Exhaust header design using individual primary pipes for each cylinder has been fairly well examined elsewhere. However, this is only applicable to engines with an individual exhaust port for each cylinder, and cannot be directly applied to many older engines with common or siamese exhaust ports such as the BMC Mini, or other in-line four and six cylinder engines such as Chevrolet 216, 235 & 261, GMC 248, 270 & 302, etc. including both pushrod OHV and side-valve types. These engines generally have the center pair or pairs of exhaust ports shared between adjacent cylinders, such as the #2 & 3 (four cylinder), or #2 & 3 and 4 & 5 (six cylinder).

    One method of compensating for this is known as the “long center branch”
design, where the individual primary pipes (from the end cylinders: #1 & 4 or 1 & 6) are merged into a pair or pairs rather quickly, as found in “tri-Y” (4-2-1) systems. Their length is ½ of the calculated “tuned length” for the RPM needed for peak power.

    The shared port has a somewhat larger diameter primary pipe. This is because the port is pressurized by the other cylinder every 360°, and added area is needed to reduce residual pressure. Based on the pipe sizes normally found in these engines and the commercially available sizes, the common pipe is about 1/8” (.125”, 3.2mm) larger OD than the individual pipe OD for smaller engines, and about 3/16” (.1875”, 4.8mm) larger in larger engines.
    The shared pipe is the full calculated length (viz., about twice the length of the individual pipes), and join the primary pipes from the other cylinders after they merge. Here’s the concept, shown above right; click on the picture for a larger image.

    To predict the required pipe size the usual rule of 300 feet per second gas speed at peak power works fairly well for the end cylinders (1 & 4 or 1 & 6) but not for the shared ports, since not only is the gas volume higher but the timing of port pressurization depends on the firing order. Shown, right, is the entire LCB header and collector system for a four cylinder engine; click on the picture for a larger image.

    The Table below shows suggested pipe OD sizes for common four cylinder engines by displacement, rounded off to the nearest 1/16” increment. The pipe ID must, in all cases, be the same size or larger than the actual port ID - never smaller. Make no attempt to taper, radius or transition the exhaust port larger to match the primary pipe ID, as an abrupt “step” is the proper shape.

Table 1: Four cylinder engine; size vs. exhaust pipe OD

Maximum size, CC

   828

   933

1,042

1,159

1,281

1,544

1,831

2,143

2,480

2,841

End OD, inches

1-1/8 

1-3/16

1-1/4 

1-5/16

1-3/8 

1-1/2 

1-5/8 

1-3/4 

1-7/8 

   2   

Center OD, inches

1-1/4

1-5/16

1-3/8

1-7/16

1-1/2

1-5/8

1-3/4

1-15/16

2-1/16

2-3/16

    The construction of a header for a six cylinder engine using the same concept is somewhat more complex, since the “natural” firing order (as well as convenience) of 1-5-3-6-2-4 separates the exhaust ports laterally into two groups: front cylinders and rear cylinders, with each group firing alternately. This means that the merge point of an end (individual) pipe with its mated center (shared) pipe cannot

have the ½-length characteristic. I suggest that ½ of the tuned length be used for the length of both pipes to their merge point, and that the full length be used to join these merged pipes.
    The common pipes (#2 & 3 and 4 & 5) should, in my opinion, be slightly larger than the proportion for the four cylinder engine, since the intervals from previous exhaust cycles are as short as 240°, and even more area is needed to reduce residual pressure. Shown, above, is the entire LCB header and collector system for a six cylinder engine; click on the picture for a larger image.

Table 2: Six cylinder engine; size vs. exhaust pipe OD

Maximum size, inches

  95

106

117

141

168

196

227

260

295

333

373

Maximum size, CC

1,557

1,737

1,917

2,311

2,753

3,212

3,720

4,261

4,834

5,457

6,112

End OD, inches

1-1/4 

1-5/16

1-3/8 

1-1/2 

1-5/8 

1-3/4 

1-7/8 

  2  

2-1/8 

2-1/4 

2-3/8 

Center OD, inches

1-7/16

1-1/2 

1-5/8 

1-3/4 

1-15/16

2  

2-3/16

2-3/8 

2-1/2 

2-5/8 

2-3/4