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Newman Cams
#11
(12-11-2020, 06:30 PM)Tom Coates Wrote:
(12-11-2020, 02:17 PM)Charles P Wrote:
(12-11-2020, 02:08 PM)Tom Coates Wrote: Thanks all, I have a stepped key on my current gear which allows for adjustment by half tooth. Depending on how it fitted I might have to try something more sophisticated, in which case I'll get in touch with Dave. Does he need the original outer part of the gear to start with?

Any thoughts on the profiles themselves? What is the max duration/lift you would live with on a car being used on the road? Not intending to do many hill starts but don't want to be burning the clutch out every few months!

Dave's are new steel gears. If it's a mag engine I'd replace the original gear - they do break.
I reckon 260 degrees will be fine in a lightweight road car and you probably don't want/need much over 270 lift. It really depends what you want the car to be. Until the development of variable valve timing technology camshafts were always an exercise in compromise. 
If you're worried about the clutch then fit cast iron linings (as per the works). Makes a very nice clutch, long lived and no nasty smell if you do slip it.  

Charles

My understanding is that extra lift beyond the point of maximum flow is still useful as it increases the amount of time the valve is open at the peak flow level.

What is the downside of extra lift? Is it just wear on the valve train and noise or are there other factors that I am missing?


The downside of high lifts is in the acceleration and subsequent loads imposed on the valve train. This can lead to increased noise, wear, risk of breakages and the need for stronger valve springs. Stronger valve springs waste power. Having lift above max flow would seem pointless to me because it's adding load without any apparent benefit.
The ideal cam would open the valve from closed to open almost instantaneously and hold it at maximum opening until it shuts like a trap. Clearly that would impose enormous forces on the drive train. Every other cam is a useful compromise between loadings and flow. A decent outfit like Newman will have done their maths to keep things in check and the three key measures are velocity, acceleration and jerk (yes, really - jerk is the derivative of acceleration). Whether Newman will share these figures I don't know but they are the standard way of comparing cams. I've never had a Newman cam to compare so can't help to measure an calculate

c
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#12
The other problem's with high lift are hitting the cylinder head and valve shrouding. Both are curable at a loss of compression.
I could not use a Whatmough Hewitt head on my car without having to do major work to allow full lift and to stop the valve edge hitting the head as it lifted.
I am now using a Ricardo Replica head.
Its difficult to get high compression along with high lift and good gas transfer.
Its all to do with the infernal triangle of side valve tuning.
I have seen Side Valve Ford blocks with a sunken inlet valve and a trench from the inlet valve to the cylinder.
I suspect the top ring was deeper down the bore than on a seven to allow this.
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#13
(12-11-2020, 09:28 PM)dickie65 Wrote: The other problem's with high lift are hitting the cylinder head and valve shrouding.  Both are curable at a loss of compression.
I could not use a Whatmough Hewitt head on my car without having to do major work to allow full lift and to stop the valve edge hitting the head as it lifted.
I am now using a Ricardo Replica head.
Its difficult to get high compression along with high lift and good gas transfer.
Its all to do with the infernal triangle of side valve tuning.
I have seen Side Valve Ford blocks with a sunken inlet valve and a trench from the inlet valve to the cylinder.
I suspect the top ring was deeper down the bore than on a seven to allow this.

The general rule of thumb for unblown engines is that valve lift doesn't need to exceed 25% of valve diameter. For a 29mm A7 valve that equates to 285 thou lift. You may argue that this doesn't strictly apply to a side valve with the shrouding on the rear of the valve but as Richard says, the limitations around CR and the transfer area between valve and piston are the significant. 


C
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