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My concern with building a new, shallower advance curve was to do with the risk of pre ignition, it'd be like setting the static at 4 degrees instead of 10 and driving it like that. It's my experience that the Sprint's "sweet spot" for timing, between pre ignition (too retarded) and pinking (too far advanced) is pretty narrow. I've even experienced badly coked up examples that exhibit both symptoms simultaneously!
In the Wikipedia entry on pre-ignition, two of the cause are given as "Ignition timing too far advanced" and "Too much air/too little fuel (octane) to withstand pressures (too much air causes too much pressure for the octane to handle, too little fuel reduces the octane to handle the pressurized air)".
It also says for pinking that it can be prevented by "retarding ignition timing" and "the use of a fuel with high octane rating, which increases the combustion temperature of the fuel and reduces the proclivity to detonate.
And that goes with what I understood: that there's no real issue, beyond power loss, to retarding the ignition timing.
I'll go and do some more research, as even I don't see Wikipedia as definitive. But this one is news to me.
But my thoughts were to keep the static timing at about 10 degrees and use a 41655 with standard 12 degree (24 at crank) stop and std 41655 primary spring, but with a shorter secondary spring. That secondary spring would be set to come in at about 2500 rpm (13 degrees crank), not the 2900-3000 (18 degrees) as std. I was also considering an even stronger secondary to hold back on the advance at higher revs. I've a selection of springs that are bigger even than the 41655 secondary (the std 41655 set should give, roughly, 1.6 degrees per 1000 rpm once the secondary comes tight, if Marcel Chichak's and the 1980 ROM data are right). Some of the more monstrous ones I have should give near half that advance rate.
I was thinking about those settings because the only problem I've seen in the last 35 years daily driving something with a Sprint engine - one at over 200bhp; IIRC 155 at rears* - is pinking with too much advance or too low an octane petrol, mostly at about 2500 rpm with big throttle openings. The first time the timing had slipped to about 20 degrees static (that engine had issues from about day 7 after I got it fitted, cos it had been round Oliver's Mount too many times), and I took it up over Bishopdale between Kettlewell and Aysgarth - not exactly like Hard Knot, but some bear steep slopes. The second time, just fast driving between Brough and York on 2 Star. Both times it cost me a set of pistons and then a set of big ends cos I didn't know better than to put new shells on the crank without taking it out to regrind it. Yes, I should have known better the second time, but I thought that the Sprint, with the timing set at 10 degrees BTDC, was okay on 2 Star and I still didn't know that re-shelling an un-reground crank could lead to it failing soon after.
But it wasn't okay on 2-Star at all, even though the timing was set right - it's been a habit of mine since the Bishopdale incident to keep an eye on it.
It might be that with a 41402 and it's fairly mild centrifugal advance stop, it would be okay. But seeing that both the 41458s I have give much more than the 18 degrees (crank) advance they should - near 30 in one case - that may be a systemic problem with that version of the 44D4. And that Sprint could have had that issue - so maybe as much as 40 degrees of advance at about 3000 rpm, even on a 10 degree static. But it's a sodding expensive mistake to make with pistons the price they are!
So, if there is no advice to be got on what max advances I want at what rpm values, then I'll stick with 98-99 RON Supers as long as I can. But I have been told often enough that the dissy rebuilders can modify the curve to make it safe to run on ordinary unleaded (before E10, perhaps before E5). But, obviously, I can't ask someone like Martin how to do that: it's his living.
So, I've worked to be able to set the advance rates from the physical dimensions of the springs - wire diameter, coil diameter, coil count, with estimates for Young's Modulus and Poisson ratio for music wire. And checked the results against the specs for the springs and the advance data from the ROM. And that's why I came here.
Graham
*And I admit that's a Dave Bogg Standard measurement.