View Full Version : ht4100 swap ideas (keeping dfi and TH200 trans)

06-25-11, 01:56 PM
I hope I got this in the right spot. I got a very nice 84 coupe with the HT4100. Runs good right now, but I guess we all know the inevitable with these engines. First, I am not looking for a hot rod. I want a good cast iron powerplant and would like to keep it all cadillac if possible. That being said, could I replace the 4.1 with a 368 and keep the digital fuel injection and TH200r4 transmission? I read where the 368 motors could be had with dfi. The fuel injection and overdrive are the 2 most important things I want to keep in mind. Any ideas? Just looking for a good reliable cruiser. Thanks

06-25-11, 02:22 PM
The TH200 will bolt to the 368, but the 200 isn't strong enough to handle the 368's power and will fail. An alternative you be getting a Chevrolet THM700 but you would need a bell housing adapter and a customized drive shaft. If you go down that route, the Cadillac 500 and 425 also had the option of fuel injection, and will bolt into your car without much trouble.

The 4100's computer probably won't work with the DFI system used in '81, but you can harvest the computer from the car you get the engine from. The fuel pump in your car should be enough to be adequate to supply the 368.

Another possible solution would be getting the full drive line and computer from a '91 or '92 Brougham. They used either a Chevrolet 305 or a 350 (both TBI), connected to the THM700.

06-25-11, 03:13 PM
The torque rating of a th200r4 shows to be 330 ft/lbs. Is the max torque rating different for the transmissions found in rwd ht4100 powered cars?

06-25-11, 04:00 PM
The 200 isn't a durable transmission and I wouldn't trust it behind anything more powerful than an Oldsmobile 307. Some people use the 200 with the Oldsmobile 350, but they usually set it for lower power outputs, similar to torque and horsepower of the 307.

06-25-11, 04:06 PM
I believe the 368 is MPI so a TBI computer wouldn't work, right? I like the idea of the 91-92 transplant. Better everything, could even swap over the digital dash.

06-25-11, 04:58 PM
The 368 wasn't port fuel injected; it used a throttle body. The cylinder deactivation used oil control valves that locked the intake valve in the closed position.

06-25-11, 09:40 PM
ok here's what i found out at rockauto.com. The many of the DFI parts between 1981 and 1984 share the same part numbers. Fuel pumps, injectors (Delco brand), fuel pressure regulators, idle air control, throttle body repair kits, fuel tanks, MAP sensors are all the same, at least in the delco brand. Couldn't find part number for the 1984 throttle body, but the injectors share the same part number for the two years. The only difference I could find was the part number for the computers. They appear to have the same type connectors, so im really wondering now if it might could work.

cadillac kevin
06-25-11, 09:51 PM
get yourself a caddy 501 family motor (425/ 472/500) and a turbo 400 trans or a chevy 350 (either vortec or tbi) and a 700r4 trans. ditch the 4100s fuel injection system.
imo the 368 is more headache than its worth (cylinder deactivation system was too far ahead of its time). if you're gonna pay the $ for an engine/trans swap go with something that either has awesome mileage or will make up for a lack of mileage with good power. the 368 is a bad mix of those 2 (mediocre/bad mileage and poor performance based on the displacement and technology in the motor)
IIRC the 200 trans used with the 4100 is actually weaker (poorer design) than the 200 used with the olds 307.
btw if you have a properly set up carb it will perform (both in power and mileage) about the same if not just as good as a basic throttle body (which is what "DFI" was back then)

06-25-11, 11:29 PM
^He wouldn't need to utilize the cylinder deactivation with the 368, which would take most of the headache out of it.

The only difference I could find was the part number for the computers. They appear to have the same type connectors, so im really wondering now if it might could work.

The programming in the computer is completely different and would not be able to operate the engine correctly.

06-25-11, 11:56 PM
I think you may be on to something fslay. It sounds like all you need to do is switch computers and hope all the sensors plug in. They may not be in the same places though. I know nothing about the 368 fuel injection but apparently it's a lot different then my 76 Seville's multi-point fuel injection. If money is no object you could get a better 200r4 to handle the brute torque of that 368. It's all too complicated for me and I would just drop in any carb engine with a th 400 or 700r4. The 90-95 Chevy engine option would probably be the most fuel efficient. Good luck and feel free to continue brain storming here.

06-26-11, 12:22 AM
If you REALLY want fuel injection, go buy a 5.3 liter LM7 v8 out of a 99-2007 Chevy truck or 2000-06 Tahoe/Suburban/Yukon. Those motors are dirt cheap at the junk yard, like fully dressed with trans and low miles for right around $1000. It'll require a complete rewire of your car, but IMO this would be the best bet for going fuel injection. Don't go messing around with 70's/80's GM EFI parts, GM really hadn't figured out what they were doing back then until they came out with TBI on the Chevy 305 and 350.

06-26-11, 12:33 AM
Aron makes a good point. I'm thinking the wiring harness will not be the same. The connection maybe but probably not the wires. I hear that 5.3 is a good overall powerplant.

06-26-11, 12:58 AM
One things for sure...I really appreciate the ideas and tips. I know the 368 only makes around 265 ft/lbs of torque (according to internet sources), but that's a lot more than the 200 ft/lbs produced by the 4.1. I was shocked to see so many of the sensors, and misc parts were the same on the dfi systems for the 368 amd 4.1. I think it would be pretty neat to have added torque of the big block with the superior gear ratios of the 4-speed automatic. I couldn't find the ht4100 throttle body for my 84 cdv on rockauto to compare part numbers to the 81 model, but considering the injectors share same part numbers, it sounds like a safe bet to me. Are the dfi intake manifolds difficult to find for the 368?

06-26-11, 02:23 AM
Like others said if you're going to do it use a better transmission and forget about the 4100s fuel injection. It won't handle the bigger engine. Best bet if you want fuel injection is to get the full drivetrain from a 90-92 Brougham like sven said. Or Caprice/Roadmaster etc which would be cheaper. If you really wanted fuel injection you could do the 360 deal and find a way to use the fuel injection setup from a 305 or 350. It shouldn't be hard finding the parts to customize it to make it work.

cadillac kevin
06-26-11, 09:52 AM
it depends on what is in the yards around you. those cars are pretty rare (by now). most got junked because of the finnicky (and cost) of troubleshooting the V8-6-4 feature. (the motor itself is bulletproof. the fuel injection not so much). theres one in a local yard here that has the complete engine (minus throttle body). It still ran when it was parked there (got plowed from the back).
I understand you want to get decent power and mileage on the cheap, but your easiest swap would be to buy an engine/trans combo (either a 350 chevy/ 700r4 or a 425/472/500 with a turbo 400. the chevy would get better mileage but the 425/472/500 have gobs of power on the low end (and would probably mess up the wimpy 7.5 rear end). personally I'd buy a tbi or vortec 350 and matching trans (can usually find a totaled chevy pickup, van, or caprice with them in it for cheap), pull the drivetrain, put it in your car, then send the other vehicle to the scrapyard. a 90-92 brougham would be ideal, but around here they're pretty costly ($3k and up) and dont seem to get totaled too often.