A lot of it is that air restriction is when the carb or carb hole is the flow limiter.
Its all able to be worked out by Techncial Collage Flow Net Physics, which I did back in 1991 with a really
good fluids engineer.
There is a lot in it, but the basics work out like this.
If you have a carb that has 3 sq inches of total carb venturi area, then you don't shove it down a hole less than 2 inches in diameter.
One 2" hole is 3.141 sq inches of area.
Anything less than that size is a Restrictor plate.
The stock 1-bbl carb is too small.
The stock 1.3", 1.5, or 1-3/4" hole is too small.
I think
Crosley's 2-bbl carb was a 1.38" venturi carb, 2.969 squ inches of venturi are serving a 200 cubic inch engine.
Or about 68 cubes per square inch.
I think that was too small, and even back in 2007, he was planning proper 4-bbl "Twin Duece" engine with two 1.38" venturi 500 cfm Holley 2-bbls for 5.938 sq inches or carb venturi area.
Which was still only the same size as the four Honda Kehin carbs Ak Miller in line six ran on the little 67 Mustang Hot Rod 125 rwhp engine.
In 2007, my beloved 25 year New Zealand Automobile Association and ex Ford Machanic mate said it all.
"Ford were so smart with the in line sixes.
Restrict air, avoid engine blow ups.
Ford knew that if an I6 was air restricted, it'd be dead realiable, and they'd never have to upgrade rod bolts, valve gear, or update con rods, crank or block. It stayed the same under the head gasket from 1960 to 1983".
Even the 1982 to 1992 250 Cross flow engine had a 2-bbl Weber carb, but it had 140 cubic inches per sq inch of carb venturi area. Still a restrictor plate engine in my book. They made 131 hp, but when Ford put a 75 mm throttle body with multiple port EFI on it in 1982, the statis was 36.5 cubic inches of engine per square inch of throttle body area. The engine only made an extra 18 hp, but it had the same camshaft as the 1964 200 cubic inch US log head.
The 1971-1974 Aussie 2V's
The 1973 to 1995 Argentine 188 ME /221 SP,
The 1986-1992 HSO Tempo/Topaz/Tracer 2.3 and 1986-1992 Taurus/Sable HSO 2.5 OHV in line Fours
the 1983 to 1992 X flow 4.1 EFI,
the 1987-1997 4.9 EFI F truck and E van engine
and the 1988-2002 SOHC 3.2/3.9/4.0
The 2003-2016 DOHC and Turbo "Barra" engines were basically the same old in line sixes under the cylinder head gasket.
These were the first US syle in line six engines to operate un restricted air flow in line six.
The 4.9 EFI truck engine has just 56 cubic inches of engine per square inch of its huge two 1.85", 5.38 sq inch throttle body.
Even the first 170 HP 2V M code 250 Falcon engines had , and the very strictive Dodge 2-bbl Bendix Stromberg WW 2-bbl was only 245 cfm, and had something like 2 sq inches of carb venturi area, about 122 cubes of engine per sq inch.
In NASCAR in the Winston Cup Series Carb 355 V8 era, a "restrictor plate" was sized to be around 127 to 146 cubic inches or engine per sqaure inch of carb venturi.
Ford sized a stock in line six to have 1 sq inch of carb venturi for a 200 to 148 cubic inches.
The 200 cubic inch to 127 cubic inch per sq inch of carb "statistic" is where the engine starts to breath. As you open the old 1960 to 1968 ish C8 and before carb up form its 1.3" hole, air flow goes up. 1.416", or a simple 116 thou hone out with a die grinder, brings the airflow up. Thats what Ak Miller did in the Horsing around with the Mustang six Hot Rod articles.
We still haven't learned the basics.
But its all good, we are learnig now!.
Actually, the World Rallye Championship 122 cubic inch engine have a restrictor plate at 81.6 cubic inches per squar inch of diffuser venturi size.
One upstream 35 mm , or 1.38 inch hole is smacked on to the air flow side of the little 2 liter turbo engines to stop them making more than 380 hp net at the flywheel.
On a naturally aspirtated engine, 1.38 inch feeding 6 cylinders won't make more than 125 hp net at the flywheel unless you do some serious compression ratio and cam work.
So even a 350 Holley with two 1.19" venturis making 2.22 sq inches of carb area on a 200 cube inch engine...thats about 90 cubic inches of engine per square inch of or carb area.
That's a restrictor plate in my book.
Every in line six, the same. So the stock carb hole is a NASCAR style restrictor plate, right out of the factory.
Ford engineers knew
exactly what they were doing.
The Offenhauser 3 carb engine with any good well sized carbs.
Just three Volkswagen carbs serves 65 cubic inches of engine per square inch of carb area. The Offy, and direct mount 500 cfm 4412 Holley are neck in neck for horsepower and air flow on a 200 cube engine.
The Faron Rhoads 14 second engine is a 25% bigger 250 engine with triple carb Offy.
It is right on a the nose at 81.6 cubic inches per square inch of carb area.