4 valve per cylinder vs 2

[Mekanix]

Which is better if all else is equal and why.

Someone recently asked me this and I didn't know what the complete answer was.

Looking for insight.

[tatu]

Which is better if all else is equal and why.

Someone recently asked me this and I didn't know what the complete answer was.

Looking for insight.

4 vlvs is better, more efficient burn, less inertial mass.

[Steve]

Yes, four valves are generally better than two. Why? At low valve lift (let's pick the time valves are just opening or about to close, they are 1-2mm open), all the air must go through a slit that is 1-2mm high and as long as the circumference of the valve. In a 4-valve engine, although the valves themselves are smaller, the sum of the circumference of the two intake valves is greater than the circumference of the single intake valve of a 2-valve engine. That allows better breathing sooner in the cycle, then, the area opened up by two valves at higher lift is again larger than the area opened up by a single valve, so they breathe better at high lift, too.

As tatu mentioned, the four-valve engines have smaller, lighter valves. The lighter valves can allow higher engine speeds or more-radical cam profiles, either one of which will make more power.

Because of the better breathing, they have a flatter torque curve AND higher peak power, what's not to like?

About the only dis-advantage is that you have twice as many valves to keep adjusted.

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[tkent02]

The way Suzuki did it, the valve components wear out much quicker with the four valve systems. With the two valve setup, all of the forces are in a straight line, pushing straight up and down. No pressure on the valve guides, no rockers to wear, no problems ever unless you let the valves get too tight.

With the four valve, there are twisting and bending forces on the valves, the valve guides wear out quicker, once the guides get loose everything up there starts to wear quicker.

I have never heard of a four valve Suzuki going over 100,000 miles without engine work, with the two valve designs it is routine.

True what Steve said about the potential to make power, but in the case of these old Suzukis, there's not much difference. A 1980 750 isn't noticeably faster than a 1979. A 1981 1100 is faster than a 1979 1000, but not by a whole lot.

[tkent02]

Because of the better breathing, they have a flatter torque curve AND higher peak power, what's not to like?

The flatter torque curve. I want a flatter torque curve on my tractor, not on my motorcycle. I want it to be fun, flat torque curves are boring like a HONDA.

[Nessism]

Lots of good into in this thread...

From strictly a theoretical standpoint, 4-valve has a big advantage in the combustion chamber shape. More efficient burn as tatu states, which translates into more power, less emissions, more mpg, higher compression ratio, etc... Just about all the modern IC auto engines are 4-valve now.

[Steve]

The flatter torque curve. I want a flatter torque curve on my tractor, not on my motorcycle. I want it to be fun, flat torque curves are boring like a HONDA.

OK, I used the wrong word.

Let's make that a BROADER torque curve, so we can keep the hump in there, but pull the sides out a bit, too.
Pulling the sides of the torque curve out will give more power sooner and keep it going longer.

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[tkent02]

Having grown up on two strokes, I'll take the entertainment, excitement and fear when it comes on the pipe over a broad smooth boring Honda curve any day.

[Steve]

Well, my Wing is the only Honda I have ever had. Yes, the torque curve almost feels like an electric motor, just a smooth pull all the way to the top, but it's oh, so reliable, and that is good for that bike.

My only experience on 2-strokes was back in the '70s, when I was new to bikes. My friends took me to a local "play" track with their CR250 and CZ250s. When those bikes came up "on pipe", my newness and lack of skills simply did not know how to handle them. Now, even with more experience, I think I would still prefer a bit of predictability in the power curve, rather than one that acts like an electric switch.

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[tkent02]

Fear is good. It keeps you from getting too old.

[1980GS1000E]

Great thread. Smart guys.

[tkent02]

Another advantage with Suzuki's two valve heads is that they had adequate cooling fins and airflow. The four valve ones had a lot less, led to the need for oil coolers and such.

[Mekanix]

Thanks guys. This is all great info.

[Nessism]

Another advantage with Suzuki's two valve heads is that they had adequate cooling fins and airflow. The four valve ones had a lot less, led to the need for oil coolers and such.

I agree with Tom that the 2V engines are more durable. One other thing is the valve pockets hold oil so the cam and shims are well lubricated. 16V GS's with scored cams and rockers are quite common after the engines gain some miles.

[1980GS1000E]

I agree with Tom that the 2V engines are more durable. One other thing is the valve pockets hold oil so the cam and shims are well lubricated. 16V GS's with scored cams and rockers are quite common after the engines gain some miles.

I had some scoring and such on my Kawasaki '84 900 Ninja cams, which uses forked followers similar to Suzuki's 4 valve arrangement. It occurred to me then how much more heavily loaded each cam lobe must be operating 2 valves through a lever system that multiplies the spring force anyway in comparison to a shim/bucket configuration.