To run power brakes, a minimum of 12lbs vacuum is a good place to be. Stall converter is variable. Lopey cams can work with stock stall converters to varying degrees of effectiveness. It's not black and white.
Yes. Vacuum advance can be disconnected and you can dial in the advance mechanically with a recurve. The tranny shifts at speed, and once performance cams are off idle, they create plenty of vacuum for the tranny.
To get a feel for cam specs vs. vacuum and driveability, check some cam catalogs like Crower's. The vendors typically break their lineup down into three segments, with the "RV" level at the top of the chart and the race-only at the bottom.
Keep in mind that bigger displacement enables bigger cam numbers.
The vital statistics are duration on the intake lobe and the exhaust lobe, typically measured from 0.006" open to 0.006" closed (aka "advertised duration") and from 0.050" open to 0.050" closed (aka "duration @ .050"); lift on the intake lobe and the exhaust lobe, expressed in thousandths of an inch, e.g., 0.480"; and lobe separation angle (LSA), expressed in degrees.
The smaller the LSA, the more the overlap, and the choppier the idle, and the less the vacuum at idle. Also, the more the duration, the more the overlap. Overlap means that both the intake and the exhaust valve are open at the top of the intake stroke, so the piston is pulling in fuel air mixture from the intake and inert waste products from the exhaust at the same time, as it moves down the bore on the intake stroke. With high duration cams the intake valve also ends up pulling on the intake valve of the next cylinder in the firing order, which still has its exhaust valve open to the outside world. In other words, a vacuum leak.
You should read this article, and also search the PHR web site for Vizard's articles on compression ratios, exhausts, and carburetors. Memorize these four articles and you will know more theory than your engine builder.
BTW, I think I detect more Chevy-itis in his latest remarks about dished pistons. I suspect this because the standard Chevy small block is 350 ci, if not 383. Crank those displacement numbers into a CR calculator, also using a 60 cc combustion chamber and flat top pistons, and I expect you will see some pretty high CR's.
Edit:
Curving a distributor means to change the amount of ignition advance you get at each rpm. With a mechanical advance-only distributor, you start with about 10 degrees of base advance. As rpm rises, increased centrifugal force causes a set of spring-loaded advance weights to swing out. The tips of these weights are cam-shaped, and as they swing out they cause the plate on which the points are mounted (or the magnetic pickup) to rotate slightly about the distributor cam (or the eluctor wheel). This rotation causes the coil to fire earlier in the compression stroke than at idle. Typically, the added advance is about 25 degrees, and is "all in" at about 3000 rpm. That means that from 3000 rpm to redline you have about 35 degrees total timing.
Recurving the distributor means to change the rate at which this mechanical advance comes in (e.g., weaker springs let the weights fly out farther, earlier) and to change the total amount of the advance, by changing the stop which limits how far the weights can fly out.
Vacuum advance is used for fuel economy reasons only. It uses vacuum to cause the mounting plate to rotate. It only works at part throttle, because at wide open throttle there should be no pressure drop below the throttle blades, i.e., in the intake manifold. The increased advance at part throttle gives you more power for a given throttle opening, which means you can maintaint the same rpm with smaller throttle openings. But it goes away at WOT, which prevents ping. At a 3000 rpm cruise on a flat freeway you can have as much as 55 degrees advance, 35 degrees mechanical plus 20 degrees vacuum. You'd ping for sure if you had that much advance at WOT going up a hill in top gear.
I`m on it man, I`ve been cramming my head full of info for this job, and it looks like these articles can make me finally understand the "physics" of the engine. Thanks man
Your distributor operates with total lack of vacuum, it is not affected by lope like power brakes or something that operates off of vacuum itself.
Cams of a given duration can also work without a stall converter.
However, just because it runs and drives without a converter, doesn't mean that it does not need a converter.
Pretty much any cam that lopes, needs a converter to operate properly.
IMO, 210*@.050" or larger should have a converter to get the best performance.
However, some 230*@.050" cams can operate without a converter.
Like said before, it's not black and white.
But it is all about matching parts to get the most bang for your bux.
I need a LSA of about 109, and a duration of 278, haven`t figured out the lift yet. I had to read it like 5 times to begin to understand what overlap was, but its a good article, little over my untechy head, but its absorbing slowly.
So you said with a duration that big I for sure need a stall, I`ll figure out how to get one.
I thought it very interesting how in the article recomended using a dual plane intake to help with vacuum, not what I`ve been told by bunch of people.
Check out flowtechinduction.com. I have had custom cams done by Ed Curtis that were right for my combination and what I wanted to do with the car. Your car will make more power and run better. You can have cams that on paper look the same, but have very different characteristics. A well designed system, will keep the lifters on the cam and prevent valves from bouncing off their seats even at high rpms. Speaking with someone like Ed Curtis will in the end save you money and will help you get the results you are looking for.
so far, with the info I have, supposed to get about 367 hp and 332 lbs torque, and I`m still tuning on paper so to speak. But I`m finding out who Ed Curtis is now.