Good aluminum radiator?

simple. you would just adjust the throttle plates accordingly with emphasis on the primary transition slot exposure. Sure the sky high initial/vacuum advance will cause the rpm to rise as engine vacuum(BMEP) goes up and up till hitting any particulr parts combo's maximum achievable manifold vacuum level(cam timing has a big impact on where you'll end up numberwise).. but that in itself does not control your idle speed. Twist the disty.. twist the mixture screws, rinse and repeat till the gauge is high and steady as she'll get. Once you start getting down to that last 5% gain it gets much trickier with air bleed tuning and ifr orifice size mods. And that is where most people stop tuning because beyond that point the carb will lean out, pop, sputter, and even surge under light steady throttle. This is not redesigning the carb.. it is simply recalibrating the fuel curve to what the existing engine combo wants. Makes an 850 Holley double pumper run good enough on a 302 and a 500 2bbl Holley make big power on a big-block or run fairly high-rpm on a spec/restrictor type motor.

Bigger cams really love heavy advance and you can nearly close the secondaries just short of sticking into the bore to keep the rpm down and not run into idle to main jet transition issues on the secondary side. Primary side is the all-important one to get right(and is the one common thing I see many novice tuners get wrong) and the remainder of the carbs secondary fuel curve just fills in from there after the boosters and all the circuits are already heavily called upon at higher rpm's. If you have a double pumper to fill in additional holes in the fuel delivery it get's even easier to close them all the way too.

 

Okay, if I'm understanding correctly, it sounds like it's easier with a big cam than with a mild one.
I was just curious -- 'never tried it. I just know that with 12 degrees initial (38 total), my throttle blades are barely open at a 900 RPM idle. I like it, but I'd have to bring it down with a major increase in initial timing. In any case, 'just wanted to touch on that. I don't wanna take this thread too far off-topic.

 

not necessarily easier.. just that bigger cams have poor flame front travel speeds at closed throttles and often prefer a larger head start in timing advance to compensate for that. Also keep in mind that new EFI setups are running very high ignition advance levels too. The cylinder pressure generated from any particular camshaft and engine combo combined with gearing and weight will decide how much you can run before detonation sets in. Iron heads hold more heat than AL heads and tolerate less timing as a result of heat soak carrying over between combustion events in each cylinder. Not to mention AL heads generally have much improved combustion chambers which can get the job done with less advance.

How much idle vacuum do you have on that motor at 900 rpm, Larry?

I do believe that this topic may be at least partially relevant to the OP's "hot runner" but maybe he already knows all this stuff and runs a high initial advance number(I myself love shooting for mid-20's/even with higher compression), very tight vac advance pot(must have it set tight at 8-12 degrees MAX advance assist) with a restricted mechanical curve down near the 10° range(harder to find these short advance curves but many Ford shafts can be rotated 180° to shorten up the curve). Feel free to PM me if you don't want to clutter up this thread.

 

It's an interesting topic, feel free to continue if you wish. I don't mind at all

 

Last time I checked, I had 11 inches of vacuum. Of course I was at 6000 feet above sea level at the time. It's a 9.5:1 motor with a performance cam (not radical though). It's always run good for me. I was just curious about the subject of running high amounts of ignition advance. Many people are unaware that more advance at idle and light load conditions actually help to run cooler temperatures, so it was good that you pointed that out.

 

I would fairly confidently say that your engine(assuming its fairly healthy yet with good valve/ring seal) should be making over 15 inches of manifold vacuum with any cam running much under 230° @.050. Sure the D.A will have impact but not enough to affect idle vacuum to larger degrees. 15 is a bit of a lowball number too.

One of the most commonly underestimated tricks in ignition curve tuning is to jack up the initial timing and simultaneously pull back/tighten up the vacuum assist on the distributors diaphragm. Many factory Ford distributors are adjustable as are many of the aftermarket units. 5/32'nds allen head, IIRC. But doing that means that you must pull back some of the mechanical advance to compensate for the extra number of degrees given to the initial timing(not including the vac advances contribution).

And here lies the basic fundamental problem with too little initial timing. First keep in mind that most cookie cutter ignition curves will use lighter initial numbers combined with fatter amounts of vac assist(sometimes over 20°!). This is great for decent idle/light cruise vacuum levels(engine doesn't care where you got the extra advance from only that it likes it and gives stronger manifold vacuum readings as a result of it being in sufficient quantities) but.. not quite as good when you go from a dead still idle to heavy/full throttle transitions. Mash the gas and all that vac assist instantly disappears in a split second(this is also true whether you use manifold or timed/ported sources on the carb) then all you have left to do is wait another second until the mechanical curve starts to centrifuge your timing number back up to the same amount you had just seconds earlier before all the vac assist fell away. This creates a soft spot and the engine labors slightly more until the carbs booster signal comes back good and strong. Toss more primary squirter at it and that will help fill the hole until the booster comes back in but what the motor really prefers is more squirter AND additional timing.

I've helped quite a few people here and elsewhere get much higher manifold vacuum readings than anticipated which results in a more responsive and wider overall power bands. Many, IIRC.. I think Scooper hit 21 inches on his moderate 230'ish) sized cam, and some others get right close to 20 inches with decent sized cams. I'm usually pretty damned ecstatic when I reach 18 inches running a performance cam. Higher compression ratio also makes it a bit easier to make good manifold vacuum with a performance cam.

Also keep in mind that vehicle weight and gear ratio has a large impact on an engines ability to rev/accelerate more quickly when under abrupt/heavy throttle. This can sometimes greatly improve the engines perceived octane tolerance because it never really gets held up/lugged for very long before rpm climbs up high enough to help ward off further detonation potential. Make more engine vacuum at idle and the carb signal is stronger/more responsive which allows it to rpm more quickly. If it rpm's/accelerates more quickly?.. you can sometimes give it even more timing without as much concern for detonation. Which then enables it to make more manifold vacuum and rpm even more quickly. Back and forth between carb, ignition, carb, ignition.. they're all cumulative baby steps that add up to bigger leaps forward. Obviously you'll reach the point of diminishing returns with ignition timing compared to the level of carb tuning work involved and most just call it "much better than it was.. and good enough for now". IIRC, Scooper ended up around 18 inches of manifold vacuum after he pulled some timing back out to avoid any more low speed carb tuning headaches. Ended up with about 3-4 inches higher manifold vacuum from where he started tuning from. He'll surely tell you that his car doesn't run worse with more initial timing. Stinkier levels of NOx usually comes with all that extra timing advance.. but much funner to bunny hop around town in. Heavy initial timing makes a motor feel bigger and come up on the cam quicker.