Rollers on the cam followers are great to reduce friction but they may require new camshafts if you want to keep the OEM valve lift profile.
The sliding surface of the OEM cam follower is not a flat plane but is curved. If you decrease the radius of that curvature (for example with roller followers), you’ll decrease how much lift is transferred to the valve at various camshaft rotation angles. Not at maximum and minimum lift, but at the points in between. To maintain the OEM valve lift profile, you would need to have new cams ground with “meatier” flanks. It’s not too difficult to calculate how the shape of the cams would need to change given an increased curvature of the follower’s contact surface. Also, if the roller followers are heavier than the sliding followers, you may need stiffer valve springs to prevent valve float at high engine speeds. All things considered, it would be a lot cheaper to just DLC coat the cams.
I spoke with another company that DLC coats engine parts. They said that the parts that slide against each other must not have any ridges. Ridges perpendicular to the direction of sliding can cause the coating to flake off and ridges parallel to the direction of sliding cause local pressure points that can also damage the coating. So if the engine has been used, the cams should be smoothed to as-new condition before being DLC coated and the followers should be smoothed as well before rubbing against the freshly DLC coated cams.
If you read that presentation “Calculation of Friction in High Performance Engines” I linked to above (Source 3), the top three sources of friction in a high performance gasoline engine seem to be:
- Piston skirt
- Cam/tappet contacts
- Crankshaft windage
Currently, it seems to me that it would be good to:
- DLC coat the piston skirts
- DLC coat the cams (to prevent wear, if nothing else)
- DLC coat the intake valves (so that carbon deposits don’t stick to the back of the valve’s head and so that the stem has as little friction as possible. I’m not sure about coating the exhaust valves because their stems have to transfer heat to the cylinder head. Are DLC coatings good at transferring heat?)
- Lap, polish, and WPC treat the cam followers
- Bore, hone, and WPC treat the cylinders
- Knife-edge the crankshaft counterweights to reduce windage and install tungsten inserts, if necessary, to get the weight back up to where it should be
- Modify the crankcase for less crankcase pumping losses (if feasible)
- Deburr and WPC treat the gears in the transmission and final drive
Getting the camshafts smoothed, lapped, and then DLC or WPC treated would probably require several weeks of downtime while the car is in a shop somewhere. It would probably make sense to get most of the things listed above carried out at the same time, during a major engine overhaul. Since my NSX’s engine seems to be in good health, that’s not going to be for a while.