Fuelly Forums - 2+2=4... But If you -2 Then??

Carbed motors with deactivated valves might get a tiny little trickle of fuel down there, but there's no air movement to suck the charge that way, so it's not going to be particularly bad. Also The valve isn't going to be very hot on a deactivated cylinder, it will likely not bake crap on.

It has lately occurred to me that on normally aspirated engines, the amount of exhaust scavenging may have a bearing on the relative success of cylinder deactivation on a given engine. This would be because the active cylinders, would in fact "suck" harder than the inactive cylinders, and may leave the inactive cylinders a little less "full" in a slight vacuum relative to manifold pressure, than the active cylinders. In a well tuned fully active engine, charge robbing between cylinders with long intake durations is avoided by good runner tuning, insuring that runner momentum is enough to avoid backflow. However, with less runner momentum due to no exhaust blowdown to scavenge, this may not be enough to stop backflow from an inactive cylinder. In an extreme case, when exhaust manifold pressure is quite high, some amount of exhaust gas could be passing backwards.

In a case where you have both strong scavenging in the active cylinder at blowdown, intake valves overlapping between cylinders, and mid-late cycle higher exhaust manifold pressure, it's possible that "inactive" cylinders are almost "running backwards" in that they are getting (relative) positive pressure just before the exhaust valve closes and negative pressure just before the intake valve closes. This would mean that the "inactive" cylinders may in actual fact contribute net energy to the system, by recovering exhaust gas energy, or at least making up for the "huge" frictional losses that they apparently cause.

To purposely set up a motor to make the most out of this effect would be rather difficult. I think you'd need an undersized yet tuned exhaust system, meaning one that offers very little flow resistance due to shape, i.e. flows very well at lower speeds and volumes, but reaches a velocity threshold at higher speeds and backs up a little. It would probably need a "shorty" header or stock manifold. You'd also need to carefully balance intake runner volume.

Most likely way to be able to do this is if you had a block used in a limited capacity racing formula, like 1600cc, and you used the 1600cc racing parts and warm cam on a 2400cc version or something.

Whether you can take advantage of the above effect or not, the first question you really want to ask yourself for DoD experimentation is "Can I find a dyno graph for my motor that shows I have a max of about 75HP at 3000 RPM" This is a "rule of thumb" figure based on rough and ready calculations, if you have a particularly small, or particularly large vehicle it might be best to figure it out exactly yourself. Basically, you need to have around 30HP available to cruise at highway speed without being hugely affected by wind and slight grades, but you need it at less than 80% of the motors capacity or you will more than likely be out the top of the peak BSFC island, and using too much gas, most charts I've seen the best zone is in the 40-80% output area near torque peak. You don't wanna be too close to the bottom or top, and you wanna have slight reserve, hence about 30HP is good, even though you'll probably only use 20HP. So I figure for unmodified vehicles with stock aero and rolling resistance, you're gonna need about a 150HP motor before DoD is worth playing with. If you're starting with a 100HP motor, you're probably going to have to whittle down aero and RR until you're sure you're using a bit less than 12-15HP at cruise. Paradoxically, this might be where you're finding your rolldown tests indicate better aero and RR, but MPG tests are getting worse or no better, yeah, you've dropped out the bottom of the BSFC map and need to put it back in. That's why I guess the more modified vehicle drivers start to pulse and glide, the motor needs more load to be efficient.