Exhaust Size Calculator

How the exhaust size calculator works

Exhaust sizing is a compromise. A pipe that is too small makes the engine push its exhaust out against back pressure at high rpm. One that is too big slows the gas down, which weakens the scavenging that helps empty the cylinder and can cost response lower in the rev range. No exact formula gives the best size, so this exhaust pipe size calculator uses published empirical rules as starting points and says where each one comes from.

The Pipe size tab is an exhaust pipe diameter calculator: it turns crank horsepower into a flow-bench capacity target and that into an inside diameter, for a single pipe or for each pipe of a dual system. The Header primaries tab is a header size calculator that estimates primary tube diameter and length from displacement, cylinder count, the exhaust opening point and the rpm where the header should work best. The Single vs dual tab is exact geometry: the pipe sizes that give the same flow area.

What to enter and where to find it

• Crank horsepower. Flywheel power, measured or a realistic estimate. If you only have a chassis dyno figure, convert it with the whp to hp calculator first.
• Tube wall thickness. Tubing is sold by outside diameter but the flow area depends on the inside, so take the wall from the tubing's specification. Header tubing often has a thinner wall than exhaust pipe.
• Displacement and cylinders. Total displacement and number of cylinders: the primary sizing works per cylinder.
• Exhaust valve opens (BBDC). From the cam card, in degrees before bottom dead center. Note whether it is the advertised or the 0.050 in figure: the advertised point is earlier and gives a longer primary. The cam calculator turns duration and centerlines into valve events.
• Target rpm. The rpm of peak torque, or the speed where you want the header to help most. The diameter formula was set up for the torque peak; using the power peak instead gives larger primaries.

Exhaust pipe diameter by horsepower

The Pipe size tab uses two figures from a well-known engine builder’s exhaust flow testing on naturally aspirated engines. A muffler that flows about 2.2 cfm on a flow bench for every crank horsepower lets the engine keep about 99.5% of its open-exhaust power, and a straight pipe the length of a muffler flows about 115 cfm per square inch of area. The calculator sizes the pipe so that a straight section of it would just reach that flow. Together the two figures say each square inch of pipe supports roughly 52 hp.

Treat the result as the size to start from, not a precise answer. Those figures are flow-bench numbers for mufflers, not a pipe-sizing law, and they ignore bends, crossovers and gas temperature. Real mufflers and bends flow less than straight pipe, so the result is closer to a minimum than to an optimum. When it falls between two tube sizes, the larger one is the safer choice for a system with mufflers, while the smaller one keeps the gas moving faster. Both figures are editable if you work with different ones.

For a dual system the calculator splits the area between the two pipes, so each needs 0.707 times the single-pipe diameter, not half of it. That is the same geometry the Single vs dual tab works out on its own.

Header primary tube size calculator

Header primaries are sized per cylinder. The diameter estimate gives each primary a flow area of rpm × cylinder displacement ÷ 88,200 square inches, a widely used starting formula. Its constant sizes the tube so that one cylinder’s displacement, pushed out during a 180° exhaust stroke, averages about 245 ft/s in the primary, a classic target gas speed for headers. It grows with the size of each cylinder and with the rpm the header targets. A primary too large for the engine's rpm range lets the exhaust pulse slow down; one too small restricts the top end.

The length estimate is the classic wave-timing formula attributed to A. Graham Bell's book on four-stroke engines: length = 850 × ED ÷ rpm − 3 inches, where ED is the angle from exhaust valve opening to TDC, half a turn plus the opening point before BDC. A lower target rpm or an earlier exhaust opening means a longer primary. Collector design, stepped primaries and the port itself all shift the real optimum, which only engine simulation or dyno testing can pin down. The formula’s sources do not say which lift basis the opening point should be on, so use one basis and keep to it. Header builders measure primary length from the head flange to the collector, and that is how the result should be read.

Header tubing comes in eighth-of-an-inch steps of outside diameter, so the calculator also shows the common sizes on either side of each estimate.

Exhaust and header formulas

Only the circle area and the single-to-dual conversion are exact. The flow rule, the primary area and the primary length are empirical, so treat each result as a starting estimate, not a measured optimum.

1. Exhaust flow capacity (flow-bench cfm) = crank hp × 2.2
2. Inside pipe area (sq in) = flow capacity ÷ 115, split in two for each pipe of a dual system
3. Inside diameter = √(4 × area ÷ π); outside diameter = inside diameter + 2 × wall
4. Primary area (sq in) = rpm × (displacement ÷ cylinders) ÷ 88,200
5. Primary length (in) = 850 × (180 + EVO) ÷ rpm − 3
6. Same total area: each dual pipe ID = single pipe ID ÷ √2; single pipe ID = dual pipe ID × √2

Two pipes with half the area each are not half the diameter. Area goes with the square of the diameter, so each needs the single pipe's inside diameter divided by √2: about 71% of it. The same geometry is why the wall matters more on small tubes: it takes a bigger share of the cross-section.

The cfm figures are cold flow-bench capacities, not the volume of hot gas the engine actually pushes out, which is considerably larger.

Worked example

Three cases, worked out with the same functions that run the calculator.

Typical exhaust pipe sizes by horsepower

Inside diameters from the 2.2 cfm per hp and 115 cfm per square inch rule. Add two wall thicknesses to get the tube's outside diameter.

Use the table to see whether an existing system is in the right range, and the calculator for your exact power and tube wall.

Common mistakes

• Mixing up inside and outside diameter. Tubing is sold by outside diameter; the flow formulas use the inside. Always add or subtract two wall thicknesses.
• Halving the diameter for a dual system. Each of two pipes needs half the area, which takes about 71% of the single pipe's diameter, not half of it.
• Using wheel horsepower. The rule works from crank horsepower; a chassis dyno figure understates the flow the exhaust has to carry.
• Oversizing for a mild engine. A pipe much larger than the engine needs lowers gas speed and can cost response low in the rev range without adding power at the top.
• Using it after a turbocharger. The flow rule comes from naturally aspirated engines; behind a turbine the pressure picture is different, so treat the result as a rough lower bound.
• Using a cam event on the wrong basis. The length formula is sensitive to the exhaust opening point: an advertised figure and a 0.050 in figure give primaries several inches apart.