[TR] distributor advance

[Randall]

I don't have my books handy, but I believe there should be a heavier, longer spring, and a lighter, somewhat shorter spring. The heavier spring should be slightly loose as installed, so that at lower rpm, only the light spring controls the advance movement. This give a fast advance curve up until the heavier spring becomes tight, then slower until the stop is reached.

Normal practice is to measure the advance curve after changing springs, so they can be tweaked if necessary to give the desired curve. Kind of like balancing tires.

If you were not getting pinging before, I would not expect a performance increase by switching to heavier springs. Maximum advance without ping is not always the best power point, but usually not far from it.
The factory had to supply an advance curve that worked under the majority of conditions, without necessarily being the best curve for any conditions 

Also, in general, a low compression smog engine can stand more advance than a higher compression, better breathing race engine can. That's why the stock "all-in" advance was so high. (Around 36 degrees IIRC) If you've done anything like shave the head or install a different cam, you might need less advance. The whole idea is to get the peak cylinder pressure with the crank throw at the best angle to turn the crankshaft.

In short I don't think there is any way to answer your question, short of comparing performance before and after. To really optimize it, you need to run tests to find the optimum advance across the rpm range for your motor, then build a distributor to achieve that.

If those lighter springs were the result of someone else doing that, then any change will likely not be for the better.

I haven't been paying attention lately, but one of the local clubs used to have a "dyno day" once a year, where you could get several chassis dyno runs for a very modest price ($40 when I was there). One way to check your advance is to try a run with more initial advance, then another with less, and see what the engine likes across the band.
-- Randall

On 27 February 2020 16:48:12 GMT-07:00, Peter Arakelian <PeterAra at msn.com> wrote:
>Have an early 71 TR6, still has the 41306 distributor with dual
>vacuums, advance and retard.  Did a little work on my distributor,
>wanted to check the function of the mechanical advance since I came
>across a new set of advance springs.  I know they are correct because
>they are Lucas and the correct part number as listed in the Lucas
>master catalog.
>the old springs were visibly different and softer.  The weight in my
>distributor was marked 13 degrees.  After I changed them, my timing at
>idle advanced 10-15 degrees.  I reset the timing to 8 BTDC.  I have
>always run there.  I use the advance 'till it pings and back off
>method. Have had it as much as 10 BTDC with no ping, but I prefer to
>run at 8. My retard works correctly, because it passes the test -idle
>increases 300rpm when pulled.  Still no ping under the test - high
>gear, full throttle 30-50 mph.  And no ping accelerating at full
>throttle from 70-80.
>Now...am I correct in thinking the car should run better at speed
>(3200rpm)?  My thinking is that the timing is more advanced at that rpm
>with the correct springs, than it was previously.
>Peter Arakelian
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