Serdi Valve Seat Cutting

Serdi valve seat cutterAre you replacing valves in your four stroke cylinder head? Or have your valves already been replaced but are not lasting as long as the original set? Ths is why.

Replacing your valves is a very important part of your engine maintenance. Valves are under a lot of stress and care must be taken when replacing them. More often than not, most repair shops will just install new valves without cutting the worn valve seats or replacing the worn valve guides.

Some shops that offer valve seat cutting are using hand cutters. The valve seat has different angles usually 30, 45 or 60 degrees. To do this cutting with hand cutters requires each angle (i.e. 30, 45 or 60 degrees) on the valve seat to be cut separately. These hand valve seat cutters do not have the accuracy needed. If the valve seat is worn out of true, the hand cutters can only follow the same contour. Valve seat cuttingThe hand pressure of the technician can vary. Trying to transition the different degree cuts needed to cut the valve seat cannot compare to the quality you get with specialist machining equipment. The exactness cannot always be seen by the eye, you need special measuring equipment. Most shops that use these hand cutters do not use a concentricity gauge to check the valve seat surface concentricity. This gauge is essential to make sure the valve seat is perfectly round or within two thousands of an inch (.002″) for reliable performance. A race engine should be less than one thousand of an inch (.001″). To put this into perspective, the average human hair is 3 thousands of an inch (.003″). If the valve seats are not cut at all or they are cut with hand cutters, the engine will most likely run, but due to improper valve seat contact, the valves will wear out prematurely and you will not be getting the maximum performance.

Valve seat cuttingThe best and most accurate way to cut valve seats is with a machine like a SERDI valve seat cutting machine. This is the machine that most professional Supercross, Motocross, F1, motoGP, and NASCAR teams use. The SERDI is a very expensive piece of machining equipment but this is required to accurately cut the valve seats. The SERDI can cut all the angles (i.e. 30, 45 and 60 degrees) of a valve seat at the same time with one pass using a carbide cutter. What makes the SERDI so accurate is its ability to float on air to find the true center in the valve seat to perfectly align the pilot before the cutting begins. This is the only process to ensure an accurate valve seat match. The last and most important step is to check the precision work with a concentricity gauge. Quality precise machining, confirmed with the use of a concentricity gauge or vacuum test, is the only way to ensure the best performance and reliability of your valves.

The end result

You can gain 10% to 15% more horsepower between an acceptable and a real race prepared valve job. The better seal you have on the valves, the higher the compression will be, especially at low RPM when the compression has more time to leak out.

For those racers that are on and off the throttle as in motocross and tight course racing the largest gain from a perfect seat is in the lower RPM range. After that the flow characteristics kick in from the angles of the seat and the porting. The valves will cool better and have less hot spots in the seat because they are touching the seat all the way around. If you pick the right multi angle cutting bit for a better flow you will pack a lot more mixture into the chamber. Carburettor tuning is more consistent because the valves aren’t constantly moving around the seats. Valve spring harmonics can also be reduced due to the way the hits the whole seat at the same time. Imagine a valve hitting the high part of a seat that wasn’t cut perpendicular to the centerline of the guide. The stem of valve actually has to flex when the head of the valve is forced to conform to the seat surface under pressure. This eventually weakens the valve and the head can break off causing catastrophic damage.

Top race tuners know that given a certain engine size and compression, the 5mm area that includes the seat and the valve is the most critical in achieving the greatest amount of power and yet it is usually overlooked. If you can’t put it on a shelf in every performance shop in the country you can’t really market it. Most mechanics and race shops don’t have the equipment to do a standard valve job so they opt to sell bolt-on performance or sell new heads if needed.

Frequently Asked Questions

Performance Head Engineering Super Flowed Race Heads specialise in development and manufacture of gas flowed cylinder heads for performance race engines.

We have a range of Performance  Head Engineering Gas Flowed Spec heads for race applications. Our heads are developed using a gas flow bench. Often, development takes many weeks to complete and is truly a black art, requiring many years of experience to develop a head which truly works.

With computer modelling tools it is possible to predict engine power results. The end result is a  head which not only performs as we say it will but is sold at a very reasonable price.

More about gas flowing…

A flow bench measures the airflow at a constant test pressure. The pressure drop is measured across the cylinder head. Using the ratio of this pressure drop to the pressure across a known (calibrated) orifice, it is possible to determine volumetric flow measured in cubic feet per minute or ‘CFM’. This is the primary measurement of the efficiency of a head.

We take readings throughout the range of valve lifts, not just at the maximum lift of the valve. The most important part of the flow frequently occurs just as the valve is closing. The last bit of air on the intake makes the difference whether the engine obtains 90% volumetric efficiency or 100% or 110%.

To begin a typical test, you set a cylinder head on top of a cylinder adapter, with a dial indicator on the valve. A threaded bolt-type device is used to push the valve open. The spring must be stiff enough to keep the valve from being sucked open during the test.

We always use a radiused inlet guide when testing a cylinder head by itself. A radiused inlet guide has a rounded shape to guide the air into the test piece. A sharp edge at the port entrance will cause the air to flow in toward the centre of the port rather than following the port walls. The edge acts like it is choking the airflow. The flow difference can be as much as 30% due to the sharp edge effect. The radiused inlet guide directs the air straight into the port with a small loss, just as it would be if an intake manifold was connected. When an operator tests a head without a radiused inlet guide or an intake manifold, his results will be very different to your results, so it is critical to use a radiused inlet guide. The radius should be about half the width of the port. The same radiused guide is used for all comparative tests.

