气门顶筒

氣門頂筒

(valve lifter)

的種類

分類：

*****

2006/09/08 11:41

Thumb

through

the

pages

of

any

performance

magazine

and

you'll

find

it

all

over

the

place--camshaft

lingo.

Lift,

dur

ation,

base

circle,

dual-pattern,

over

lap,

rocker- arm

ratio;

w

hether

you're

new

to

hot

rodding

or

a

seasoned

gearhead

raised

on

Duntov

cams,

fuelie

heads,

and

Muncie

rock-crushers,

you

have

to

admit

that

sometimes

all

that

camshaf

t

jar

gon

can

get

confusing.

But

there's

good

news

ahead.

In

the

follow

ing

pages

we'll

explore

var

ious

aspects

of

camshaft

operation

and

review

some

of

the

more

frequently

used

ter

minology.

In

the

end,

you'll

not

only

be

able

to

speak

fluent

camshaft,

but

yo

u'll

also

be

armed

and

ready

to

select

your

next

bumpstick.

The

Fundamentals

Our

camshaft

discussion

centers

on

typical

small

-

and

big-block

Chevys,

which

have

the

camshaft

located

through

th

e

top

centersection

of

the

engine

block

and

incorporate

the

use

of

16

respective

lifters,

pushrods,

and

rocker

arms

t

o

actuate

valve

movement.

Unlike

many

moder

n

engines,

the

small

-

and

big-block

Chevy

is

not

an

overhead

camsh

aft/valve

design,

but

simply

an

overhead-valve

engine.

The

camshaft

in

C

hevy

V-8

engines

is

driven

by

the

cr

ankshaft

and

spins

at

half

the

speed

of

the

crankshaft.

By

d

esign,

the

camshaft

has

separate

lobes

for

each

valve

(16

for

Chevy

V-8s).

At

the

lowest

point

of

each

camshaft

lo

be

is

the

base

circle.

This

is

the

place

where

each

valve

will

rest

in

the

closed

position

and

is

the

point

w

here

all

v

alve-lash

settings

are

made.

Above

the

base

circle,

the

lobes

are

ground

in

an

eccentr

ic

fashion

w

ith

ramps

that

initi

ate

the

lifting

and

descending

movements

of

each

lifter.

As

the

camshaft

rotates,

each

lobe

tur

ns

under

its

respective

lifter

while

the

pushrod

is

forced

upw

ard

by

the

valve

l

ifter.

Next,

one

end

of

the

rocker

arm

is

moved

upward

by

the

pushrod

while

the

opposite

end

of

the

rocker

arm

pi

vots

downward

to

push

the

valve

open.

As

the

camshaft

continues

to

revolve,

the

lifter

moves

downward

on

the

lob

e

and

the

valve

is

closed

w

ith

a

valvespring.

Alter

ing

a

camshaft's

character

istics

can

result

in

more

pow

er.

Let's

se

e

how.

Lift

Here

One

way

to

change

an

engine's

power

profile

is

to

increase

valve

lift.

Valve

lift

is

the

amount

(

measured

in

thousan

dths

of

an

inch)

that

the

valve

is

lifted

off

its

seat,

and

lobe

lift

is

the

amount

that

the

cam

lobe

raises

in

radius

ab

ove

the

cam's

base

circle.

Maximum

valve

lift

is

determined

by

multiplying

the

maximum

lobe

lift

by

the

r

ocker

ratio.

For

example,

a

cam

w

ith

0.350-inch

lobe

lift

yields

0.525-inch

valve

lift

with

a

typical

small-block

Chevy

1.5:1

rocker

arm

(0.350

x

1.5

=

0.525