1987 Deville 4.1 to 4.9 Conversion
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HT4100 4.1, 4.5, 4.9 Discussion, 1987 Deville 4.1 to 4.9 Conversion in Cadillac Engine Technical Discussion; This car was the last year that the infamous HT4100 was used. It's got 270k miles on the clock and ...
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    Edahall's Avatar
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    1987 Deville 4.1 to 4.9 Conversion

    This car was the last year that the infamous HT4100 was used. It's got 270k miles on the clock and everything on the car (paint, interior, transmission) is still in good condition except the engine. The engine has had a noisy valve train for a long time but lately, it has been getting progressively worse to the point that everybody could hear it coming from several blocks away. Also, the rated 125 hp was more like 50 hp. The engine would struggle in 2nd gear with the pedal through the metal going up Snoqualmie pass and passed by semi's that were carrying a full load. By the time you got to the top of the hill, 25 mph was as fast as she would go. We did open up the valve covers to inspect the rocker arm support fixtures for cracks and they were fine. So most likely, the camshaft and cams were wiped out. Also, average fuel economy had dropped down to 12 mpg. Even my 1990 3/4 ton Suburban could actually do better than this.

    Anyways, long story short, we were given a 4.9 out of a 1992 Seville with 80k miles on it. The decision has been made to put this engine in place of the 4.1. The plan of action is the following:

    1. Block off all the injector ports on the 4.9.
    2. Bolt down the 4.1 throttle body to the 4.9 intake manifold
    3. Transfer all the accessories over to the 4.9
    4. Use 350 Chevy injectors
    5. Bolt the stainless steel 4.1 exhaust manifold onto the 4.9
    6. Use the 4.1 distributor
    7. Modify exhaust system to have 2 glass packs in series and replace the back muffler with a chambered muffler.

    Replace this



    With this 4.9

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    Re: 1987 Deville 4.1 to 4.9 Conversion

    I would try to find the alternator brackets and all other accessories from the 4.9L and use them instead. The tensioner, idler, brackets from a Deville are plentiful in junkyards. You won't need the AIR pump, 4.9's didn't have them, and I don't think there would be an advantage to having one on there. The Seville's had 2 O2 sensors, the Deville of your era had only one O2 sensor. I don't know how the 4.1 manifolds work on the 4.9, here again I would look for a pair from a 91-93 4.9 at the junkyard. And grab the crossover exhaust pipe also. You might be able to use the distributor that is in the 4.9 instead of using the 4.1. You will have a better oil pump in the 4.9 and the gear on the distributor should be in better shape, I'd leave it in there. If you do anything to the 4.9 before you drop it in, replace the valve cover gaskets. Good luck and anxious to see how it turns out.

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    Re: 1987 Deville 4.1 to 4.9 Conversion

    You've done this before, IIRC. Also, I'm sure you already read through my conversion but here's a link anyway

    Quote Originally Posted by Edahall View Post
    1. Block off all the injector ports on the 4.9.
    I thought long time about doing that and experimented with some options but eventually decided to use the 4.5 manifold for simplicity. One thing I remember for some reason is the PFI valve covers are different from the TBI valve covers, and they are setup for different accessory mounts, I dont remember if its important it just sticks with me.

    2. Bolt down the 4.1 throttle body to the 4.9 intake manifold
    Should be no problem

    3. Transfer all the accessories over to the 4.9
    I had to use the 4.9 upper brackets. The accessory bosses on the 4.5 TBI manifold were different and will be different for the 4.9 too. Also the alternator in the 4.5/4.9 is a CS-144 instead of the SI-17 that was in the 4.1, and that has a different bolt pattern on the back, and a different wiring connector too. You might be able to keep the old brackets but I went ahead and used the 4.9 brackets and alternator, with a wiring adapter, since this lets me use CS-144 alternators instead of the old SI-17 alternators.

    Also the smog pump setup will not perform any kind of functional purpose on a 4.9 engine since the block does not have the ports for the exhaust gasses. It will just be dead weight, a useless part that can only cost money, etc.

    Rest of it should be straightforward

    4. Use 350 Chevy injectors
    Do you mean 305 injectors? The 4.9 is about 299 CI, so 305 TBI injectors are pretty close. Even those are rich at cold start. 350 injectors will probably flood it out.

    5. Bolt the stainless steel 4.1 exhaust manifold onto the 4.9
    Yup same port sizes.

    6. Use the 4.1 distributor
    Yup keep it simple--some of the wiring changed for the 4.9 distributors


    edited to fix a couple of comments

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    Re: 1987 Deville 4.1 to 4.9 Conversion

    Engine yanked out! Bye Bye HT4100. Anyone want it? Might be good for a riding lawn mower.


