View Full Version : 4100/8-6-4 Swap- Help!

03-28-04, 02:44 PM
For all you fellow masochists out there ('80s fans):

I have a little situation only I could find my way into, and need some help / technical advice. I have (among other things) a 1983 Coupe de Ville (Cream/Cream/Tan) that I bought with a blown 4100 (the body, interior, and astroroof were well worth it). I had the motor and tranny yanked out figuring later I would drop a 350 in it, as well as some other things. Recently, an older gentleman I used to work with told me he was parting out his 81 Eldo and I could have whatever I wanted. Knowing it had a perfectly good running 6-8-4, I jumped all over it. Yesterday I went to check the car out, and was pleased the motor ran great, but displeased to find out that car was front wheel drive (Eldo's not being my specialty, I didn't know this). Sooo. . .

This motor does not appear to be transposed unlike the 4100 motor in their respective front wheel drive cars. I know the 6.0L is the preferred motor to have in the RWD cars I fancy, and need to know if this motor can be bolted to a RWD tranny and dropped into my car. I can get the ECM and all that no problem. I am also not worried about motor mounts, exhaust, etc. If anyone is familiar with these motors, please let me know if it can be done. It will just help me get this car running sooner, rather than waiting for something else RWD to part out. For credibility's sake, I also own an 84 Coupe (Silver/Blue, wrecked and bad 4100- nice car though), an 86 FWB de Elegance (triple black), a 91 Brougham de Elegance (triple blue), and a 93 FWB (Black/Tan)- just so you all know I'm not THAT nuts.

Any help or advice would be appreciated.


"Fleetwood" Max
Richmond, VA

03-18-07, 10:23 PM
You could probably use the V-8-6-4 in the 83 Coupe de Ville, but you will need many RWD parts in order to make it fit. Your biggest problem will be with the tranny. The transmission mated to the HT4100, which has a lock-up tourqe converter, will be too weak for the larger engine. If you decide to re-use the existing tranny, you will definitely need a shift kit. Also, on the FWD-configured engine, the flywheel, shift linkage and other parts will be different. You may also run into computer problems. If you decide to go through with this, you will need to search salvage yards and hope to find an 80-81 RWD Cadillac with the same engine and harvest the necessary RWD parts from that car.

My recommendation is to replace the HT4100 with an Olds 350 or 403 gasoline V8. A small-bock Olds V8 is the perfect choice, because many Cadillacs from the early 80s were built with the GM 350 diesel which was based on the Olds small-block design. The 403 was widely used in many full-size RWD Pontiacs, Oldsmobiles and Buicks in the late 70s, especially in station wagons. Hopefully, you can find a complete engine-transmission combo from one of the afore-mentioned C bodies. Any parts you my need to make the Olds V8 fit in your car (including an entire transmission), could be found on an 81-84 RWD Cadillac equipped with the 350 diesel. Whatever you do, do not attempt to use a Chevrolet V8. While Cadillacs, Oldsmobiles, Buicks, and even some Pontiacs shared many similarities, Chevorolet products were vastly different from those of the other divisions up through the late 80s. A Chevy small-block will not go into your car without major modificaions

I hope this helps!

Conshohocken, PA

03-18-07, 10:42 PM
I disagree, the 200r4 should be well up to the task of a stock 368 -V8-6-4.
those engines were kinda underpowered compared to their older siblings and since you can easily get one go for it. you will probably need the oil pan from a rwd BBcad engine for the 368 and there are some other mods that may be necessary try searching the forums for more info. I personally would look into new exhaust/manifolds to eliminate the restrictive stock setup.

