Variable displacement V8 - thoughts?

[autoteach]

Well, like my students, this has gotten unreasonable. Good luck, I will wait to see the patents.

[bestclimb]

The way I look at it even if you get past the valving and ECU O2 sensor issues you have a couple hurtals to over come.

The balance of the engine is engineered for all 8 jugs functioning, it may/will run rough. (this may or may not be an issue mechanical and would be up to you if it is a comfort/annoyance issue)

The biggest one efficient wise is that you are still accelerating and stopping and accelerating a bunch of mass in the pistons and the of scrapping the rings along the cylinders of the deactivated jugs several thousand times a second.

Got anything against pulse and glide?

[t vago]

Originally Posted by bestclimb

The balance of the engine is engineered for all 8 jugs functioning, it may/will run rough. (this may or may not be an issue mechanical and would be up to you if it is a comfort/annoyance issue)

I expect to feel some difference from the engine if 4 cylinders are deactivated. However, I don't think this is such a big issue from a mechanical longevity standpoint, as the Chrysler 5.7L MDS system shuts off the exact same cylinders as I had considered, and the only thing I have read about is that some drivers complain of a low vibration between 1100 and 1800 RPMs when MDS is engaged.

Originally Posted by bestclimb

The biggest one efficient wise is that you are still accelerating and stopping and accelerating a bunch of mass in the pistons and the of scrapping the rings along the cylinders of the deactivated jugs several thousand times a second.

It's called friction losses. Can't get around that.

However, it's not much of a concern. If the Big 3 can make variable displacement work on their production engines, and can get significant improvements in fuel economy, even with friction losses of the deactivated cylinders, then it's not much of a problem.

I think you meant to say "minutes" instead of "seconds," though. If my V8 were somehow able to make its pistons reciprocate thousands of times per second, I don't think it'd live for much longer than a few seconds.

Originally Posted by bestclimb

Got anything against pulse and glide?

It's unrealistic for a daily commute.

[MechEngVT]

Originally Posted by t vago

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.

Try the following:
Chrysler / Dodge / Plymouth Dakota Rear Differentials Chrysler 9.25 Ring & Pinion Sets Auto Parts: YUKON GEAR,YUKON AXLE,YUKON,RING PINION,RING,PINION,GEAR,DIFFERENTIAL,CROWN WHEEL,CROWN PINION,CROWN,CHRYSLER 9.25,CHRYSLER 9 1/4,CHRYSLER,RICHMOND GEA

They list a 3.21 gearset for the 9 1/4 axle for less than $300, and I seriously doubt a shop would charge more than $300 to do an install. I have that ratio in my 1988 Ramcharger with 235/75r15 (and 275/50r15) tires, but it's in an 8 1/4 axle. The 9 1/4 in my Ram has the 3.55 like yours, but with 265/65r17 tires.

I honestly think you're barking up the wrong tree if you want cost-effective efficiency improvements that will be reliable. As you state the valve arrangement of the 4.7L isn't suited to tried-and-true methods of variable displacement. Any home-brewed solution for the existing valve arrangement is likely to be insanely expensive or unreliable enough to cause major engine damage, or likely both. You may be-LIE-ve you can do it, but I wish you luck while recommending you divert attention elsewhere. Maybe modifying the heads to accept the double-rocker style actuators would have potential for reliable success, but again it would be quite expensive.

[Big Dave]

This is soooo beyond the shade-tree mechanic on so many levels.

If you want displacement on demand, go find a 5.3 or L92 GM engine. Their system works really well. Don't expect more than a 3-5% improvement in MPG.

You want mileage, swap in a Cuymmins 4BT3.9 diesel. There are now commercially available kits for that swap. Expect a 25-50% improvement in MPG.

[t vago]

In order to test whether variable displacement will or will not work on my engine, I will move ahead on my own. I will fabricate spacers out of 16 ga Home Depot sheet steel that will simulate the closure of the intake ports of cylinders 1, 4, 6, and 7. I will then install said spacers onto the engine of my truck, with gaskets. I will then electrically disconnect the fuel injectors for cylinders 1, 4, 6, and 7. I will then perform an A-B-A test with this setup. I will then figure out what fuel economy gain I received, if any, and will report back here.

I do appreciate the advice and suggestions you all have given, though. Thank you. However, I do ask that from now on, that any advice or suggestions given in this thread directly relates to supporting this project.

[some_other_dave]

It will be interesting to see the results! It's a neat project to tinker with, regardless of how much or how little it saves you.

-soD

[roflwaffle]

Originally Posted by t vago

In order to test whether variable displacement will or will not work on my engine, I will move ahead on my own. I will fabricate spacers out of 16 ga Home Depot sheet steel that will simulate the closure of the intake ports of cylinders 1, 4, 6, and 7. I will then install said spacers onto the engine of my truck, with gaskets. I will then electrically disconnect the fuel injectors for cylinders 1, 4, 6, and 7. I will then perform an A-B-A test with this setup. I will then figure out what fuel economy gain I received, if any, and will report back here.

I do appreciate the advice and suggestions you all have given, though. Thank you. However, I do ask that from now on, that any advice or suggestions given in this thread directly relates to supporting this project.

Try it w/o the spacers first. I have a feeling putting spacers to close off the intake port will simulate the highest pumping losses you could see for those cylinders (throttle is not open). Just disconnect the FIs for whatever cylinders and see if there is any improvement. Make sure you can keep test conditions as consistent as possible. For instance if your Dakota stays in third w/ the TCC unlocked after the FIs are disconnected during a highway run, when you're doing the run w/ the FIs connected do it in third w/ the TCC unlocked.

[cr45]

Originally Posted by roflwaffle

Try it w/o the spacers first. I have a feeling putting spacers to close off the intake port will simulate the highest pumping losses you could see for those cylinders (throttle is not open). Just disconnect the FIs for whatever cylinders and see if there is any improvement. Make sure you can keep test conditions as consistent as possible. For instance if your Dakota stays in third w/ the TCC unlocked after the FIs are disconnected during a highway run, when you're doing the run w/ the FIs connected do it in third w/ the TCC unlocked.

Just disconnecting the four FIs will not work as the oxygen sensor will pick up on a lean AFR and the engine management will try to adjust by adding extra fuel.

[comptiger5000]

Even if 3.55s are the tallest factory gearing, there may be other gearsets available to fit your axles. Do you know offhand what axles are in that truck?