Ignition Timing Education

[olddog]

Ok the more dense the fuel air charge the faster it lights. This explains why the vacuum advance is important for part throttle operation. It makes sense that the more dense the charge the closer the molecules and the faster the flame can travel. It also explains why you need to back off the timing in a blown application.

However if this were the main factor, then why did the old mechanical advance operate on RPM alone, when at WOT, and the fuel injection engines pretty much do the same thing. Volumetric Efficiency increases with rpm to a point and then drops off again. Take a stock 5.0 for instance, it quits breathing around 3800 rpm and VE starts dropping like a rock. Then the theory should be that as the rpm goes up and VE increases the timing should be backing off. Then at above 3800 rpm as the VE drops off the timing should increase again.

I'm not trying to be argumentitive here. I would like to understand this.

[Jac Mac]

Originally Posted by olddog

However if this were the main factor, then why did the old mechanical advance operate on RPM alone, when at WOT, and the fuel injection engines pretty much do the same thing. Volumetric Efficiency increases with rpm to a point and then drops off again. Take a stock 5.0 for instance, it quits breathing around 3800 rpm and VE starts dropping like a rock. Then the theory should be that as the rpm goes up and VE increases the timing should be backing off. Then at above 3800 rpm as the VE drops off the timing should increase again.

I'm not trying to be argumentitive here. I would like to understand this.

Your spot on with this analysis-- the reason most if not all manufacturers dont bother to take advantage of it is that for most of the motoring public the cars will never be operated in a manner that requires it & the few that do are probably buying performance models with improved VE at upper RPM levels.

I know when I was building motors for a Restricted class of car here in NZ ( Holden 202ci 6 cyl breathing thru one (1) single barrel carb with a 1.25" dia venturi ) that I was able to make power by using the vac advance hooked to manifold vac so that when it was up around 6000rpm + it pulled in another 5° advance over the mechanical total. Worked fine until the techs banned the vac advance, which meant that we had to then play with the mech adv to achieve the same thing which was a lot harder to get consistant results.
Now obviously thats one situation where the carb creats the point of restriction that limits breathing , but in most cars with reasonable size carbs the point of restriction is usually @ the inlet valve / seat or throat ( evidenced by the fact that a bit of work in this area on most production cyl heads will show the most gains ), therefore the vac advance trick wont help much until the carb becomes the restriction.

Most electronic ignition systems available now also retard ignition timing by about 4° at high RPM and the 'original' Morse type timing chains also tended to do the same to the cam timing at high rpm as centrifugal force made them ride higher on the sprocket on the tension side ( as long as there were no torsional effects making it snake ) which would retard both cam/ignition timing@ high rpm.

That should get you thinking-- gotta go watch the last Aussie V8 race right now.