I never claimed to be a welder (aka Don's Turbo Build)

[CincinnatiFiero]

3900 is a PITA. My buddy did it in his Fiero and has had some serious regrets... he says with the money he spent he could have done a v8...

And been slower, get him to post here! I have a friend considering replacing his 3.4pr with one, who'd love to see some pics.

[Honest Don]

Thanks for the input, guys!

I'm hitting the u-pull-its this weekend to try and score a 3400, however they seem to be a bit of a rare bird around here. I'm probably going to end up with a 3100, as they are common as dirt and less expensive. I'm building this car to more or less beat the shit out of it, so being able to aquire an extra motor on short notice is quite a bonus.

Wiring is going to be not much of a problem. I'm thinking my main concerns are what to do about the "hot" parts I've already made, and the whole dogbone/alternator/? issues.


also, will the th125 convertor bolt up to the newer flexplate or do I have to use something else?

[CincinnatiFiero]

Its more work but it may be worth looking at the overdrive automatic bolted to the 3100. Get you some better mpg and the later trans' tend to be stronger.

[The Dark Side of Will]

If you want abusability, the TH125 is not the transmission you want.

They last FOREVER at stock power levels (My Pontiac 6000 AWD has over 240K on the original transmission). They fail quickly at greater than stock power levels (As many of Rcheee/JP Transmissions customers can attest).

[Honest Don]

If you want abusability, the TH125 is not the transmission you want.

They last FOREVER at stock power levels (My Pontiac 6000 AWD has over 240K on the original transmission). They fail quickly at greater than stock power levels (As many of Rcheee/JP Transmissions customers can attest).

what usually dies on them?

[The Dark Side of Will]

Front what I've heard, the 2nd gear band doesn't have the oomph to engage 2nd gear with a powerful engine at WOT.

[Honest Don]

Front what I've heard, the 2nd gear band doesn't have the oomph to engage 2nd gear with a powerful engine at WOT.

oh ok, so its not shattering planetary gears or anything?

[The Dark Side of Will]

Correct, BUT, I've also been told that no amount of servo enlargement or accumulator reduction can make the band grab hard enough to actually engage the gear. Even at full apply pressure, it just slips and cooks itself and the intermediate drum.

[Aaron]

3900 is a PITA. My buddy did it in his Fiero and has had some serious regrets... he says with the money he spent he could have done a v8...

YAH AND THEN HED BE FAST CUZ ALL V8S ARE FAST DUH THATS WHY ARHCHEE MAKES MONEYS!

Ask him if his V8 would even hold the bumper of a 3900. He'd need more than a TPI or LT1, and LS1s are definitely not cheap.

I'm also interested in knowing why it was so hard. It's the same basic block as every other 60*. He may have had to move the alt err nate err, but that's no big deal really, just some fab. Run it on a 7730 ECU, and it'd be good enough. What's so hard about using the factory PCM though? THey came with a stick, all that VVT shit just means more wires, but the wires go from the PCM to the engine, so it isn't like they're hard.

[The Dark Side of Will]

I'm also interested in knowing why it was so hard. It's the same basic block as every other 60*. He may have had to move the alt err nate err, but that's no big deal really, just some fab. Run it on a 7730 ECU, and it'd be good enough. What's so hard about using the factory PCM though? THey came with a stick, all that VVT shit just means more wires, but the wires go from the PCM to the engine, so it isn't like they're hard.

The CAN bus ECM's don't like to run without input from the BCM via the CAN bus. The BCM is actually the bus master.

[Aaron]

I was thinking it may have had something to do with the BCM. And I guess it would be impractical, if even possible, to give the BCm all of the inputs it needs (ie make the car think it's a G6, like Rickaddy or whoever did with his 3.5 DOHC).

[Honest Don]

Correct, BUT, I've also been told that no amount of servo enlargement or accumulator reduction can make the band grab hard enough to actually engage the gear. Even at full apply pressure, it just slips and cooks itself and the intermediate drum.

interesting...

still, if its not hard parts that are breaking, there's a gotta be a solution there somewhere.

[fieromadman]

I dont want to rob this thread but he had issues with getting other things to work too.

The returnless fuel system he decided to turn into a returned fuel system. The stock crank sensor that the 3.9 has is the 24x res so he bought the 60* v6 stores crank angle sensor that mounts by the harmonic balancer. The bracket that mounts the sensor to the engine is not meant to fit on the 3900, only one bolt lined up so getting that to work was kinda a PITA because the serpentine belt tensioner had to be moved/removed. He's running the 91-93 3.4 DOHC ecu right now because he had one from when he had a 3.4 DOHC in his old car and he thought that it would work well. Apparently the car still wont start with a tune from pcms for less on there.

He's running a 4t65e with that and for whatever reason the ecu cannot control it so he either had to buy a trans controller or come up with a more cleaver way of doing it... which he did although we dont know if it works yet. Also he bought a northstar throttlebody for it because it was drive by wire from the factory. He bought the northstar throttlebody adapter from 60 degreev6 which also had to be modified. He also had to rig the VVT to a locked postion (he just advanced it for more top end power) since the 3.4 DOHC ecu couldnt control the VVT. I dunno what else to say, but its been a pain for him, just little things battling him the whole way.

I'll see if he wants to make a build thread over here, its a really cool car actually, lambo orange homebrew choptop with some big ass tires on it. It should be fairly quick when its done too.

Also forgot to mention that he has done a 3.4 dohc swap, and a 3400 swap and they were both much easier for him than this engine has been so far. He noticed that the 3400 was slower than the 3.4 dohc though. He ran a factory 3400 ecm with the 3400...

