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Cam question... what do they actually do to one?
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Author:  poeee [ Thu Dec 09, 2004 6:44 pm ]
Post subject:  Cam question... what do they actually do to one?
With a "regrind", what do they actually do to one?
Author:  drmini in aust [ Thu Dec 09, 2004 6:59 pm ]
Post subject: 
Pretty hard to describe without CAD and I'm CAD illiterate!
Bear with me.. note I am generalising a bit here.. :wink:
Cam lift:
If you look at a cam lobe with the pointy bit (`nose') at the top, it is taller than it is wide. The difference in measurement is cam lift. Multiply that by the rocker ratio (1.25 approx) you get valve lift.
Duration:
The time (in degrees) that the valve is open. Measured from where the tapered bits on each side (flanks) meet the round bit at the bottom (base circle) and *2 because it's measured in crank degrees. Varies from around 240* (stocko) to 300* approx for a race grind. Grinds that are good for street use are usually around 260-280* or so.
With me so far??
OK..
when they regrind a cam, they get more lift and duration by leaving the top of the nose pretty much alone, but grinding the base circle smaller, and altering the shape of the flanks and the nose. It's not as simple as it looks- the cam shape must give smooth acceleration and deceleration to the lifter, while getting as much valve lift at different points as possible. If the nose is too small the cam will wear `rather quickly'.. :cry:

Vizard's yellow book is pretty good re what works and what doesn't. 8)
Author:  poeee [ Thu Dec 09, 2004 7:17 pm ]
Post subject: 
Ah yes... I get ya. So cams of a different grind wont have the "point" in a different spot, but the leading and trailing edges of the "point" will differ?
Author:  drmini in aust [ Thu Dec 09, 2004 7:21 pm ]
Post subject: 
True. The point will be a different shape though, blended into the flanks. And note the 2 flanks are not always the same shape as each other. RE-13 grind is `a bit tricky' in that area.. :P
Author:  poeee [ Thu Dec 09, 2004 7:22 pm ]
Post subject: 
Makes perfect sence. Now the term "grind" makes PERFECT sense!

Sounds like too much trial and error to me!
Author:  drmini in aust [ Thu Dec 09, 2004 7:26 pm ]
Post subject: 
poeee wrote:
Makes perfect sence. Now the term "grind" makes PERFECT sense!

Sounds like too much trial and error to me!


These things are reground on a machine tool designed for it. A bit like a crank grinder, they are a copy grinder, using a master cam profile shape which is then transferred to the cam being ground. Inlet and exhaust lobes are usually different profile, for an A series anyway. :wink:
Author:  poeee [ Thu Dec 09, 2004 7:28 pm ]
Post subject: 
I imagined something like that. However, I mean't when designing the cam to start with. So much that can be done, only way to really test it is to put the thing in an engine!
Author:  drmini in aust [ Thu Dec 09, 2004 7:30 pm ]
Post subject: 
poeee wrote:
I imagined something like that. However, I mean't when designing the cam to start with. So much that can be done, only way to really test it is to put the thing in an engine!


Which is what Vizard did. He had an engine dyno to test with. And Graham Russell in Sydney grinds cams and has an engine dyno, too. 8)
Author:  JC [ Thu Dec 09, 2004 7:47 pm ]
Post subject: 
Informative topic.

Changing the shape of cam lobe would really only change the rate it which it would open, i though? Let's assume you are looking at the lobe square on, round bit a bottom and pointy bit at top. The only way you are going to get more lift out of the cam is by making the lobe taller and more duration making the lobe wider, right? I thought to get more lift/duration they welded 'stuff' onto the cam and then ground it back to specs.

Or am i on the wrong track.
Author:  drmini in aust [ Thu Dec 09, 2004 8:31 pm ]
Post subject: 
Whitestriped63 wrote:
Informative topic.

Changing the shape of cam lobe would really only change the rate it which it would open, i though? Let's assume you are looking at the lobe square on, round bit a bottom and pointy bit at top. The only way you are going to get more lift out of the cam is by making the lobe taller and more duration making the lobe wider, right? I thought to get more lift/duration they welded 'stuff' onto the cam and then ground it back to specs.

Or am i on the wrong track.


No to get more duration the bottom bit is made smaller and the flanks are then longer, giving more duration. Duration is measured from the base circl/flank intersect point, up over the lobe and down to the other base circle/ flank intersect.

With OHC motors they do weld cams up, because there's little meat between the base circle to the cam body..

On an A series cam there's lots of nice meat in the base circle, just right for reshaping.. but the lobe nose cannot be higher than stock or it won't fit thru the cam bearings!:P
Author:  JC [ Thu Dec 09, 2004 10:09 pm ]
Post subject: 
bottom half of cam lobe would represent valve being closed right? If you reduce the size of the lower half of the lobe the valve can't close any further or there something else that is carried out the same time as 'grinding' the cam? I don't get how you can reduce the circumference of an object and have something travelling around it go further (that it did when the lobe was 'bigger').

This isn't arguing kind sir but rather me tring to get my head around it.
Author:  NAV [ Thu Dec 09, 2004 10:44 pm ]
Post subject: 
Whitestriped63 wrote:
bottom half of cam lobe would represent valve being closed right? If you reduce the size of the lower half of the lobe the valve can't close any further or there something else that is carried out the same time as 'grinding' the cam? I don't get how you can reduce the circumference of an object and have something travelling around it go further (that it did when the lobe was 'bigger').

This isn't arguing kind sir but rather me tring to get my head around it.


What happens is when they take the meaty bit away the distance from the base circle to the tip of the lobe increases and you adjust up your tappets to take up the slack and viola more lift and/or duration depending on the grind style. Another way to get more valve lift is to fit 1.5:1 ratio rockers
Author:  gafmo [ Thu Dec 09, 2004 11:00 pm ]
Post subject: 
NAV wrote:
Whitestriped63 wrote:
bottom half of cam lobe would represent valve being closed right? If you reduce the size of the lower half of the lobe the valve can't close any further or there something else that is carried out the same time as 'grinding' the cam? I don't get how you can reduce the circumference of an object and have something travelling around it go further (that it did when the lobe was 'bigger').

This isn't arguing kind sir but rather me tring to get my head around it.


What happens is when they take the meaty bit away the distance from the base circle to the tip of the lobe increases and you adjust up your tappets to take up the slack
Increases :?: Adjusting the Tapet I under stand does it not shorten the distance from the base to top of the Lobe
Author:  JC [ Thu Dec 09, 2004 11:02 pm ]
Post subject: 
what is that ratio (1.5:1) a measurement of?
Author:  gafmo [ Thu Dec 09, 2004 11:06 pm ]
Post subject: 
It's the Rockers themelves..as you can get differnt Cams you can get different Rocker arms...1.2 1.3 1.5
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