Porting KLR650 cylinder head is a must for any real power gains. The standard cylinder head runs out of steam at 323 CFM and .350″ valve lift. A ported head with standard valves will run out of steam at 332.6 CFM and the Small PoKLR650 will deliver 346 CFM at 0.350 Lift.
Even if you are able to increased flow by another 50 CFM by porting the KLR650 cylinder head aggressively, the major restriction is the air box. It will choke flow down to a measly 282 CFM. An aggressive air-box modification will be addressed later.
There is a difference between porting and hogging.
Porting KLR650 Cylinder Head with bigger valves
The KLR 650 cylinder head will actually flow less air with the valves removed than with the valves in place. Changing aspect ratios will help to change that but it will have other unwanted consequences. Changing the aspect ratio from 86% to 92% will cause the air speed to slow down at low lift. This will cause a situation where all the power is made at high RPM and the engine will be sluggish a bottom end.
We want to to increase power low down but yet at the same time make good power at high RPM. Bigger valves and an aggressive port job is the answer.
Increasing the intake valve from 1.495 – 1.535″ (38 to 39 mm) and increase cam lift from 0.340 – 0.360″ (8.63 to 9.15 mm) will increase curtain are from 2.702 to 3.344. An increase of 23.5%.
To often the porter will change the valve sheet shape to what he/she thinks it should be. Many times this leads to all sorts of problems such as loosing low lift flow etc.
The current KLR650 valve seat has a very good shape and therefore the first order of business was to make a copy of it. An imprint of the valve seat was made with epoxy. This will allow, if needed, for the creation of a profiled cutter of the exact valve seat shape.
Although the KLR valve seat has an excellent profile, a gamble was taken with a slightly different profile. A new profile to accommodate a 1 mm oversize valve was cut. This caused the intake side of the head to really come alive. Cutting the aspect ratio of the port to 86% of the valve size gave another nice increase in flow. The bowls were reworked and the port area leading up to the bowls was reshaped. All of this lead to a 27% overall gain in flow and zero lost of flow in the low lift areas. The flow in the “Ported 1 mm over” section were achieved with the BMW throttle body and K&N air filter connected to the cylinder head.
|Ported 1 mm over
Click to see porting KLR650 cylinder head 39mm intake valve graph
The valve side of the oil flow return galley cause a nasty valley in the roof of the port. Filling this valley and other unwanted areas with epoxy will result in an even better flowing head. We will report on this at a later stage
Porting the KLR650 Cylinder head exhaust side.
Except for seat width, the exhaust valve seat profile is the same than the intake profile. It was decided to change the exhaust profile to a profile that had a little more radius in it. Combined with porting , the exhaust flow responded very well to this mod. Exhaust flow increased by 20%. Disaster struck after the aspect ratio was corrected to a standard 85% of valve size.
Whaaaaaat !!!!!!Flow fell out of the bottom and it did not matter what was done we could not get the flow back to where it was.
New valve seats were installed and re-profiled. This time the aspect ratio was kept the same size than original factory. Magically the lost flow was back! This was a troublesome situation because it is the first time in 40 years of porting that there was a negative response to aspect ratio correction.
De-Shrouding when porting KLR650 head.
Changing out a valve seat is not that much of a job so it was decided to slowly increase the throat of the port. At 83% aspect ration the flow started to drop again. The throat was left at an aspect ratio of 83% but it was decided that de-shrouding the exhaust valve might help.
Aggressive de-shrouding of the exhaust valve / seat indeed helped and resulted in an comprehensive gain in overall flow.
|Aspect ratio screw up
|33.5 mm exhaust valve
|De-shrouding exhaust valve
Click to see porting KLR650 cylinder head 33.5mm exhaust valve flow graph
To be continued. Attention was turned towards a Land Speed Record event at Bonneville where a customer of ours is participating in.