type26owner wrote:Stuart,
The exact timing is not all that important as far as getting the MBT (maximum brake torque) out of the engine. Here's a graph which shows the typical MBT curve deviation from 100% torque due to timing errors. As you can see it's not a big deal and trying to achieve the perfect timing curve is an exercise in quickly diminishing returns. What is more important is preventing detonation or pinging.
http://www.daytona-sensors.com/tech_tuning.html
Despite the reference document being somewhat respected I'd take that information with a huge chunk of salt especially when applied to the real world non-steady state conditions.
Assume you intend to time to an optimised MBT but you then have a 10 degree timing error, which according to the graph, gives approx 95% of max torque. That might possibly be the case at full throttle with a fixed load but in no way does that replicate real world driving conditions. As many others will have observed 10 degrees timing error imposed on the timing curve, either advance or retarded can quite easily make the engine undriveable - sometimes totally.
type26owner wrote:Those folks which are doing the recurving MUST know that the slope is not all that important. Shame on them to lead you down the path just to pick your pockets clean.
Unless your engine only ever operates in a very limited rev range by definition the slope of the timing curve *has* to be important. OK so you might need the maximum advance to come in rapidly on the twincam but attaining that advance has to be controlled, and that control is essential to keep the engine from running into detonation. Part throttle conditions are also significant (particularly for road use) so a better solution might be to add vacuum advance as well.
A totally standard engine using an optimised mechanical distributor setup when converted to a mapped ignition (measuring throttle angle, revs and manifold pressure), would always, without exception, increase the area under the torque curve, be more driveable and ultimately quicker on both the road and the race track. The distributor is a mechanical bodge, albeit a relatively good one.
I had considered getting a copy of Taylors book but after seeing that snippet I think I might stick to Heywood instead!