camshafts.pdf

fast tech

/ TECH / CAMSHAFTS /

fast tech

ABOUT TIME

Camshaft timing: what it means, and how to get the best from it.

at what a camshaft

consists of and how it operates.

This month we’ll cover what the

duration figures on a camshaft

mean (234, for example), plus

how altering the camshaft

timing can affect your engine’s

performance — for better or

for worse!

Commonly, a camshaft

manufacturer will give you

timing figures for his cam as

follows: 25-69 inlet and 69-25

exhaust. Now whilst this may

initially sound baffling, it doesn’t

have to be if you sit and think

carefully about what it means.

Let’s start with the inlet cam

lobe figures 25-69.

25 is simply the point that the

inlet valve starts to open before

the piston passes the top. This is

known as 25 degrees before top

dead centre (BTDC for short). The

69 is the amount of degrees after

bottom dead centre that the valve

closes again — in this case 69

degrees after bottom dead centre

(ABDC for short). If we add the two

ﬁ gures together and then add 180,

we get the cam’s duration. So for

the inlet we have 25+69+180 = 274

degrees of duration.

With the exhaust cam lobe

ﬁ gures we start at 69. This is simply

the point that the exhaust valve

starts to open before the piston

reaches the bottom. This is known

as 69 degrees before bottom

Cam manufacturers should

list duration ﬁ gures, making

choosing a new cam easier

Words: Stewart Sanderson

dead centre (BBDC). The 25 is the

amount of degrees after top dead

centre that the valve closes again,

in this case 25 degrees after top

dead centre (ATDC). The duration of

this cam can be worked out exactly

the same way as the inlet, and in

this case is the same 274 degrees

of duration.

Interestingly, using that 274

degrees of duration we can now

ﬁ gure out where peak lift should

occur — all we have to do is take

our duration ﬁ gure and halve it, so

274/2 = 137.

If we now remove the amount

of degrees the inlet valve was

open for before the piston

reached top dead centre (25),

we are left with the figure for full

valve lift on the inlet — in this

case 112 degrees ATDC.

Are you all with me so far?

Good, because not only can

we ascertain full lift from those

ﬁ gures, we can also ascertain how

much overlap the cam design

will have. We simply need to add

the inlet valves opening and the

exhaust valves closing positions

together, so in this example we

have the inlet valve opening at 25

degrees BTDC and the exhaust

valve closing at 25 degrees ATDC.

So 25+25 = 50 degrees of overlap

where both valves are actually

open, thus overlapping.

the various timing ﬁ gures mean,

what differences do different

timings make to the actual

performance of the engine?

Well, we tend to ﬁ nd that in

most scenarios a good spread

of power is found by timing our

peak valve lift area to occur at

the point of maximum piston

speed. This speed is determined

by the stroke of the engine but is

usually found between 105 and

118 degrees after top dead centre

in the majority of four-cylinder

engines. You normally ﬁ nd that the

manufacturer has timed his cams

close to this area.

From here we can advance or

retard our cams. Let’s say we have

our inlet cam dialled, as per the

earlier example, so that peak lift is

occurring at 112 degrees ATDC. If

Measuring valve lift helps

you to work out how much

overlap the cam design has

CAM TIMING FOR

PERFORMANCE

So, now you know how the

camshafts work and what

0108 NOVEMBER 2006 FAST FORD

FAST FORD NOVEMBER 2006

0109

ABOUT TIME

LAST month I looked

the various timing ﬁ gures mean,

fast tech

/ TECH / CAMSHAFTS /

fast tech

Decreasing the lobe separation

angle will generally increase the

torque while dropping the power

band somewhat and damaging

the idle quality, not to mention

the fact it will normally lead to

more chance of detonation.

It’s all experimentation and

no ﬁ gures are available to

suit all engines; they all vary

and results depend upon

many things including

other modiﬁ cations like

exhaust, cylinder head

and turbocharger work.

As mentioned earlier,

the camshaft will only

work perfectly at one

engine speed, you just

have to choose that

particular speed and

where the compromises

must lie... Or do you? No.

There is a solution.

CAMSHAFT TERMINOLOGY

SINGLE OR

DOUBLE?

