3900 fuel saver mode experiment

[Guest]

I've been discussing this engine quite a bit but am not sure if I will invest in the version with the extended fuel management option. As an experiment I pulled one of the injector fuses this morning on the hwy (in my Fiero) to see what the effect would be and amazingly the car continued to run and maintain speed which was only 60 MPH at the time.

Since the valve train was still working unlike in the 3.9 where it shuts down for the affected cylinders, I'm not sure if the vibration which really wasn't bad at all would smooth out even more if it were possible to disable the valves on my current engine. Since I'm not sure of what any potential side effects would be I didn't run the car that way for an extended time.

I believe GM deactivated the valves to reduce the load on the valve train by the non contributing cylinders, however there maybe other dynamics at play such as thermo which would be different with valves opening on a dead cylinder allowing cooler air in compared to them being closed on a dead cylinder where the temperature in the cylinder as well as the pressures would be more stable with the valves closed during fuel saver mode.

If I were not so far away from home I would actually disable one bank on the hwy for a couple of days to see what my fuel savings if any would be like since I travel nearly 100 miles round trip a day at the moment. It's only feasible at hwy speeds since the purpose is to take advantage of a steady state, interestingly enough I didn't have to apply excessive additional throttle to maintain 60 MPH and as those who have driven an engine with a misfiring cylinder would know, the higher the rpm the smoother the engine gets.

This is something to think about because if there is no appreciable downside you can install a switch and shut down half the engine by cutting off one bank of injectors on a long trip and set the cruise control to hold speed, provided there is a significant enough fuel savings which I believe there would be running on 1.7L in a 3.4L engine.

Again the vibration really wasn't that bad and in a larger car maybe even less noticeable.

[geoffinbc]

The reason you want to close the valves is to prevent compression which would rob power and would also contribute to the rough running. If you leave the valves closed, the cylinders are essentially idle.

[IsaacHayes]

But if you dont have compression you'll have vacuum.... They gotta open a certain way to keep it breathing without much pressure...

[geoffinbc]

It is ok to keep the piston sealed off. It will have compression and vacuum but it is just like a big spring. All the DOD engines that GM has right now effectively shut off the lifters on the dead cylinders.

[IsaacHayes]

That's what I thought, but I'd think that if they could keep them open at the right time that it would reduce resistance. Like on the compression stroke and power stroke if it could open the exhaust valves so it just pushed the air right back out and then back in or something.... But that would require individual valve control like electronic valves or something... BTW didn't Mercedes have a motor with electronic valves? I think it didn't even have a TB and it just controlled the valve lift for throttle.... I think it had direct injection too. I remember hearing about it, don't know if it's a production motor though.

[Joseph Upson]

That's what I thought, but I'd think that if they could keep them open at the right time that it would reduce resistance. Like on the compression stroke and power stroke if it could open the exhaust valves so it just pushed the air right back out and then back in or something.... But that would require individual valve control like electronic valves or something... BTW didn't Mercedes have a motor with electronic valves? I think it didn't even have a TB and it just controlled the valve lift for throttle.... I think it had direct injection too. I remember hearing about it, don't know if it's a production motor though.

After further consideration I figured out as well as someone else that leaving the valves functional will screw up the air fuel ratio, 3 cylinders pumping fresh unburned air over the O2 sensor is going to send a very lean signal resulting in a very rich fuel mixture to compensate. So either the valves have to be shut or the the O2 sensor has to be limited to the running bank of cylinders.

[IsaacHayes]

Yeah I had thought about that too, but forgot to mention that. The ECM would have to calculate the expected change in O2 levels based on MAF and air temp. But likely the O2 sensor would be way out of it's operating range to get a good reading anyways so that won't work either I bet. Running only one bank at a time would probably cause vibration issues, I believe the DoD motors alternate across the whole motor to keep it smooth.

I guess a separate O2 sensor for each cylinder would allow you to run with valves open for free-er rotating assembly that's not compressing a dead cylinder.

I'm sure something could be designed like I said but it would be costly but who knows maybe the mpg would be very high?