Mid Length or Long Tube Headers - Impala Tech
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post #1 of 5 (permalink) Old 06-04-2016, 06:05 PM Thread Starter
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Mid Length or Long Tube Headers

Asking opinions on any noticeable performance and sound differences between each on a 68 BBC, Muncie; 3.73; convertible operating at 4,000 rpm max. A true cruiser.
Only modification is slight cam increase, Comps 268H and 2.5" exhaust exiting behind wheels. Will be purchasing within a few days. Thanks all!
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post #2 of 5 (permalink) Old 06-04-2016, 07:13 PM
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No difference in the noise may that is attributable to lack of mufflers and big tubes. Performance difference is longer tubes build more bottom end shorter tubes make more peak horsepower at higher RPM. With primary tube size the bigger the tube the more peak horsepower you will make the smaller tubes make installation far easier and build more bottom to mid range torque.

Big Dave
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post #3 of 5 (permalink) Old 06-04-2016, 08:12 PM Thread Starter
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Understood. I may have posted words incorrectly. Asking the difference in mainly torque as I'll be mainly below 4k rpm. I'll never spin high enough to see peak hp. numbers. And is the primaries long enough to moderately scavenge. Hard to find full length's with collector's tucked up closer through the off the shelf aftermarket's without cutting and welding. Also maybe looking at a 1" drop in future, to be determined though.
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post #4 of 5 (permalink) Old 06-04-2016, 09:26 PM
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If you are capped up there is no scavenging. This is because wave theory requires an open tube (uncapped) to generate a standing wave. So short or long they will both scavenge exhaust gasses equally poorly if you remain capped up.

In addition to wave theory another thermodynamic principle is in effect with tubular headers. That is exhaust pulses or plugs of high pressure gas entering and exiting the header. f the header is about 18 inches long from port to collector merge and back to the top of the next port it can segregate these individual plugs of gas and use them to pull exhaust gas out of the chamber. It isn't as strong of a pulse as you get with a standing wave, but it is better than nothing and it has one other important benefit. One that adds up to twenty horsepower with a capped header.

This is the ability to move exhaust gas away from the head and prevent the high pressure discharge from cylinder seven entering cylinder five before the exhaust valve closes. This can happen with a stock cast iron log style exhaust manifold as is used on a BBC. This could dilute the fresh air in the cylinder five as the valve closers acting like a long duration cam powered EGR valve.

This effect plus reduced weight are the only reasons to consider a set of tubular headers.

Big Dave
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post #5 of 5 (permalink) Old 06-04-2016, 10:14 PM Thread Starter
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Most mid length headers have primaries longer that 18", coupled with a low hp. 402 and possible vehicle drop, sounds like a 3/4" intermediate coated header is best overall for my app. Thanks Dave.
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