I tell ya, the thinking stuff pours in every now and then. So here is my latest thought! We all are aware of the high idle mod and moparmans IAT sensor mod that helps MPG's. BUT, what about a way to modify the whatever (ECM, PCM, etc.) to allow the engine to only inject fuel in the cylinders on demand?Some have the 3cylinder high idle, and that helps in warmups. But what if there is a way to allow the engine to only run on 3 cylinders or less or any amount desired to cut down on fuel and raise MPG's? Surely someone around here is smart enough to know whether or not it can be done?Some of the newer vehicles do the same thing. They run on 3-4 until a certain speed or throttle and then kick in to 6-8 for the full power when on demand. We really need an emoticon to express chin scratching...

The problem is that although the VP44 or anything newer can deactivate certain injections, it cannot deactivate valves. Cars that deactivate cylinders to save fuel also open the valves on the dead cylinders, otherwise you would be wasting power on the compression stroke.

Correct... You would have ot find away to gain control of the valve train and hold the exhaust valve open during compression stroke.

Hmm... good point. So some of the energy from the active cylinders is leeched away to help drive the inactive cylinders to overcome the compression stroke, right?This in turn would cause poor performance and higher temps, right? Just trying to get an overall idea of how it would work. Just thinking along the lines of the 3cyl high idle mode.I suppose tweaking the IAT sensor would be a better way to make better MPG's...

Hmm... good point. So some of the energy from the active cylinders is leeched away to help drive the inactive cylinders to overcome the compression stroke, right? This in turn would cause poor performance and higher temps, right? Just trying to get an overall idea of how it would work. Just thinking along the lines of the 3cyl high idle mode. I suppose tweaking the IAT sensor would be a better way to make better MPG's...

During the 3 cylinder high idle, energy from the active cylinders is used to overcome the compression stroke on the dead cylinders, correct. That is the design of it, to create a load so that heat is created which is why it kicks in on the coldest days. It is very inefficient, inefficiency creates heat which warms the engine up faster. Oddly enough, the pressure that it builds is released in the opposite manner on the power stroke of the dead cylinder. Since there is no injection and no valves open on the compression or power stroke, that means that the energy used for the compression stroke is given back during the power stroke, much like compressing a spring and then releasing it. If the engine opened the exhaust valve at the end of the compression stroke so that the power would not be returned, then the engine would have a harder time and get hot even faster. In Mike's article http://articles.mopar1973man.com/index.php?option=com_content&view=article&id=51:high-idle-mod&catid=26:engine-systems&Itemid=107 he has a table towards the bottom. You can see the different loads and such. With 3 cylinder enabled, it runs around 600F, with it and the exhaust brake enabled, it runs 800F. The exhaust brake makes really does nothing for the dead cylinders though it would make them harder to go up since they are trying to push air out, though they would not have much air pressure in relation to the active cylinders. It is mainly the active cylinders that are straining the engine a lot with the exhaust brake on because all the pressure they created from combustion is trying to get out but since the brake is basically like keeping the exhaust valve closed, the piston has a hard time going up since it is basically compressing an ultra hard spring. Then the exhaust brake relieves pressure at a predetermined point so that the engine doesn't completely fall on it's face. Now this car deactivation stuff opens the valves in the dead cylinders so that those cylinders really don't take much power away. Most of the energy used in turning an engine is used on the compression stroke, the pistons are not very hard to go up and down by them selves. The interesting thing about that and diesels is that diesels don't really have the same problem as gassers. The EPA says they gain 5.5-7.5% efficiency with the deactivation going. That means the trailblazer that GM used it in at first in the new millenium that is rated at 19mpg highway 13 city on a 2wd model, can potentially get 14 to 20.5 mpg. Now mind you this is a 5000lb trailblazer (if that) with a 5.3.... To prove my point, I have a 6500lb truck that has never seen under 21mpg unloaded for the 100k miles I have owned it. Now do you see why diesels don't need it? Question is, WHY do they not need it! There are guys with 4BT's that can only pull off 30mpg in little SUV's. I saw a 6.2L V8 chevy diesel (POS) in a corvette that got 45mpg. All I can figure out is that diesels are very good at using their power efficiently. If a little SUV with a 6BT can only get 5mpg better than me then I think we are at the top end of what these engines can do efficiencywise. I could probably deactivate 2 cylinders and be fine 90% of the time but would the engine actually be more efficient? Is it really using more fuel driving 6 cylinders than it would with 3 or 4? If you have a load (driving down the road) then it is going to take X amount of fuel to have enough energy for the load. Now it is true that X amount of fuel is generally thought of as being split into 3rds. 1/3 of the fuels energy goes out the exhaust (but partially regained with the turbo). 1/3 goes to the radiator, and 1/3 is actually used for power. With fewer cylinders and more fuel to each cylinder, it makes sense that less energy would be soaked up by the cylinder walls since your total surface area is down (only using 4 cylinders with same amount of fuel). This would theoretically gain you some efficiency, (perhaps "5.5-7.5%"). This is also why we have an advantage over powerstrokes and duramaxs since more surface area is there to soak up (and waste) more energy. The combustion process is however thought of as adiabatic, which means the pistons are going so fast that no heat from combustion is soaked up by the cylinder walls. We do still have 2 cylinders less which means we need more fuel per cylinder for the same power as a V8 diesel. Eh there are so many variables and I can't explain them all without explaining why some things seem contradicting.

mmmmm... I see what your saying! Thanks for the little schooling! :smart:Diesel is still relatively new to me, so I must try and wrap my head around the inner workings, as I have been taught in the gasser world. Applying the volatile principles to compression engines.