IMO
for a streeter with a big street cam
By running more advance, giving the engine what she wants, you are lighting the fire early enough that the pressure spike gets an earlier transfer to the piston. Now all the fuel gets burned up in the chamber, and more of the pressure is transferred to the crank. So by the time the exhaust valve opens the pressure has fallen in there, to a lower level. Now,as the exhaust valve approaches closed,and during the overlap cycle, there will be a lessor pressure pulse leaking back into the intake,past the now-opening intake, which formerly reduced the vacuum in the intake. The more overlap your cam has, the lower the intake vacuum will be, and especially at the lowest idle speeds, because this allows more time(in degrees) at piston TDC(overlap), for the burned up exhaust to travel into the intake, when the intake valve pops open. As an example; the 292/292/108 Mopar cam has 76* overlap, just at advertised duration. Whereas a 268/276/110 might have only 52* at advertised; that works out to the larger cam having ~50% more overlap/ 50% more TIME,in degrees.
So your first defense is increased idle speed, and the second is increased idle-timing.
There is nothing wrong with a low-vacuum idle, if you can tune for it. which with a carb is not all that hard. But with EFI that depends on a MAP sensor, it gets tricky.
So like has been said, start with a higher rpm and more idle-timing, then as time goes by and you get a handle on your tune, you can experiment downwards.
How much idle-timig to run is somewhat dependent on your TC if automatic, and on the very low-rpm jumpiness if a standard trans.
Generally, with a big cam, and a regular mechanical advance distributor, you want to run a lot of idle timing, because this increases the low-rpm responsiveness. But with a low-stall TC or especially with a manual trans, the more powerful pressure pulses can make low SPEED driving a little jumpy.With a factoryD you are limited to a two-step curve at best.
With computer-controlled timing however, you can take care of all that with programming. You can advance or retard anywhere you want.
Say with a manual trans and 3.55s;
You can have a start retard to say 5*up to 450rpm, then have a smooth idle with 15/20*beginning at say 700, and cut back to say 8* at 900, so you can idle along at 7 or 8 mph, and not have the car start bucking like a wildhorse. Then you can bring it back in to say 28*@2800 for that low-rpm responsiveness. Then slow it down to 34* at 3400, so you can burn 87E10 with aluminum heads and stay out of detonation. Your options are limitless.
With an automatic, and a 2800 or better TC, you can just about do whatever you want cuz the TC will suck it up. But there is no good reason with a TC to run just 10* with a big cam; you're just sending a lot of unburned fuel out the exhaust; OOps that's carb-talk,lol. With EFI,and a MAP sensor you're just complicating the fuel tune.
IMO
