I always thought it went through the back of it and into the tranny. However, I've been looking at the TH425 (FWD tranny from toronados and caddys in the late 60s and 70s.) and it doesn't look possible with that setup. The engine is still longitudinal with the TC bolted to the back of it. A chain goes from the TC to the tranny which is alongside/under the engine, and also runs front to rear. How does this work?
Thanks, Don.

 

I could come on this site and ask for the weight of the blinker module and I think somebody would know. Somebody here's gotta know how the fluid gets in and out of the TC. Help me out. I need to know for my street legal dune buggy.

 

I've misplaced my THM 425 service manual, but basically it's aTHM 400 with the TC offset. The valve plate which connects the TC and engine area to the input shaft (where the chain runs) basically directs the fluid to the TC the same way it would be in the THM 400.

The 425 was a neat transmission, It was used in the GMC motor home also, which was WAY ahead of it's time, IMO. In that application they eventually went to straight cut (vs. helical) planet pinions for durabilty, even though helical are stronger. The straight cut pinions don't have end thrust like a helical. More than you wanted to know, right? Well, maybe not, because if you are using a HIGH hp engine and runing in 1st and second gears a lot at WOT, the motorhome 425 might be preferred. FWIW that one might have had lower final drive gears also.

I've always thought the THM 425 was the basis for a killer RWD sports car.

So what kind of power is going into that street legal dune buggy? If it's less than 400/400 the 425 is overkill. There was a smaller one based on the 200, I believe.

 

Thanks for the reply. I'm having trouble visualizing how this works, though. I've only seen pics of the setup. When I've taken the TC off of a normal tranny, I didn't see any valve plate between the TC and tranny or engine. The TC just slid onto the input shaft and had to spin a little to get seated.
Here are the 2 ideas I'm batting around regarding the layout for the buggy. 1. Weld the front clip from a late 60s or very early (before the compression drop) 70s toronado onto the back of the buggy. Convert to coilovers and maybe try to find severe duty cv joints. I don't know if they'd take N2O or not. The low weight of the vehicle should help. I'd hate to lose one while pulling a wheelie at 90 mph. 2. If I can figure out how to pump fluid from the converter to a cooler without having the rest of the tranny mounted ( I guess if I understood this plate you are talking about, that could solve my question of how to get the fluid out to a cooler and back), I could use a chain like the 425 does, only run it to a gear on a shaft that is connected to the front of an old 4 speed. That idea has a lot of interesting possibilities. I could have the fun of shifting through the gears, but not have to hold down a stiff clutch at the lights. I'd get the torque multiplication of a TC. It should be a lot easier to launch. The rearend should be protected from the harshness of a typical hard launch with a clutch. I'd be using a regular rearend, so I wouldn't have to worry about the cv joints giving out on me under power. The buggy would then be mid engine, so I shouldn't have quite so many wheelie concerns. Rebuilds on the tranny should be much cheaper. The only problem is that mid engine buggies tend to look too big. Oh, well. I guess that's the price I'll have to pay. But first, I have to figure out how to pump fluid to a cooler and back so I don't fry the converter. Any ideas? Can you find a picture or exploded diagram of how the fluid gets in and out?
There are a number of people who have made cool cars with the 425. One guy has one in the back of a fiero. Another guy used one to put a caddy 500 in the back of a bug.
Too much info? No way. I love finding out things like that.
I'll definitely start with a big block and maybe add some N2O later. I'll probably set it up a little, too, just to grab some attention. Maybe a high-rise with a really tall air cleaner, also. Or a scoop over the carb. Through some headers at it and it should look huge on there.
The good thing about the type of setup I'm thinking of is that it is kind of modular. Add another gear to the shaft, set the engine off to the side with a longer chain and lookie there!!!!!! There's room for another engine on the other side!!! And you know how nature hates a vacuum................ As long as I fab the rear with that in mind from the beginning, there shouldn't be any problem that an angle grinder and a welder couldn't take care of. I wonder if the cage would pass tech at the strip. I have no idea what they look for in a cage.
As you can see, I'm not looking for the most practical or efficient solution to going fast. I just want to do something different and as "in your face" as possible. Sort of like walking around with .45 on your hip, but it's overshadowed by the shotgun in your hands, the grenades strung across your chest and the rocket launcher on your back. I'm kinda evil that way.
I just need to get the fluid from the TC to a cooler and back.
BTW. Do you know who made the strongest manual tranny? I can't really get into the 6 speed viper ones or anything, so I'll have to stick with a 4 speed. Can one be had for any sort of a reasonable price? Can any of them be built to take the kind of power I'm talking about?
Thanks, Don.

 

Shades of the "automatic" transmission in the Chaparral racecars Jim Hall fielded in the 60's! In spite of all the hype, they merely used a manual trans with a fluid coupling or torque converter in place of the standard clutch.

Some fluid couplings are self contained, sealed units, but they aren't meant to transmit the kind of hp you are talking about. To use the 425 setup you still need the oil pump which is in the transmission input end just like a 400. The chain gets the power from the TC to the trans input/pump shaft. The connecting plate is also full of oil passages to get the oil in and out of the TC.

If you want to shift yourself, just use the entire 425 transaxle and put in a full manual valve body.

The 425 CV joints were some of the strongest ever used in a car. They should work just fine if you don't let them get to extreme angles.

As long as you are biting off such a big project, how about a dual engine buggy with a 425 drive in the front and one in the back. Yeah, that's been done.

Good luck.

