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So, your headporter tells you that you should get a 3 or 5 angle valve job done. You don't know what this is and want to learn some of the basic terms and why this should or should not be done. You've come to the right article.

Before starting here please read my Headporting Article entitled "Gotten Head Lately" at linked below, the "Ideas: Flow Velocity, Flow Capacity, Flow Quality" Article in this website's Articles, Joe Alaniz's "Basic Headporting Terms" website page linked below, and Standard Abrasive's Basic Review of Headporting Terms, if you are unfamiliar with headporting basics.:

Here is a general overview of overhauling the valve seats, valve guides, and valves in aluminum heads:

Introduction: Types of Cylinder Head Port/Seat Design


Cylinder head ports fall into two basic designs:

the Straight Shot port and the High Approach port.

In Straight Shot port, the design allows for a line of sight from the inlet directly to the front opening of the intake valve. Sometimes this design gives a lower flow value but because it is straight, we attain higher velocity of fuel/air entering chamber. It also creates a turbulent spin or swirling in the combustion chamber, yielding a more efficient and more complete burn.

A High Approach port is where a turn is necessary in the port design. The term "high approach" comes from angle of the last section of the port relative to the valve - it is more in-line with the valve stem.

We take advantage of this to get a full volume flow path in the complete 360° circumference of the valve. Making a turn to an airflow is problematic but we caress the turn - making it as moderate and efficient as possible - to use it in our favor. The straight shot into the opening yields a fuller, more uniform flow around the entire valve curtain. The advantage is that the complete valve opening is used more completely.
Remember that the pre-requisite of the flow arriving at the valve seat should be that the flow velocity at any rpm maintains atomization of the fuel (fuel remains suspended as a mist in the air and does not "rain out" in big droplets onto the cylinder head wall).The flow in the intake port should not separate and form vortexes before it turns down into the combustion chamber.

from 1996 Motorcyclist Magazine Cylinder Head Tech Article

When flow in a duct (an intake port, for example) arrives at a bend, it loses any semblance of orderly behavior. Particles on the inside of the bend travel the shortest distance (offering the least resistance to flow), so they tend to maintain speed in the downward turn to the valve seat. But flow in the top of the port slows relative to the floor, creating a large velocity gradient. Pressure in a moving fluid varies inversely with it's speed, so the velocity gradient creates a lower pressure at the port floor than at it's roof.

This differential causes air at the sides to move upward and the midstream air to move down, with the resulting flow stream made to divide into to contrarotating vortices where the port bends. Add to this the invisible "smoke ring" vortex forming beneath the opening intake valve and you have enough disorder to confound even the best of minds (or computers).

Port and valve configuration (both shapes and angles) can profoundly influence combustion efficiency as well. Jack Williams' AJS 7R made it's best power with an intake port shape that compromised flow in favor of creating more combustion chamber swirl and redirecting incoming fuel droplets away from the cylinder walls. I am reliably informed that Keith Duckworth has settled on the intake valves leaned 15 degrees from the cylinder axis, and ports at 30 degrees from the valves in a similar trade-off between flow and combustion.

Intake flow influences combustion because both carburetors, and fuel-injection nozzles deliver fuel in liquid form. The best you can hope for is a fog of droplets small enough to stay suspended in the air while evaporating; big drops are centrifuged out of the air stream, splatting against the intake port and cylinder walls, which is bad for power, fuel efficiency and emissions. Fuel can't burn until it evaporates; if you have raw fuel still trying to burn when the exhaust valve opens, it goes out the pipe, wasting your money and polluting the air.
Tapered ports and valve seats aim to re-accelerate the air-fuel mix like a venturi effect, after it has slowed down and made the turn down the bend to the throat. However, there is more to it than that.

A. Stock Valve Seat Angles On Integras

Here is what the stock valve seat angles recommend. Honda uses the 30-45-60 degrees discreet angles configuration for their valve seats in their performance-oriented cylinder heads, as seen in the GSR, ITR, and CTR. They are called "discreet" because they stand out on their own with distinct sharp edges or borders and are not blended smooth into each other like radiused seat angles seen in domestic heads. It's important to cut the 45 degree angle valve seat surface location precisely (using dye to mark if the seat sits properly).

