Cylinder Head Porting Fundamentals
1 - Intro
This information was gathered from practical experience as well as written resources from industry leaders in various fields.
The purpose of this is to provide the basis for the common man, the bike owner, the amateur engine tuner, the weekend racer and the ABUSO customer. The focus here is on motorcycle cylinder heads, particularly those of more classic years, from 1970 to 2000.
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2 - Why Porting?
We want to GO FASTER so we need more powerful engines. Power is a function the Air that the engine can consume so we must do all we can to increase air flow – Bigger Engine, Hotter Camshafts, Bigger Carburettors and of course Increase Cylinder Head Flow
One of the coolest thing about older engines is how far from perfect they are and how much we can improve them with more modern materials and understanding. If you start with a 200HP bike, there isn’t much we can do to improve it, but an older Air-cooled or Oil-cooled Suzuki is a different matter altogether.
Suzuki GSX 1100 Ported and Ready for Action
3 - Basic Principles
Principle 1 – Air is Good
Engine maximum horsepower is dictated by the maximum airflow that it can move so Air is Good, Air is Great!
Principle 2 – Too much of a good thing
It is possible for a head to flow to much air for a given engine – just like overly large carburetors, when ports are too big the result is too low air velocity, bad Low RPM behaviour, poor flow atomization, bad throttle response. All VERY bad in a motorcycle engine.
Indiscriminate enlarging and reshaping of ports can result in a loss of flow / performance, particularly at low RPM.
Even the BMW Factory must make compromises… A Big torque dip between 6K and 8K RPM
Principle 3 - Objective
Our objective must be to increase air flow at all valve lifts.
If our porting and other modifications produce an engine that has very high flow only when the valve is fully open we have made this engine worse.
If we produce a cylinder head that has higher flow for all valve lifts, this engine will have improved performance under all running conditions.
Great improvements across all valve lifts for both Intake and Exhaust
Principle 4 – Sharp Turns
Air doesn’t like to take sharp turns so all our porting work must be guided by this principle of avoiding sharp bends or transitions.
Principle 5 – Smooth, not Polished
A lot can be said about port surface finish but let’s not complicate things too much and say that we should not polish the ports to a mirror like surface. They should be smooth but not polished – this is great news as polishing a port is not a lot of fun
Formula 1 (V10 Era) Ports are smooth but not polished
4 - Port Anatomy
The port is made of many different areas with different needs, so it is important to first establish a common terminology around this topic.
I will use the terms in the following picture to identify the various areas of the port.
Port Anatomy - Should be taught in School
5 - Porting Procedure Step by Step
Step 1 – Removing Old Guides
The first step in any grinding procedure is removing the valve guides. These guides are considered a wear item and cannot generally be replaced. This is a good time to get some quality ABUSO Manganese Bronze valve guides.
Some people grind the whole tip of the guide in an effort to gain flow and avoid the work and expense of removing the guides. This is far from ideal as shortening the guide remove support from the guide and diminishes the guide’s ability to cool the valve, both very important factors in valve and seat longevity and overall long term engine performance.
Furthermore, grinding away the guide produces minute improvements in port flow and these small improvements do not justify the many drawbacks.
Removing original cast iron guides on Honda CB 750 SOHC (there are better ways…)
Step 2 – Throat Area
Apart from the valve seats that we will address later, the most important area in any cylinder head is the Valve Bowl or Valve Pocket right after the valve seats. The thoat is where the actual port begins...
Throat Area is the tightest area of the Port
Generally, the throat of the valve and the recess where the valve seat is inserted are machined simultaneously by the factory, which often leaves behind sharp edges and bumps that inhibit flow and create unwanted turbulence.
If you have a stock cylinder head with the valves removed, run your finger just above the valve seat where it merges into the port you will probably find a stepped and uneven transition that is not good for flow.
This is why, these area demands careful attention to smoothly blend the valve seat and the port, always removing as little material as possible, eliminating any bumps and ridges.
Stock head with poor transition after the Valve Seat
This area is always smaller than the valve diameter and can be enlarged, always avoiding going too large. A good rule of thumb is sizing the valve throat and valve seat (internal diameter) to around 85% to 90% of the valve diameter. If you install Oversize Valves this area can be enlarged accordingly. This is why oversize valves can improve port flow when the head is ported.
Step 3 – Short Turn Radius
The second zone in the Valve Bowl / Valve Pocket that demands careful attention is the short turn radius.
Modern cylinder heads that are designed for maximum NA horsepower (Modern 1000cc Sportsbikes, F1, Moto GP,…) have very steep ports (as close to vertical as possible), improving intake port flow and high RPM performance. This steep ports guarantee a straight flow path that is directed at the back of the valves, avoiding any sharp turns.
F1 Cylinder Head (V10 Era) with very straight ports, mainly on the intake side
Most older style heads (Honda CB750 SOHC, Honda CB900F Boldor, Suzuki GSX 1100, ….) have much more horizontal ports, meaning the air has to make a sharp bend right before the valve, which reduces port flow as the air separates from the short turn radius (tighter side of the bend).
