A quick summary of what I obtained from reading and research is that for best
peak numbers a correctly tuned and designed 4 into 1 header is the best choice. To obtain the highest
average power numbers the 4 into 2 into 1 is the best choice. The hole engine is a package with every piece effecting the other. I feel that on a street or road car a flat broad powerband that gives up a few peak horse power is the way to go. I talked with the guys at Burns Stainless, Kooks, and Loren Barns with S&S headers. I am building the headers for my ChevyII due to the many one off peices and no off the shelf headers come even close. Most mass produced tri-y's have short primary pipes for fitment which sacrifices a lot. The Doug Thorleys on may pickup have 12" to 14" primarys pipes that are way to short for proper scavenging. You will want,depending on your engine PACKAGE, somewhere between 30" and 36" primary tube length. The June 2008 issue of Circle Track has a dyno based header comparison that includes the 4/2/1 design. Another mag article that I held onto is "Exhaust Science Demystified" in the May 2005 mag Popular Hot Rodding. This article gives a good foundation to get you thinking on your own and asking good questions. To get the advantage of the 4,2,1 headers the correct cylinders must be paired with each others within the packaging limits of a car. On a 18436572 sbc engine you would pair 1&5, 3&7, 2&4, 6&8 at the primary collectors. There is definitely some varying "beliefs" and theorys out there so beware and ask for proof (Dyno) of there beliefs. A lot of header guys tell you that you must have equal length primary tubes. But in my humble
opinion(belief) definning that exact primary pipe length for a street car with full exhaust that operates from 2k to 6k rpm under limitless differant throttle postions and loads is not possible and again your tuning that length for just one peak rpm. Packaging alone makes an equal length design impracticle and if they are all the same length with out running through the middle of your steering shaft they now have so many restrictive bends and tight bend radiuses that the flow has been compromised. Smokey Yunick built many an engine with differant ignition and cam timing specs on groups of cylinders to broaden the power curve. He blue printed each cylinder to make max power each not to be all the same. On a street/road car 4" to 7" primary tube length variances is not going to have a major negative effect as long as the paired tubes are keep close in length and that length is not too short or long for the operating rpm of the engine design. You again may loose some peak power but will increase your area under the curve. The engine masters competion demonstrates this thinking well. A car with a long broad power band is wonderful to drive on the street or out of a hard corner and down a staightaway. Read, ask questions, listen, and most of all think for yourself!
Brian Hobaugh SCCA National Tour June 2014
First Hemi 'Cuda Convertible Ever Built
Short clips: Goodguys Pleasanton autocross and pit videos
Header design question
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