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Old 07-12-2012, 07:41 PM
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Olds Toronado THM-425 and Vette

Hey Guys,

I am working on a mid-engine car using a Toro THM-425 transmission. Currently it has the std. factory Olds cast iron differential in it. I would like to adapt one of the later model Corvette aluminum differentials AND flip it so that I could reverse the drive train and have the tranny chaincase at the front (middle of the car) and retain 3 forward gears with the differential flipped and pushing in the correct direction.

I have been told by a Vette shop that the Vette differential absolutely won't work in this configuration? Any opinions and facts would be appreciated!

Thanks,
Swiss

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Old 07-18-2012, 03:54 PM
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Well, first I have to say that anything is possible. Now, here's some of my thoughts on this subject. The Turbo 425 differential bolts to what would normally be called the tailstock of the trans. Your late Vette rear axle is fed by a standard driveshaft and the latest Vettes with the transaxle is another matter entirely. So, no, it won't work unless you spend extensive money and time having an adapter made. And there would be no gain as far as I can see. I was trying to figure out why you wanted to turn the engine around and the only thing I can think off is to move the differential rearward making the engine sit in front of the rear axle instead of straddling it like it would if you used a stock Toronado setup. If that is the case... I'm not certain but you may be able to install a Turbo 400 tailstock and output shaft on the Toronado trans, run a short driveshaft and then install a flipped differential of your choice so that you don't end up with 1 speed forward and 3 reverse with your rearward facing engine. If the parts interchange it should be a fairly easy job. Now, flipped differentials shouldn't have any lubrication problems but not a lot of people have run them that I am aware of so I can't say anything about durability. Yeah, I've also thought about this before but it is probably easier to get a proper transaxle, like a Porsche, and use a ready made adaptor. Good luck, and put up some posts when you get going, sounds like my type of project.
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Old 07-18-2012, 05:10 PM
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Thanks ol'rodder,

Well, first I have to say that anything is possible. Now, here's some of my thoughts on this subject. The Turbo 425 differential bolts to what would normally be called the tailstock of the trans. Your late Vette rear axle is fed by a standard driveshaft and the latest Vettes with the transaxle is another matter entirely. So, no, it won't work unless you spend extensive money and time having an adapter made. And there would be no gain as far as I can see. I was trying to figure out why you wanted to turn the engine around and the only thing I can think off is to move the differential rearward making the engine sit in front of the rear axle instead of straddling it like it would if you used a stock Toronado setup. If that is the case... I'm not certain but you may be able to install a Turbo 400 tailstock and output shaft on the Toronado trans, run a short driveshaft and then install a flipped differential of your choice so that you don't end up with 1 speed forward and 3 reverse with your rearward facing engine. If the parts interchange it should be a fairly easy job. Now, flipped differentials shouldn't have any lubrication problems but not a lot of people have run them that I am aware of so I can't say anything about durability. Yeah, I've also thought about this before but it is probably easier to get a proper transaxle, like a Porsche, and use a ready made adaptor. Good luck, and put up some posts when you get going, sounds like my type of project.

First of all, it is not a std. old style differential, it is one of the nice aluminum boxes that bolts on the back of a newer Vette (like a C3) transmission housing. So it is already basically the same kind of set-up as the Toro differential, a bolt on. Second it is TONS lighter than the cast iron beast that the Toro came with. Third, although it makes a little difference, by turning the system a** backwards it moves some of the weight of the large heavy chain drive and the transmission forward which would be helpful in this ultra light rear weight biased car! Forth, it shifts the low hanging chain case from the extreme rear of the car to closer to the middle which means the various crank driven belt drives are now at the rear and tucked up under the body. And last, there isn't any room for even a short shaft drive, this already short Toro drivetrain extends from just behind the driver's seat back to the rear bodywork with the lower chaincase actually showing about 2" into the rear pan as it sits now.

I appreciate your support and will certainly do some photos when it is ready to go. Any body else got any exerience/advice? Been waiting for Chip Foose to sneak this thing out of my shop for years now and evidently he doesn't know about me! Loved his show and how FAST they could turn around a Beater into a Show car! HA!HA!

Swiss
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Old 07-19-2012, 10:40 AM
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Is there a reason your wanting the Corvette diff? I think the stock unit could be flipped and no special adapter required and just redrill the mating flange, correct? (I'm a closet mid engine olds builder wanna be)


Oops, never mind, I had missed the middle of your second post
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Old 07-19-2012, 04:07 PM
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You will have improper gear oiling. In the upright position, part of the pinion gear, and part of the ring gear sit in a lube bath. which instantly oils both gears as they contact each other. From there, everything else is splash oiled.

If you flip the housing, the ring gear will be the only thing in the lube bath, the contact point will only be oiled by what the ring gear can carry, with very little lube left for cast off.

