Lots of initial timing scoop?

[scottj]

Originally posted by PJS50


I am glad to see you guys talking about this because I have a lot of questions about this topic also...

1st, how the hell does the carb "know" what the initial timing is set at and why does it make a difference unless the carb is running rich on the idle circuit? Looking into that 4 seconds flat website, I kinda understand the advanced initial timing/idle concept but it seems to me that the advanced timing issue is just dealing with an "overly rich" mixture in the first place. Couldn't you get the same results by leaning out the idle mixture and backing off on the timing a bit? I think that I am missing something.....

A large cam with alot of overlap is very inefficient at idle and at low engine speeds. All that overlap causes the intake charge to be contaminated with exhaust gas and as a result the mixture burns much slower than an uncontaminated mixture. In order for this slow burning mixture to have time to complete combustion, the engine will need alot of advance at idle. AND, in order to smooth out the idle and eliminate off idle surging, large overlap cams also require a very rich idle and off-idle mixture. If you don't run alot of advance your going to dump alot of that unburnt fuel out the exhaust.

Re: Demons liking alot of advance; it's not really a Demon thing in general but, a Demon (or any carb) that is calibrated for a bigger cam. Really, your matching the carb calibration and timing to the cam.

Also, many people think that if you keep advancing the total timing, power will keep going up, presuming no detonation. This is not true. The engine will make peak torque at a certain amount of advance and for maybe a couple of degrees more. If detonation prevents you from reaching that advance and maximun torque, then you have a problem with fuel octane or a problem with the engine combination. But, if optimum timing for max torque can be achieved, then advancing it further, without detonation, won't make more peak torque. My street engine made peak torque at 35* and at 36* torque was down. My builder timed it at 35* because that is where it made the best combination of torque AND hp under the curve. Because of my cam I can run it locked out at 35*. If I had a smaller cam with less overlap I could run into detonation problems at lower engine speeds with it locked out. By comparison my other engine with the good heads makes peak power at 31* advance.