4WHEELOFFROAD.COM 4-WHEEL & OFF-ROADNOVEMBER 2019 71
BY Trenton McGee
[email protected]VORTEC 8.1L CLUTCH
Q
I currently have a Vortec 8.1L in my
GMC with an SM465, a NP203/205
Doubler, and 4.11 gears. I’m running 41.5-
inch tires. The stock replacement Luk
clutch I installed not long ago already
slips in high gear while lugging up a grade
on the road. I don’t feel like I’m that hard
on the clutch. Upon further investigation
I found out a stock-style clutch is rated
for 365 lb-ft of torque and 280 hp, while
the 8.1L is factory tuned to 450 lb-ft and
330 hp. Which clutch has butter-smooth
engagement and pedal feel, but can still
handle the power and torque? Should I
stick with an organic clutch disc and a
stronger pressure plate, or step up to Kev-
lar or a dual friction disc?
ROBERT K.
Via [email protected]A
The 8.1L Vortec big-block is an often-
overlooked option when considering
engine swaps, and we’re not really sure
why. They make gobs of power and torque
but aren’t quite as responsive to perfor-
mance modifications as their small-block LS
counterparts, so that could be one reason
why. But they can also be had cheaper than
a small-block LS, and even in stock form
have more than enough grunt for just about
any 4x4 that isn’t involved in competition.
Though you didn’t mention it, we’re curi-
ous what combination of components you
used to run the SM465 behind the 8.1L.
That engine was never available with an
SM465, though it was available for a short
time with a ZF six-speed transmission. The
late-model big-block will physically bolt to
the earlier bellhousing, and even the earlier
flywheel will work with the later starter as
long as you’re using a 165-tooth flywheel.You’d have to do a little tricker y with the
clutch to use the later flywheel, so we’re
guessing you used the flywheel, clutch,
bellhousing, and clutch linkage for the
SM465 with the later big-block.
We took your questions to the resident
clutch experts at Centerforce (centerforce.
com) to see what they had to say. The first
thing they mentioned was to make sure
that all of the geometry is correct within
the bellhousing. We found conflicting
reports on whether or not the crank offset
of an 8.1L matches the earlier small- and
big-blocks. Some references stated that
they match the earlier small- and big-
blocks, while others claim they mimic LS
small-blocks (which are different from the
earlier engines). We weren’t able to get
a definitive answer. Assuming you have
the engine that came out of your truck, it
would be worth comparing the distance
between the surface of the crank and
the block’s bellhousing mating surface
between the two engines. If there’s a differ-
ence, this means the face of the flywheel
is in a different spot within the bellhous-
ing than it’s supposed to be, which could
cause problems. The thought process here
is that the throw-out bearing could be
preloaded slightly on the clutch fingers,
which reduces the pressure plate’s mount-
ing force. This could cause the slippage you
describe when the clutch is heavily loaded
but otherwise function normally. One
way to check this is to see if there’s any
free-play between the top of the pedal’s
movement before you feel resistance. The
pedal should have an inch or so of free-play
before starting to engage the pressure
plate. This can be dif ficult to discern with
hydraulic linkage, so another way is to see
if the clutch fork at the transmission has a
little free-play when the linkage is at rest. If
there’s no freedom of movement, this could
be your problem. It would al so be wor th
noting whether the clutch engages near
the top, middle, or bottom of the pedal’s
movement. It should be right in the middle
and pretty smooth overall.
If the geometry checks out, then the next
step is taking a look at the clutch itself.
There were actually three different clutch
diameters used with the SM465: 10^1 ⁄ 2 , 11,
and 12 inches. The larger the clutch, the
stronger the holding power, as the larger
clutch has more leverage as well as sur-
face area to engage the fly wheel. For your
application you should definitely be using
a 12-inch clutch. Even though you claim to
be easy on the clutch you are actually put-ting a lot of stress on it with 4.10 gears and
42-inch tires, so it’s understandable that
you could be driving through the clutch on
a long grade in Fourth gear. Centerforce
said you are likely at the ragged edge of
what an organic disc can handle, even with
one of the company’s upgraded versions.
Kevlar is an option, but instead Centerforce
recommended stepping into a dual friction
clutch, which offers more holding power
than a conventional clutch while maintain-
ing the smooth engagement you expect.
Centerforce spends a lot of time tweaking
its dual friction products to minimize the
engagement complaints you often hear
about with more aggressive clutches, and
we’ve used the company’s dual friction
clutches in a few different high-torque
applications with success. Beyond a dual
friction clutch would be a multidisc clutch,
which is both expensive and likely overkill
for your application unless you’ve put some
serious power-adders on the big-block that
you didn’t mention. A quality dual friction
clutch should offer you all the holding pow-
er you need while maintaining the smooth
engagement you require.RELAYS VS. SWITCHES
Q
I’m confused about when to use
a relay versus just using a switch
when wiring electrical accessories. Isn’t a
relay and a switch pretty much the same
thing? It seems like a relay just adds an
extra level of unnecessary complication
to wiring. Why do the car companies use
both, and sometime fuses and other times
circuit breakers?
BRIAN F.
Via [email protected]A
You are correct that a relay is basically
just a switch, but it’s a switch operated
via remote control, so to speak. A regular
s witch control s electrical current by con -