Older Technology Suzuki GSX1100 – Horizontal Port / Curved Port
New Technology BMW S1000RR – Vertical Port / Straight Port
In these older engines, our objective is to reduce the sharpness of this turn, increasing the radius of the short turn of the port.
Production Cylinder with tight short turn radius – not great for flow
By smoothing and possibly resizing the valve throat area and improving the short side radius, we have addressed the most important areas of the cylinder head, the areas where most of the gains will be attained and where most of the attention must be spent.
In many cases we should stop here and jump straight into machining a multi angle valve seat leaving the remaining port stock. This is because the biggest restriction in any port is the valve area and so we have achieved most of the possible gains with relatively low risk and trouble.
If we choose to stop now, we have made what is generally called a “Pocket Port” because we only ported the Valve Pocket or Valve Bowl Area.
If we are after maximum performance, then we must also do the steps described bellow…
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Step 4 – Valve Guide Boss
Since we have removed the valve guide, we have good access to the valve guide boss, which is the material that surrounds and supports the valve guide.
Here there are various options but the general idea is to remove unnecessary material, leaving behind an adequate support for the guide.
Properly Ported Guide Boss
Step 5 – Port Entry
Before taking the grinder to the Port Entry we must understand that this work will only improve airflow in the high valve lift area. When the valve is operating in the low lift area (where it spends the most time), the maximum airflow is dictated by other factors such as the valve diameter, the valve seat, the throat and the short side radius. This is the reason why these areas are the most important and should be addressed with the greatest care.
The port entry can improve high lift flow and if we improve this area we will improve engine horsepower. As always, we must understand the engine and its purpose because if we go too far with porting, we can end up with a severe torque dip in middle the powerband, resulting in poor performance and drivability.
So, the cautious approach for the amateur engine builder is to improve on the work of the factory, enlarging the port in proportion to the HP increase expected. If you start with a 65HP Honda CB750 and want to achieve 100HP after modifications, your porting will have to be more extensive than when starting with a 150HP Yamaha FZR 1000 that is looking to reach 180HP…
Before and After porting on Honda CB750 SOHC
Another aspect that is important with classic bikes is that most of them have curved ports which are obviously far from ideal and we should try to straighten them as much as possible.
Honda CB 900F Boldor is a bad example of bent ports
The problem with this is that the air will flow predominantly on one side of the port, reducing its effectiveness.
There are very extreme solutions such as machining out the original port entry and complete replace it but this is very hard, costly and risky so the other solution is to work within the factory constrains, using the available material to straighten the port as much as possible, minimizing the issue.
Honda CB900F by Vince & Hyde – now with straight ports (Endless Work and Moneys €€)
Step 6 – Combustion Chamber
I can hear you thinking – The combustion chamber is not part of the Port! Well, as far as the air is concerned it is. The chamber does influence the airflow because if we have material that shrouds (surrounds) the valve, this will reduce airflow.
This area is very critical and should be taken with a lot of care and attention. Most of the times we should stay away from the combustion chamber unless we’re chasing every last possible HP.
Our work here is to smooth the chamber and remove as little material as possible while creating a nice airflow path around the valve. Of course, this work must be VERY consistent between all chambers to maintain the same combustion ratio balance.
Old Valves in place, “porting” can be done without damage to the valve seats
By removing the sharp edges in the combustion chamber that come from the factory (using sandpaper rolls for example) we also reduce hot spots that can result in detonation that we must avoid at all costs.
Step 7 – Valve Seats
The valve seat is one of the most important areas of the cylinder head along with the valve pocket/bowl area and should be done last, after reinstalling new valve guides (hopefully improved bronze valve guides when available).
Today, a three-angle performance valve job is standard procedure, and the first angle cut is the throat cut, which is typically around 60-70 degrees. This helps ease the air’s transition to the seat cut, which is the second cut and is generally done at 45 degrees. This is the surface that the valve actually seals against and is also responsible for cooling the head of the valve.
The third and final cut is called the top cut, which is normally 20-30 degrees and is made immediately after the seat. This cut helps reduce valve shrouding of the airflow past the valve (or before the valve on the exhaust side) as the valve starts to lift off of the seat.
3 Angle Valve Seat from a Formula 1 Engine
There are other high performance valve jobs that incorporate up to five different angles, including in some cases a 70- to 75-degree throat cut, depending on the port and its flow characteristics.
5 Angle Valve Seat – Looks pretty!
Conclusion
Porting is as much science as it is art in the sense that sometimes the ideal port shape is counter intuitive and may even contradict some of the principles expressed here.
We must also give credit to the factory design because countless engineering hours went into the study and development of any cylinder head. This means that our work should focus on improving the original design instead of trying to move as far away as possible from it, unless we have solid evidence to support this.
In essence, go slow, be conservative while focusing on the purpose of the engine, maybe opting to start with a Pocket Port (Throat + Short Turn Radius + Performance Valve Seat) along with some oversize valves (if required) and then check the results before going for the whole porting procedure.
Either way this is the fun part of modifying older style engines and bikes in general – they have many opportunities for improvements to satisfy our inner engineer. So my advice is GO FOR IT and Have Fun!