To warm or not to warm? (not too warm this time)
Discussion
I have always been led to believe that you should let the TV RV8 warm up before driving:
Yesterday someone who knows much more than me about this said (on another topic) that idling / warming up causes harm - cam damage etc.... How many of us have been labouring under a misapprehension and how much have we added to global warming by letting our cars idle for 10 minutes before setting off?
M :smoking:
Yesterday someone who knows much more than me about this said (on another topic) that idling / warming up causes harm - cam damage etc.... How many of us have been labouring under a misapprehension and how much have we added to global warming by letting our cars idle for 10 minutes before setting off?
M :smoking:
Gentle driving is always better than tickover. Apart from the fact it takes longer to warm up, I seem to recall reading somewhere that the wheely things on the followers (techie talk here!) don't start spinning until >1500rpm so can cause excess wear on them and subsequently the cam etc. Something like that anyway.
cold start warm ups!! firstly i would just like to say the oil in your engine doesnt really start protecting untill it gets warm, the best you can do is run an ester based oil like silkoline or motul etc... but even these dont offer the world when cold. secondly cam wear is instigated by slow moving of the cam follower over the cam lobe (nose), at slow speeds ie idle, you have a heavy valve spring of lets say around 250 ftlb of pressure on the lift of the cam and around 70 ftlb on the base circle of the cam, so at idle that poor old cam lobe is suffering 250 ftlb of pressure for lets say a second with no real oil protection, if those revs increase to normal driving conditions ie 2000-3000 rpm when driving the cam lobe only feels the same 250 ftlb for a fraction of the time, because the follower is up and over the lobe much quicker, in plain english in part of my karate training we used to have to lay on the floor and let the girls walk across our stomachs, fine when they just steped on you and carried on but if they stopped for a few seconds it hurt much more! and that is just a way cam feels!!
v8 racing said:
in plain english in part of my karate training we used to have to lay on the floor and let the girls walk across our stomachs, fine when they just steped on you and carried on but if they stopped for a few seconds it hurt much more! and that is just a way cam feels!!
i really like that analogy
mongoose said:
in plain english in part of my karate training we used to have to lay on the floor and let the girls walk across our stomachs, fine when they just steped on you and carried on but if they stopped for a few seconds it hurt much more! and that is just a way cam feels!!
Ooops.... Steak's ready must run
>> Edited by seasider on Wednesday 17th May 07:44
v8 racing said:
cold start warm ups!! firstly i would just like to say the oil in your engine doesnt really start protecting untill it gets warm, the best you can do is run an ester based oil like silkoline or motul etc... but even these dont offer the world when cold. secondly cam wear is instigated by slow moving of the cam follower over the cam lobe (nose), at slow speeds ie idle, you have a heavy valve spring of lets say around 250 ftlb of pressure on the lift of the cam and around 70 ftlb on the base circle of the cam, so at idle that poor old cam lobe is suffering 250 ftlb of pressure for lets say a second with no real oil protection, if those revs increase to normal driving conditions ie 2000-3000 rpm when driving the cam lobe only feels the same 250 ftlb for a fraction of the time, because the follower is up and over the lobe much quicker, in plain english in part of my karate training we used to have to lay on the floor and let the girls walk across our stomachs, fine when they just steped on you and carried on but if they stopped for a few seconds it hurt much more! and that is just a way cam feels!!
Surely when the engine's running faster the load on the lobe is greater as the acceleration of the follower is higher (as it must move through the height of the cam lobe in a shorter time; F=m*a and all that). Hence why not to rag the engine cold - before the oil can offer proper protection. Also if at low engine speeds the lobe is under pressure from the valve spring for a relatively long duration (due to the slow rotation of the cam), it also has a relatively long time off (as the base circle takes a similarly long time to rotate). Overall it'll still be under pressure for the same proportion of the time (e.g. 25% - 90 deg of a rotation or whatever) - hence the same total time over e.g. a minute.
I don't dispute cold tickover causes engine wear (overfuelling, high oil viscosity etc), but I still don't see why cam wear in particular should result. Is the RV8 the only engine to suffer high cam wear at cold tickover or do other engines?
I can't dispute the girls walking on stomachs analogy though!
griffter said:
Surely when the engine's running faster the load on the lobe is greater as the acceleration of the follower is higher (as it must move through the height of the cam lobe in a shorter time; F=m*a and all that).
The inertial effects are why the loads go down with revs. The peak pressure occurs at peak lift when the valve spring is fully compressed. THe more revs you have the more acceleration the valve train is under at this point, so the more inertial effect there is unloading the valve. (If you red line the engine the cam follower will be virtually floating at this point.)
The other effect is one that Rob reported a little while ago - the cam followers are designed to rotate slowly in their housing but they don't do this until the revs get high enough. I'm sure this is a critical factor while bedding in the cam. I don't know how critical it is subsequently but it may be a factor in cam wear. I know somebody who had a show car that was left running for hours at a time and the cam died relatively quickly, the car had barely turned a wheel.
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