Oops, too long the first time.
> Ok, this is probably more about this subject than you really want to know.
>
> Here is a graph constructed from Cam Doctor data on a stock TR4 cam -
> http://home.swbell.net/cartrip/TR_StockB.gif. You can measure the
> lift curve with a degree wheel and dial indicator. It just won't be
> this accurate. The velocity is the slope of the lift curve, which
> could be calculated in a spreadsheet. The acceleration is the slope of
> the velocity or the curvature of the lift curve.
>
> You will notice there is a constant velocity ramp at the opening and
> closing ends of the lift curve. The purpose of the ramp is to avoid
> smacking the cam too hard. You want the valve to start opening at the
> point I have indicated. If you want to know what valve clearance to
> use and you do not have a cam card (or maybe even if you do) you have
> to consider both the rocker ratio and the flexibility of the linkage.
> Once the cam contacts the lifter, there will be 0.002 to 0.004 in of
> cam lift taken up by compression of the linkage (pushrod, etc.) before
> the valve starts to open. If you look at the graph, the point I've
> indicated for valve opening occurs at about 0.010 cam lift. Using
> 0.003 for compression, the valve clearance should be set at (0.010 -
> 0.003)*1.5 = 0.0105. You would not want to use less valve clearance,
> because the valve would be just cracked open for several degrees. The
> cam designer can make the ramp end at any lift, but I don't know why
> you would ever design a cam for more than 0.015 valve clearance.
>
> As Bill and others have mentioned. Once the valve starts to open, you
> want to open it as quickly as possible. Upward curvature of the lift
> curve is acceleration. Unfortunately, high acceleration means high
> force (remember mass times acceleration). High acceleration also
> triggers high amplitude vibrations in the valve train, which can lead
> to valve float, etc. The graph shows a maximum acceleration of about
> 0.0006 in/deg/deg which is typical for a high performance pushrod
> engine with solid lifters. The increased stiffness of an OHC permits
> cams with about twice as much acceleration, i.e. much quicker
> opening. This is the biggest advantage of an OHC. Stiffness is more
> important than weight.
>
> As I mentioned in my previous post, 3 cams specified a valve clearance
> which caused opening on the flank, and the TR-666 was one them. It's
> ramp ends at about 0.01 lift also. At a valve clearance of 0.028 you
> are up on the flank at high velocity. My conclusion from all this is
> that most of these cams were never designed for use in a Triumph.
> They are probably recycled from some other application.
> Larry
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