Cam/vacuum

[MomsDuster]

Yeah I had a hard time trying to make it comprehensible too! lol. I knew what I wanted to say, but I spent some time in editing to get as good as I could. The Key is in the overlap.
There is only one way to make more power with any given engine size and that is to put more fuel and air thru it. The fuel is easy.
There are only three ways to put substantially more air thru a given engine; more head flow,super charging, or more rpm.
More rpm by itself is not the answer; the engine has to process more air, and that requires ever increasing valve-opening times.
Your engine needs more and more intake/exhaust durations to make power at those ever increasing rpms. There are only a given number of degrees to work with; namely 720.To get what you need, you have to steal it from the compression and power degrees. And when that runs out, you have to start at the other end; and when you do that, the overlap grows and grows. Eventually it becomes a fifth cycle, and becomes a mini supercharger at mid to high rpm.
But in so doing,the cam almost destroys the engine's ability to idle at a street-normal sub-900 rpm, as the intake and exhaust get all mixed up in where they are supposed to be going.
So while a lot of overlap at higher rpm,the result of the long durations, can make a powerful engine, it is lousy at idling.
And while most SBM engines make best power at 34 to 36* of power timing, this is generally waaay to much at WOT/lower rpms. The use of hi-stall TCs has opened up the opportunity to give the engine the idle-timing it wants, yet not detonate itself to death at stall rpm.
As an example, on a distributor engine, you can often advance the idle timing to 40/50 degrees with accompanying ever increasing rpm;but with a lo-stall factory TC it might rattle like a diesel on take-off.
And it is possible to cruise 65mph with as much as 55/60* timing, but it better drop out like lightning when you go to pass someone.

thanks, it’s starting to become clearer.

 

[MomsDuster]

Ok guys quick update. Went out this morning to fiddle with the FiTech and timing. Found that my handheld already set to 18.9* idle advance. So I checked actual timing with my light, It showed 10* and was jumping around. Fitech states that it will do that +/- 6* as part of idle control. I have the old software so I have to bring the car to just above idle to verify actual timing. At aprox 1500rpm it was showing a steady 10* while hand held showed 19*. So per Fitech directions I had to "sync" the timing by adjusting the distributor to match the handheld. Not an easy task by yourself LOL. Bringing the rpm up above idle with one hand, looking at the handheld, checking the timing marks and turning the dist with the other! Anyway, got them pretty darn close. Set the idle to 850, idle timing at 20* and reset the IAC steps and boom! 11.9-12.2" of vacuum at Idle. Seems to be a tad more responsive when i blip the throttle too. Also at a steady 2000rpm 18" of vacuum. I understand that to be because of the lower amount of "time" in the overlap event or 5th cycle as @AJ/FormS explained. (correct me if I'm wrong). So needless to say you guys @AndyF @crackedback @RustyRatRod and AJ were spot on! Thank you all for your help and guidance, much appreciated! Looks like I have to get a Windows based computer, I have the Fitech software update that will among lots of other things allow me to lock the timing at idle so i can get the phasing spot on. Unfortunately Fitech software doesn't support Mac. One again thanks to all!