 |
PM |  | |
Quote | | |
Reply | | |
Full Topic | |
| Joined: | 04-07-2022 |
| Location: | Oregon USA |
| Posts: | 88 |
| Status: |
Offline
|
sorry
I meant at which valve clearance is the normal opening and closing figures of the valves given?
Best regards
Kim
Old thread, but I thought I would answer this question for any future readers out there.
There are two ways duration is rated. The industry standard (in the US) is 0.050: lobe lift is where the duration is measured. I believe in metric terms, 1mm is the lobe lift point most commonly used (0.040").
There is another measurement called "advertised duration" which has no standard, but is typically quoted when the VALVE lift (not lobe) is 0.005" off the valve seat. This number depends on the rocker ratio in overhead valve designs (1.5:1 in a small block Chevy for example). HOWEVER it could be measured "seat to seat" or 0.001" valve lift to as much as 0.010" valve lift. This means that comparisons of advertised duration can be meaningless.
For our engines, there is no rocker ratio to confuse things and the numbers quoted are advertised durations. The lift and duration are measured AFTER the valve lash has been taken up by the cam. So valve lift will be 0.010" less than the lobe lift because there is a 0.010" clearance between the cam and the valve bucket that needs to be taken up before the valve starts to move.
The most meaningful measurement is taken at 0.050" lobe lift because the valve is not really flowing much until the valve is significantly off its seat. Also, the difference between advertised duration and 0.050" lobe duration tells you something about how modern the cam is. Modern designs will have about 60 degrees difference (or less!), but old school cams can have up to 100 degrees, meaning the cam is "lazy" and will not perform as well as a cam that opens and closes the valves more rapidly. You can infer how modern the design is by looking at the lift for a given duration. If two cams have the same advertised duration, and the 0.050" duration is unknown (as with our cams) then the cam with more lift is the more modern design.
For a street driven car, 270 degrees advertised or 210 degrees 0.050" duration is about the limit. More than this idle suffers, low end torque diminishes, and fuel economy and emissions degrade. How much things degrade depends on how tight the lobe centers are, the design of the engine, number of valves, etc. David Vizard has observed that the optimum lobe center spacing (for power only) depends solely on the overall engine design. Optimum power may not be acceptable for the other considerations (driveablility, economy, etc).
All of the above explains why modern engines go to the trouble of having variable valve timing and/or phasing. If you want good economy, emissions, driveability, and power (the whole enchilada) the cam timing at 6,000 RPM will need to be much different than at 700 RPM. Incidentally, this is how a modern car can get 100HP/liter with excellent economy and emissions. The ECU adjusts the cam parameters depending on temperature, throttle setting, and RPM.
Vance
Last edited on 04-19-2025 07:15 pm by vnavaret