Here's some more on DoD.

My eyes must be playing tricks on me then , because what I read in that article was --

¨GM estimates fuel economy savings of 8% when driven on the standard fuel economy test routine and up to 25% increased economy for some driving conditions¨

25% under some conditions , is not 25% gain.
The average FE gain is as clearly stated - 8%.

Focussing on that 25% is just the same as believing some of those incredibly high instantanious reading ones in here get on their Scangauges.

Great numbers , , just not reality

But a DIY economy minded driver who puts this system in is not going to drive according the the EPA driving schedule. And the system, which is very touchy and switches on/off quite a bit, will be on most of the time. The point I'm making is that if GM says the average EPA benefit is 8%, and the max from the system is 25%, then someone interested enough in economy to rig their own version will probably use it most of the time and see way more benefit compared to the EPA cycle computer controlled benefit that switches on/off a lot and isn't on as much. I think a 20% increase in FE isn't that hard to imagine when some people here see upwards of a 50% increase just by changing driving habits. I mean, what you're saying is that those numbers won't reflect reality, which is probably true for GM's version, but I think they can if someone here is driving to take advantage of their own DIY system.

have you taken a look at how honda works their cylender deactivation? it's simaler to how they do there variable valve timing, only with differnt rocker arms, the first time I held one of honda's heads with cylender deactivation in my hands I was confused, then I realized what it was.

I'm checking it out now. I found this on cylinder deactivation for a CBR1100 prototype , and it looks like Honda is being way more aggressive with their system.

Just pulling the fuel for say, two out of four cylinders won't have as big of an effect because there are still the two inactive cylinders pulling in air, but I really think cutting fuel on two out of four cylinders on an inline four could result in a 20% increase in mileage. Here's something else to chew on

So with the air springs/closed valves, pumping losses drop by nearly a half, halved, aka 75%, with the difference probably being increased pumping losses for those cylinders that are carrying a bigger load, so 65% real world. I'd wager that the DIY version via a fuel cut to two cylinders could reduce pumping losses by up to ~40%, and power would be available by turning the two injectors back on from the cab. Now there's also the question of whether or not the engine/mounts can handle this... I wonder if the variable displacement engines share the same bottom end as their "normal" versions?

Going back to the bike, if something like a twenty year old CB650SC can pull ~45-50mpg in normal riding, and probably ~70-80mpg in fuel efficient riding, it may be possible to run ITBs/SAFI and have a bike that can run low 11s quarter miles while getting nearly 100mpg@55mph with two cylinders deactivated.

I think you guys may be missing the point WRT hypermiling...an _automatic_ cylinder deactivation, like an automatic transmission, won't affect fuel economy as well as a manual one because the cylinder deactivation will happen at the wrong time to have the greatest impact on efficiency.

Automatic cylinder deactivation is designed to reduce fuel consumption only while cruising, not during acceleration. If you're using 10% of your V4's available power instead of 5% of a V8, that's not going to give much of an engine efficiency improvement compared to the 80% vs 40% you'd get
during acceleration. I'd expect GM's ECU would activate the deactivated cylinders long before you'd get to that point.

For cylinder deactivation to have the greatest impact on FE, the deactivated cylinders should only be activated for emergency acceleration (and perhaps starting, as suggested above). Only the driver can determine when emergency acceleration is needed, not the ECU.

As a side note...my first (and previous) car was an `78 Chevy Malibu we bought in `85 with a (carbureted) 305 V8. On its 2000 mile trip home, it averaged about 20 MPG. Several months after we bought it, we had the engine rebuilt. It had a soft cam and a few of the lobes had worn completely off, so the engine had effectively only been running on six cylinders. After the rebuild, which effectively reactivated all the cylinders, the best FE I ever got was 15 MPG.

As I said, I'd rather just make do with 60 HP instead of 120 all the time and just avoid situations that require emergency acceleration (e.g. pulling into a short gap in fast-moving traffic). The extra time spent idling while waiting for a longer gap would be more than offset by the greater overall efficiency.

you are right and an economy minded driver doesnt drive a v8

Reviewing this thread again, and thinking about what it would take to effectively reduce a 4-cylinder PFI to 2 without major modifications, I've reached a few conclusions:

- If you want a 4-cylinder engine to start on 2 without major modifications (ie. tearing the engine apart), you need to allow fluid to freely move through the cylinder, as with an unmodified valvetrain. If fluid is going to move freely anyway, making it move as freely as possible would reduce the losses. Hence, removing the spark plug and perhaps the injector makes the most sense for the small-time DIYer. If you can tear apart an engine, removing all the extra parts for the dead cylinders makes the most sense, and this has been done in the past.

- If extra air is introduced into the exhaust, the O2 sensor will sense it and richen the mixture to compensate. This would probably negate any net savings in efficiency, so it appears essential to block the exhaust ports -- either the valves or the manifold ports -- of the deactivated cylinders to keep fresh air from reaching the O2 sensor.

Any thoughts?

Good point about the behavior of the O2 sensor, probably why most manufacturers use the air spring approach. Obviously, running in open loop mode would avoid a really rich condition, so killing the O2 sensor and injectors may be required. Also, the ECU may only richen the mixture up so much, so the increase in efficiency could still be greater than the decrease from a rich mixture, or the ECU may even automatically drop into open loop when the injectors are turned off. This would be a great project for someone with a flashable ECU/SAFI and a SG.

I think you mean marginally more power. ;)