It is worth noting that

there are various camshaft

configurations used in

engines today, but most

notable and common are

the single (SOHC) and

double overhead camshaft

(DOHC) configurations

engine with only one camshaft

per cylinder head.

Engines with two heads such

as the V6 and V8 will generally

have two camshafts ﬁ tted (one

for each head). The camshaft

lobes act upon rocker arms that

in turn press down on the valves,

thus opening them. The valves

are closed again by extremely

strong springs ﬁ tted to them.

Most modern engines are now 16-valve twin-cam engines

Vernier

pulleys will

help you get every

last bhp out, and get

the timing 100 per

cent spot-on, too

VARIABLE VALVE

TIMING

A system was developed many

years ago that allowed us to have

the best of both worlds — a nice,

lazy torquey cam profile for low

engine speeds, with a nice race-

type cam for high engine speeds.

Its operation varies from

manufacturer to manufacturer

and has many different names

such as VVT, VTEC, VVI and VANOS,

not to mention DOUBLE VANOS.

The systems around vary

somewhat from the simple ones

that simply adjust the camshaft

timing by acting on the pulleys

in some manner, to the far more

useful and advanced systems

that actually change the lobe

being used dependant on rpm.

As you can imagine, these

systems are the future for

obvious reasons.

follower and it’s the spring that

keeps the valve in contact with

the follower at all times. This is of

most importance when the valve

is supposed to be closing at high

speed as this is largely down to the

ability of the spring.

DOHC ENGINES

This particular conﬁ guration is

an engine with two camshafts

per cylinder head. Again, a V-

conﬁ guration engine would have

four cams — two for each head. It

is normal practise to use a double

overhead camshaft conﬁ guration

with an engine that has four or

more valves per cylinder as a

single camshaft simply cannot

operate this many valves,

normally due to the spacing of

them around the cylinder.

It is worth noting though that

there are a few double overhead

camshaft conﬁ gurations that,

rather oddly, only operate two

valves per cylinder such as the

Ford DOHC engines found in

some Sierras and Granadas of

the early ’90s.

overhead camshaft conﬁ guration

retard the

exhaust as

well perhaps?

Instead of just altering

the points of peak lift and

keeping the overlap the same, we

could start moving both cams in

opposite ways. This will have the

effect of increasing or decreasing

the lobe separation angles.

Increasing the lobe separation

angle will generally broaden

your power band while reducing

peak torque and lowering your

idle quality, but it’s also a great

solution to decrease detonation.

Single overhead cam

engines only have one

camshaft. Funny that

SOHC ENGINES

This particular conﬁ guration is an

These springs are extremely strong

because the valve is accelerated

open incredibly quickly by the

solution to decrease detonation.

Even cylinder head mods can affect optimum cam timing ﬁ gures

we advance the cam 4 degrees,

we will move it backwards from

112 to 108 degrees. This will make

the valve open to its highest point

earlier on in the intake cycle,

and this tends to improve low-

end torque.

Don’t forget that by moving

the peak lift point of the valve

4 degrees you are also moving

the opening point of the valve

to 4 degrees sooner too, thus

increasing the time your valves are

overlapping. This can become a

power struggle as overlap affects

low-end torque in a negative way,

so often you will have to move the

exhaust as well so it also hits its

peak lift sooner, thus keeping the

overlap the same as it was before.

If we retard the inlet, we would

move it forward 4 degrees to

116 so it now reaches peak

valve lift some 4 degrees later

than standard. This will normally

improve the top-end breathing

ability of the engine while also

decreasing valve overlap.

However, you will recall that

overlap is often beneﬁ cial at high

engine speeds so we have another

battle on our hands... Time to

CONTACT

Stewart Sanderson co-owns

Motorsport Developments

in Blackpool

01253 508400

www.remapping.co.uk

NEXT MONTH

The Racing Puma and Focus ST170 both have variable

valve timing that comes into effect at a certain rev range

Late-model 2-litre Sierras use a twin-cam engine, but it only has eight valves

Everything you need to know

about engine management.

0110 NOVEMBER 2006 FAST FORD

FAST FORD NOVEMBER 2006

0111

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: