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2K views 10 replies 6 participants last post by  carnut 
#1 ·
Alright, here I am again, I passed on the ht4100 Coupe DeVille I posted about earlier, and have since spotted another car. This one is also a Coupe DeVille, this time a 1981, white with a white landau top. It also has the V8-6-4 engine.

Now, I've read about the problems in the V8-6-4, and was curious if there were any workarounds. Aside from simply deactivating the cylinder-reduction feature, that is. How does one deactivate the feature (assuming it isn't already, which is highly likely)? Also, are there any other common problems with the engine?
 
#2 ·
The ONLY problem that the 8-6-4 368 had/has is that the engine was out of balance briefly when it switched to 6 cylinder operating mode. It was only in this mode for a brief time before it switched to four on the highway. The problem was, Cadillac owners were used to butter-smooth big blocks, and complained about the vibration. So, dealers were authorized to disconnect the 8-6-4 feature. It is activated by a wire from the trans to the ECM.

My 81 has it disconnected, but I plan to try and reconnect. However, I am going to add a switch to keep it turned off, then only activate it on the highway so I have only 8/4 operation (avoiding the unbalanced 6 mode).

Check the current "variable displacement" V8s from GM and Chrysler. They are 8 and 4 cylinder operation only.

The 81 is a great car. Last year (other than limo) of the factory big block Cadillac. You also get the bulletproof TH400, and the beefy 8.5 rear axle. The 4100 cars got the weak 7.5 Chevy axle.
 
#3 ·
Jay, are you sure you can engage it while the car is in drive, or even on? I'd just be afraid of making changes while in motion. Maybe you could hack the computer and have control when and when not to turn cylinders off. What kinda mileage will she get on only 184 cubic inches? I mean, are the other 4 getting "dragged" along for the ride?

Greg
 
#4 ·
The stock setup engages it while the car is in motion. You aren't dragging those extra cylinders along - soleniods activate to keep the valves closed so those cylinders act as an air spring... just like in modern cars.

Here's the 1981 brochure for a little more reading...

I don't have the fuel data handy, but I will see what I can find. It is supposed to be good, though, in 4cyl mode...
 
#5 ·
All the ECM does is provide a ground path to engage 6 or 4 cyl mode. A simple toggle switch can be used to interupt this ground, so with switch de activated, it wont switch at all. With it activated, it would switch. I too like the 81 Cad 864 powertrain. They used plastic valve covers!! so make sure they dont leak because of cracks.
 
#7 ·
Alright, I'm going out to meet this guy tomorrow. In the meantime, another question:

Let's say, for argument's sake, that I wanted to give this car overdrive. How does the 4L80E or 4L85E compare to the THM400? From what I've read it's the THM400 with overdrive and some electronic controls.

This wouldn't be done for some time, however, as transmissions are expensive and changing one without dire need isn't high on my list.
 
#8 ·
The 4L80E will be plenty strong. It will likely require floorpan clearancing, a modified crossmember, and lower rear gears. The stock 2 series gears will lug the engine once it is in OD. You would want to go to at least a 2.9x, or a 3.xx. However, I am not aware of gear sets for the axle that is in there. It isn't the same as the later 8.5 that came in the L05 and LT1 cars.

Another issue is that you would need a separate stand-alone management computer to control the trans. Available from GM, at pretty much the same price as you would pay for the trans itself. A couple thousand dollars will buy quite a few tanks of gas...

Honestly, give it a try as is. They are set up well from the factory to get decent mileage without OD.
 
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