REELAV8R

One of the first modifications I made to my 77 L48 corvette was to get rid of the restrictive stock exhaust system in it's entirety. I installed Hooker headers with side pipes and baffles. I liked the look and also the sound.
Then I rebuilt the stock q-jet carb using Cliff Ruggles' book to improve the fuel flow to the engine.
These two modifications made the car run much better and added some pep to the engine. However it still felt like something was holding back the engine. So I began to look at the intake side of things. I knew more exhaust flow necessitates better/more intake flow.
The stock intake system is still in place with the single snorkel forward of the radiator for cold air intake. This looked to be restrictive to me. I mean the L82's had two intake snorkels and this L48 only had one. In order to explore this I hooked up a monometer set up like this.
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It is nothing more than colored water in a 1/2 inch vinyl hose taped to the passenger side window and a strip of tape with inches marked off to see how far the water level would drop according to the vacuum created in the air box.
Then the hose was ran forward to the air box under the hood through one of the knockouts in the fire wall just aft of the hood latch. Then I hooked it up to the where the valve cover vent usually went into the air box.
First I tried it with the Fram air cleaner that was in there. At cruise power (no secondaries) The water level remained at the top. When I floored it and opened the secondaries this created enough vacuum to draw the water down about 8 inches in the tube. Too much. My target was about 1 1/2 to 2 inches, or zero lol.
Next I removed the air cleaner and this time when I floored it the engine drank all the water. Siphoned it right out instantly.
I concluded the air cleaner itself was a serious restriction.
Installed a K&N air cleaner.
This time when I floored it, with the air cleaner in, it drank all the water again. At least now the air cleaner was not a large restriction.

I calculated the total intake area on the air box snorkel (cold air intake) to be 12.14 square inches (4.5 inch by 2 inch rectangle plus 2 inch hole, to account for the rounded ends of the intake snorkel). This didn't help me much as I didn't now how to equate this to the air needed by the engine but would be a good point of reference. Next I decided to drill as many 1/2 inch holes in the bottom of the air box out side the area of the air filter that I could around the perimeter of the bottom of the air box. Like this;
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These holes I calculated to be about 20.4 square inches if my math is correct (3.14 x .5 squared x 26). So I increased the amount of air available to the engine by better than 100%. It's not cold air so it is less than ideal, but hot air is better than no air and it would mix with the cold air to get something in between. The plus is that it retains the stock look.
I next decided that I didn't want this hot air available to the engine all the time only when the engine demanded it. So I fashioned some "valves". These were nothing more than strips of aluminum from an old liscense plate cut into the proper shape to cover the holes. I welded in two pins on each end of each "valve" so that it could move float up and then settle back down over the holes. The pins had caps on them so that the valves could not float too high and come off their guide pins. It looks like this;
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Here they are "floated" up;
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Now when on the primary circuit of the carb the valves sit down over the holes and the engine uses only cold air. Soon as enough vacuum is suffecient in the air box the valves float up and allow more air to be drawn in.
My last goal was to fine tune these valves with springs so that they floated only when the secondaries were actuated. I did this by hooking up my manometer system again and testing until I achieved 1 1/2 inch drop in water level. The "caps" on the pins are vacuum tube cut in half and a small hole punctered in them so they could slide down the pin. Not easily but with enough friction so that they would not move once placed. Using this method and very light springs I was able to get 1 1/2 inch water drop at 5000 rpm (max rpm I ever use). In this way I can get the advantage of cold air when on the primaries and still get all the air the engine needs while on the secondaries, plus it retains the stock appearence. Fun project

BKbroiler

I like the monometer test, no doubt a very accurate test. I used some empirical data to prove the same thing. I made back to back 1/4 mile runs on a drag strip with my car using the stock L48 air cleaner plus all the cold air ducting, then using a 14x3 open element air cleaner. The car went 4 tenths quicker with the open element. (This is with a fairly mild 383 engine).

REELAV8R

The monometer idea wasn't actually mine I got it from one of David Vizard's books. It did work well. The holes and valves for the air box are mine though.
4 tenths with an open element is quite a difference. Just shows how restrictive that stock set-up is. Wonder how much could be gained if it had a cold air system with suffecient air supply.

7t9l82

you mentioned intake and exhaust changes, don't forget to put a curve in the distributor. there is some performance to be gained there

REELAV8R

Your right. I did that too and did see some performance gains.

Also got rid of the stock heavy duty clutch fan for electric fans for an estimated gain of 13 horses freed up for the powertrain.