Quote:
Originally Posted by Old Tele man
...contact the SAE for papers on (A) GM's variable (V8-V6-V4) engine (fiasco) and (B) Chryslers's Multiple Displacement System (MDS) Hemi engine(s).
...and, I'll bet, FoMoCo has dabbled in this arena too.
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The Cadillac V864 was actually a beautiful design that was plagued by a woefully inadequate engine computer. This idea wasn't tried again until Mercedes experimented on their V12 engine at the turn of this century. GM recently reintroduced their AFM system, while Chrysler has MDS, and Ford has VDE. Honda and Mitsubishi also have variable displacement systems.
The systems appear to fall into two categories, which are fairly easy to implement. The first system is suited for pushrod engines. Allow selected lifters to completely collapse, and the corresponding valves just won't open. The second system is suited for overhead cam engines where the rocker arms are fixed in the center by a pivot bar. Each valve to be controlled gets two rocker arms for that valve. The first rocker arm follows the cam, and the second rocker arm operates the valve. Normally, the two arms are mechanically linked by a pin. However, remove the pin, and the two arms are no longer mechanically linked, and the valve again won't open.
However, neither system is suited for my engine. This overhead valve engine has the rocker arms underneath the cam The rocker arm pivots at one end on a hydraulic lash adjuster, is actuated by the cam at the center, and actuates the valve at the other end. It's held in place solely by placement on the valve stem and lash adjuster and cam. Remove the cam, and the rocker arm lifts right out.
Quote:
Originally Posted by 5speed5
That said, I'd be willing to bet that you'd be better off just changing out the rear gears to something taller. It would have a similar effect...reducing the number of cylinder fires per mile.
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Fine and dandy, but where would I find a decent diff gear that will fit my truck without me having to do a lot of machining work to make fit, and without having to spend a total of much more than $1000? I've got the tallest available 3.55 gear already.
Quote:
Originally Posted by Frank Lee
A lil mind game: figure at best it won't match the claimed +8 to 20% and at worst it won't be any worse than stock. 19 x 1.08 = 20.52; x 1.2 = 22.8. My WAG is, if it works at all, don't expect more than 1 to 2 mpg. @ +1 you might save 57 gal/yr; +2, 115 gal/yr.
You could find an econo beater that pays for itself in less than a year.
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Um... Your post above highlights the widespread fallacy of using MPG as opposed to L/100km. I actually happened to do this mind game, and I got substantially different results.
235.2 / 19 MPG = 12.4 L/100km
Now, assuming an 8% improvement, that means I used 8% less gas to go the same distance. In other words, I went the same distance using 92% of the fuel I would have normally used. So, 12.4 L/100km * 0.92 = 11.4 L/100km. And, 235.2 / 11.4 = 20.7 MPG.
Doing this with an wildly optimistic 20% improvement: 12.4 * 0.8 = 9.9 L/100km. And 235.2 / 9.9 = 23.75 MPG.
Therefore, I could save anywhere from 1.7 MPG to 4.75 MPG with this mod. And that's assuming 19 MPG as a starting point. With 21 MPG, which is what I'm shooting for now with my aerocap, the gains are a little more.
Finally, keep in mind that my goal is to improve the fuel economy of my existing truck, while retaining its existing power output capability.I don't want an econobeater, and spending thousands of dollars transplanting a different engine and transmission is similarly not an option. An econobeater can't haul a ton of gravel, nor can it haul 4 or 5 cans of recyclables and mulch at a time.
I realize that variable displacement has had a checkered past here on this board, and that you've seen your share of snake oil here. I do happen to believe, though, that I can get variable displacement to work on my truck. As an aside, I do appreciate the thoughtful arguments against this idea.