Hi Simon,
All cams are timed to close the intake valve somewhat after the piston
has reached bottom dead center & started back up on the compression stroke.
Cams with more radical timing usually close the intake valve even later
after BDC. This means that the later intake valve closing lets some of
the cylinder charge escape back out before the intake valve closes. At
higher rpm there is enough intake mixture inertia to prevent this
reversal & loss of pressure.
This means that the ignition timing can be advanced at low & mid rpm to
compensate for lower cylinder pressures but the maximum advance at high
rpm must remain the same or even be reduced a bit. The optimum timing
changes will depend on the particular cam's timing, engine compression
ratio, & port velocities. Thus the rolling road dyno tests or actual
road tests with a lot of experimentation.
Most distributors have a two stage centrifugal advance where the initial
advance is fairly rapid to somewhere around 2500 engine rpm, & then a
slower rate of advance from 2500 to 5000 rpm. The primary spring has
less tension & the secondary spring has more tension, but has slack in
the spring mounting so that it does not exert tension during the first
part of the advance travel. Thus the two stage.
A typical stock cam ignition advance might be something like the
following, all in ENGINE degrees:
With 10 degrees static advance;
1200 rpm = 14 degrees = 4 degrees centrifugal
2500 rpm = 24 degrees = 14 degrees centrifugal
5000 rpm = 36 degrees = 26 degrees centrifugal
A more radical cam might require the following timing:
With 10 degrees static advance;
1200 rpm = 18 degrees = 8 degrees centrifugal
2500 rpm = 30 degrees = 20 degrees centrifugal
5000 rpm = 34 degrees = 24 degrees centrifugal
The above numbers are for illustration only & your particular engine
might require something different.
The distributor changes are made by adjusting spring tensions & altering
the maximum advance stops. An old fashioned distributor machine is very
helpful.
The only down side to leaving the timing stock will be reduced power &
throttle response at low & mid rpm.
You could make up for some of this loss by increasing the initial
advance timing BUT may run into silent detonation at high rpm due to
over advance.
Regards,
Dave Russell
Simon Lachlan wrote:
> Can I try this on you all again............?
>
>
> These are the 2 little springs in the distributor that are part of
> the advance/retard mechanism. These things are no longer available as
> straight off the shelf parts. I was advised (thank you list) that
> Denis Welch has them. They sell a set of three pairs..stronger,
> normal, weaker. I spoke to them, At Denis Welch, regarding which set
> to fit to my slightly tuned BT7 MkII. These are my notes QUOTE
> :-According to Jeremy Welch, regarding their selling distributor "bob
> weight springs" in packs of 3:- Strong pair retards more
> Medium/normal pair is standard Weak pair advances sooner. He suggests
> the Medium pair for my car with its DWelch "Fast Road" cam as the
> start point, otherwise trial & error on a "rolling road". Hoho!!
> -:UNQUOTE
>
> On reflection, that advice doesn't leave me a hell of a lot wiser! Do
> any of you gurus out there have any tips or even generalities on this
> topic about which I know little and seem to understand less?
>
> Simon
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