A short study on ignition timing and combustion

' In a real (not model) engine, effective burning would typically occur evenly across TDC, where we'd expect to see lots of flame action before, at, and just after TDC, with the after burn continuing during piston descent. '

I think the pressure vs time curve is asymptotic from spark to max pressure, and with higher octane fuels the curve happens at a slower rate. Thus you can use more static advance. I don't believe the combustion event changes much in the total time taken relative to the revs. Most of the required ignition advance curve is due to the change in acceleration of the piston near TDC as the revs rise, while the time for the combustion event is pretty much fixed. In all cases the time that max. pressure occurs must be somewhere in the region where maximum leverage is applied to the crank. And if the exhaust valve opens too early with an open exhaust a loss of power with more noise happens. You will notice that with old British bikes, the really fast ones are usually relatively quiet even when fitted with a megaphone.
The approach I've suggested previously was to calculate the ideal curve to suit the geometry and an estimated fixed combustion time, then jet to suit it. The time for the combustion event can be calculated from the known preferred advance of 29 degrees at 3,000 RPM and a guess to the point when max. pressure should occur, for the standard commando rod and stroke lengths. Then you would need to do an error analysis by substituting different values for the max. pressure timing. That would give an idea of the variation in degrees if you get your guess wrong. In any of this stuff there is usually a lot more latitude permissible than you might think - especially with cam timing. The ignition timing only gets critical if you are leaning off the jetting to the extreme, as you do with two stroke racing engines to get them going. I don't believe it is wise to jet first then adjust the ignition timing.

Here's a teaser from 8 page article on scope of combustion ignition with some swirl vs tumble tossed in.

One thing to understand is that detonation is not necessarily destructive. Many engines run under light levels of detonation, even moderate levels. Some engines can sustain very long periods of heavy detonation without incurring any damage. If you've driven a car that has a lot of spark advance on the freeway, you'll hear it pinging. It can run that way for thousands and thousands of miles. Detonation is not necessarily destructive. It's not an optimum situation but it is not a guaranteed instant failure. The higher the specific output (HP/in3) of the engine, the greater the sensitivity to detonation. An engine that is making 0.5 HP/in3 or less can sustain moderate levels of detonation without any damage; but an engine that is making 1.5 HP/in3, if it detonates, it will probably be damaged fairly quickly, here I mean within minutes.

Click to expand...

http://www.streetrodstuff.com/Articles/ ... etonation/

I want 1.5 hp per cid, who doesn't, so grasping at straws and stumbled on this swirl and tumble ignition detonation article example of factory porting a V8 for lots better flow lost its chamber turbulence which lost a lot of power while greatly increasing proneness to detonation. Don't know if applies to Norton hemi's though. it where above 8 pg article was found. Bing Bang BOOM

http://www.eng-tips.com/viewthread.cfm?qid=149346

hobot said:

Here's a teaser from 8 page article on scope of combustion ignition with some swirl vs tumble tossed in.

One thing to understand is that detonation is not necessarily destructive. Many engines run under light levels of detonation, even moderate levels. Some engines can sustain very long periods of heavy detonation without incurring any damage. If you've driven a car that has a lot of spark advance on the freeway, you'll hear it pinging. It can run that way for thousands and thousands of miles. Detonation is not necessarily destructive. It's not an optimum situation but it is not a guaranteed instant failure. The higher the specific output (HP/in3) of the engine, the greater the sensitivity to detonation. An engine that is making 0.5 HP/in3 or less can sustain moderate levels of detonation without any damage; but an engine that is making 1.5 HP/in3, if it detonates, it will probably be damaged fairly quickly, here I mean within minutes.

Click to expand...

http://www.streetrodstuff.com/Articles/ ... etonation/

Click to expand...

If you take jetting and ignition timing to the extreme situation where the combustion process is always very close to detonation, you tend to get the best power from the fuel, however it comes with the risk of engine damage. With Norton engines, we don't usually get that close to destruction. However if you tune a commando engine as you would a racing two stroke engine, you end up in the same expensive domain where you need to keep an eye on the weather. When you talk about V8 car engines in this context, I suggest the situation is less critical, their state of tune is usually far removed from the two stroke example.

Yep Sir Alan its stupid and immature to seek risky thrills on two wheels with fragile engines and pilots, so how lucky to ya feel punk to make life time memories or kill something trying? Keep in mind how well tuned comnoz prior 'puterized injection and ignition system was yet recently discovered he was running with mild detonation for dozens of 1000's of mile prior to recent knock sensor revelation-revolution. Egotistic competition rules the world so best wishes maxing out. Peel's 'safety' feature is anti-detonation fluid spray both inside and outside for my ego boosts seeking to out do others short of 200 mph bee lines. Will even try magnetic flux on fuel molecule activation with same charge ionic repelling and in tank hydro carbon chain breaking catalysis and plasma ignition via diode arrays that seemingly tap into other dimensions EMF back lash energy flows. I'm a nut case like the rest of you already ahead of me, just not in denial is all while trying to catch up.