Quote:
Originally Posted by mechman600
This is making me think that disabling 1/2 of the intake valves would be a stupid idea. It would make the vacuum on the piston much higher and more efficiency would be lost than gained. What we need is a very good breathing engine with an extremely short duration camshaft....like 200/200 degrees seat duration or less, as opposed to the 280/290 or so as is. Peak torque @ <2500. That would rule. Would my engine management system know the difference? Or would the CEL come on, throwing this whole thing into an open loop nightmare?
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Honda seems to think it's a good idea to close half the intake valves. Their VTEC-E engine uses one intake valve for low speed operation and both intake valves for high speed operation. I'm not sure what RPM activates the second valve. I would think it might be about 4000 RPM. This engine is very fuel efficient and I think the closed intake valves give it an economy boost. The faster air flow into the cylinders helps to speed up combustion and that will cause more of the energy in the gasoline to make mechanical work instead of exhaust heat. The extra restriction doesn't do much to change the vacuum on the piston except at full throttle and high RPM. A little extra restriction at the intake valves can be compensated for by opening the throttle a little more. Then you don't have a higher vacuum at the pistons. At high RPM you need the other intake valves working to get maximum power.
A short exhaust duration will help low RPM torque and fuel economy but a long intake valve duration combined with an increased compression ratio can give a big improvement in fuel economy, especially with a large engine. The late closing intake valve reduces low RPM torque but may increase high RPM torque if it isn't very much later than normal.
I remember reading a story in Hot Rod magazine about this concept about 1982 or so. They built a Chevrorolet 350 engine with a very high compression ratio by using taller domed 327 pistons and then grinding the heads so there would be enough room for them. I think the compression ratio was about 14.5:1. They also got a custom camshaft with very late intake closing events. That allowed a normal cranking compression pressure. They used the engine in a tow truck pulling a heavy trailer. The result was a big gain in miles per gallon. I think it went from about 9 to about 16. In another issue they installed the Crower Mileage System in a Chevy V8 engine and put it in an old Hot Rod, something like a 1929 Chevy. The axle ratio was high, maybe 4.11, and they used extra fat rear tires. That seems like a big mistake if they were interested in fuel economy. They reported that the car would often achieve over 25 MPG. That's a very high number for a car with a V8 and an automatic transmission without lockup torque converter or overdrive.
I think I still have those magazines and could probably dig them out. I remember that they did a dynomometer test and reported some amazing BSFC (engine efficiency) numbers at low RPM. The power was lower than normal, probably a little over 200 HP. Torque was pretty low but increased at higher engine speeds.
Changing camshaft timing, especially the intake valve closing event much will disturb the fuel injection calibration if your engine uses a MAP (manifold absolute pressure) sensor. It probably will turn on the check engine light. On my Honda Civic with the worn timing belt I needed to connect a resistor to the MAP sensor output and ground to avoid a rich fuel mixture. If you have fuel injection with an air flow sensor (like Bosche systems on european cars) then it will be more compatible with altered valve timing. All the engines I used for camshaft timing adjustments had carburetors. Carburetors may run a little rich at low RPM when the intake valves close late because of a more pulsating air flow.
It would be possible to reduce compression pressure by having intake valves close very early instead of very late but then the power would be reduced severely. This would only be practical for an engine running with a limited speed range or an engine with variable valve timing.
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