hey i have a few questions regarding the 3spd how does it hold up under boost and and what kind of 1/4 times do people see under totaly stock conditions manualy operating the trans

I bet it doesn't hold up to boost at all.


the 125C/3T40 is a weak transmission with a horrible final drive ratio


1997 Pontiac Sunfire
1993 Lexus ES300
1982 Kawasaki KZ550 LTD

Actually the TH125C/3T40 has proven to be quite durable. Yes, the stock final drive suck, but it was set up for economy. There is alot that can be done to them. There's three gear ratios for the Differential, four deive chain gear sets, shift kits, limited slip differentials and conversion kits and over a dozen different stall speeds for the torque converters(which is one of the most durable cores, used in most racing converters).

For an example of a build up see Event's post. The pics may not be available any more, but the info is still there.

And, Yes, it will stand up to boost.

the 125C/3T40 is prone to suddenly loosing reverse, and fail quite regularly behind 3.1 Multiports in w-bodies. Boost would only make things worse.




1997 Pontiac Sunfire
1993 Lexus ES300
1982 Kawasaki KZ550 LTD

boost on 3 speed =being laughed at

Daniel, Mike Teets (Scrufdog) runs a 3sp with nitrous, which hits allot harder than boost. It's all a matter of doing some research and knowing what the weak points are and correcting them.
Kevin, as far as the problems with the TH125C's, the bigest problem I've found has always been the lack of fluid changes(bad for any tranny) and some port and valve problems, which have been addresseed by the TSBs(pump seal leaking, TCC port gasket causing TCC non-lock-up, bypass port enlargement to eliminate TC shudder). The failures I've seen deal with the lack of preventative maintainance and the soft, slow stock shifts on cars driven hard. The stock shifts cause allot of band an clutch slippage, which can glaze them, causing more slippage. The only reverse failures with them came from the failure to stop the car completely, causing excessive wear and heat on the low/revese clutches, steel and housing, something my Ex had problems with.

MadJack wrote:Actually the TH125C/3T40 has proven to be quite durable. Yes, the stock final drive suck, but it was set up for economy. There is alot that can be done to them. There's three gear ratios for the Differential, four deive chain gear sets, shift kits, limited slip differentials and conversion kits and over a dozen different stall speeds for the torque converters(which is one of the most durable cores, used in most racing converters).

For an example of a build up see Event's post. The pics may not be available any more, but the info is still there.

And, Yes, it will stand up to boost.

You sound like you know alot about the 3 speeds I have a 98 sunfire with the 3t40 that was behind the 2200. Curently its bolted to a 4.9 caddy v8. I was thinking about switching to the 4t60 but i didn't know there where so many options for the lil 3 speed. What are the sources for the parts you listed.


85 fiero GT 3800 sc
98 Sunfire 4.9 swap in progress
85 S-10 turbocharged 3800
83 Citation x-11 turbocharged 3.1 with AWD 6000 running gear SOLD
94 Bonneville SLE daily driver
94 Bonneville SSEi wifes pearl pink daily driver
93 Bonneville SSEi I duno why
94 Bonneville SSEi parts for the rest of them

85 fiero GT 3800 sc swap in progress
98 Sunfire 4.9 and AWD swap in progress
85 S-10 turbocharged 3800
83 Citation x-11 turbocharged 3.1 with AWD 6000 running gear SOLD
94 Bonneville SLE daily driver
94 Bonneville SSEi wifes pearl pink daily driver
93 Bonneville SSEi I duno why
94 Bonneville SSEi parts for the rest of them

how long has that 3T40 been behind the 4.9?


I wanna see pics when it grenades itself!


1997 Pontiac Sunfire
1993 Lexus ES300
1982 Kawasaki KZ550 LTD

lol people have been running 125's and 4t's behind 3800 sc's northstars and SBC's in fieros with very little failure. Granted the fiero is light and i don't know much about the trans, but it seems to me to be a pretty durable little unit.

85 fiero GT 3800 sc swap in progress
98 Sunfire 4.9 and AWD swap in progress
85 S-10 turbocharged 3800
83 Citation x-11 turbocharged 3.1 with AWD 6000 running gear SOLD
94 Bonneville SLE daily driver
94 Bonneville SSEi wifes pearl pink daily driver
93 Bonneville SSEi I duno why
94 Bonneville SSEi parts for the rest of them