[The Dark Side of Will]

Yes. Hard part failures in automatics are virtually nonexistent. Autos have 3-5 times as many mesh points as manual transmissions, so mesh failures are much rarer. Because of the torque convertor, the transient loading with an auto is much smoother than with a stick, so they have much less tendency to shear or twist input and output shafts. Mostly it comes down to the ability of the clutches and bands to hold the torque going through the transmission.

[Honest Don]

Yes. Hard part failures in automatics are virtually nonexistent. Autos have 3-5 times as many mesh points as manual transmissions, so mesh failures are much rarer. Because of the torque convertor, the transient loading with an auto is much smoother than with a stick, so they have much less tendency to shear or twist input and output shafts. Mostly it comes down to the ability of the clutches and bands to hold the torque going through the transmission.

exactly.


Although, I have seen a few of those "virtually nonexistent" failures before, usually just on drag cars though.

[Shaun41178(2)]

I was just thinking that if you decide to stay auto, dennis lagrua built up a th125 tranny and has since gone to a 4t65e with his 3800 swap. he still might have the tranny laying around which would be a straight drop in, and would hold the power better then a stock tranny that you have.

if you weren't going with a 3100 aluminum head motor I would say your tranny would be fine as I don't think an iron head could really make all that much power to really destroy a stock auto.

but a 3100 turbo would probably wear the clutch bands out rather quickly. I would ask dennis if he still has it. but it probably won't be cheap. Or run the auto tranny you have now, and just keep the boost low untill you can afford to put a better tranny in. or just don't go boost with the 3100

[chuck]

I installed the 3900 and yes, in hindsight I would have went with a 350 or 383. especially since the 4t65ehd seems like it would be geared better for a v8. Keep in mind all of the problems I ran into I solved so I don't need A-wrong telling me how easy it is. I also realize that some of the problems are due to my choices in how I wanted the swap done so you can ignore those if you want.
I chose 4 point mount it to eliminate the dogbone. This caused issues with exhaust clearance so I rerouted out the back. I had to clearance the frame for the oil filter. I installed a mobil one filter after the work was done and it's longer than the penzoil one that was on it so I can't remove the oil filter without loosening the engine mount and lifting the engine. The engine has an oil cooler which needed to be bought and mounted. Some of you are going to think that's fine, and it is, but it's also more money into this bottomless pit. The returnless fuel rail was modified so I could keep the stock style fuel system. After a lot of searching I found a bosch fuel regulator for a decent price. The forward rail has a rediculously small hole that I imagine someone would want to enlarge anyway if going turbo. It was any easy fix for me since I needed to loop the rail on that side anyway. Like mentioned already, the throttlebody adapter needs to be modified and they don't specify which year throttlebody to use, at least they didn't at the time, and it didn't come with the bolts as advertised. I already knew the external crank sensor would only have one bolt that lined up, I decided to do it anyway, but I did need to grind at the sensor mount to get the sensor closer to the reluctor wheel. The 3800's have a different torque converter than the 60degrees so that was more money at zzp. They did have fast delivery though. I had to make alternator and A/C mounts to keep everything 3.4tdc for easier parts search and troubleshooting. The decklid needed to be cut to clear the alternator. My 3400 needed the same thing. The driverside decklid spring doesn't leave any room for an intake so I had to remove that. I had to flip the throttlebody because the decklid hit that. Locking the cam was more work. Sort of unneccesary or necessary, your choice. I ran into trans troubles due to poor advise on axle selection and poor buisness practices by TCI. So I decided to ratchet shift it. The fiero frame has a hump for the exhaust on the passenger side. This gets in the way of the belt routing. Routing is still easy enough to overcome but I had to eliminate the tensioner. The throttle cable was easy. I still haven't done anything with the variable intake. Maybe I'll get an rpm switch or take it out completely. Not high on my list right now. The oil filler is too tall and needed to be removed. The coolant fill hits the passenger side spring so I made a filler with a jet bleed in it so I can fill from the rear and bleed the air if I ever figure out why it won't start. A/C hoses still need to be made. They might fit with all the oil cooler lines, exhaust, wiring and sensors in the way already. Various small fabrication to use all 3.4tdc sensors. I'm sure that's not everyting but you get the idea. The 3400 was rediculously easy aside from the anti theft. If I were to do another 3900, I wouldn't. If you were to do one. I would recommend the stock computer and still fix the fuel rail. Hopefully after I get it on the road I'll have a differenty opinion.

[Shaun41178(2)]

pics?

[Honest Don]

I was just thinking that if you decide to stay auto, dennis lagrua built up a th125 tranny and has since gone to a 4t65e with his 3800 swap. he still might have the tranny laying around which would be a straight drop in, and would hold the power better then a stock tranny that you have.

if you weren't going with a 3100 aluminum head motor I would say your tranny would be fine as I don't think an iron head could really make all that much power to really destroy a stock auto.

but a 3100 turbo would probably wear the clutch bands out rather quickly. I would ask dennis if he still has it. but it probably won't be cheap. Or run the auto tranny you have now, and just keep the boost low untill you can afford to put a better tranny in. or just don't go boost with the 3100

Does he post on here? I'm curious as to what he did to his. I've gone through a few turbo 350s & 400's and know a couple tricks to make them live longer. I imagine certain methods can be transferred over.

[CincinnatiFiero]

Dennis' sig on OE says that that built TH125 is sitting behind his 3.4L turbo (the PR). So it sounds like he is using it.