In this video we are doing a detailed comparison of two different cylinder heads, a performance cylinder head represented by the high revving Toyota 4AGE and an economy cylinder head represented by its more modest cousin, the 4AFE engine. Both of the engines from which this heads come from share an equal displacement and an almost identical cylinder block design, which means that the key differences between the engines lies in the heads, making this a great example to demonstrate how cylinder head design influences engine character, power, torque, efficiency and more.

Although both our heads are DOHC and have the same number of valves, they differ in the way their camshafts are driven. In our economy head only the exhaust camshaft is directly driven by the belt, but in our performance head both camshafts have their own cam gears and are directly driven by the belt. In stock form this doesn't play a big part but the performance head has a key advantage when it comes to tuning. It gives you the opportunity to install adjustable cam gears on both cams and retard or advance cam timing independently for intake and exhaust.

Both heads feature a shim over bucket valvetrain. A staple of many Toyota engine designs the shim over bucket is easy to maintain, compact, lightweight and low friction but it's limited in the amount of cam lift and duration you can run with it. Too much lift and duration can pop shims off buckets and high rpms. Which explains why Toyota employed a roller rocket based valvetrain on its more modern 2ZZ-GE engine which features very high lift and duration on the aggressive cam lobes of its variable valve timing system. Honda's K20 is similar example.

But our 4AGE and 4AFE are pretty old engine designs. Neither feature any sort of variable valve timing so their camshafts must strike a compromise between high and low rpm operation. When it comes to valve sizes it's interesting to note that the economy cylinder head doesn't fall behind the performance head, largely due to the fact that it's a more modern design. The performance head has thicker and stiffer valve springs to prevent valve float at the higher rpms that it's capable of reaching.

A key difference that defines the character of the two heads is the valve included angle. The included valve angle is the angle of the intake and exhaust valves against the cylinder head's center-line. Our performance head features a wide valve included angle, while our economy head features a more narrow valve included angle.

If you look at other more modern engine designs you will notice that many of them incorporate a more narrow valve included angle. A narrower valve included angle also has the benefit of improving combustion efficiency, because the air comes into the chamber at a steeper angle it tumbles more which promotes better mixing or homogenization of the air fuel mixture improving combustion and helping the engine squeeze more energy out of the air and fuel mixture. Most modern engines are also able to maintain a good scavenging effect despite the narrow valve included because many have variable valve timing and cam phasing.

The combustion chamber is in our performance cylinder head is a typical pent-roof combustion chamber design. It's a staple the design of the vast majority of performance oriented dual overhead cam engines and it is a good design. As you can see when the valve opens the area around the valve is for the most part free and unrestricted allowing the air freely flow, this promotes good airflow and cylinder filling which enables the engine to breath better and perform better, especially at high rpm.
The combustion chamber in our 4AFE cylinder is obviously very different. As you can see almost as much as 50% of the valve area is actually shrouded and the valves are set deep in the combustion chamber. It seems that the goal here again was to tumble the air against the walls of the chamber as much as possible with the same goal of maximizing homogenization for better combustion efficiency. The 4afe engine doesn't rev very high at all which is why high airflow that would be need at high rpms was sacrificed in favor of combustion efficiency or economy.

00:00 Intro and specs
01:04 Basic anatomy
02:57 Shims and buckets
04:17 Variable valve timing
06:55 Valve sizes
09:09 Valve included angle
14:59 Combustion chambers
16:16 Intake ports
18:32 Exhaust ports

A special thank you to my patrons:
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Peter Della Flora
Daniel Morgan
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Brian Durning

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