1983 Coupe deVille HT4100 to 425 swap

[BigBlock83]

Like the title says I am replacing the 4100 in my 83 Coupe with a 425 from a 78 Coupe deVille. The poor little 4100 soldiered on for 33 years and 115000 miles before succumbing to all the usual 4100 stuff. Many people in the recent 4100 thread asked me to do this so I'll try my best. I am not doing the work myself, don't have the space or equipment so I have a shop doing it. This being the case I can't really speak for exactly how all this is getting done, I'll simply be explaining what parts were necessary and what the outcome is.

I'm keeping the whole 83 drivetrain save for the engine. I know all about the 200R4 and it's reputation and I don't care. Keeping it is easier in terms of parts gathering and I like the overdrive. I'm counting on having to do a rebuild of it. I'm fairly confident it will be fine if not abused and I don't plan on driving this car like that. If rebuilds become an annual ritual I'll start look for a Turbo 400, driveshaft and rear axle.

The work just got started so I don't have a whole lot to report. The 4100 is out and laying in a corner, where it belongs. The 425 is in. It fits like it's factory. I used the 425 motor mounts, I've heard the 4100 mounts will work but didn't even bother trying. It is not necessary to swap out oil pans or exhaust manifolds with 368 parts as I have read in some places. A 425 from a RWD 77-79 will drop right in to a RWD 80-84 (and beyond I suppose, I'm only going to speak for model years I know well).

To make the 425 work with the 200R4 you will need a universal flexplate. I got one from Cad Co, MTS also makes one. Once you have one of them the engine and trans bolt right together. My mechanic has decided to use the mechanical fuel pump, when I know all the details about how he is doing it I'll update. I also got a torque converter lock up kit he will install since the car is no longer running off the computer. Hopefully he makes some progress over the weekend and I have some more to report Monday.

The biggest headache I'm predicting is the AC. I honestly have no idea how that one will play out. If anyone has any suggestions or tips I'm all ears. I'm also curious what will happen with the idiot lights in the dash. I'm guessing since the computer isn't talking to the engine anymore they will all light up. If I have to take off the dash pad and pull out the bulbs that isn't a big deal.

Hopefully you guys find this informative and helpful. When there is more significant progress I'll update the thread.   

[35-709]

It's not the TH200-4R you need to worry about, it's the TH200 --- there is a difference.  If you already have the 200-4R you are in much better shape. 

[corey raub]

Thank you very much for the well written post.  I always wondered what was involved in swapping out a 4.1 liter engine for a 425.  One would have to assume there are a lot of owners of Cadillacs with 4100 engines wondering the same thing.  I personally prefer the level of detail you provided.  You are informative but not boring.  I will be following your progress.  Thanks again.  Corey   

[Scot Minesinger]

The dash warning lights worked by making ground if the oil pressure got low, temp got too hot and etc.  It may not be too bad of a job running power to the correct warning light with ground back to sensor in 425 engine for it to make ground if problematic. 

Agree the climate control may be the issue.  The only real interface with a 1983 engine/compressor is the refrigeration hoses and turning on compressor I think - hopefully not much more.  Maybe you could install an ignition on only power source that enables a/c compressor clutch via manual switch in series with ambient switch so a/c would never be on if below 50'F (or so - adjustable).

[BigBlock83]

Thanks for your replies and compliments.

I have the 200-4R. I forget there are two THM 200 versions and I get lazy and forget to mention the 4R sometimes. I'm really not too worried about it. I know in stock form the 425 is pushing the envelope of what it's designed for. But that same trans was used in the Grand National and other "high performance" G bodies so there has to be a way to build it so it can handle more power. Like I said in my first post I have no problem with and am half expecting to do a rebuild.

The warning light scenario sounds like it's worth looking into. I was planning on installing auxiliary gauges. I don't like the telltale lights, I've always preferred gauges. But I also don't like how a lot of aftermarket gauge installations turn out. I'll have to chew on this one a little more, but its nice to know there might be an option to keep it all stock.

Thanks for the insight about the AC. I'm sure I'll get it worked out but right now it's not priority one, the AC can be dealt with later. I've kind of gotten used to not having it in that car anyway, the 4100 always felt like it towing a boat with the AC on so I hardly ever used it. The problem will be addressed and solved and all your input will be appreciated, but right now I'm not too concerned with it.

[Chas]

Big Block........this is like the answer to my prayers! I have a gorgeous '83 Coupe that was bought new by my late Dad. 4100 equipped, and on its last legs. Also have a 425 laying out in the shed. I'll be watching this thread closely!

[35-709]

The 200-4R can be built to handle a lot of HP.  I am using one behind a 325 HP 350 SBC in the Mark V Jaguar sedan I am building.  The tranny was built with HD parts but no "shift kit" --- I dislike being kicked in the butt everytime it upshifts and I am not going to the drag races with it.  It's the wife's car.   

[BigBlock83]

You're confirming what I was thinking. They'll never be a THM400 but you can make them respectable. My dad had this transmission rebuilt about 25 years ago and it has a shift kit. It actually chirped the tires going into second once, my 4100's greatest performance achievement. He can't remember if it had any other go fast goodies put in it. It shifts pretty tame too unless its under heavy throttle.

[TJ Hopland]

I would think as long as you drive it like a Cadillac would typically be driven the rest of the drive train would hold up reasonably well.    A stock 425 isn't that amazingly powerful.  IF we were talking about a built 500 things or heavy towing things would be different.   Even a stock rebuild on a transmission like the 200 4R these days improves them quite a bit over how they were when they originally left the factory.

I think where the exhaust changed was for 77, they basically flattened the 500 manifolds to give more clearance the new frame/suspension design.

