I recently made a couple of 'sprinkler' style venturis for my MVVS 49s.  The stock venturi seems to work well at 10,000 rpm or so, but the engine 'stumbles' with larger loads at lower rpm.  After doing a bit of on-line research, I found Claus Maikis' venturi cross-section recommendations.  The 2 venturis I made are within his recommended range for an engine of this displacement.  With the 12sq. mm venturi the engine turns a 13x5 Zinger at around 7500 rpm in a solid 4-stroke with no stumble.  The 14 sq.mm venturi allows a bit more rpm.  It remains to be seen if I can turn that much diameter and pitch, but it's worth trying.  The smaller venturis certainly have better fuel draw at lower rpms, however air testing will determine if the thrust is sufficient.

Interesting that MVVS prop recommendations for the preceeding .51 stunt varied from 12 x ? to 14 x4.  That's a heavy load for an 8cc engine!  However, my engine did not get very hot turning the 13x5.

Here's a You Tube flick of the test:



Fred

Gordon,

5% nitro, 10% castor, 10% synthetic.  Plug is a Merlin 2004.  I probably added 1 head shim last year and forgot.  I'll have to pull the head and check.  The muffler is a chambered mousse can attached to a MVVS supplied header pipe with screws and JB Weld.  I have tested the MVVS RE F2B muffler as well (last year).  It is a bit more restrictive and the RPM was a 100 or so lower with the 12x5 and 12.25x3.75 props.

It's a good thing I purchased a 2nd engine from Alberto Para.  MVVS recently stopped production on most of their glow engines.

The plane (nearing completion) is a modified KA-10.  I started with a Dixon kit and modified the fuselage profile to resemble Andrey Yatsenko's 'Classic 2'.  Eventually, I will post some pics. 

Fred