The Iron Duke Resource Site



Welcome to my newly redesigned Fiero 2M4 resource site.  The original site recorded over 12000 visits,  hopefully this one will also prove as useful.
Ira Crummey


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


Code Axle Ratio 1st 2nd 3rd 4th 5th Rev
1984 econo 4 speed MY8 3.32 3.53 1.95 1.24 0.73 -- 3.42
1984 perf 4 speed M19 4.10 3.53 1.95 1.24 0.81 -- 3.42
1985/86 V6 4 speed M17 3.65 3.31 1.95 1.24 0.81 -- 3.42
1985/88 4cyl 5 speed* MT2 3.35 3.73 2.04 1.45 1.03 0.74 3.50
1986/88 V6 5 speed** MG2 3.61 3.50 2.05 1.38 0.94 0.72 3.41
*  Isuzu 5 speed
** Muncie/Getrag 5 speed

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