I have a 302 GMC powering a 40 Chevy. I have made two different single 4 into 1 headers over the 20 years I have owned the car. The engine has 9.5 cr pistons and a old ISKY 3/4 cam.
My first attempt was tho simply match the stock GMC head with 1 5/8 and 2 inch tubing into a 3 inch collector. These tubes give mismatched gas flow rates into the collector as the siamese ports carry 2x the gas of the single ports yet the siamese port area is only 1.55x the single port area.The car ran 84 mph in quarter mile. Next, I had the head ported but I ran the old mismatched and undersized header and the car did 89 mph in the quarter mile.
Next I built a bigger header to match the enlarged single ports and equalize the flow rate. This was 1 3/4 inch and 2 1/4 inch tubes into a 3 1/4 collector. These tubes have 2:1 areas to give equal the flow rates into the collector. The car turned 86 miles per hour. The car has less pickup!!
Questions.
Does anyone understand why the larger header with equalized flow rares into the collector slowed down the performance?
Why did GM undersize the siamise ports?
What is the best tube size for a a ported 302 GMC head with port areas equivalent to 1 3/4 and a 2 1/8 ID round tubes? What is the best collector size for this combination?
Does equalizing the flow rate into the collector by making the siamese port tube areas 2x the single areas make any sense?
