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What is the point/benefit of 4 valves/twin cams?

I understand that:

  1. valves #1 open to let in the fuel/air mixture,
  2. spark plug ignites the mix, and the piston is pushed away.
  3. When the piston comes back up, the valve #2 opens to let the piston push out the exhaust.

What is the point of having 4 valves doing this?
Any helpful links that explain some basics?


Basic reason is that 4 valves have more area available for flow of the intake and exhaust gasses, allowing more efficient filling and emptying of the cylinders, than 2 valves.

Why not just use 2 bigger valves?
(But those valves go to 11?)

Because the valves need to fit in the space provided by the cylinder. The more valves you have, the more efficiently you can pack openings into the cylinder head.

OK, let’s put the 2 biggest valves that’ll fit in there. There will still be room for two smaller valves in the area not covered by the big valves, right? And once you start optimizing that situation you end up with 4 valves, with two matching intakes typically slightly larger than the two matching exhaust valves. This also allow for a centrally-mounted spark plug, which improves combustion performance.

And while the size of the valves is important, it’s actually the circumference, not the valve area, that matters. The circumference times the valve lift = the area open to flow, the more, the better (within reason).

Why not just use 2 bigger valves?

Draw two circles to represent 2 cylinders. Now draw the two biggest circles you can inside the “cylinder” #1 circle. Then, inside the #2 cylinder draw the 4 biggest circles you can inside the circle. Shade the area inside the 2 circles in cylinder #1 by coloring them in. Next, shade in the area inside the 4 circles in cylinder #2. It should be easy to see there is a lot more shaded area in cylinder #2 compared to #1.

This means you can get more fuel mixture into #2 and you can the exhausted burned fuel out more completely from #2. More fuel is more power potential, better exhausting means less pollution.

More surface area in cylinder #2 means the valves don’t have to open as far to get in the same amount of mixture and the valves don’t have to be open as long to get in the same amount of mixture as compared to cylinder 1. This give engine designers more options to make the most power in the most efficient manner and reduce pollution.

Not all engines need 4 valves per cylinder. There are some 5 valve per cylinder engines in a few motorcycles. It all depends on the application and demands placed on the motor. Higher reving motors, high performance motors, and high horsepower from smaller displacement motors often result in 4 valves per cylinder.

The round shape of the cylinder, and the round shape of the valves, means making more valves is more effective than just making larger valves.

Some additional points to consider. With the smaller diameter of the valve heads with 4 valves per cylinder the closed spring pressure can be less that with a larger diameter valve.

An advantage of twin cams is that the moving mass to open the valve is less. If the cam is directly over the valve, the cam follower cup, the valve retainer, part of the valve spring, and the valve itself have to be moved. With a single cam there is a rocker arm to move unless all the valves are in a line which brings us back to 2 valves per cylinder. In the case of a 2 valve, push rod system, there is the valve lifter, push rod, rocker arm, valve spring retainer, 1/2 valve spring, and valve to move.

With the small circumference of the smaller 4 valve heads, heat can be transferred to the valve seat more efficiently. This allows the valve head to run cooler.

Another advantage of the 4 valves per cylinder is that the valves can be arranged to place the spark plug tip in the 4 sided area between the valves. A lot of the 4 valve cylinder heads are arranged in a penthouse shape with the intake valves canted on one side and the exhaust valves canted on the other side. Not only does this allow a larger valve to be installed but also allows a short path from the manifold to the valve and allows the intake charge to drop directly into the cylinder. With the spark plug tip in the center of the combustion chamber the amount of spark timing advance is reduced to reach maximum pressure at TDC. The reduced flame travel distance also reduces the tendency to detonate allowing a higher compression ratio.

The 4 valve per cylinder arrangement allows an engine design that can safely rev to a higher RPM than a heavier 2 valve system. Individual intake valves do not have to lift as high as would a single intake valve to get the same volumetric efficiency. A 4 valve arrangement can easily buzz to 7-9 grand without valve float where a two valve push rod system requires massive closed and open spring pressures to keep the valve train in contact with the cam lobes.

Draw two circles the same size on a piece of paper. Now, draw the two largest circles you can in one of them. In the other, draw the four larges circles you can fit in. You’ll discover that the one with the four circles takes up more of the space in the original circle than the one woth the two circles can.

In valvetalk, that means that more area in the top of the cylinder can be opened to let fuel in and exhaust out with four valves, opening two at a time, than with two valves. That allows the engine to breath better and that makes it more efficient and capable of more power. It also reduces the amount of energy wasted pulling the piston down on the intake stroke and pushing the exhaust out after the gasses are burned.

Twin camshafts serve a different purpose. Again keeping in mind the goal of providing as much valve area as possible, two cams allow the top surfaces of the cylinder (under the head) to be slanted, like the sides of a triangle, making those sides larger respective to the base of the triangle (the cylinder size). That allows bigger valves.

Hmmmm…I started to write before Uncle Turbo and Researcher posted, posted mine after, now see theirs…and I like theirs better. A lot better. Go there.

Just drawing some pictures and measuring the results, it looks like 4 equal valves have about 1.66 times more total circumference than 2 valves. So there’s one basic benefit.

Now for the rest of the story. When a manufacturer selects a cam profile, it is a compromise between a mild profile for good idle and low end torque vs. a “radical cam” (lots of lift and lots of duration) for high RPM horsepower.


At high RPM, you have less time to fill the cylinder with the fuel air mixture. You can increase the “curtain area” (lift x circumference) or the duration (number of degrees of rotation that the valve is open) to pack the cylinder. Increased duration causes a rough idle and poor low end torque.


So assuming that texasas is correct, and I believe he is, if you have a four valve design, the manufacturer can use a milder cam for good low end torque and have adequate “curtain area” for high RPM horsepower.

Also, the 4 ports can be configured to achieve better flow than the ports used for 2-valve heads…

Having said all this, there are old engine designs that have been developed over decades that can produce phenomenal performance using the 2-valve push-rod design…

Where the high-tech designs excel is in small displacement, high output applications that also demand decent fuel economy…With high-efficiency engines, you can have your cake and eat it too…

Great replies! Thanks.