Honda K-series Cylinder Head Information
Firstly there are two main types of cylinder heads on the K-series engines , one is a lesser spec and is used on the lower bhp models - CRV K24 and Accord with the K20a6 or K20z2 engines. These heads do not have 3 lobe camshafts on either side of the head. They have a 2 lobe inlet camshaft and a single lobe exhaust camshaft. This basically means the engines only have vtec on the inlet camshaft and is a very mild vtec engagement. On these heads there are no lost motion springs which means it is not as simple as just swapping over a set of 3 lobe rockers and camshafts.
Cylinder Heads -
* These images show the camshaft design and no position to install the lost motion assembly springs.
On the K20a6 / K20z2 engines there was an article online some years ago which we read where a tuner had swapped the pin placement on the rockers of the 3 lobe vtec rockers and installed the assembly with a pair of 3 lobe camshafts. What this meant was there was no need for the lost motion spring as the camshafts were ''always in vtec''. Ok so at the time we tried this and here is our feedback.
We found this didn't work properly and wouldn't recommend it for a few reasons
1. As vtec is always engaged the idle and below 3000 rpm area is very rough.
2. Due to the lower compression of these engines they did not like the bigger 3 lobe camshafts and vtec wanted to engage very late.
3. We didn't check but would be very confident that the engine would not pass an emissions test.
The second main type of head is the 3 Lobe vtec head which comes on the higher bhp models. This type of head also has several different versions , here they are listed below , there may also be more types but these are the ones we have found on European / JDM engines and also a flow chart to show how each head performs.
1. PRB version - This version is the most popular and comes on the Honda EP3 Civic Type-R and the Honda Integra DC5 Type-R. This version is one of the best heads for flow in standard factory form.
* The most noticeable difference on this head is the large coolant port opening on the right hand side of the picture.
* Here you can see the RBB cylinder head codes. Also on this design head the top coolant pipe running to the top of the radiator comes out at the side of the head and not at the front of the head like the PRA version. This makes it a little harder for people installing the engine into a RWD cars.
3. RSP version - This version comes on the Honda Civic Type-R 2006-2011 FN2. This version flows the best of all the standard heads and is an excellent upgrade for a high output K24 engine. This head looks very similar to the RBB style head with some factory porting and obviously the casting stamp is different. Below is a dyno chart showing the power change from a K20z4 engine to a K24a2 engine with a RSP head change.
3. RBC version - This version comes on the Honda Accord CL7 JDM Euro-R. This version flows very well and is an excellent upgrade for a high output K24 engine. This head looks very similar to the RBB style head with some factory porting and obviously the casting stamp is different. The RBC head comes on a K20A JDM engine which produces 220bhp from factory so it needs to have very good factory flow as the rev limit on this model is close to 9000rpms.
Valve Springs -
Some K-series heads come with Dual Valve Springs. These are normally the JDM models with the K20A 220bhp engine such as the Civic EP3 , Integra DC5 and Accord CL7 Euro-R but not all years came with dual valve springs so it is a case of checking the head manually. Dual valve springs can be identified by looking through the large valve spring and seeing if there is a smaller valve spring positioned inside the larger spring. The Dual valve spring setup was installed on the K20A engine versions as the camshafts are larger and also they rev higher than the other K20 engine versions around the world.If you are changing the valve springs to Dual valve springs you will need to check the valve seats to make sure they can accept Dual valve springs.
* Above is an image of a Dual Valve Spring seat.
VTC GEARS -
On the Honda K-series engines there are 2 different types of VTC gears which sit on the end of the inlet camshaft . This gear allows the engine control unit to change the timing of the inlet camshaft while the engine is running.The first version allows for 25 degree advancement in timing while the second allows for 50 degree advancement in timing. Normally the Type-R engines from the EP3, DC5 , FN2 , FD2 and the CL7 Euro-R all come with the 50 degree VTC gear. When upgrading or tuning it is recommended to install a 50 degree VTC gear if it is possible to do so with valve clearance. It is also recommended that if you check your piston to valve clearance and there is not enough clearance to allow 50 degrees advancement that you modify or buy a modified 25 degree VTC gear as sometimes the VTC gear cam overshoot the degree command from the ECU , it is much safer to have a mechanical end point in the gear.
* The 50 degree version can also have the code RBC.
* Depending on how much clearance you have you will need to machine the VTC to suit.