Variable displacement V8 - thoughts?

[user removed]

Originally Posted by 5speed5

Sounds like a cool idea, but also quite involved.

My wife's '09 Impala SS has GM's 5.3L with cylinder deactivation and it does seem to work nicely on the highway. We averaged 29 mpg on a recent 1100-mile round trip. That was with two adults (and I weigh 240), 150 lbs of kids and the trunk loaded full-to-bursting with coolers and luggage going 75-80 mph the whole way (speed limit is 75).

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.

Best answer so far. Reduce revs per mile and increase load.

The add some aero to improve even further.

regards
Mech

[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.

23,000m/yr @ 19mpg = 1210 gal/yr; @ +1 you might save 57 gal/yr; +2, 115 gal/yr. 1210g @ 2.80 = $3388.

As an example: $500 30 mpg car yields 767 gal/yr @ 2.80 = $2148.

You could find an econo beater that pays for itself in less than six months then it's all gravy after that.

[t vago]

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.

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.

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.

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.

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.

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.

[cr45]

Originally Posted by t vago

I had thought of doing just that, in order to completely shut off one bank. Ideally, in order to have a more-or-less evenly running engine, you'd want to have -X-X-X-X, where the "-"s correspond to deactivated cylinders, and "X"s correspond to active cylinders. This is to maintain, as much as possible, the existing engine harmonic arrangement.

However, the problem with this approach, of shutting off a whole bank, is that my engine has a firing order of 1-8-4-3-6-5-7-2. That translates to a LRRLRLLR pattern. If I were to shut off one bank (e.g., the left bank), so as to facilitate using the O2 sensor on the other bank, I would have -RR-R--R, which looks very uneven, and could shortly lead to destructive bearing wear or even a snapped crank.

One extreme measure would be to run the oxygen sensor on one cylinder only and spark cylinders 1-4-6-7. Alternatively, if you had a 4-2-1 manifold you could run the oxygen sensor of two cylinders such as 6 & 7.

[Phantom]

Instead of sucking air in or blocking the intake as that would introduce a restriction making the other working cylinders work harder you could try one of these options.

1. Reroute/route exhaust to the deactivated cylinders controlled by a butterfly valve, the exhaust would be ran from just a little after the O2 sensor before the cat. This would allow the O2 sensor to be happy as no new O2 in being ingested. The issue that might happen is leakage of exhaust gas when running on all 8 that could be reduced if the new line is added as a T with an additional valve to block the exhaust.

2. Is the opposite idea of the first. Let the engine take in all the fresh air it wants but modify the exhaust on the deactivated cylinders to vent to air. Once again use a butterfly valve so you can change the exit of the exhaust this way when deactivated the valve will move blocking off the original exhaust and direct the air out of a short pipe. This option should take care of the O2 sensor problem also as it will not see the unburnt air since it never makes it to the sensor. Doing this you might need to have the firing order even so the O2 does not have a large gap with out a reading. The other issue that there might be is fresh air being pulled into the exhaust or worse exhaust being vented to air. Air being pulled in would be easy to fix with a flapper at the end of the pipe but I doubt that will be an issue so you could add a second butterfly valve to the exit of the pipe to help ensure that is is sealed.

[Frank Lee]

^

[Frank Lee]

Originally Posted by t vago

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.

Perhaps a slightly taller tire would help?

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.

I wouldn't call 20.52 vs 20.7 substantially different. Besides, if the 8-20% figures assumed benefit are given based on the mpg system, then that is where they're "accurate" yes?

I didn't use the wildly optimistic 20% because it's... wildly optimistic.

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.

If you are seriously hauling that much, the vast majority of the time, then OK. Color me amazed though!

[RobertSmalls]

Originally Posted by t vago

I don't want an econobeater

Fair enough, and that's the end of that.

Originally Posted by t vago

An econobeater can't haul a ton of gravel, nor can it haul 4 or 5 cans of recyclables and mulch at a time.

Sure it can. Get a wagon with a 2000lb tow rating, and a braked 4'x8' trailer. Unhook the trailer when your only payload is a body or two.

Now if you're hauling heavy cargo the vast majority of the time, then I agree with Frank's assessment.

[t vago]

Originally Posted by cr45

One extreme measure would be to run the oxygen sensor on one cylinder only and spark cylinders 1-4-6-7. Alternatively, if you had a 4-2-1 manifold you could run the oxygen sensor of two cylinders such as 6 & 7.

Or I might go the route of a standalone engine controller running open loop, and cut the truck's engine computer out altogether during the time variable displacement mode is activated.

Originally Posted by Phantom

Instead of sucking air in or blocking the intake as that would introduce a restriction making the other working cylinders work harder you could try one of these options.

The idea is to make the running cylinders work harder. This will raise average pressure inside those cylinders, allowing them to have higher output, and making them more efficient in the process. Keep in mind that I am blocking the intake ports of individual cylinders, not the whole manifold.

Originally Posted by Phantom

1. Reroute/route exhaust to the deactivated cylinders controlled by a butterfly valve, the exhaust would be ran from just a little after the O2 sensor before the cat. This would allow the O2 sensor to be happy as no new O2 in being ingested. The issue that might happen is leakage of exhaust gas when running on all 8 that could be reduced if the new line is added as a T with an additional valve to block the exhaust.

Sounds like a fancy way of adding EGR to the engine. Not sure it'll work, if I understand you correctly. You're trying to find a way to not use butterfly valves in the intake ports of the deactivated cylinders?

Originally Posted by Phantom

2. Is the opposite idea of the first. Let the engine take in all the fresh air it wants but modify the exhaust on the deactivated cylinders to vent to air. Once again use a butterfly valve so you can change the exit of the exhaust this way when deactivated the valve will move blocking off the original exhaust and direct the air out of a short pipe. This option should take care of the O2 sensor problem also as it will not see the unburnt air since it never makes it to the sensor. Doing this you might need to have the firing order even so the O2 does not have a large gap with out a reading. The other issue that there might be is fresh air being pulled into the exhaust or worse exhaust being vented to air. Air being pulled in would be easy to fix with a flapper at the end of the pipe but I doubt that will be an issue so you could add a second butterfly valve to the exit of the pipe to help ensure that is is sealed.

This option would still cause the engine to do needless work, as it would simply become an air pump as far as the deactivated cylinders are concerned.

[Frank Lee]

This option would still cause the engine to do needless work, as it would simply become an air pump as far as the deactivated cylinders are concerned.

You don't know how many freekin times I've said that already.