In my quest for the best four stroke performance I could squeeze out of my Saitos I discovered they run at the best power level for stunt with a somewhat restricted venturi.  I found the engines I have (on high wing loaded airplanes) both run the best at about 1/3 the opening the RC carb has at full throttle. My stunt experience with two strokes has been pretty much with plug-n-play engines, ST 46, OS35s Brodak 40 etc. Never had the need to really play with 2 stroke venturies.

Wondering if there is any correlation between what I discovered with 4 strokes and the way everyone is running the big 2 strokes? I just cant imagine the power a 72 two stroke would put out if it was ran wide open. So you guys that know the big block 2 strokes how would you compair the size of the venturi relative to some reference, say a ST 60..

There are a couple of things I can think of.

1. At the same rpm and displacement, a two-cycle ingests approx. twice the air of a four-cycle. (No wonder they are fuel hogs!) Two cycle has an intake cycle at every rev. compared to a four strokes every other rev.. A arger venturi area for a two-cycle makes sense from a flow restriction point of view.

2. A four-cycle will produce significantly more pressure differential at the intake for a given change in volume (cylinder volume) than will a two-cycle (case volume). Conversely, a four-cycle's intake pressure differential is very sensitive to flow so some restriction (smaller cross-sectional area) would be of benefit.

Just to add a little.....Four strokes tend to be a lot more efficient mainly because the exhaust valve and the intake valve are both closed during combustion. In a two stroke the incoming fuel charge is what pushes the old burnt fuel out so some of the new charge goes out with the old. It can't be helped it's just the nature of the beast.....Grins

HI Greg

The 4 strokes  actually are  NOT a lot more efficent, your  just looking at apples and  oranges, if you peak out a 2 stroke they will make  more  power than the same size 4 stroke, just most people do not run them that way, there is no free lunch and you have to burn fuel to make power.... compare a Nelson 40 to a Saito 40, or  a  PA 75 to a 75 sized 4 stroke. Aero Tiger 63 to the same sized 4 stroke, the 2s have a power edge running peaked. Notice  I did not compare a  plain bearing FOX 35 to a AAC ball bearing 4 stroke, there are  exceptions
 What people give up in the 2 stokes  are the run, most would rather run teh engines rich or in a 4 cycle and  get  the nice engine run with a  4-2 break.

Regards
Randy

Didn't ask the question to get into a 4 stroke - 2 stroke power discussion was really wondering if the big (above 60) two strokes were also running relative small venturies or not. No question a two stroke of the same displacement is capable of cranking out more power but do we use it with big block 2 strokes?

My Saito 40a will happily turn an 11-7 at 10 grand but I wouldn't want to be flying stunt with it at that setting.