Lynn.

What's your take on the runner length for 6,000? I believe you add the plenum length to the tube length for the total length, So I guess the longer/ bigger the plenum the shorter the tube has to be.

George (Down under)

I was hoping nobody would ask me that until I have tried a few on the dyno. I need to go back to square one and get out some books, and have my wife teach me Algebra and Trig again.

 

The problem in kind of a dumb way is that almost anything will work well enough to power nearly anything you could want to fly in. Look at the McCulloch Drone engines. 79 pounds, 72 HP. No inlet tract, no runner length no air cleaner, no injection, carb a 4" tall length of pipe with a needle valve stuck through one side and a gas hose through the other. Bolted solid on the crankcase. Actually very well designed with a built up crank in needle races needle bearing rods and wrist pins.

 

Designed to last less than 10 minutes and get shot to pieces during that time. The stud spacing for the intake tube was the same as an old 6 cylinder Plymouth carb. So getting one running was no trick at all. So is 4" just right for 5,500 or whatever those things ran?

I know the props were not very long. Probably not. Maybe 8" or 10" would have made a bit more HP. Was the square cut end of the tube a lack of thinking? Or was the vena contracta so produced, there to make a depression so the lack of a venturi would still get the carb to work. If you put a nice bell shape on the top it would flow more air but have no differential pressure to flow fuel. So the unhappy drag of a squared off tube becomes a carb choke.

 

The two rotor Mazda (It isn't really a Wankel) is two 4 stroke Otto Cycle engines running on the same crank, and sharing a iron segment to save weight. The piston controlled valving feature makes the intake tune just like a piston port 2 cycle engine of the same displacement. About 575 CC per engine.  So for 5,000 to 7,000 it aught to look kind of like a good running 500CC single dirt bike engine? And in airplanes we see whopping big diameter tubes running over the engine and clamped to a big plenum chamber with a big TB on one end or the other. Enough induction system for over 500 HP. Just to wring out a paltry 160 (Maybe) HP?

 

The first good Chevy engine I built was a 302 for a CanAm car. It had a Crane roller cam and gear drive. It made a wonderful gear howl when it ran. The intake was Endrel manifold and tubes looking like a pipe organ. Not very large in diameter, and shorter than others were using. It was a dribble system like the Bendix systems on Continental engines but even simpler. The engine driven pump metered fuel to a distribution block and then into nozzles and into the intake all of the time. There was a line back to the fuel tank that went through a selector block where a number of return jet sizes could be selected by the driver to adjust gross mixture.

And I do mean gross. The real smart guys had other ways of tailoring parts of the band but we didn't. We didn't have a clue. When the car was sold, the new owner put that little cast iron Chevy on Jerry Mong's dyno and got 575 HP. It was nothing like the big guys had, but in a 1300 pound evil handling Lola, it was enough to keep the driver hysterical. Waking up at night screaming, drinking too much. The usual stuff. But those tubes were so small compared to what others were running. The intake valves were only 2.02" so how big a diameter was correct. ??? I never found out.

 

Anyway the runners on our store bought (Daryl Drummond) race engines are very nearly stock diameter. The very slightly modified stock manifold gasket is used. The high mid range is important in a race car. The closer you get to 2,000 RPM of power the better.

 

This is to have power when a gear change pulls RPM down further than was expected. If you pull it down too far the time spent accelerating again would have been saved with a downshift to the next lower gear.

 

So in an airplane, it is good to have power enough to at least stall the prop a bit on power up from stopped. Rapid acceleration to lift off with less time for gusts to upset you. Less time with poor aerodynamic control, and so on. All the good stuff. So airplanes should have a good mid range although lower than the race car, and the mid range should be strong. With increasing power as the revs come up right through cruise to top speed.

 

So what makes good mid range? runner velocity. More is better. Smaller diameter is better than larger. Straight is better than curved. And if injectors are closer to the port, slick and shiny is better than anything else. The inlet end must be protected by a free flowing air filter, and no vena contracta at the ends. Bell shapes only, always.

 

Other things to think about. Without a dyno and an adjustable length runner system the chances of hitting the ideal length for best top power and a good mid range, is zero.

 

But how about this. A log manifold is a convenience so you can run a single TB and cable system. So two engines are miss tuned by interconnection, for convenience. How about two smaller TBs? And it is two engines, not 4 engines, why 4 tubes? How about two tubes?

 

Either with a junction close to the engine or just bite the bullet and go Pport with two tubes and 2 TBs. Much less space involved, and less complex with much more power. You can get the length and diameter wrong and still make more power than a side port engine. Notice Tracy's short runner intake? Running real well.

 

More later.

 

Lynn E. Hanover