ESE's works engine tuner

[Frits Overmars]

... we have a situation where at 1000 rpm below peak power, the pipe - now too long ?? for the tuned rpm, acts to create an even sooner, greater depression at the port.
Thus it makes me think that at BDC, there will be a large, unavoidable span of rpm where even with an inefficient diffuser, we will have excessive short circuiting, simply due to the pipe geometry....

Wob, I noticed that you tend to use a somewhat smaller header length percentage than what I am used to. The difference is maybe just 1 or 2 %, but it could have a noticeable effect.

Short-circuiting due to too short a pipe is somewhat self-regulating due to the much lower exhaust gas temperature resulting from the escaping cold mixture, as Neels points out.
I think this was a reason for provoking deliberate short-circuiting and/or high-speed loop-scavenging in yesterday's direct-drive kart engines.

[Storbeck]

I'm not a Viking but I am pretty sure he's using 670 Rotax cylinders that look a lot like this (pictures from ebay ads I found on the interweb).

The experts can opine if those are lift shafts, to me they are not the MOST squared off lift shaft transfers I've ever seen....but they don't look much like a teacup.

[wobbly]

The Eagle Eye of Mr FOS strikes again - of course the pipe is too short at the lower rpm - dumbarse.
And yes, I had not considered the effect of the short circuiting regime on the EGT - again an obvious error, thankyou.
The short header and short steep first diffuser section is aimed specifically to peak and overev generation, then using a steep longer last diffuser section to regain the front side.
Especially in engines with no PV, that ameliorates many evils.