why change your ignition advance curve..

[grumpyvette]

why change your ignition advance curve, on a high performance engine?

 

Ill over simplify it for you, as the load rate on the engine increases the rate of accelleration of the piston away from TDC tends to slow as does the rate of increase in the number of strokes per second, cylinder pressures build rapidly from the time the spark ignites the compressed mix untill peak pressures are reached,but not nearly instantly, it takes TIME for usefull pressure to build up, as the pressure in the cylinders before or at TDC does nothing to produce power ,in fact it tends to work against the engines efficiency, but because theres a lag between ignition and the useable pressure over the piston you need to advance the timing, so that the majority of that pressure peaks slightly after TDC and maintains that positive pressure over the piston for as long as possiable, that pressure curve and the necessary required ignition advance required to get it varies with the resistance or LOAD the engines working against and the RATE at which the piston moves away from the TDC point. if that load slows the piston to much it gives the cylinder a chance for secondary ignition(detonation) to occure, as the pressure and time both have an effect. you can get away with a faster advance if the piston moves rapidly away from TDC,IE ligher loads or a higher gear ratio that allows the engine to build rpms faster, factors like, octane used,air temps in the cylinder,quench ,and valve timing, even the rod length/to stroke ratio, effect results here

things to read

 

http://hcs.harvard.edu/~jus/0303/kuo.pdf

 

http://superchevy.com/technical/engines_drivetrain/accessories_electronics/0411sc_crane/

 

http://www.erc.wisc.edu/modeling/multi_dimensional/ModelingMtng2004/7-NSMBpaper.pdf

 

 

look here

 

http://www.iskycams.com/ART/techinfo/ncrank1.pdf

 

now compare your cam choices and notice that the valves open to bleed off pressure and the valves close to maintain pressure while the piston is a good deal closer to TDC tham most people realize, most of the effective pressure occures in the upper 1/2 of the stroke, with very little useful pressure in the cylinder after 90 degrees past TDC

 

heres a cam I use alot

http://www.cranecams.com/?show=browsePar...lvl=2&prt=5

notice when the exhaust valve opens, and compare that to the location of the piston in relation to the peak cylinder pressures as they relate to the pressure curve over the piston, youll quickly notice that the peak pressure occures between tdc and at most 30 degrees past TDC and has dropped off drastically by the time the exhaust valve opens

[blueovalz]

Thank you for the weekly enlightenment. It was good for me!

[331CI 280z]

Grumpy,

 

I have a complete MSD setup, distributor, coil, 6AL box and timing retard for startup and above 4,000 rpm. Can you briefly discuss advantages of retarding the timing over 4,000 rpm for more top end power?

 

I have not used the retard yet. I currently have a 0 degree chip installed. I know that it is useful for NOS, but I am running all motor. It has helped me on startup (I also installed a Ford solenoid and high torque starter). I no longer have hot start problems. The part number of the controller is MSD-8982. I'm not really sure where to start for tuning the spark retard on the top end.

 

I've dialed in the advance curve, and it made a substantial difference.

 

Thanks

Ian

[grumpyvette]

sure! as the engine rpms increase the velocity and frequency of the piston strokes increases and the time between those strokes decreases

example at 1000rpm theres 500 intake strokes per minute or 8.3 intake strokes per second, at 7000rpm,youll naturally have only 1/7th the time, the pistons moving 7 times faster so you get 58 intake strokes per second, now up to about 3000-4500rpm (depends on factors like port size, compression ration,etc.)the ignition timing needs to advance to allow time for the cylinders to build the necessary pressure curve, but above that rpm range the combination of the fuel/air mixs vastly faster movement in the cylinder, turbulance in the cylinder and heat in the chamber increase the burn rate, and the higher the rpms the faster this becomes (within limits) so more advance in the ignition becomes counter productive. now remember its the TIME it takes from ignition to get the pressure built, not the degrees in advance of TDC, its the peak power that we are attempting maximize by trying to regulate the burn so that the pressure curve over the piston does the maximum work and the power will be lower if the piston must try and compress a mix where the pressure resists the pistons upward movement against it.

now at some point, lets say, an example of ,(6000rpm) the burn with the ignition fully advanced is going to produce a good amount of pressure BEFORE TDC as the pistons still in its upward movement, that will tend to work against your total power output, simply because a good deal of potential power is wasted compressing a mix thats been ignited and burning fast enought that youll be wasteing a good deal of that pressure curve. retarding the timing moves the pressure curve at this rpm range back to the past TDC where it does you some good.

[331CI 280z]

Thanks. I'll try some degree chips, starting small. I don't know how pronounced the change will be. I realize that a dyno is probably best, but I'll have to road tune for now.