meaning the higher the

gear, the more power will be lost

through the tyres. How can that be

expressed as a deﬁnite percentage

at the end of the run? Surely, it would

lose proportionally signiﬁcantly

less power at peak torque (say 40

mph in fourth gear) than it does at

peak power (perhaps 110 mph in

fourth gear) so your graph loses out

on torque because the operator

wanted to say you lost X percent

everywhere... Not ideal.

How do you work out a blanket

percentage for that? Express a

different percentage for each gear?

You can’t. Most power runs are done

in fourth due to the 1:1 gearbox ratio

so that it has no multiplying effect

on the ﬂywheel torque, but I know of

at least four chassis dyno operators

in the UK that use third and a few

that use ﬁfth, so again, these dynos

cannot be cross-compared with each

other as far as ﬁgures go.

THE IMPORTANCE OF COOLING

Cooling is something that does not

seem to get taken seriously enough

by dyno operators. I have seen lots

of dyno set-ups in all different kinds

of installation and I can honestly say

that in my opinion over 90 per cent

of the dynos out there have totally

inadequate cooling systems. A small

2-foot wide fan that you can talk

over is OK for maybe one power run

and it will hopefully stop the engine

overheating, but of course that isn’t

all that it is there for.

The engine bay in a modern car

is designed to be force-fed with air

at high speed. The airbox needs

to see pressurised air and the

intercooler, radiator and transmission

components are all designed to be

cooled by high-speed air. This is

why they pretty much all have

ﬁnned casings.

As a ﬁnal test to convince yourself

just what sort of a fan is required

to replicate real-life motorway

acceleration, try sticking your

head out of the sunroof next time

you are a passenger at 100 mph.

It is very windy indeed. That’s the

kind of airﬂow your engine and

transmission was designed to

generate full power in. There is quite

a lot of heat to get rid of, and get rid

of it you must if you are not to risk

TRANSMISSION LOSSES

AND POWER AT THE

FLYWHEEL MEASUREMENT

A chassis dyno measures the power

produced either at the wheels or

the hubs. Now, given the fact that

there will always be more power

produced at the ﬂywheel, how do

we determine how much power

was lost between the ﬂywheel and

the point of measurement? Well,

this is where it all starts to get quite

complex and some very educated

people seem to argue needlessly

because to me is quite simple:

gearboxes, differentials and wheel

bearings will all consume a certain

amount of power as they go about

their work. However, the way that

some dyno operators calculate the

losses is ridiculous.

One thing to remember — and

this is quite important, so pay

attention — is that the calculated

loss can never be 100 per cent

accurate unless it was by pure

luck. There will always be a little

guesstimation in there.

How much so varies between

systems. Here are the main two

GEARBOXES

The losses experienced in gearboxes

are similar to the losses experienced

with differentials... We are generally

dealing with gear sideloadings and oil

drag losses. These losses are related

mainly to rpm but are inﬂuenced by

load to a certain extent. Also don’t

forget that the losses in a gearbox

will differ depending on what gear

is selected, especially those with a

1:1 fourth gear that transfers drive

straight through the box with no

gears involved at all.

I hope the above examples allow

you to understand my line of thinking

when I say that no ﬂywheel output

ﬁgures can be totally accurate and

some are indeed wildly inaccurate

due to them taking an overly

simplistic approach to a difﬁcult task.

Go by what the rollers measured

at the point of measurement, as

these ﬁgures are going to be far

more accurate due to not being

subjected to wild guesstimations. So,

power at the hubs or wheels is king

as far as accuracy is concerned

when using that type of dyno.

PERCENTAGE LOSS

CALCULATION

Some dyno operators just add on

a percentage to the wheel ﬁgure

they measured, and that method of

calculating losses is so wrong that it

is almost criminal!

You probably wonder if I am willing

to back this up when so many UK

tuners say it daily? Yes I am... As far

as I am concerned, there is no way a

powertrain can ever be regarded as

consuming a percentage of power

from anything in a linear fashion.

For example, with an average

transmission system comprising of a

gearbox, differential, wheel bearings

and a set of tyres, you are looking at

roughly the following variables:

methods used to calculate power

lost through the transmission...

DIFFERENTIAL LOSSES

The loss in an average differential is

not only road speed-related, but also

torque-related as well. All your gear,

bearing and internal windage losses

will increase as road speed rises.

Once again these particular losses

are not related in any way to engine

rpm or even necessarily bhp...