On the cylinder head, the 30 degree is closest to the piston or combustion chamber side and is called the "top cut". Next is the "seat angle" proper which matches up to the valveface's 45 degree seat angle. The 60 degree angle below the seat angle is closer to the intake port/IM and is called the "throat cut".

Figure 1. B18 Stock Valve Angles and Locating Valve Seat Area

B. Typical Areas Targeted By the Headporter's Grinder

Figure 2. Traditional Target Areas for Material to be Removed by Headporting (lighter shaded parts): Roof around valve guide, Floor's short turn radius, and valve seat. The area from the exposed area of the valve guide down to the short turn radius or port floor is called in headporting terms the port's "bowl area".

Figure 3. Reducing the lowest valveface angle (called "backcutting" the valve) and the altering the head's throat cut & seat angle to modify the "flow cone" shape at the low-mid valve lifts are where the major gains on a 3-angle valve job are made. Narrowing the seat angle's width also improves low lift flow. Top cut alterations and bowl blending improves mid-high valve lift flow.

Example of a Backcut Valve (Left):

Figure 4. From the Standard Abrasives website linked above:

(A) In this production intake port, air starts into the port flowing smoothly. When it encounters the factory casting flaw on the floor of the port, smooth flow breaks into tumbling and turbulence. This causes restriction to the overall airflow in the port.(B) turbulence in the airflow becomes more severe as air passes the sharp edges of the short side radius in this drawing. Smoothing the radius and removing (certain) casting bumps and flaws (not all of them) reduces turbulence and increases flow.

C. Why the Valve Seat?

Figure 5. Casting Flaws (indicated by the 4 red arrows) below the valve seat and the ridge above the valve seats in the combustion chamber are typically smoothed out to allow for a more homogeneous flow to get swirl filling (of the air-fuel mix) into the cylinder. Some suggest that the "underhangs" below the valve seat (bottom 2 red arrows) should be left alone, since they help create swirl as the intake valve opens (from Endyn's B16A head porting article).

Most headporters would agree that the prime "bang for the buck" area to gain performance from Honda headporting is at the valve seat angles and in the transition from the port's bowl area into the valve seat. This is where Honda focused it's attention on the venerable b16a head to turn it into the impressively improved b18c5 (type R) head. This fact may surprise the novice who may have thought that the port entrance (mating to the IM) would be the main area for performance improvement.

The goal of the flow at low rpms in DOHC layouts, as it passes by an opening intake valve seat, is to have swirl filling or reverse tumbling or a combination of these 2 cylinder filling methods rather than just a conventional tumble cylinder filling that the overhead valve layouts prefer. In this way we achieve layers (called a stratified charge) of air:fuel ratios which become progressively lea and leaner towards the bottom of the cylinder. This stack of air:fuel layers with different air:fuel ratios (leanest at the bottom by as much as 28:1 air fuel ratio) is the basis of a compact combustion chamber's lean burn theory. It allows for better gas mileage, emissions, and of course, power by combusting or burning more rapidly (and more completely) than just the tumble fill of large combustion chambers (in old domestic V8's), when the air flow speed is low. At mid to higher rpms, swirl-tumble filling with higher air flow speeds achieves a homogeneous intake charge to achieve an efficient complete burn when there is less time for the combustion stroke event. The valve seat angles are critical in achieving swirl-tumble fill and prevent reversion (reverse flow back up the intake port throat).

Figure 6. Mitsubishi Combustion Chamber With Direct Fuel Injection Into the Chamber Instead of the Port Helps Induce Reverse Tumble (Swirl) Filling versus Conventional Tumble Filling

see the movies of these 2 types of cylinder filling by going to the link at mitsubishi of this image above:


Check out the .avi files of Swirl Filling Versus Tumble Filling! it. A quick primer on valve seat angles.
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