Because the 1981 computer coudln't keep up with the system it had to control.
A 90* V8 will run smooth on 8 cylinders, and even on 4 cylinders (in the case of select cylinders being shut down) but NOT V6 mode... no matter what cylinders you shut down. (Notice how none of the new...as in todays systems are V8/6/4, just V8/V4)
So when the computer would tell the engine to run on 6 cylinders, it would start to shake alot and wasn't very smooth.
The computer would knock down cylinders based on engine load... usualy by throttle input and manifold pressure... but that system is flawed because if you are coasting down a hill it would knock it down to V4, but then go to hit the gas and it would stumble around a bit before it switches the V8 mode to accelerate. Same thing for around town, it would switch between V8-6-4 alot.
It would ONLY deactivate cylinders when in high gear, so it woudln't do it in stop and go traffic... but even at 35mph, its in 3rd and would start cutting cylinders back.
There are a few more things as well.... all pretty much about the same thing.
Well, another thing was, the V8-6-4 feature was never counted in the EPA fuel things, so on paper, it didn't get much better mileage then prevoius versions. So the whole idea behind it.... better fuel mileage while still having V8 power was now kinda shot down on the window sticker thanks to the EPA.
Notice how none of the problems were due to the selonoids themselves or the physical deactivation method, because both are about as simple and solid as they can get.
Fast forward 25 years and we now have the system coming back. Instead of messing with the rockers, they are now in the valley and change the lifters a bit... why? the V8-6-4 was ugly looking, humps in the valve covers... with none of that up there, they now have a sleek looking engine again.
No more V6 mode... GM/Chrysler now realize you can't make a 90* V8 run smooth in V6 mode, so they stopped it all together. V8 or V4.... keep it simple. Plus these new engines have the power to push V4... the 368 needed the V6 mode to act as a buffer as it was somewhat low on power already. Which is why on even a 425, the V6 mode wouldn't be used either, there is enough power to push 4cylinder mode.
Finally, the computer systems today are far more advance and can handle it so it is now seamless operation.
The Cadillac Sixteen has V16-8-4 mode.... they can get away with that, because the V16 WILL be smooth in V8 mode.... it's all about balance on the crankshaft.
In the case of what I want to do, I want to wire up all 4 selonoids to an on/off toggle switch, mount it somewhere on the dash, so its either V8 or V4... when I want... either crusing on the highway, maybe when stuck in traffic etc... if there is enough power in V4 mode, I'll use it... otherwise I have V8. It is a very solid system, I am glad I got the parts off atleast 1 of them so far.
It worked by simply changing the rotation point of the rocker arm. Instead of it piviting in the middle to open/close the valve, the pivit point was effectivly raised, letting the rocker itself move up and down. Both valves would stay closed.
The dead cylinders were hardly any extra work on the rest of the engine, the same fuel/air kept getting compressed, all the energy used to compress it was gained back.
It was an amazing system, the theory behind it is great, and is now being used again.
One of the reasons I like it so much... and want to adapt it onto my engine.
It was very ahead of its time, and I give Cadillac & GM TONS of credit for inventing it. I just wish they would have held on to it longer and refined it, and possibly made the 8-6-4 optional to the HT4100. That would have been cool!
Its a very fragile motor. It was ahead of its time for 1982, but it was under engineered and rushed into market unready. It seems that they have problems with coolant leaks, headgasket not sealing, intake manifold not sealing, the oil pump was faulty, and IIRC, the water pump was faulty also. Most dont last past 60k miles, but some do last to 150k miles.
Oh yeah, and they are VERY underpowered. 125-135hp for a 4100lb car
I don't mean to piss any HT4100 owners off, this is just what I have heard.
Very fragile motor???? No man, my dads 4100 has 170,000 miles and has plenty of power and he doesn't drive it like a baby. We were 1 1/2 hours late to airport for my uncle, and my dad beat the living shit out of that motor. I could swear that the rpms were maxed out. Still runs like a clock and we never had ANY gasket issues!! Take off is great also. My dad drives the caddy like he drove his M6. Tranny is also original, the car just needs engine mounts. I took out the plugs from bieng 8 years old, and they looked new. The 4100 isn't bad in the FWD cars, 82-85 RWD cars, that engine is useless. That is where you start to blow everything.