Hirth F23 exhaust length

[Orv Judd]

I have a Kitfox LITE with a fuel injected F 23.  I am interested in finding out the proper length of the exhaust from engine to exhaust port on the muffler.  Is there any one who can help me with this question. I can not find this info in any of my engine manuals?

[facthunter]

Seems a Pretty Plain system  with No expansion chambers  I don't think length is critical but someone can Measure theirs.  Use thin wall tube for weight saving. 2 into 1 to go underneath, fairly long before it Joins. at small included angle. The cylinders are staggered so expect some vibration, I'd go for an absorbtion Muffler. You might fit a spark arrestor if there's Grass around (Krizmann?) There was plenty of USA ones at one time but in California 2 strokes have been banned.  Nev

[turboplanner]

4 hours ago, Orv Judd said:

I have a Kitfox LITE with a fuel injected F 23.  I am interested in finding out the proper length of the exhaust from engine to exhaust port on the muffler.  Is there any one who can help me with this question. I can not find this info in any of my engine manuals?

You don't have space to fit the Hirth system?

 

If not are you looking at pipes only or pipe and muffler?

[onetrack]

Quote

There was plenty of USA ones at one time but in California 2 strokes have been banned

 

I believe Orv is in Eagle Creek, Oregon, so he just needs to keep out of Californian airspace, and he'll be just fine with his Hirth.

 

Edited November 9, 2025 by onetrack

[facthunter]

It may affect the availability, in the US. Nev

Edited November 9, 2025 by facthunter

[Jack Jones]

On most Rotax 912/914 installs in Kitfox airframes, there isn’t a single “factory set” exhaust length from engine flange to muffler inlet. It’s typically a custom fit within the Kitfox exhaust system geometry. What matters most is keeping smooth bends, proper slip joints, and adequate support so there’s no stress on the heads or cracking at the flanges. If you can share which muffler system you’re running, someone may be able to compare it to a known Kitfox F23 installation.

[Jennifer P]

Hello, the length of pipes and diameter is determined by where you want the power band..   What I mean by this is what rpm range do you want to be your max VE? 

here you go..  this might get you going in the right direction.. 

 

To calculate exhaust header size and length for optimal scavenging, use formula \(L = \frac{850 \times (360 - \text{EVO})}{\text{RPM}} - 3\) for length (inches) and determine primary diameter based on engine cylinder displacement and target RPM, usually targeting high-velocity flow. Key metrics include the exhaust valve opening (EVO) timing and peak torque RPM. [1, 2, 3]

Exhaust Header Formulas

• Primary Pipe Length (HPL):
  \(L=\frac{850\times (360-\text{EVO})}{\text{RPM}}-3\)
  • EVO: Exhaust Valve Opening angle before bottom dead center (BBDC).
  • RPM: Desired peak torque RPM.
  • Alternative: \(\frac{850 \times \text{Exhaust Duration}}{2 \times \text{RPM}} - 3\).
• Primary Pipe Diameter (HPD):
  First calculate desired Primary Pipe Area (PPA) (sq. inches) = \(\frac{\text{Cylinder\ Volume\ (cc)}\times \text{Gas\ Velocity\ (ft/s)}}{3370\times \text{RPM}}\).
  Then use: \(D = \sqrt{\frac{4 \times \text{Area}}{\pi}}\).

[facthunter]

You are unlikely due to weight and space considerations to be able to gain the flow benefit of tuned length Pipes in Our aircraft.  Concentrate on safety and no Leaks. Nev