The Ape Man
03-19-07, 11:26 AM
Good advice has been given mixed with opinion. Here is mine based on what I've learned swapping out about a dozen HT-4100s. The Olds swap was much more popular but you already have a good running 8/6/4. The 8/6/4 will swap in and the electronics conversion will be minimal due to the fact that both engines use DEFI. This isn't the case with a Chevy or Olds conversion. With the 368 swap your exhaust and engine mounts will have to be sourced elsewhere as these 2 cars don't have enough stuff to make that happen. The HT-4100 exhaust from the catalyst back might work but you will need a Y pipe and cat converter from a 1981 RWD. These should be available new. I dont think the exhaust manifolds from the FWD '81 will work for you without welding but dunno for sure. I'd check with a muffler shop that does fabrication to see if they can make the thing work by welding the hacked up '81 Y pipe to what's left of the HT-4100 pipe after you cut it. Changing exhaust maniflods on this engine should be avoided as the bolts will be prone to breakage AND the manifold design is part of the calibration of the fuel system. I would not swap anything here except some small tube headers which would be expensive.
Engine mounts, oil pan and pickup from a 425 or 368 RWD will work for your application.
The sticky part is the transmission. The 2004R will bolt up to the 8/6/4 but the flexplate (flywheel) bolt circle will not work. I've modified 4 in the past with no problems with hand tools and a little careful measuring. The transmission in stock form is weak and it's just a matter of time until it pops. A good trans rebuilder can work around that but you have to see if it's worth the price verses alternatives. If you stick with the 2004R then you will need to fabricate or buy a lockup controller and (shift) detent cable linkage. When it's all done, you will have a car that outperforms a RWD 425 vehicle but gets a little better fuel economy. That extra gear really wakes things up. This assumes that you ditch the modulated displacement which IMHO was largely a gimmick and will not work well with overdrive and a lockup torque converter. It can be kept as an excercise in vibration and displacement schitzophrenia (http://dictionary.reference.com/go/http://www.reference.com/search?r=13&q=Schitzophrenia). The modulated displacement parts can simply be left in place as they were quite reliable. The final gear ratio with the HT-4100 running gear including overdrive will be very close to what the 8/6/4 car had. Too bad Cadillac didn't make them like that back in the day.
One other alternative is to use a TH-400 and 2.41 rear axle just like the original RWD. Trouble free parts but you just lost a lot of roadability due to no overdrive. A third alternative would be to grab a 4L80E from a diesel vehicle with stand-alone powertrain controller for a bullitproof overdrive. These show up on ebay. Google reading required.

03-21-07, 11:59 AM
The flywheel shouldn't be an issue. While this may be overkill, it is compatible with the big block Caddy engine and the 200-4R tranny:
http://cgi.ebay.com/ebaymotors/Cadillac-472-500-SFI-flexplate-new-Maddog-FP500_W0QQcmdZViewItemQQcategoryZ33616QQitemZ32009 3676873QQrdZ1QQsspagenameZWDVW

The HT4100 computer should be able to run the fuel injection for the DFI on the 368. The 368 uses a larger throttle bore for its throttle body than the 4100. If using the 4100 computer, your modular displacement will be disabled. The 200-4R (that the 4100 uses) can be strenghtened to handle the torque of the bigger engines (think turbo Buick).

The Y pipes for RWD 368s are available online (I can't remember where I got mine, but they do exist for around $40. The manifolds for FWD Eldos will definitely NOT be a direct bolt on to the RWD Y pipe setup.

You may want to pull the cable bracket from the 4100 and modify it to bolt on to a spot on the intake of the 368 (measure the location in relation to the throttle body on the 4100 so that you don't mess up the cable length too much, it will still need to be adjusted).

You may need to heat the exhaust manifold bolts to remove them to replace with RWDexh manifolds.

The Ape Man
03-21-07, 07:02 PM
All great info except the manifold bolts. Those bastards don't come out even when heated. I think it would be better to source an 8/6/4 DEFI compuker. One time I swapped a 368 into a car that had a 4.1 and left the stock computer. The thing ran like crap at part throttle. I even fooled it by turning up the fuel pressure but it wasn't enough. Usable though.

01-20-11, 02:15 PM
would the frame mounts be the same? i want to do a 500 swap in a 80s eldorado. i know the 368 would be the same but i just found a 1983 eldorado for 500 with the 4.1. i can do other modification but don't really want to hack the frame.

01-20-11, 11:23 PM
Be aware, any 79-85 Eldo/Riv/Toro engine can be used in RWD, but the slightly smaller dia FWD flexplate must be changed. The whole oiling system & exhaust manifolds generally change as mentioned. But a 500 will bolt in where a 368 was. I wouldn't try using a fuel injection system on an engine it wasn't designed for, or you're opening a whole new can of worms. Bruce Roe

01-21-11, 10:27 PM
If you take the motor, you will need a different oil pan... A 425 oil pan and pick-up tube will fit.... They are eazy to come by.. The oil pan & tube on that motor is worth $100 plus... I'll bet you could get someone to trade and pay all shipping...425 exhaust manifolds and Y pipe will work just fine on the 368.. But you will have to leave off a sheetmetal plate between the head and manifold... If the trans is good it will stand up to the 368... the 368 is a slug....