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You're right that none of the engines I mentioned had DOHC. I'm not convinced that advancing only the intake camshaft will increase fuel economy. This would increase valve overlap and reduce reversion of gas flow (from cylinder to intake manifold) at the beginning of the compression stroke. The reversion helps to lower intake manifold vacuum when the engine runs at low speeds and low loads. This causes a reduction in pumping losses. Advancing the camshaft increases the vacuum at a given power level and therefore causes increased pumping losses. Increasing the valve overlap is also likely to reduce fuel economy by increasing the amount of exhaust gases mixing with the intake mixture. Too much EGR slows down combustion.
Do you know how the intake valve timing is controlled in the new Dodge Hemi? Does it depend only on engine speed or is it also affected by the throttle position? What is it at idle? I would think that the most efficient way to control it to leave it retarded except at low speed and high load.
That's interesting about how the valve lash affects the industrial engines. Part of the idle speed change may be caused by the engine pulling in more fuel by increasing manifold vacuum. I can believe that they also more efficient at low RPM with more valve lash.
You reminded me of an experiment I did on a lawnmower engine. I removed the engine's thick head gasket and put back the cylinder head with some sealant. This raised the compression ratio. The engine ran faster than normal even though the throttle was at the minimum. I didn't get to use it very long. It broke a connecting rod. I guess it was running faster than the redline. If it didn't break I would have had a very fuel efficient lawn mower.
I've read that the Honda VTEC-e engine keeps half of the intake valves shut during low speed operation. I think this increases turbulence in the combustion chamber and speeds up combustion. Both of the exhaust valves work all the time. I agree that choked intake flow could increase pumping losses but that would only happen at high RPM.
To garys_1k: a lot of what you're saying is true but I believe mechman600 when he says he can get better fuel economy in the hills then he can get normally. His engine is more efficient at high loads than low loads so it is better to use a high load part of the time and no load the rest of the time than to use a low load all of the time. It's basically the same as pulse and glide (or burn and coast) except that hills let the speed remain relatively constant. If the engine was small enough to run at maximum efficiency at highway speed then the hills would probably reduce fuel economy by causing the engine to run at higher power than the level that gives greatest efficiency.
In general it's true that lower power demands help fuel economy. Keep in mind that mechman600's car still has about the same average power demand in the hills as on a level road.
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