Does gas flow through the head really make that much difference? Is it really the cam that determines the flow or is it the airflow capacity of the cylinder head?

The inlet port of a head has a single job – to allow the passage of fuel/air mix from the fuel injector/carb at one end to the inlet valve at the other, through which it passes to the cylinder. In order to do this as smoothly and efficiently as possible inlet ports need to be as straight and obstacle free as the design of the engine allows. The port should also promote efficient fuel/air mix by swirling and tumbling the mixture as it flows in to the cylinder.

The shape and size (volume) of the port determines the air speed and length of the column of air/fuel that enters the cylinder (measured in cubic feet per minute) and creates the essential venturi effect which affects gas velocity and mixture.

Modern cylinder heads work pretty well on the whole, they are well made, well designed and flow gas well for motors in a standard state of tune. But the limitations of mass production and cost of manufacture, combined with other factors such as fuel economy and performance which is acceptable to the market, means there are many compromises in the manufacture of a standard cylinder head, particularly in the port shape and surface finish

If you are looking to improve the power of an engine, the first step might be to fit a free flowing exhaust and adjust the fuelling to suit. After that, the next stage is to have the cylinder head ‘gas flowed’.

Engine power is proportional to airflow and within certain ranges, it’s directly proportional to airflow. The exception to this is that if you make the port too large for the flow level, you lose some inertia supercharge effect, because you’ve reduced the air velocity. Frequently, when flow is increased, you have to change the cam timing and the engine will move into a higher speed range. And if you move into a different speed range the engine will require shorter intake and exhaust lengths. There are relationships between all of these parameters.

By measuring the airflow, you can determine the potential power of the engine. If you measure 100 CFM of airflow, you can predict that that engine can only produce a certain amount of power on fuel with a defined octane rating. It’s a simple equation:

For pump fuel at 10″ water pressure: CFM x No of Cylinders x 0.43 = BHP

For example: 135CFM x 4 cylinders x 0.43 = 233 BHP

Other factors must be taken in to account (that’s why Port Flow Analyzers demand so many parameters and measurements) but this is a general rule of thumb for the potential of any head. The figure 0.43 is an average co-efficient seen between CFM and eventual horsepower.

It doesn’t matter what else is done you won’t exceed a certain power. It’s just like the throttle on a carburettor. Open it to a certain point and you get more power, and open it a little more and there is more power, and finally you end up wide-open. But, if you can flow more air, it’s just the same as being able to open the throttle another 10 or 20 percent.

You can also predict the speed at which the engine will develop maximum power. As an engine draws air into the cylinder it causes a pressure loss across the valve. The engine is pumping air into the cylinder. At a certain point the work that it takes to pump the air in through the valve exceeds the power gained by the extra airflow that occurs.

Engines tend to peak when the maximum velocity through the valve at the maximum point in the intake cycle is about 550 to 600 feet per second. At greater velocities, the power drops. The engine will keep getting more and more power until it reaches the point where the ‘Mach’ is 0.55, then the power drops. You have to increase the airflow, and reduce the Mach number, in order to get power at a higher speed. Mach is ratio of air speed to the speed of sound, about 1080 feet per second at sea level.

‘Gas flowing’ involves re-profiling of the inlet and exhaust ports in the cylinder head to allow the fuel/air mix to flow in and the exhaust gases to flow out more efficiently. Essentially it is a redesign of the port based on extremely accurate measurement of size and flow rates and how changes to the port affect the flow.

Of course, hidden behind the word “redesign” is a great deal of detail and pure experience of what works and what doesn’t. What is too much and what is not enough This development process relies heavily on the experience of the expert doing the job and the measurement tools available to him.

The development process of new performance heads starts with a base line measurement of the standard head. The gas flow engineer then begins to modify that port by identifying possible points which could be modified by removing material and changing the surface finish and then making that change by hand using accurate hand controlled tools. This can only be done using experience It is entirely possible to make changes that do not work or break through the head material to the water cooling circuit during development. Sometimes we have to scrap a head and start again. Knowing what works and what doesn’t is where ‘experience’ comes in.

The changes made can then be measured and a further cycle of modification and measurement can be carried out until gas flow improvements (accurately measured) reach a point where the head can be said to be optimised (within the parameters of the head design, valve size and lift). The process is iterative and very analytical. Very subtle changes in the port make a large difference to the CFM measurement. To assist in achieving the optimum design, we use the latest gas flow bench and software to measure and record changes.

Changes made to Performance  Head Engineering Race Spec heads are mainly achieved by removing material from the head which changes the shape and dynamics of the flow through the port. We also add material (by welding or using epoxy resins) to the port to change its shape in terms of flow improvement.

The Port flow analyzer software used is designed to “streamline” the recording, calculating, reporting and graphing of cylinder head flow bench data. It makes flow testing faster and more accurate and assists the experienced gas flow engineer in achieving the optimum design. It is important to realise that this technology does not do the job for the gas flow expert. In many respects it makes the job more difficult because of the sheer amount of information it can provide. However, properly used with the intuitive science of our gas flow guru, the results are outstanding.

Power improvements at the ‘cutting edge’ of racing come from minor improvement in details. For cylinder heads, these details not only include flow, but port velocity, swirl and tumble motion. The Port Flow Analyzer software we use has options for port velocity mapping with a pilot tube, swirl and tumble measurement (using our swirl meter and our tumble adapter), port stability, etc. Using these features and the processing power of the computer (and some fairly advanced physics contained within) we are able to predict the potential power of an engine fitted with this head from the flow bench. This power prediction is amazingly accurate to within 1%

For more information please give us a ring on 01225 791182