    I got the engine bay pressure washed. It was a terrible mess from all the oil that the HT4100 leaked out.



    Instead of switching distributors, I took some of the guts from the 4.1 distributor and put it into the 4.9 distributor. I left those 2 connectors (at the bottom of the picture) disconnected because the 4.1 doesn't use those. I hope it'll work.

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    Re: 1987 Deville 4.1 to 4.9 Conversion

    If you look at the aftermarket HEI parts, you'll see they are different for 4.1 and 4.9. From what I remember, wiring from the coil was different at the connector. It was something like yellow and red were reversed, I don't remember the exact details, but they were basically incompatible electronics. You might be able to get away with swapping the innards, I decided to just buy a new 4.1 dizzy to reset the maintenance clock and keep it simple.

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    Re: 1987 Deville 4.1 to 4.9 Conversion

    Quote Originally Posted by Edahall View Post
    Also, the rated 125 hp was more like 50 hp. The engine would struggle in 2nd gear with the pedal through the metal going up Snoqualmie pass and passed by semi's that were carrying a full load. By the time you got to the top of the hill, 25 mph was as fast as she would go.
    I had forgotten just what a gutless wonder those engines where. I can remember mine (I actually had 2) downshifting with the cruise set and driving into a strong headwind. I think both of mine had permanent creases in the seat cushions from my attempts to pass on a 2 lane highway.

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    Re: 1987 Deville 4.1 to 4.9 Conversion

    I worked for a guy years ago that had an Eldorado with a HT4100 in it. Wasn't bad enough it didn't have power enough to pull a sick whore off a pisspot, let alone all the times I had to drain the cheap gas and water in his fuel system. Most of the time I always thought the water in his gas was the problem until I went on a road test with him and he told me that is how it is supposed to run.

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    Re: 1987 Deville 4.1 to 4.9 Conversion

    Here's what was partly responsible for the terribly low power. Hint... Take a look at what has come out of the end of the pipe.



    Plugged up all the injector holes on the 4.9 with some bolts and RTV. Those injector holes don't need to be used since I've converted it over to TBI.



    The distributors are different between the 2 engines but wanted to avoid swapping them. So I swapped the innards instead. I hope it will work.



    4.9 engine ready to be installed.


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    Re: 1987 Deville 4.1 to 4.9 Conversion

    A clogged CAT will certainly kill performance.

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    Re: 1987 Deville 4.1 to 4.9 Conversion

    Got it running. I'm currently working out some of the kinks. The exhaust was a major success. It sounds very good and produces just the right level of noise.

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    Re: 1987 Deville 4.1 to 4.9 Conversion

    good to hear... pretty simple swap once the mystery is taken out of it

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    Re: 1987 Deville 4.1 to 4.9 Conversion

    Here's how the 4.9 engine sits in the car. It runs very smoothly and has tons more power than the HT4100. So far, I'm very pleased with the results and I'm glad that I saved this very well built car from being sent to the crushers and recycled into a Smart car. With only 80k miles on the heart transplant, it should last for a very long time. Also, my homemade exhaust fits like a glove and sounds like some of the more expensive exhaust systems out there. It was easy to do with the engine out.

    I don't know if anyone else is interested in saving an old car like this but if someone is, I would be willing to write something up on how to do this. The job was not difficult but there was someone planning and homework that went into this to make the conversion successful.



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    Re: 1987 Deville 4.1 to 4.9 Conversion

    What did you do about the accessories and brackets?

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    Re: 1987 Deville 4.1 to 4.9 Conversion

    I ended up using the bracket (that supports the alternator and power steering pump) from the old 4.1 engine. It fit pretty well except 1 bracket feet bolt hole in the back (of the bracket) did not line up and I had to put some washers under the front two bracket feet bolt holes. I didn't have the accessories from the 4.9 so for me, this was the preferred route to take. The bracket is plenty stout enough even with the 1 feet bolt hole not bolted down.

    I also used the 4.9 distributor with some of the guts transferred from the 4.1 distributor. This was a lot easier than swapping distributors. I never even had to set the timing.

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    Re: 1987 Deville 4.1 to 4.9 Conversion

    Did you happen to get a picture of the block on the side down behind the starter? In 92 they were supposed to have changed the casting with ribs on the side and a thicker oil pan rail. Glad to see you got it running.

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