Discussion
basically when you first fire it up hold the revs above 2500 rpm for around 20 mins, if you need to stop for some reason to top water up etc kill the motor DONT LET IT IDLE, once you have done this drive the car down the road in fourth gear and accelarate from 2000 rpm to 5000 rpm twenty times, from this point just drive the car as normal, but try not to let the engine labour or idle for too long
The way a cam and followers work is the follower spins so that the cam doesnt wear the same part of the follower, if the follower doesnt spin the contact patch between the cam and follower is the same and eventually over heat and start to wear a grove in the bottom of the follower, as soon as this happens the case hardening is removed and excessive wear is provoked, at idle the cam isnt spinning fast enough to make the follower spin, the follower starts to spin at around 1500 rpm so always best to keep above 2000 rpm untill properly bedded in, even then it is not a good idea to let the car just idle as this can shorten the cam and followers life
The story told to me was that the peak load on the cam lobe occurs at peak lift, and as you increase the revs the inertia of the valve train reduces this peak load. But the spinning lifters explanation makes sense too. How on earth did anyone work out what rpm the lifters start spinning at?
GreenV8S said:
The story told to me was that the peak load on the cam lobe occurs at peak lift, and as you increase the revs the inertia of the valve train reduces this peak load. But the spinning lifters explanation makes sense too. How on earth did anyone work out what rpm the lifters start spinning at?
Surely, the peak load will occur at the point when the acceleration of the valve train is at a maximum (force = mass x acceleration). This will vary according to cam profile but will not occur at maximum lift. The acceleration of the valve train increases with increasing revs.
HarryW said:
T......How on earth did anyone work out what rpm the lifters start spinning at?
There's some guy out there with oil splatters all over his face who can testify it's true.
Not got a scooby myself
basicaly yes i got covered in oil! i cut a big hole in the valley to watch what was happening!
profstoff said:
Surely, the peak load will occur at the point when the acceleration of the valve train is at a maximum (force = mass x acceleration). This will vary according to cam profile but will not occur at maximum lift. The acceleration of the valve train increases with increasing revs.
But don't forget about the spring though. The max opposing force from the spring is at max compression. Take into account valvetrain acceleration aswell then the peak load is just slightly before max lift.
wheeljack888 said:
Surely, the peak load will occur at the point when the acceleration of the valve train is at a maximum (force = mass x acceleration). This will vary according to cam profile but will not occur at maximum lift. The acceleration of the valve train increases with increasing revs.
But don't forget about the spring though. The max opposing force from the spring is at max compression. Take into account valvetrain acceleration aswell then the peak load is just slightly before max lift.
Good point, but nevertheless, spring force will not vary with revs but valve train accel;eration will so the load on the valve train will increase as the revs increase.
But I'm not sure how relevant this is to running in speeds; the thing about spinning followers seems more credible.
The spring load is higher than the inertial load of the valve train and the peak force on the lifter occurs just before full lift at lower rpm. But I'm sure that Rob's explanation of the lifters rotating is the true reason why we must avoid low revs while bedding in the cam. I'm trying to imagine what it would have been like to run an engine with a hole in the valley gasket while trying to peer in to see what the lifters are doing, I imagine that would get quite messy!
I think there are lots of reasons really, also another big factor is the slower the cam turns and dont forget it is only half the rpm of the crank, the longer in time the cam lobes are subjected to high spring loads, at idle the follower slowly goes across the lobe so if the spring is say 250ib the cam has to suffer this for a lot longer in time than if the engine was at 7000 rpm
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