Tuned Exhaust Pipe Length Calculation For Single/Twins/Fours.

You only have to put ONE cylinder value in,

Note from Eliot: The formulas on this page were taken from "Four-stroke Performance Tuning by A Graham Bell". The book was published 25 years ago (and is still published). My dad paid me pocket money to take the formulas and make them into a program on our Sinclair Spectrum, then our Amstrad PCW8512 and then into Java you see here.

If you want to learn more about the subject - search for the above book on google.

Exhaust Open BBDC.
Exh close ATDC.
Engine R.P.M
CC of one "Cylinder"
Exhaust valve duration in Degrees.
Carburetor induction Length in Inches .
Exhaust valve correctly timed full lift withdegrees after TDC .
Overlap in Degrees.
1, Tuned length From the Valve Head................"P"
2, Primary inside Diameter is in Inches .
3, Secondary Pipe Length in Inches ............."P2"
4, Two Secondary pipe inside diameter.
5, Fours Primary Pipe Always (15" Inch)Long.........."P1"
6, Collector box Length in Inches........ "CL"
7, Tailpipe Inside diameter in Inches.
8, Tailpipe Length in Inches............ "TL"
Tuned exhaust length start from valve head,this know as "Down pipe"
The induction length start from valve head to end of the belmouth, center line of the port or as You know it the "Stacks"

  • For Single engine use:# 1,2,8
  • For Twin engine use:# 1,2,6,7,8
  • For Fours use:# 2,3,4,5,6,7,8.......(1)+(6)+(8)
  • For 4-2-1 use:# (2,5,) (3,4,) (6,7,8,)

    Notes 1
    For road , you'll be best off with a 4-2-1 system on a four cylinder engine.
    For high performance & race use, the only way to go is a 4-1 system.
    The 4-2-1 system gives good all-round power, with no special power peaks or dips, while a 4-1 system give the best power at higher revs - At the expense of low end power.

    Speaking of 4-2-1 extractors, this is the WRONG WAY to do it on a four cylinder engine! A 4-2-1 system must have cylinders #1 & #4 joined up, and #2 & #3 joined up, then further down those two pipes join into one.
    This is because four cylinder engines have a 180 crank, whereas virutally all V-8's have a 90 crank (the angle between the crankpins) and so the order in which the exhaust pulses come down the pipes is different.

    Notes 2
    A reduction in primary pipe Diameter of 1/8 of an inch will move the torque peak down by 650-800 RPM. Conversely an increase in diameter of the header will raise the engine speed at which max torque occurs,by approx the same RPM,for each 1/8 of increase.

    Adding length to the primary pipe will increase low speed and mid-range power, with corresponding reduction in power at max RPM.

    Shorter primaries give an increase in high speed power, at the expense of reduction in the midrange. However,there will be little change in peak torque or the engine speed at which it occurs.

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