Large-displacement, high-revving engines adore this occasion short, fat headers to embellish high-rpm ability.
If your engine sees mainly street duty, subtract 3 from the rpm at which peak torque occurs. For a race engine, subtract 3 from the rpm at which peak horsepower occurs. We'll call this Figure B. Then, divide Figure A by Figure B and you have the pipe length in inches.
Header influential tube tuning has two basic parameters: airflow requirement and exhaust pulse tuning. Tubing calibre is dictated by flow requirement, which in itself is decided by engine displacement, engine rpm and the Appendix of a supercharger, turbo or nitrous. Header salient length has to create with timing the exhaust pulses so that the exhaust from one leading helps to suck the gases from an consequent cylinder. There are a character of differential "established" formulas, and Everyone offers a at variance flat of precision and utilize specialization.
Adjusting Displacement for Boost
This is something that most formulas don't story for, on the other hand it's constitutive whether you're using a turbo, supercharger or nitrous. Most formulas (including the one accustomed here) apply green cylinder displacement, however actual displacement depends on volumetric efficiency (how still fuel-air charge gets used vs the engine's displacement). Inceptive, decrease the engine's complete displacement by the cipher of cylinders to receive the cylinder displacement. Then, multiply that by 0.80 for a typical street engine or 0.90 for a tuned race engine. For boost pressures under 14 psi, calculate using the exact displacement. For boost levels over 14 psi, divide the actual boost level by 14 and multiply that by the displacement.
Adjusting Displacement for Nitrous
Nitrous is a bit easier, since exhaust requirements go up linearly with the amount of nitrous. First, divide the nitrous kit's horsepower level by the engine's horsepower without nitrous. Then add that figure to your volumetric efficiency (0.80, 0.90 or whatever the boost turns out to be if you're running a supercharger with nitrous). Multiply your nitrous-adjusted volumetric efficiency by the individual cylinder's displacement (engine displacement divided by cylinder count) to arrive at your final adjusted cylinder displacement.
Calculating Pipe Length
Look at your camshaft specs and find out how long the exhaust valve opens in degrees at 0.50-inch lift. Subtract this number from 360, then multiply that by 850 (we'll call this Figure A).Commensurate most matters automotive, exhaust header and process plot is a science unto itself. You could applicability empirical info gathered from others to drive the finest feasible header influential length and breadth -- and this is probably the easiest groove -- or you could calculate it yourself liable some basic confidence approximately your engine combo.
Considerations
The formula looks like this: ((850 x (360-EVO))/rpm -- 3 where "EVO" equals "Exhaust Valve Open duration at 0.050-inch lift."Diameter Calculation
Multiply your Volumetric Efficiency-adjust single cylinder displacement by 16.38; we'll call this Figure C. Add 3 to your calculated length from Step 4, then multiply that by 25; this is Figure D. Divide Figure C by Figure D and you'll have the header tubes' inside diameter in inches.
Example One - Mild 350
For this example, we'll use a 350-cubic-inch, naturally aspirated street V-8. The exhaust duration at 0.50 checks in at 212 degrees, and peak torque occurs at 2,800 rpm. Being a street engine, it has a volumetric efficiency of 0.80. We'll begin the length calculation buy subtracting 212 from 360 (equals 148), then multiply that by 850 (equals 125,800). Then we'll subtract 3 from our peak torque (equals 2,797). Dividing 125,800 by 2,797 and we end up with a final primary tube length of 44.9 inches.
Next, we'll adjust displacement by dividing 350 by 8 (equals 43.75 cubic inches per cylinder) and multiplying that by our 0.80 VE (equals 35). Multiply 35 by 16.38 (equals 573.3, Figure C). Now, well add 3 to our calculated header length (equals 47.9) and multiply that by 25 (equals 1,197.5) to derive Figure D. Finally, we'll divide Figure C (573.5) by Figure D (1.197.5) to arrive at an inside tubing diameter of 0.479 inch, or about 1/2 inch.
So, for our mild-cam, torque-heavy street 350 we'll need headers with primaries that measure about 44 inches long and 1/2-inch on the inside of the tube. Bear in mind that most header companies market their pipes by outside diameter; after accounting for the thickness of the metal tube, this actually comes out to about 3/4 inch outside diameter.
Example Two - Supercharged and Nitrous 350
While the 1/2-inch primaries given above may seem very narrow, it's appropriate for that particular engine. Lets take that same engine and add a supercharger (bumps the VE up to 1.10) and 100 horsepower worth of nitrous (adds an additional 0.25 VE for a total of 1.35), then add a longer duration camshaft (235 exhaust degrees at 0.50) and calculate header size based on the peak horsepower rpm (6,500).
Substituting these variable for those of our mild street 350, we end up with a header primary length of 16.35 inches and an inside tubing diameter of 1.96 inches. Those 16 1/2-inch by 2-inch primaries line up fairly well with about what you'd expect for a full-race 350.
