One of the recent podcast puzzlers got me to thinking what motivated the Toyota engineers to decide to put 16 valves in my Corolla’s 4 cylinder engine? What are the advantages and disadvantages of 16 vs 8? I can think of these possibilities
16 is better than 8 b/c
8 is better than 16 b/c
Are there other serviceability, driveability, or engine robustness issues, 16 vs 8? Emissions?
Probably better efficiency and pollution control with the 16v engine.
More valves equal more power since Puegeot did that in a 3 liter 4 cylinder in 1913.
Conceivably you could get the same power from 8 as 16, just make the 8 valves bigger. Or isn’t that feasible b/c of space conflicts? Or if the big- 8 idea remains feasible, does it cause other problems?
Nope, see what the total circumference of four circles vs two are that fit inside a single circle.
If 2 valves were just as good, don’t you think they’d use them?
Ok, that’ll be my Christmas puzzle. Do you mean “area” rather than “circumference”?
He means area and circumference… curtain area. Valve lift times the circumference. Much greater with 4 valves than 2.
Also, the lighter 4 valves can be revved higher making more hp.
Nope. The area for air flow is approximately the valve circumference times valve lift.
@Mustangman beat me to it.
Harry Miller made huge advances with DOHC 4 valve supercharged engines in the 1920s, evolved into the Offy engine that won dozens of major races.
39 times in 57 tries at the Indy 500!
Harry Miller was sent the Peugeot for repairs in 1915. He liked what he saw… as did everyone else! DOHC 4 valve engines exploded in racing from that point on.
Sunbeam built a 24 valve 6 in 1923 (?) as the first road car to use it. Then Duesenberg with their straight 8.
Is the 4 valve per cylinder design what allows Toyota gasoline engines to be non interference?
No, 4 valve engines could be either interference or not. It depends on the valve lift, valve angle from vertical, compression ratio and the reliefs in the piston tops.
High compression engines generally have trouble being non-interference.
No. Many older 2-valve per cylinder engines were non-interference, and the 1.5L Tercel engines with 3-valves per cylinder were interference. Usually, a non-interference engine is designed so the valves do not extend past the bottom of the cylinder head, and the pistons are designed with valve reliefs, or a flat top that is recessed slightly to allow a “margin of error” for carbon buildup and thermal expansion.
The biggest difference between the 2 valve and 4 valve heads is the curtain area. The curtain area is the circumference x valve lift. 4 valve heads have about a 50% larger curtain area than a 2 valve with the same bore dimension.
4 valve engines can both flow more air and at the same time use a milder cam profile so that the engine can have a broader torque curve, higher HP, smoother idle and higher rev limit.
Another issue facing 2 valve designs is that the wall of the combustion chamber is often very close to the edge of the valves, especially when you are pushing for the maximum size valves. The problem that occurs here is that increasing the lift does not increase air flow. Increasing the lift should increase the curtain area but with the edge of the valve so close to the combustion chamber wall, the gap between the wall and the edge of the valve determines the curtain area and not the valve lift. This can affect about half the circumference of the valve. Using a smaller valve can actually increase the curtain area and thus the air flow.
2 valve designes are cheaper to make.
Less valve float.
How is the Corvette still a 2 valve per cylinder pushrod engine?
Because Chevrolet can pull tons of power out of a 7 liter 2 valve pushrod engine with titanium rods and intake valves and somewhat less out of a 6.2 liter engine with ferrous parts
Corvette offered a DOHC 4 valve engine in 1990 to about 1995 in the top ZR1 model. Designed by Lotus and built by Mercury Marine.
Plus, the new top of the line 2023 Z06 C8 Corvette is now a DOHC 4 valve flat plane crank V8.
Because the technology has been refined for many, many decades. The engineers know how to design cylinder heads that can flow air and use materials that allow the engine to turn the RPM to utilize it. There are entire industries built around American pushrod V8s - the GM LS engines in particular. Think about how much easier and cheaper it is to swap a single camshaft in a pushrod V8 vs. 4 camshafts in a DOHC V8. You have hundreds of camshaft options for LS engines. Racers and hot rodders gravitate toward the most HP for the least money and you can’t beat American pushrod V8s for that. Any time you have that much human capital contributing to an industry for decades, you’re going to see significant improvements.