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The Iron Duke Resource Site |
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Welcome to my newly
redesigned Fiero 2M4 resource site. The original site recorded over
12000 visits, hopefully this one will also prove as useful.
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The
Pontiac Fiero has been available with three differently geared Muncie 4
speeds, an Isuzu 5 speed and the Muncie Getrag 5 speed. The 3
speed model 125 is the automatic transmission (with lock up torque converter).
Here is a brief run down of what was available and what it means.
Transmission:
Simple working definition, a device for transmitting engine power to the the
final drive (and then on to the wheels).
Transmission
lore is a mixture of fact and myth. Let us explore some of these points
with an eye to the Fiero. (Note: emphasis on issues of importance to potential
engine swappers)
Automatics,
in the case of the Fiero a TH125 three speed with torque converter lockup. A
reliable transmission, if not exactly an exciting one. The automatic is
a good choice with V6 models, or sufficiently beefed up for a 3800 or V8
engine swap.
4
Speeds,
again the four speeds are an excellent choice for the high torque, low rpm
powerplants. A 1984 economy 4 speed (option MY8) would make a very good
high speed cruiser with a Chevy V8, allowing decent fuel economy, relaxed
cruising (about 2000rpm at 60mph) and plenty of performance because of the
flat torque curve of the typical 305 or 350. The 3.65 ratio 4 speed used
in the V6 (M17) would also prove a good choice giving better acceleration but
a rather busy highway cruise and lower fuel economy, this transmission would
be a better choice for a 3800 V6. The 4.10 4 speed (M19) is best left to
the drag racers (or as an acceptable but less than ideal unit for the high RPM
twin cam
engines.). There is no real difference between the 4
and 5 speeds other than the fact that the 5 speed has more intermediate ratios
which allows you to keep the engine in its powerband.
That's
it!
As a matter of fact when Porsche built their first Turbo 911s they actually
replaced their normal 5 speed with a beefed up 4 speed since they could build
a very strong 4 speed which would be lighter than an equal 5 speed (Their
normal 5 speed was not strong enough). They reasoned correctly that the
increased torque of the turbo made the extra ratios unnecessary. Marketing
however showed that people saw a 4 speed as being "low tech" so the
5 speed soon returned BUT ONLY BECAUSE PEOPLE WHO KNEW LITTLE ABOUT CARS
DEMANDED IT. Just as a 5 speed V8 combination is unnecessary in a Fiero unless
you plan on competitive road racing. This does not mean that a 4
speed is preferred over a 5 speed, just that if you have a perfectly good 4
speed you may not NEED a 5 speed unless you are racing.
5
Speeds,
The Isuzu 5 speed attached to 85-88 4 cyl Fieros is not a good choice for a
high torque swap, it is not strong enough. This transmission may cause
problems because of weak cast alloy (pot metal) shift forks, which could break
if abused especially at high rpms. Careful shifting can make these
transmissions survive since it is the shift forks, and not the gears, which
causes the problems. The gearing of the Isuzu transmissions seems to be a
better compromise than the Getrags, but the reliability issue can cause some
concern. This leaves the Getrag, although not perfect it is probably the
best transmission used in the Fiero. It is strong enough for the high torque
V8s and V6s but with the better ratio spread necessary for the cammier engines
such as the 3.4 twin cam V6, the Quad Four and some V8s tuned for top end
power. If you have one this is the transmission to use for any swap.
Now
you may ask what is the point of this little discussion. Simple, many of
you may be contemplating a V8 swap but are concerned about the added cost and
complexity of finding and installing a Getrag. If you have a V6 with 4 speed,
or an 84 with the economy ratios, you don't need the Getrag. Also decent 4
speeds are available for much less than the typical Getrag ( All Citations
with manual transmissions used this 4 speed, but with different combinations
of ratios, such as the 2.5 with 3.32 final drive but 0.81 4th gear). In
an extreme case, the 6 speeds Chevy sells in the Camaro and Corvette are
mainly meant to allow tall gearing for good fuel economy and as a marketing
gimick (no flame please, I would love to own one I just want to point out that
a 9000rpm Honda VTEC engine would need that transmission, a 6000rpm 5.7 litre
V8 does not). With big torquey engines 4 speeds are fine. Now the Iron duke
2.5 like mine, it could use all the help it can get, but since I drive an 84 I
have the four speed.
The
moral of the story is, if your perfect Fiero must have a V8 than a 4
speed is fine, but if you long for the high pitched scream of an engine on the
far side of 6000rpm the 5 speed is the logical choice. The wide ratios
of the economy box in my 1984 suits my slow revving iron duke just fine.
Ira
Crummey
Manual Transmission Ratios
Note: All indications are that NO V6 ever left the factory with the 4.10 ratio 4 speed, it was a 1984 only option.
Automatic
Transmission Final Drive Ratios:
84-86
L4 had 3.18 final drive.
85-86
V6 had 3.06 (86, at least the GT haven't seen any in SE had 3.33 optional)
87-88
L4 had 2.84
87-88
V6 had 3.33 (2.84 optional)
All
will interchange
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Transmissions