- Joined
- Mar 12, 2013
- Messages
- 317
JS
remember the spintron is operating cold, and not experiencing the heat of a running engine, consequently clearances set will not change on the spintron, but they most certainly will on a running engine.
Your experience with the tappets on a BSA is common. Clearances could be set to nil, but with the pushrods free to rotate. This positioned the valve train to start taking up the slack while the follower was still on the early part of the ramp, and it would be in a similar position when closing.
When the engine is hot and working at high rpm, the flexure and bending of valve train components will ensure that the valve will open when the cam reaches the point where the velocity curve starts to suddenly change from constant velocity to begin pushing the valve open - unless of course the tappets have been set too wide to start with, then the ramps, however well designed, will have no effect, and the cam on opening will crash against the follower causing tremendously high stress in valve train components, particularly pushrod, and will also close the valve in a similarly violent manner.
An examination of a cam profile as graphed for cam lift via software such as Cam Analyser or similar, will show the ramps in detail, and it will be readily apparent that a good starting point for consideration of initial tappet setting is by using a clearance that will place the opening point about halfway along the ramp. From the ramps shown in earlier posts, it is clear that all ramps are not equal, and depending on the cams in question, can vary in both rate of lift, and duration of ramp.
The foregoing at best can only be a generalisation. All engines do not respond in the same way, even those of the same make and model.
It has been said that one should consider all engines as being made of materials that , although called steel, aluminium, titanium or whatever, can in fact be best considered as being some peculiar kind of rubber, because every single item that goes to make up an engine, if subjected to sufficient force, will distort, bend, deflect, twist, or in some way deviate from it's unstressed condition, and when the force is removed, hopefully, it will return more or less, to it's original form.
In a way this ideally explains the workings of a pushrod valve train.
remember the spintron is operating cold, and not experiencing the heat of a running engine, consequently clearances set will not change on the spintron, but they most certainly will on a running engine.
Your experience with the tappets on a BSA is common. Clearances could be set to nil, but with the pushrods free to rotate. This positioned the valve train to start taking up the slack while the follower was still on the early part of the ramp, and it would be in a similar position when closing.
When the engine is hot and working at high rpm, the flexure and bending of valve train components will ensure that the valve will open when the cam reaches the point where the velocity curve starts to suddenly change from constant velocity to begin pushing the valve open - unless of course the tappets have been set too wide to start with, then the ramps, however well designed, will have no effect, and the cam on opening will crash against the follower causing tremendously high stress in valve train components, particularly pushrod, and will also close the valve in a similarly violent manner.
An examination of a cam profile as graphed for cam lift via software such as Cam Analyser or similar, will show the ramps in detail, and it will be readily apparent that a good starting point for consideration of initial tappet setting is by using a clearance that will place the opening point about halfway along the ramp. From the ramps shown in earlier posts, it is clear that all ramps are not equal, and depending on the cams in question, can vary in both rate of lift, and duration of ramp.
The foregoing at best can only be a generalisation. All engines do not respond in the same way, even those of the same make and model.
It has been said that one should consider all engines as being made of materials that , although called steel, aluminium, titanium or whatever, can in fact be best considered as being some peculiar kind of rubber, because every single item that goes to make up an engine, if subjected to sufficient force, will distort, bend, deflect, twist, or in some way deviate from it's unstressed condition, and when the force is removed, hopefully, it will return more or less, to it's original form.
In a way this ideally explains the workings of a pushrod valve train.