 

I'm going to have to do a search on those cars. They sound interesting. How did they get past the problem of disengaging the trans from the engine when they wanted to shift? Did they have a trans that was specially made to shift under power?
My biggest problem is trying to figure out how to pump the fluid in and out of that TC. If I knew how to route lines to it, I could easily use an electric pump to pump the fluid to the cooler and back. I just don't know how to "tap into" it. I'm figuring the fluid goes into the rear center of the TC where it mates to the input shaft of the trans. I just can't figure out how to attach lines to it. I wonder if I could take the input shaft and pump from a trans and mount that up somehow. What do you think?
This seems like the hard way to get 2 engines to one trans, but I really like the idea and the benefits. What you said about the manual valve body would work and probably be much easier, but I like the whole thing of running up to redline, putting in the clutch, yanking the shifter and dropping the clutch back out. Way too fun to pass up if I can have it. Also, a converter on each engine means they are not connected mechanically, but with fluid. If one engine takes a dive (say a nitrous valve sticks open) at least they aren't connected with gears. Even with something as minor as normal vibrations, I'd like to avoid having them mechanically connected.
Now for what to do if I just can't get this to work and I end up going with the 425.
I'm not thinking of putting the cv joints on an angle. I had thought of doing that to move the engine forward for weight distribution, but that can be accomplished in other ways. Maybe I could mount the engine somewhat forward of the trans on the buggy. I could connect them with a shaft and as long as they are mounted rigidly in relation to each other it should be fine. I don't know if it's possible to make it rigid enough, but it might be worth looking into.
As far as the front and back idea, I've thought about that with a truck before. Interesting concept, but if the tires aren't all exactly the same size, the 2 engines end up fighting each other and putting a lot of strain on both drivetrains. I also thought about having the engine in front of me, but then traction becomes more of a problem and the heat from the engine would blow back over me. Florida's hot enough, thanks. I think mid engine is a good way to go, Twin big blocks side by side (admittedly later) would look incredible and probably run even better. Hhhhhhhmmmmm..... twin big blocks on nitrous....
If I do have to use an auto, I'd consider the 425. I just don't know if the axles and differential would be strong enough for my type of abuse, even highly modified. I could replace the chain drive with a gear driven unit. That's been done before. It's not cheap, but at least it should survive.
Isn't the 425 basically the same as a 400, but with a different layout? If that's the case, then the transmission itself should be capable of being built to withstand what I will throw at it.
Are there any 4 speeds that could take that if they were built correctly? This will all take time and a lot of money even if it's done with a lot of my own work and boneyard parts, but if I can learn all this now, I can at least go with one plan through the whole build and subsequent mods and not pay to do things twice.
Thanks for all your input, OldSStroker. And for not calling me crazy, too. Even if I am.

 

Well, I was talking to a guy who used to rebuild transmissions and found out there's no way I can use an electric pump. He says that the pressure going into the converter is probably the same as what is going through the rest of the trans. (1500 to 1800 psi) I don't know of any electric pump that could do that with the power avaliable on the buggy.
I guess when the time for twins arrives, I'll have to use a gear drive to connect them to whatever trans I decide to use. In the meantime, I'm still wondering if I can get an old 4 speed to put up with this. I also wonder if the TH425 can be modified to take the power from a modified and nitrous fed 455 or 500. Do they use the same gears in the diff that a normal rearend uses?

 

Chaparral: to shift you just lift a little to unload the trans, shift and get back on it. You can do the same with most manual trans. Starting engine was tricky. It was in gear. They used 1, 2 and 3 speed transmissions as engine power and tire traction improved. Remember theis was from about '65 thru '70 or so.

Listen to your friend about not using an electric pump. Use the 425 as it is or slightly beefed. It's a 400 just turning in reverse. Final drive gears are very strong. 2.73's in most, I think. The GMC motorhome used the 425 also, and had some stronger internal parts. Probably lower gears too, but I don't remember.

My advice is to not try to reinvent the wheel (or the transaxle). Just bolt it it. I was suggesting one complete system in the front and one in the rear if you wanted a twin. Don't couple engines together. Even it they don't shift exactly together, which they would if you used manual valve bodies, it won't be a huge problem.

I never said this wasn't a crazy idea!

Good luck!

 

I guess if I don't decide to go with a mid-engine 4 speed setup then I'll just use the 425 as is. I'd like the fact that it would be a lot shorter if I did that, and I'd get the fun of the auto trans kick-down. Launching should be a lot easier, too. If weight distribution becomes a problem, I could always move the engine forward a couple of feet. I'd have to make sure that there was no flex between the engine and the trans, then I could connect the 2 with a shaft between the crank and the TC which would stay connected to the trans. I don't know how much that would help, but it might be worth it.
What are your thoughts on gear ratios for something like this? I would think that I should use numerically low gears and tall tires to keep the engine in it's powerband, but I don't really know. That's why you are here, so people like me can pick your brain and avoid having their project turn out to be more irritation than fun. WHich reminds me, I've really appreciated your help so far.
Next question. If the 425 turns backward compared to the 400, wouldn't it use a different valve body? If it does, that might make it kind of difficult (and very expensive) to find a fully manual one. Same for a shift kit, if I don't go fully manual.
I'm wondering why it's such a bad idea to link engines. What kind of problems crop up? I understand that it would be easier to put one in the front, but I'd like to have them both in the back if it's possible to do for less than the national debt. Wouldn't the back engine lift the chassis and take traction from the front engine? Also, the heat coming back from that on a hot summer day in Florida could kill a horse. Don't let me give the wrong idea, though. Two engines are most definitely better than one, regardless of how it's done.
I know you didn't say this is a crazy idea. You'd have to be crazy to say that. I just said you haven't called ME crazy. Or told me it can't be done.