Glen's Machine Shop

Designing the cylinder head and valve train


This is the revised version of 'Designing the cylinder head'. The first version was hard to follow even by me. I hope to have an easier to understand version here.
NOTE: All measurements are inches

Before I start let me remind you that I am not an engineer nor a mathematician, so take this information for what it's worth.

Update Jan. 26, 2008
I have learned that the recommended valve lift should be 1/4 of the face diameter.
From Sept-Oct issue of Model Engine Builder magazine, pages 22-24

When I started making the valves for this engine, I started thinking about how big they should be, and how much lift a valve should have. Then I started trying to figure out how much lift would be needed for a certain size port. I started working these calculations after I had already made a prototype cylinder head with valves. The original valve sizes were: intake 0.475 exhaust 0.375 . After checking these valve sizes with my calculations, my exhaust valve is too small. The exhaust valve area should be 80% of the intake valve area. Also note that all valve calculations should be made on the size of the valves port diameter, not the total diameter of the valve. Because the valve laps over the port to close it, the amount of the lap should not be considered in the calculations.

I also decided to make larger valves. I may have to make a 'hemi' or 'penthouse' type head. The prototype head has been returned to the design stage, as it is totally unacceptable as it is.

I first thought that I would just make the valves as big as possible, with the valve lift as much as possible. But after thinking about all of the sizes of valve, valve lift, diameter of the ports, etc. I thought that each must be related to the other. After carefull consideration, and a few calculations I decided:

  1. The size of the valves is limited to the size of the cylinder.
  2. The size of the port is limited to the size of the valve.
  3. The valve lift is limited to the size of the port.


Figure A

The Valve Sizes and Port Sizes

To get maximum flow into the cylinder the valves should be as big as possible. The ports should be as big as the valve will allow. The valve lift only needs to open as much as the port itself can pass. Too much lift can easily be attained. Too much lift will do little or no good, and will only make the parts work harder.

I decided that I could get a large enough valve in a flat cylinder head and decided against the penthouse type head. The penthouse type head being much more difficult to make.
Lets start with #1 above, the valve sizes. We want the valves to be as large as possible and still fit inside the cylinder area. Keep in mind that there also needs to be room for a spark plug in there somewhere. Figure A shows how I layed out the valves and spark plug. Each valve is as large as possible while still allowing clearance to the cylinder wall and space between valves for the valve seats. Ideally the valve should be well clear of the cylinder wall for good flow. But with the limits of the cylinder I can't be too picky, and both valves must go really close to the cylinder wall.
Since the exhaust gasses are less dense and under more pressure, the exaust valve can be smaller than the intake valve. The exhaust valve area should be 75% to 80% of the intake valve area. The following sizes were chosen so that they will fit within the limits of the cylinder and fit the requirements that the ex. valve be 75 to 80% of the intake (area). Note that this is AREA of the valve and not diameter.

    80% of the diameter of 0.500 = 0.400
   80% of the area of .500 = 0.441
(which is a significant difference.)

To get the valve sizes, I picked a starting size for the intake. I started with the intake port dia of 0.500 and calculated the area for it.

    a = pi x r2
    a = 3.14159 x 0.250 x 0.250
    a = 0.19635 sq. in. of intake port

We have the size of the intake (the larger of the 2), now figure 78% (I use 78% because it is about half way between 75 and 80) of it.

     area of ex. port = 0.78 x 0.19635
     area of ex. port = 0.15315 sq.in.

This gives us the area (in sq.in) of the exhaust port. We need to convert this to a diameter. To get the diameter we use the following formula:

    d = 2 x sqrt(a/pi)
    d = 2 x sqrt(0.15315/3.14159)
    d = 0.44158  diameter of exhaust port

We now have the port sizes for each valve. 0.500 intake port, and 0.442 for the exhaust. I decided to adjust the port sizes to a standard drill size for ease of construction. Here are the valve sizes that I will use for the BECO 2.0:

    Intake valve dia.:  0.550"    port dia.  0.500"
   Exhaust valve dia.: 0.488"    port dia.  0.4375"

These are the sizes that I have decided to use. You may be able to get a larger valve if you push the limits a little. I am looking for construct-ability first (within my own personal limits), then reliability and last power.

We now have the size of the valve and the size of the port. What is left is the valve lift that will allow maximum flow for the port. Before we continue lets take note that the valve stem is also blocking a small part of the port area. If the valve stem only runs through the center of the port, we could subtract the area of the stem (area of 0.125" dia.). However the port crosses the stem when it comes into the cylinder head. So we must allow a little more for it. The stem is 0.125 dia. the port dia (which the stem crosses) is 0.500 (intake port). The area here appears as a simple rectangle, so the area that is blocked is:

     a = 0.125 x 0.500
     a = 0.0625 sq. in.

