Header length formula

[mtiberio]

I'm looking for a simple header length formula. Something based on Bore stroke rpm cam specs, etc

[jacksoni]

Get something that does that and a whole lot more= Pipemax by Larry Meaux. = Maxracesoftware. Best bang for few bucks there is.

[WOODY@DDLLC]

What Jacksoni said! 
Meantime: From the Dynomation-UsersManual - Download here: http://www.motionsoftware.com/
The book Performance Tuning In Theory and Practice by A. Graham Bell, has excellent empirical formulas for header pipe length and diameters that provide reasonably accurate estimates for most applications. These formulas work well because they incorporate EVO timing within the calculation:
HPL (Header Length Inches) = ((850 x (360 - EVO)) / RPM) - 3
HPD (Header Diameter Inches) = ((SCID x 16.38) / ((HPL + 3) x 25)) x 2.1Where EVO is Exhaust Valve Opening Timing measured in the 720-degree system. SCID is the displacement of a single cylinder in cubic inches. HPL and HPD are lengths and diameters in inches. Solve math within nested parentheses first.
https://www.amazon.com/Performance-Tuning-Theory-Practice-Strokes/dp/0854292756/ref=sr_1_2?dchild=1&keywords=Performance+Tuning+In+Theory+and+Practice+by+A.+Graham+Bell&qid=1601647386&sr=8-2

[wobblywalrus]

The Pipe-Max program gives a good idea of an optimum setup.  The Dynomation program looks at how header, collector, and other pipe dimensions change the power curve.  This was an advantage for me.  Packaging concerns meant that the optimum dimensions could not be used and some compromises were needed.  The Dynomation program told me what changes from optimum could be made without seriously impacting performance. 

[panic]

Many end with "then add 6" to the primary length, and shorten to suit by testing".
I defer to Gordon Blair as to why there is no simple formula.

[WOODY@DDLLC]

You will need a calibrated hack-saw! 

[wobblywalrus]

The speed of the pressure wave is directly related to the exhaust gas temperature.  The waves bounce out of the exhaust port and travels down and back the length of the header and primary before they enter the enter the exhaust port when the valve is open and they do useful work.  A drop in exhaust gas temperature slows these waves.  Gas temperature drop through the system has a big effect on the optimal pipe length.  It is very difficult for the folks writing the programs and the equations to take your specific temperature situation into account.

One time I made a thin wall exhaust system that worked good during the calculations when hot exhaust gasses were assumed.  In practice the peak power rpm was much lower than intended.  This was on a bike with wind blowing on the pipes.  Some more calculations with the slower wave velocities in a cooler exhaust gave a lower peak power rpm. 

Dyno testing has a similar problem.  The pipes get hotter in the dyno room than they would be on the track because the little dyno fan can not move enough air to mimic real use.  This makes the pipe lengths that perform good on the dyno to work differently on the track.

What I do now is to minimize the exhaust gas temperature drop so the wind has less of an effect on tuned length.  Thicker wall stainless steel pipe is used throughout with tape wrap on the headers and collector.  This keeps the exhaust gas temperatures, and wave velocities, up during use so the pipes perform as designed.           

[John Burk]

An important part of exhaust tuning is using the good cycle and avoiding th bad ones  . The exhaust pulse changes from positive pressure to negative or from negative to positive every time it returns from the pipe open end . The 1st and 3rd cycles help by pulling exhaust out of the port and the 2nd and 4th cycles hurt by pushing the exhaust back in . First cycle tuning would need a very long pipes so most headers are tuned to the 3rd cycle . The goal is to time the harmful 4th cycle to a lower rpm where it doesn't matter and the harmful 2nd cycle above the shift point . This puts the beneficial 3rd cycle where you need it . I remember the big change with my small block gas dragster going from 30" pipes (bad 2nd cycle at 7000 rpm) to 24" (good 3rd cycle at 7000 rpm) .

[wobblywalrus]

My reply six is sorta confusing.  The graph shows how the speed of sound changes with temperature.  The best header length is related to the sonic speed so it is also optimum at a certain temperature.

Stainless steel has a thermal conductivity of approximately 15 watts per degree kelvin per meter.  Carbon steel conducts heat three times better at 45 watts per degree kelvin per meter.  The "per meter" is the thickness of the conductor perpendicular to the direction of heat flow so a thicker pipe conducts less heat than a thinner one.  Sixteen gage stainless with a wrap is what I use to minimize heat transfer.

The idea is to keep the heat in the pipe during the run down the salt so its performance is similar to what it shows on the dyno.

My best results are also to tune to the 3rd harmonic, like John.  Are these headers for a four valve per cylinder engine?     

[wobblywalrus]

Here is the graph.  A side benefit of pipe insulation is a cooler cockpit and engine intake air.

[mtiberio]

My reply six is sorta confusing.  The graph shows how the speed of sound changes with temperature.  The best header length is related to the sonic speed so it is also optimum at a certain temperature.

Stainless steel has a thermal conductivity of approximately 15 watts per degree kelvin per meter.  Carbon steel conducts heat three times better at 45 watts per degree kelvin per meter.  The "per meter" is the thickness of the conductor perpendicular to the direction of heat flow so a thicker pipe conducts less heat than a thinner one.  Sixteen gage stainless with a wrap is what I use to minimize heat transfer.

The idea is to keep the heat in the pipe during the run down the salt so its performance is similar to what it shows on the dyno.

My best results are also to tune to the 3rd harmonic, like John.  Are these headers for a four valve per cylinder engine?   

2 valve hemi head

[wobblywalrus]

The Pipe-Max and Dynomation formulae should work OK, base on my experience with that type of engine.  The Victory Papers have some info, too.

[Rex Schimmer]

Duke and I used the service provided by Burn's Stainless. They have an online form that you fill out with all of the engine specs and what you are looking for, in our case it was maximum hp at 12,500 rpm, and then they run it through their program and provide you with the header dimensions. All free.

Rex

[kiwi belly tank]

My reply six is sorta confusing.  The graph shows how the speed of sound changes with temperature.  The best header length is related to the sonic speed so it is also optimum at a certain temperature.

Stainless steel has a thermal conductivity of approximately 15 watts per degree kelvin per meter.  Carbon steel conducts heat three times better at 45 watts per degree kelvin per meter.  The "per meter" is the thickness of the conductor perpendicular to the direction of heat flow so a thicker pipe conducts less heat than a thinner one.  Sixteen gage stainless with a wrap is what I use to minimize heat transfer.

The idea is to keep the heat in the pipe during the run down the salt so its performance is similar to what it shows on the dyno.

My best results are also to tune to the 3rd harmonic, like John.  Are these headers for a four valve per cylinder engine?   

2 valve hemi head

2 valve hemi head bike engine, that's interesting! What are you working on, an N/A nitro engine?
  Sid.