Do multivalve engines have any real advantages for the average motorist?

Dear Car Talk

Dear Car Talk | Oct 01, 2002

Dear Tom and Ray:

Do "multivalve" engines have real advantages for average motorists? Many years ago, I enjoyed reading Smokey Yunick's articles in Popular Science. In an article on engines, Smokey said that there is no advantage, to most drivers. He said that multivalve engines have value only at high rpm (i.e., for people who drive very fast). What's more, gas mileage doesn't seem to have improved with all of these multivalve engines out there now. What do you think of Smokey's comments? -- Charles

RAY: What a coincidence; Smokey Yunick is what we call my brother every time he lights a cigar.

TOM: In this case, I don't happen to agree with Smokey. Valves are all about breathing. And the more valves you have -- up to a point -- the better your engine can breathe. By breathing, we mean getting a fresh charge of gasoline and air into a cylinder and then getting the spent exhaust out of there.

RAY: The easiest way to see this is to draw a circle. That represents the top of a cylinder. Now draw the two largest circles you can fit inside that first circle -- those are your two valves, intake and exhaust.

TOM: You can see that there's a lot of wasted space. Now, if you draw four circles (i.e., valves) inside that first circle, you can fill up a lot more space, open up a lot more of the top of the cylinder and get a lot more stuff in and out. That means you get more power out of each stroke.

RAY: And while it certainly helps most at higher rpm, because that's when you're asking the most of your engine, it makes a difference at moderate rpm, too.

TOM: And while, in and of themselves, extra valves don't improve your mileage, they do increase the amount of power you can get from the same engine. So if you wanted to, you could then decrease the size and weight of a car's engine without giving up power. And THAT would increase your gas mileage.

RAY: But alas, car manufacturers haven't done that. Instead, they've gone horsepower crazy. Instead of using all the increased efficiencies over the years to improve mileage, they've used them to increase power.

TOM: This is so we can all drive faster into bridge abutments while we're talking on our cell phones, instead of conserving gas to decrease our dependence on foreign oil.

RAY: But that's not the fault of the valves, Charles. It's the fault of car companies. We could easily have higher-mileage cars if we all insisted on it.

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