The KLR650 Small Port Head makes an already good cylinder head even better. It came about when it was realized that the port volume is actually to large thus slowing the airspeed down. To understand the relationship between cylinder fill, port volume and airspeed one has to understand the effect the one has on the other.
The simplest way to explain this relationship is to look at two pipes. One pipe has an inner diameter of 2 inches and the other has an inner diameter of 1 1/2 inches. Both pipes are fitted with a one inch nozzle that can support a flow rate of 25 gallons per minute. The flow rate in the 2 inch pipe is 10 gallons per minute and the flow rate in the 1 1/2 inch pipe is 15 gallons per minute. Which one of the two setups will fill a 55 gallon drum the fastest? It is easy to see that the 1 1/2 inch setup will fill the 55 gallon drum the fastest.
If the same principal is applied to a port in a cylinder head the same effect can then be expected. Increasing the port speed too much can have the opposite result.
How to Increase Intake Flow with a Small Port Head
At 0.350″ and 0.400″ of valve lift, the stock intake ports, without the carburetor attached registered 323 and 312 CFM respectively. Note that the CFM started to drop after max valve lift of 0.350. A classic example of either turbulent air or slow airs speed in the port or intake valves that are to small.
With the intake valves completely removed from the head the flow bench registered 304 CFM. If there was no port turbulence or airspeed was sufficient this figure would have increased because the restriction in the port in the form of the intake valves were removed. Since the ports are fairly quite during flow bench testing it can be assumed that turbulence in the port is fairly low. The culprit then must be low air speed in the port.
Before modifications were started the aspect ratio of the port was checked. Aspect ratio is the ratio between the intake valve diameter and the throat diameter of the port. Aspect ratio will determine how the engine will respond to throttle input. Too big and all the power will be made in the higher RPM range. Too small and the engine will respond very quickly to throttle input with most power made on the bottom and nothing up top. The ideal aspect ratio for a good performing street engine is 85 – 88%. The KLR650’s aspect ratio is as close to perfect as one can wish for. In the case of the small port head the aspect ratio, valve and seat angles and cam lift was left alone. Apart from valve and seat angles and aspect ratio the next operation to perform to increase flow is porting.
There is some areas in the intake port that can be improved but porting now becomes the catch 22 situation. Removing more material from the intake will increase port volume which in turn will decrease air speed.
First port volume was measured and it checked in at 123.4 cc. Flow bench figures for the head at 28 Inch of water, without the carburetor attached to it came in at :
Porting the KLR650 Cylinder Head
Basic porting was started by removing all the ridges in the bowls. The bowls were then blended into the rest of the port. The short side of the port was radiused and blended into the floor of the port. The valve guide supports were slightly reshaped and all casting marks were removed from the port. Total port volume increased to 136.8 cc. Back on the flow bench. A significant increase in flow was recorded at .400″ lift
Basic porting helped the head flow better and CFM increased at 0.400″ lift but overall port speed is still down because when the head was tested without the intake valves in place, CFM was only at 314.5
Reshape the intake with clay to achieve a KLR650 small port head.
To achieve the end result of the KLR650 small port head clay, was used to reshape the floor, the short side and portions of the roof of the port. This was a tedious and painstaking slow process but the end result was well worth it. Final port volume was 106 cc.
Final flow bench clearly show that an increase in airspeed will help the port flow better:
Why is airspeed important for cylinder fill?
When the piston turn on bottom dead center during the intake stroke the intake valve is still open and continue to stay open until 69° ABDC. The question now becomes how do you prevent the piston from pushing air out of the cylinder after it turned at BDC and start with the compression stroke. The answer is air speed. The speed at which the air moves down the port continue to fill the cylinder after the piston turned at BDC and eventually block the air from flowing backwards in the intake port until the intake valve is close. Once the intake valve is closed the flow of air built up a very small positive pressure behind the intake valve. At the next intake event the small positive pressure immediately starts to flow thus helping cylinder flow. In the performance industry this is better know as intake pulls.
Too much air speed can be bad!
To much airspeed can also have a negative effect on engine performance. The fuel that is mixed with the air is heavier than the air. When the air/fuel mixture turns to enter the combustion chamber the fuel in the air tends to separate from the air and go straight, eventual forming puddles against the long side of the port. This cause all kinds of problems.
Final testing of the KLR650 Small Port Head.
Dyno testing for the test mule was done when it was bone stock and after the following mods KLX needle, 142 main jet, airbox mod and the addition of a FMF Power Bomb and PowerCore4.
Not only did the KLR650 small port head achieve an overall gain of 10% but throttle response is crisp and power comes on smooth and stay longer.
Stock KLR650 compared to first stage mods consisting of KLX needle, 142 main jet, airbox mod and the addition of a FMF Power Bomb and PowerCore4.
Stock KLR650 first stage mods consisting of KLX needle, 142 main jet, airbox mod, FMF Power Bomb, FMF PowerCore4, and a KLR650 small port head
|Specs: 2014 Kawasaki KLR650||Standard||After Porting||KLR650 Small Port Head|
|Port Volume||124.4 cc||138 cc||112 cc|
|Combustion Chamber Volume||64||64||64|
|Intake Valve Size||38 mm||38 mm||38 mm|
|Throat Diameter||32.4 mm||32.4 mm||32.4 mm|