NZ Engine builder looking for advice

Greetings from NZ,

long story short over the last couple of years I sand cast my own crankcases and built a V Twin 144cc motorcycle engine in my garage.

this is raced in a formula class (F4) only governed by a max displacement of 158cc for 4 strokes 100cc water cooled 2 stroke and 125cc air cooled. Thankfully the bike has been reliable and finished every race but is a bit lacking in the power department when compared to my friends 140cc single cylinder pit bike motor (YX140 with Z40 camshaft). Our chassis are identical and I am 4-5 seconds a lap faster on his bike vs mine (60 seconds per lap on the faster bike). Top speed is also up 10km/h at the end of the straights.

more about the engine is on my YouTube channel

[url=https://youtube.com/@OddsandEndsMachining?si=djnNLIEasEhzcbL7]https://youtube.com/@OddsandEndsMachining?si=djnNLIEasEhzcbL7

The engine I am using is an 80* v twin. The carbs are the same front and rear although the inlet tracts are different lengths due to packaging. The exhaust headers are also a different length due to packaging/time constraints.

Obviously equal length would be better, so I am wondering could this one change in isolation make that much of a difference? I also don’t know where to start in terms of how long to make the header pipes before they join together, I’ve only really worked with 2 stroke exhausts in the past.

Any help with exhaust design lengths and diameters would be much appreciated otherwise I will just buy an off the shelf motor although developing my own engine would be more rewarding to get right.

Thanks in advance.

Jonny

Jonny,

This sounds like a great project, thanks for sharing.

I think monitoring lambda on each cylinder individually would give you the data to optimize each carb and see what that does for you before you get into creating new custom parts, and the data would ultimately be useful if you end up changing parts as well.

There are software packages which can help determine optimal runner lengths, but with a vtwin bike exhaust setting up a sliding extension to vary the length might net you some data pretty quickly.

And here's some other potentially helpful info I found in a prior post I made:

Creating your own exhaust manifold can range in scope quite a bit depending on the number of cylinders, chassis, packaging, whether turbo(s) are involved, what tools you have access to and more, but HPA's Motorsports Fabrication Fundamentals course and Practical TIG Welding course include information on fitting, cutting, welding etc. which covers a lot of the process.

https://www.hpacademy.com/dashboard/courses/motorsport-fabrication-fundamentals

https://www.hpacademy.com/courses/practical-tig-welding/

In terms of design, if you're creating a turbo manifold, I found some great info in this HPA podcast with a guest who has been doing it for quite some time.

[url=https://www.youtube.com/watch?v=zK0Mke-vyG8]https://www.youtube.com/watch?v=zK0Mke-vyG8

Beyond that, there's a 3d modeling course if you want to CAD model your manifold so you can buy exactly the pipe you need and reduce fabrication time:

https://www.hpacademy.com/courses/3d-modeling-and-cad-for-motorsport/