Header Length Calculation

Below is my formula for finding the correct header length in millimeters (from piston to begining of diffuser cone) if you know the exhaust port duration, target maximum rpm, and speed of sound in the header (affected by the temperature). It is based on the belief (with evidence from the real world) that at maximum rpm the negative return wave from the end of the header should arrive back at the exhaust port a certain number of degrees after BDC, depending on the maximum rpm. That value is determined by the max rpm. It's between 0 for 5000 rpm and 25 for 10,000 rpm.

THL=(((((ED/2)+((MRPM-4999)/200))/(MRPMx6))xMPS)/2)x1000

THL: Tuned Header Length (piston to diffuser) in Millimeters (divide by 25.4 to get inches)

ED: Exhaust Duration in Degrees (from opening to closing of port)

MRPM: Maximum RPM of Engine

MPS: Meters Per Second Speed of Sound in Header. This formula works perfectly for a Grubee bicycle engine with 135psi compression and 104 degrees ATDC for the exhaust port opening. (Less degrees results in a higher temperature.) For speed of sound (in mps) use 600 if you are within 400 feet of sea level, or use this formula to for your altitude: mps=600/(1+((alt/1000)x.032)) where "alt" is the altitude in feet. For 1000 ft altitude the mps would be 581.

Below is my formula for finding the maximum rpm for a known length of header (piston to diffuser in millimeters).

MRPM=((((ED/2)+((IRPM-4999)/200))*MPS)/(12*HL))*1000

MRPM: Maximum RPM the Header is tuned to.

IRPM: Intended maximum RPM. The max rpm you desire if you are designing a pipe, or the top rpm you think the engine is producing with said pipe.

HL: Header Length in millimeters from piston/port to beggining of diffuser cone.

Automatic Calculation via Microsoft Excel

I've put these formulas in an Excel file to make it easy for you (somewhat). Click here to download the header2.xlsx file and open it with Excel. Enter the correct #'s under each column heading and press Enter and the calculated values should appear. The second table lets you enter an existing header length along with other details to get the maximum rpm of the header powerband. The first column is "Exhaust Duration". If you don't see it then scroll the page to the right. The header length is critical but too long a diffuser length can limit the maximum rpm also. But this calculation only concerns the header length.