You'd need to move the vent too, but thats easy compared to fixing the lube issue.

I'd also want to check the torqure rating on that axle, Olds torque output is notorius for ruining under rated axles.
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Old 07-19-2012, 04:13 PM
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Thanks, I think the Torque limit will be the tires. Will be running Street Radials, wide but not slicks and the total car weight will be under 2500lbs. I am aware of the 500-600lb. Torque outputs of the big Olds engines!

Swiss
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Old 08-16-2012, 12:03 AM
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TH 425 fan

I have a rear v8 powered fiesta with that trans heres a link http://www.overbeckmachine.com/Fiest...naIMG_5844.jpg

anyway I installed a 31 spline trutrak and 3.70 gears and with a little metal removal the whole rear-end weighed 78 pounds.
Not too bad for all that iron..
Remember this is a spiral bevel diff, don't know about the vette but I suspect its a hypoid. Way less efficient than what you have and the pinion will be lower or higher by a fair amount.

if its a hypoid flipping it will lower the pinion and therefore raise the whole drive-train. If you don't flip it, I don't believe the axles will clear the crankshaft.

I suspect moving the engine to the rear like that will be making the weight distribution far worse than it already is after all the toro 425 complete with the rear end weighs about 290. If your talking V8 (and you better be or go find a far lighter drivetrain) you will be moving 425 - 500 lbs to a place I wouldn't want it.

The stock configuration works
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Old 08-16-2012, 01:07 AM
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Actually by turning the engine/transaxle around, the engine weight doesn't move towards the rear at all. Since the drive axles are about in the center of the engine the engine weight stays in the same location. Only thing swapping is the entire transmission is now just forward of the axle centerline, a good thing which moves weight towards the center of the car and not back to the back end. And the heavy chain assembly is also swapped to the front of the engine (relative to the car) and is also closer to the center of the car. Relatively light weight pulleys end up being at the back of the car for the alternator and water pump.

Yes, using the Olds 455 engine, heavy but strong. And it is not so bad compared to a small block Chevy for weight. Also plan on some aluminum heads to shave a couple of lbs. off the engine weight.

Swiss
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Old 08-16-2012, 01:59 AM
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Fiesta

Impressive build on the Fiesta. I see that you are building a 2nd Full Track version with the engine moved radically forward.

Have to ask, how was your performance with the first build? Handling problems? Straightline numbers? Engine build? Fuel economy? HA!HA!

What made you decide to build it a second time and go so radical (relatively speaking) with it? Lots of build photos!! What was your running weight for the 1st build? I will be pouring over photos for the next day or two!

Swiss
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Old 08-16-2012, 10:35 AM
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Here is a cutaway pic of the C5 differential.



Swiss
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Old 08-16-2012, 10:36 PM
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Your right on the lack of weight change when turning the unit around, my engine was spaced 5" and yours isn't shows what I know.

I have looked high and low for a different rearend for the th425 but that pinion is awful short and of course special splines on it. A GMC motor home guy went to India and had 3.70 and 4.11 gears made so I bought the 3.70 and while I was at it make a 31 spline trutrak fit. That said I was still looking for a rumored aluminum third member that Mickey Thompson had made for the th425 and that is what I was searching when I found your thread.

Now to answer your questions, built the car in the 80'S It was in hot rod in 1986 sat from 1990 to 2009. Started autocrossing and thats what initiated all the additional mods.

At the drags with 245 tires and 3.08 rear end it ran low twelves our drag strip closed and I always had better traction on the street but after the recent mods and 3.70 according to a gtech device it runs low 11's will take it to sears point sometime and see.

I build a vega it was in hot rod also in the 70's it also had a th425 with the engine on the trans and it handled I thought fine. the fiesta had a 5" spacer to put the axle under the next main cap and it handled MAYBE better. I recently ran the numbers and moving a 450 lb engine 5" on a 90" wheelbase car only moves 25 lbs from the rear wheel to the front. Thats why I am moving the engine an additional 12"

prior to starting this latest mod steady state cornering it would pull 1.25 G on hoosiers and 1.38 g peaks

Correct suspension geometry go's a long way in taming the rear heavy beast.

mileage? no Idea

Wayne
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Old 08-16-2012, 11:57 PM
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No problem on the 5" spacer, lack of weight change idea. Move into a topic and not understand exactly what is being said makes it easy to imagine something that isn't there. Since I am building this as a street car I have looked at the idea of gas mileage! HA!HA! Some HotRods get fairly good mileage and looking at old Olds Toros and Caddys I saw highway mpg numbers of over 20mpg. Not bad for a "******" toy. Everyone has always told me two first statements (if you can have two of them!) You won't be able to keep the front wheels on the ground! And with all of that weight in the back you won't have any decent handling and will spin the rear end out when trying to corner! I countered with the same answer, Tires! You need enough to conteract the rear weight bias in order to handle and you need enough to spin rather than "flip the front end"! I guess it is a real balance and of course having good suspension to work with it helps.