However, some differential losses

are indeed load-related as well, such

as the losses experienced in helical-

cut gears and thrust bearings due to

the sideloadings they experience, so

road speed as well as power play a

huge part in the losses at this

particular component level.

Remember, in ﬁfth gear you will

present far less torque the differential,

but lots more rotational speed and in

ﬁrst gear we would present masses

of torque, but very little rotational

speed. (A gearbox is merely a torque

multiplier remember.)

COAST DOWN

Most systems use the coast down

method. This system has the

operator depress the clutch and

allow the transmission to come

to a halt on its own while the

system measures its resistance to

rotation. The system measures this

resistance and creates what you

might call a negative bhp graph.

This resistance is then simply added

on to the power we measured at

the wheels/hubs and given to us

as ﬂywheel power. As an example,

if we make 200 bhp on the power

run, and our rollers measured that

the transmission consumed 30 bhp

when slowing down, it would give

you a power reading of 230 bhp at

the ﬂywheel. Simple...

A decent dyno cooling set-up is

vital to prevent component failure

TYRE/ROLLER INTERFACE

For any tyre and measuring drum

the resistance value will be quite

high when accelerating from rest to

movement, and as the tyre (road)

speed increases the friction from the

SUSPICIOUS POWER LOSS AND FUDGED FIGURES

Most rolling roads use the coast-down method to

calculate an at-the-ﬂywheel ﬁgure (see above)

NEXT MONTH

Nobody likes to think

dyno ﬁgures can be any-

thing other than perfectly

accurate, but I’m afraid

the system is open to

abuse, although some

more than others. For

example, the coast-down

system is easy to fudge

ﬁgures with, all you need

do is brake gently and

the transmission power

loss will be increased sig-

niﬁcantly, thus increasing

the predicted ﬂywheel

power output too.

Alternatively, adjusted

tyre pressures will result

in different ﬂywheel ﬁg-

ures for pretty much the

same reasons.

If you want a reason

to be suspicious of per-

centage claims, as well

as coast-down claims,

consider this...

A 4wd car generat-

ing a genuine ﬂywheel

power of 400 bhp was,

in my presence, recently

quoted as losing 35

per cent of the ﬂywheel

power through its trans-

mission. Now while to

the uneducated this may

seem a realistic ﬁgure

due to its host of extra

components over its

2wd brethren, let’s look

at that ﬁgure a little

more closely:

35 per cent of 400 bhp

is 140 bhp... To convert

bhp to Watts we simply

multiply it by 745.7 to

give us 104,398 Watts.

So, you are being told

that over 104,000 Watts

(KW) was lost as heat in

your transmission? Your

lubricant would have

boiled and your alloy

casings would likely have

been near to melting

had you lost that sort of

heat into them! Just think

what these 1000 bhp

engines would do to a

poor gearbox when they

had to cope with 261-

plus KW of heat every

time you used the power.

261,000 Watts of energy

lost in your transmis-

sion? Unlikely! To give

you some comparison, a

large industrial gas space

heater generally has

an output of between

10-20 KW of heat, so its

suggested our transmis-

sion is akin to ﬁve space

heaters, all burning away

under the car.

Now, I am sure that

the real power loss can

be expressed as an aver-

age percentage, but the

point I want you all to

understand is there is no

way on Earth the trans-

mission loss will ever be

accurately measured by

a rolling road, and nor

can it be expressed as

a linear power loss per-

centage across the rev

range, due to the torque

loading having more

effect on the mechani-

cals. The higher that load

is and the road speed

having more effect on

the tyres, the higher that

road speed is.

I personally think you

would struggle to lose

much over 20 per cent

no matter how com-

plex the transmission

system and bhp of the

engine, especially given

the proven fact that the

more power you have,

the less you will lose per-

centage wise through the

transmission due to the

fact some of the losses

calculated are pretty

much ﬁxed and are not

affected by power, such

as tyre drag and various

bearing frictional losses.

Ever wondered why bhp is so

called? What’s the difference

between power and torque?

Find out next month.

CAN TRUE

TRANSMISSION LOSS

EVER BE MEASURED?

Yes, and it’s actually very

easy. Whip the engine out

and measure its power

on an accurate engine

dyno, install it back in

the car and run it again

on your chosen accurate

chassis dyno. Use the

power at the wheels

ﬁgure only and subtract

one from the other. Hey

presto, pretty accurate

transmission losses.

0124

OCTOBER 2007 FAST FORD

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