The dash lights should be fine, they all have their own sensors so its just a matter of swapping over the sensors or finding a similar sensor that will thread into the 425.  I think the 4100 had metric threads in a lot of places.    Look up http://www.digi-panel.com/  as an option for gauges.   Its not especially cheap but much less out of place than the typical round aftermarket gauges.   I think that company is a one man show so be sure to ask the expected turn around time to ship your order. 

For the AC even if you can find a bracket from a 368 that will let you mount the original R4 compressor I doubt the hoses and lines will still work so you will be looking at some custom hoses.   If it was me I think I would get a Sanden compressor and custom hoses.  MTS has a bracket that will fit any of the intakes and was where I bought my compressor.   They had them for the same price as everyone else did.  The R4 is cheaper but not by much.  The Sanden has a better rep for performance and reliability than the R4.   The A6 off the 425 would be an option too but those are getting expensive and a little hit miss for quality plus people say they just don't work well with the rest of the system where it cycles the clutch.

[BigBlock83]

TJ, I want to make sure I am understanding correctly. If I get the compressor bracket from a 368 I can use the compressor from the 4100 (R4) with custom lines and hoses. And everything will work from the 83 climate control head like it always has. Is that correct? This honestly is my ideal scenario. I know the original compressor is good. I have no clue what condition the compressor on the 425 is in. Plus I would really rather prefer to not have to install switches and such on the dashboard.

This has all been good information guys, thank you. Stay tuned for more updates.

[V63]

There is a significant weight difference in these motors...the front coil springs might need to be replaced with specifications that can compensate?

[BigBlock83]

You are correct. I'm comparing part numbers between 79 and 83 and the springs will need to be changed. Shocks are the same though. The next phase of this project will be suspension work so I anticipated this already.

[TJ Hopland]

IF your R4 is good then getting the 368 brackets should let you keep it.   The 368 did use the R4 and it was more or less in the same location as the 425 and 4100 for that matter so maybe you could get lucky and just need slight mods to the lines.   

Changing the compressor doesn't effect the controls.  The climate control does tie into the ECU but from what I have read apparently doesn't care if the ECU is missing, it still works.   

Cruise apparently did get some info from the ECU so you need a diesel or 80 module to make that work.   These days just going to an aftermarket system would likely be easier and more reliable than trying to find working OE stuff.   You can for sure still use the OE set button.  I would have to look at a diagram to see if you can use any of the others.   On my 81 diesel set is all I can used because that was still a mechanical system which had a really unique switch arrangement. 

[BigBlock83]

Thank you TJ. This is the route I'm going to go. I really like the Sanden idea but right now the budget dictates I should stick with the R4.

As for the cruise control, well I'm going to go with the delete option. I think I can count on my fingers how many times I have used cruise control, on all my cars. It's not remotely worth my time, money or effort to restore something I barely use.

[Carl Fielding]

If you are increasing the front spring rate , you should also increase the rear. - Carl

["Cadillac Kid" Greg Surfas 15364]

If you need the 368 bracket set up PM me.
Greg Surfas

[BigBlock83]

I just paid the shop a visit and things are looking good. He predicts we are a day or two away from start up. Right now he is reinstalling the radiator and all the engine accessories. Next is running all the vacuum lines and electrical hook ups. The rat's nest of wires coming out of the firewall is mostly irrelevant now because the majority of them are for the sensors from the fuel injection system.

So far everything has been going smooth with a few minor, though note worthy, modifications needed. The first two involve the location of the starter. On the 425 its on the passenger side, the 4100 it's on the drivers side. This means the battery needs to be relocated to the passenger side and he needs to modify the torque converter cover to clear the starter. Neither is a big deal.

We are going to use the original 83 radiator. I forgot to ask which year's hoses he ordered, I'm assuming 77-79. You will also need the 77-79 fan shroud, which I took from the parts car I got the engine out of. And speaking of parts from 77-79 you will also need the exhaust Y pipe.

My mechanic decided to use the mechanical fuel pump, which I'm fine with. I've not encountered any yet but I have read the electric pumps from those years could become problematic and hard to diagnose. He dropped the tank and removed the electric pump. In case you are wondering the gas gauge sending unit is a separate part so removing the pump won't affect it.

The next puzzle to solve is hooking up the throttle and kick down cables from the old throttle body to work with the Quadrajet. I don't see him having huge problems making this work. As I have said earlier we are scrapping the cruise control. It's just too much work for a feature I don't use. I might leave the parts in there for appearance's sake or maybe remove them to unclutter the engine compartment. I'll deal with it later on. Once its running and back on the road I'll swap the springs to handle the extra weight.

I hope you guys are finding these posts informative, I'm not a professional (barely an amateur) mechanic so sometimes I feel a little out of place trying to explain this stuff. And I truly do appreciate all of your input.

[BigBlock83]

The work in progress.



And here is an older picture of the whole car. Looks the same today except the fillers have rotted away while the 4100 was dying. I have new ones waiting to get painted. Also, digitals cameras have come a loooong way.

[V63]

If I were doing such a conversion, I would opt for the 'drop on' fuel injection units. The price is now reasonable on them. FItech is one I'm installing now on a 500 CID, and I put a FAST brand on a 365. The ease, performance, smoothness, starting, emissions and efficiency is absolutely outstanding! You may never want to go 'carbonated' again! I know I don't! They are generally a one day conversion, it's that easy!

I mention this now as you have the tank out to install the In tank elect fuel pump which is recommended over the rail pump (noisy too)

[NickD]

Hey big block, I am enjoying your progress reports. I may attempt the same type of conversion project in the future. I have a question will the 83 radiator, have a large enough coolant capacity to cool the 425 engine?