I never did say what the car was but it is an Opel GT. I always wanted a mid-engined sports car and years ago figured that it would be neat to build a "Sports-Street Rod".. As you said, no real big product lines to find a nice aluminum housing bolt on differential so I am left to figure out how to make my own. Same with cylinder heads, Olds engines don't come cheap like building small block Chevy or Ford stuff. If I can take off around 100lbs with a set of heads and the differential swap that will help the *** heavy situation.

I appreciate the performance numbers. I have seen lots of Street Rods built with the Olds THM-425 but never any kind of performance numbers on them. You can join the THM-425 mid-engine group on Yahoo and check out some of the photos and info. I used to have a stack of articles pulled from various magazines through the years and back in '95 I went on vacation to Florida/Disney World and lost it somewhere along the trip. It was my boredom fighter that I could open up and re-read the articles. I am sure that I had your car articles in them, it seems very familiar.

THM-425 : TurboHydramatic 425 Conversions

I stuck a link to your photos on the Group Link listings.

One of the Fiero guys built a gear driven replacement for his chain drive case that allowed him to reverse the input to the transmission and reverse the trans/turn the engine around. Others have flipped the stock Toro differential. I just figured it made sense to try something lighter and modify that to work.

Interesting on the weight change and the front/rear bias. Turning the engine/trans around will shift the couple of hundred lbs. of the transmission maybe 18" or more from the rear of the rear axle centerline to the front of it. Compared to that a manifold change from cast iron Toronado to Aluminum Edelbrock won't be a big change since the location won't shift, just a weight loss.

1.25g steady state is pretty damned good for a street toy! I will probably run non-performance 2.70-1 type ratio. Still with an ultra-light overall weight it should run fairly well.

Swiss
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Old 08-17-2012, 01:15 AM
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OK here's a link to a calculator, put in your wheelbase, weight of what your going to move, and run it for each location (before move and after move) and then find the difference its usually lots less than you would expect.
If your not sure how it works try it with some simple numbers Beam Support Force Calculator


Everyone wants to know if it does wheelies, no it doesn't even try, It does pick up a inside front on hard acceleration corner exit but don't notice it while driving only see it in the pictures.

On the Vega I modified the trans for hard shifts 110 mph 2-3 would chirp the tires, after a while it broke the chain. So on the fiesta I left it stock and its been bullitproof.

There is a very very very remote possibility I may model the 3rd member and machine 1 or a few out of aluminum will let you know if I get serious as I said remote.

An opal is a nice looking small car should be fun!

What are these drive-trains going for these days less engine?
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Old 08-17-2012, 06:26 PM
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Not sure what the going price is, but of course a lot of them have been sent to the crusher. Getting harder to find, but because they have been used in the big motor homes for years, there is still a fair supply. You can find things like cast aluminum oil pans and such, but a cast pan is going to weigh as much or more than the stock steel pan does. Motor home supplier had some altered chain drive ratios but not something for performance.

What was your engine set up for your old Fiesta build?

Swiss
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Old 08-19-2012, 03:47 PM
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Quote:
Originally Posted by waynger View Post
Your right on the lack of weight change when turning the unit around, my engine was spaced 5" and yours isn't shows what I know.

I build a vega it was in hot rod also in the 70's it also had a th425 with the engine on the trans and it handled I thought fine. the fiesta had a 5" spacer to put the axle under the next main cap and it handled MAYBE better. I recently ran the numbers and moving a 450 lb engine 5" on a 90" wheelbase car only moves 25 lbs from the rear wheel to the front. Thats why I am moving the engine an additional 12"

Wayne
Wayne,

One option rather than the long tubular spacer and drive coupling and such that you are going through would be to reverse the engine/trans, have the engine/trans/chaincase weight all forward of the rear axle and run a TM400 tailshaft housing with a short driveshaft to a differential (aluminum of your choice and flipped for rotation or don't know if a 4WD front differential has the correct rotation?) and your rear suspension. This moves the entire engine AND trans forward rather than just the tranny. Your limit to the forward setting is how much room you have and with the engine offset to the passenger side of the car you can basically center your tailshaft output and drive shaft. With your current 17" or maybe a little more you could possibly clear the front of the engine (now at the rear) with your independent drive shafts to the wheels?

Of course that means re-engineering all of the extension work that you have already done!! HA!HA!

It has been mentioned that there is a way to reverse the transmission output shaft rotation by swapping in some 400 series gears/parts into the THM425. But I haven't found anyone that has actually done it, just rumors so far.

Swiss
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