Wow. Been busy, just catching up.
Yup, with low vacuum you should use lower step-up springs. The general rule is that the step-up springs should be half your idle vacuum. So if you have 18" of vacuum at idle, like I do, you use the 8" springs and stretch them a quarter-inch to get them to be more like 9". If you are running a lopey cam with 8" of vacuum at idle, run the 4" springs, and so on.
But the best (only, as far as I am concerned) way to properly tune a carb is get an AF/R meter and tune it to the numbers. Yeah, I know, it's $300 bucks. But people get on the forums, and complain about performance, and want to know about swapping cams and carbs and even heads, or putting in new engines, and Guess what? If you don't tune that newly built engine right, it will still suck. Thousands of dollars later, and it will still suck, where for hundreds you could get a decent AF/R meter and make what you have perform as best as it can, and then decide. And if you then spend the extra money on bunches of new parts, you will still have a good AF/R meter to set that up properly and get the best out of it.
I guess that's a lost cause when you ask people complaining about their truck's performance what their timing is set at or what vacuum they are pulling at idle and they don't know because they don't have a $20 timing light or vacuum gauge. Criminy.
And then there's the people who recurve their vacuum advance and mechanical advance, and then run crappy wires and the wrong plugs. No, "hotter" with regard to plugs does not mean high performance. The stock GM distributor curves are actually pretty good, and the recommended plugs are -- surprise! -- generally the right ones. You actually want to run about the coldest plugs you can without oil fouling, because hotter plugs solve oil fouling but will aggravate pre-ignition problems. Ask the guys at the dirt tracks what they run in SBCs and they will all tell you Autolite 24s. Wires? Taylor, period.
And don't get me started on ported vs. vacuum advance. I remember when ported advance was introduced in 1968, and we were all like "You gotta be kidding me." 4 degrees BTDC base timing? We laughed. 16 degrees BTDC base timing, 15 degrees of vacuum advance on manifold vaccum, 20 degrees of mechanical advance is the sweet spot for SBCs, and has been since 1955. I know, we had a 1955 Chevy 210 with the 265 SBC when I was a kid, with a 4.11:1 rear end and an electric overdrive for gas mileage on the highway. Dad put glass packs on it and replaced the carb with a downdraft and a flatter air cleaner to get it under the hood, and that thing would move.
OK, OK, rant over.
Yeah, MO, the Edelbrocks have air flaps on the secondaries. They are weighted closed and operated by air velocity in the secondaries. I may get a Qjet and swap it in on my truck and see what difference a correctly tuned Qjet dialed in by the numbers makes over a correctly tuned Edelbrock dialed in by the numbers, just for giggles. I've sent a query off to SMI. We'll see. If so, I'll let you all know.
On the crate engine question, one problem is that we don't know the history of our engines. How were they treated before we got them? The nice thing about getting a base 350/260 GM crate engine is that those issues are off the table, and you have a known blank slate to start from. Replacing the cam is one step up, replacing heads is another. You can drop the bottom and swap out pistons as well if you want to go whole hog, but at least you know what you are starting with, and it won't kill your wallet. Going with a roller-cam crate engine is another way to start from a known point.
Starting from a known point like that makes things a lot easier, because the paths from there to some specific performance numbers are so well-trodden and the write-ups on setups from those starting points, like my setup from the 350/260 as a starting point, are all over the net.