This is the blockage (0.0625 sq. in) when the port crosses the valve stem. Since the blockage of the port running co-axial with the stem is so small I will not worry about it here. This blockage must be subtracted from the port area of the intake valve for valve stem obstruction.

     0.19635 sq. in.   intake port area   
   - 0.06250 sq. in.   valve stem obstruction
     -------
     0.13385 sq. in.   effective intake port area



     0.15033 sq. in.   exhaust port area
   - 0.05468 sq. in.   valve stem obstruction (exhaust port is
     -------           smaller than intake port)
     0.09565 sq. in    effective exhaust port area

With the new port areas calculated, how does the 75 - 80% of the intake area work out for the exhaust area? My calculations show that 75% of 0.13385 effective intake port area is 0.10038. This compares really close to the 0.09565 of the effective exhaust port area. (I figured this out and the exhaust port is 0.007" dia. too small. I think I can live with this.)

Figure B

The Valve Lift

I always thought that the valves should open as much as possible. So I tried to figure how much lift I could get from my cam. The absolute maximum lift of the cam lobe could never be more than 0.2" and I really don't think this would be practical. After a few calculations the math shows a different story. Let's use 0.188" for the starting calculations. That will give us an area of opening at the valve of:

    circumference of the ex. port is 1.37444"
    lift of ex. valve is to be 0.188"

    1.37444 x 0.188 = 0.2584 sq in area around     
                             the open valve.

    effective area of the port:  0.09565 sq. in.
    area  of  the  open valve :  0.2584  sq. in.

This shows that the area of the valve fully open is more than twice the area of the port. This means that we can use less lift on the valves, much less. There is no sense in having a door bigger than the hallway.
If we use a little algebra we can calculate the lift so that there is no wasted area.
Figure B shows the lift and the circumference of the valve. The lift multiplied by the circumference is the area of the opening.

The Exhaust Valve

To calculate the exhaust valve lift:

    lift = area of port / circumference of port
    lift = 0.09565 / 1.3744
    lift = 0.0696

I want to add a couple of thou. to the lift for slack in the valve train. lift = 0.0696, add for slack in push rods, rockers, etc. = 0.006
So the total lift is: 0.0696 + 0.006 = 0.0756

Round this to 0.076" lift (at the cam) for the exhaust valve.


Intake valve

port size is 0.500"
effective area of the port is   : 0.13385
circumference of port : 1.5708

   lift = area of port / circumference of port
   lift = 0.13385 /  1.5708   
   lift = 0.08521

add 0.006" for valve train slack and we get:
Total intake valve lift (at the cam): 0.091"


The Cam Shaft and Rocker Arms

This takes us through the head assembly, but we need to think about the cam shaft also. The lift on the valves comes from the cam. So if the valve has 0.091" of lift then the cam needs 0.091" of lift, Right? Well maybe not. I would like to have the rocker arms to be a slight multiplier, so that the cam lobes can be a little smaller. Some full scale engines use a 1.5:1 on the rocker arms. Meaning the valve opens 1.5 times as much as the cam lift.

This means that the intake valve can be opened with a cam shaft lobe of only 0.091 / 1.5 = 0.061". This should make the mass of the lifters and push rods move much less, which in turn would use smaller springs on the valves. Well..., OK. I'm not to sure about the springs, but I like the idea of a smaller lift on the cam.

The intake valve:
    The total lift for the intake is 0.091"
    divide by 1.5 (for the rocker arms) = 0.06066"
    and round this to 0.061" for total cam lobe lift.

The exhaust Valve:
The total lift = 0.076" (from above)
    divide by 1.5 (for the rocker arms) and we get 0.050"
    this will be the maximum lift of the cam shaft lobe for 
    the exhaust valve.

These are my thoughts on valve and camshaft design. I plan to put them into action as soon as possible. I will add an update when I have proven the design.
Note: After initial test runs, I can safely say that these values have worked well for me.


Here are the final results:
    Intake valve  size: 0.550" dia.  lift @ cam: 0.061"  lift @ valve: (0.091" - 0.006") = 0.085"
    exhaust valve size: 0.488" dia.  lift @ cam: 0.050"  lift @ valve: (0.076" - 0.006") = 0.070"

    Don't forget the 0.006" valve train slack. This amount is added to the cam lobe. When you
    measure the lift at the valve most of the .006" should be gone.

Update Jan. 26, 2008
I have learned that the recommended valve lift should be 1/4 of the face diameter.
From Sept-Oct 2007 issue of Model Engine Builder magazine, pages 22-24


If you have any comments about this feel free to send me Email


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