What is SPI?

Dave P. wrote:Now I get it!  That makes perfect sense. So that means that the bottom of the exhaust port is slightly lower than a non-SPI cylinder?

Peter Chinn on Engine Test Cox 049 QZ wrote:Cox engineers Bill Atwood and Dale Kirn therefore set about designing a new cylinder for the QZ. Firstly, they eliminated sub-piston induction by raising the lower edge of the exhaust ports. This presumably, slightly reduced power compared with the standard Bee and, since the silencer was bound to reduce output still further, two more cylinder modifications were adopted. These consisted of adding an extra transfer flute and fitting a trumpet-type cylinder head in place of the stand hemispherical head. Both these are features of the very high performance Tee-Dee 049 contest engine and the overall effect has been to make a very significant difference to the QZ's power output.

Eddy wrote:If it is of any interest, the OK Cub .19 was designed with SPI. Of the several I have, all have SPI. According to Model Airplane News 1954 test of the Cub .19. It ran an 8X6 prop at 12,200 RPM on 25% Nitro, not all that bad for that era. Eddy

Eddy wrote:If it is of any interest, the OK Cub .19 was designed with SPI. Of the several I have, all have SPI. According to Model Airplane News 1954 test of the Cub .19. It ran an 8X6 prop at 12,200 RPM on 25% Nitro, not all that bad for that era. Eddy

Oldenginerod wrote:SPI is nothing new, or indeed exclusive.  Many radial port engines use it.  Back in my early Taipan diesel days I recall them having SPI as well.  The need for muffled engines killed the idea.

ticomareado wrote:However, Cubs don't really have all that great of a metallurgy and this engine would probably be best used for short motor run r/c assist free flight, if at all. It would probably wear out in less than two gallons of even high castor oil fuel.

ticomareado wrote:If your "W" heads are still good my advice would be to clean them up real good and replace them with regular heads. The "W" element heads are rare as they were a patent infringement and are somewhat collectable.    

Dave P. wrote:So after learning what SPI is all about and the differences in the muffler cylinders, I checked out my Medallions.  I have a couple with throttle rings and a few without.  The ones with throttles have #2 cylinders the ones without are #3s. Both types have SPI, the ones with throttles have dual bypass ports.  They also appear to have tapered cylinders based on the tightness at TDC on the lower time one.

CEF wrote:Slit Cylinders: Description
(note: some engines such as Black Widow, QRC, Medallion... had slit cylinders on later models)
Medallion: 2 bypass, 1 boost port on each bypass, top fin milled. no SPI (RC version ground for use with throttle ring)

Dave P. wrote:Another thing I noticed on both the Medallion throttle rings and the ones I got from Bernie, are that the port is much smaller than the exhaust port on the cylinder. I'm thinking that's because if it was the same width, it would take a quarter turn or more to get any throttle action, is that about right?

Peter Chinn wrote:However, whereas the manufacturer's normal recommendation for the Medallion series is the medium nitromethane content Cox standard glow fuel, the use of Cox Racing Fuel is suggested after fitting the throttle control (except during very hot weather). This has the effect of raising torque at low speeds and thereby partially compensating for the restricted breathing at high rpm. Because the torque curve drops off very much more rapidly as rpm are raised, effective prop sizes are apt to be larger.

Our test of the Medallion 049 with throttle, indicated that the prop size, that would enable maximum power to be developed in the air, would not be smaller than 6x4. A 7x3 Top-Flite, with between 1/8 and 1/4 in. off each blade, might be even better. The same engine without the throttle unit, however, would get nearest to its peak power (in the air) when turning a 6x3 or 5-1/2x3 prop. The difference is accounted for by the fact that, although the non-throttle equipped engine on standard fuel was more powerful than the throttle-equipped engine on racing fuel, the throttle system cut bhp peaking speed by over 5000 rpm - i.e. from nearly 18,000 to 12,500.

In the case of the 049, this drastic change in the shape of the performance curves is, in fact, quite acceptable. Compared with a Half-A free-flight, a Half-A R/C ship is usually heavier and has higher drag and a tiny prop spinning at high rpm is not the best way to move such a model through the air. Thus, although the Medallion 049 without throttle - even on less powerful fuel - had a 25 percent higher power output and, for example, turned a 5x3 prop 3,000 rpm faster than the same engine with throttle unit fitted, the actual performance on suitable R/C model props was not so very different.

RknRusty wrote:
How SPI(Sub Piston Induction) works

The upstroke: On the piston upstroke the crankcase pressure is relieved by the action of the piston moving up to compress and burn the previously introduced charge. In a reed valve engine, the drop in crankcase pressure releases the reed which finally opens, fully drawn by the vacuum created by the displacement of the rising piston. Fuel/air is drawn into the crankcase through the venturi behind the reed. In a rotary valve engine such as a Tee Dee, the intake valve opens while the piston is rising and the vacuum it creates draws the air/fuel mix through the carburetor venturi into the crankcase.

TDC and the downstroke: As the piston goes through top dead center, the piston skirt clears the bottom of the exhaust port, momentarily opening a gap exposing the crankcase to cold air(Not recommended for engines with mufflers). The crankcase vacuum created by the rising piston causes fresh air to be pulled into the crankcase adding additional air as the now descending piston compresses and forces the freshly inducted air to combine with the existing fuel/air mix.** This gives the crankcase a belly full of extra pressurized f/a mix ready for the next combustion cycle. The increasing pressure of the descending piston forces the mixture to scoot up through the bypass ports until the crankcase pressure is diminished. It starts over again with the upstroke and Kaboom, it's a wild cycle. With SPI more fuel/air mix is available for combustion power. It only has its full effect at high RPM. Usually a small prop, less than 6' is needed to get to useful RPMs for SPI to have its effect.

**Obviously, the bonus air pressure added to the existing air from the venturi dilutes(leans out) the mixture, so that the needle valve must be opened more to let in extra fuel to balance the mixture. You can call it "Richer" but it's really the same optimum F/A percentage, just slightly more volume of it.

In summary, the crankcase is charged with fuel/air mix from the venturi plus the SPI inducted air and then compressed, mixed, and injected up the bypasses by the descending piston. All of that, plus the extra fuel from the "richer" needle setting to balance the F/A ratio, and now you have a higher volume of fuel charge moving from the crankcase into the combustion chamber than if there had been no sub piston induction.
Rusty Knowlton

Here's a good read by Paul Gibeault, a winner of many Mouse racing championships. You can download it here:
https://www.coxengineforum.com/t753-the-revised-2011-gibeault-mouse-race-program.

ticomareado wrote:RE: "W" element glow head-- I must have gotten misinformed here:

"•#??? "W" element Standard Head - Hemispherical shape - Low Compression - Larger dia. threads. The concept was borrowed from either Atwood or Holland engines and Cox lost the ensuing patent fight therefore the production run was very short due to the legal situation."

http://www.wikiwand.com/en/Cox_model_engine  

Atwood & Adams Super Champion .62 cu in wrote:William E. Atwood (1910-1978) was one of the most prolific and successful engine designers in the 1930's to the 60's. He designed his first engine, a water cooled 30 cc design in 1932. The next year he designed the successful "Baby Cyclone" of which over 20,000 were sold. His Torpedo engine design was sold to and built by John Brodbeck of K&B engines. The Champion and Super Champion were made from 1946 to 1948. In 1948 he designed the Triumph engine line and the next year with Bob Holland produced the Wasp .49 which later became the Atwood .49. In 1960, he joined Cox Manufacturing where he designed the successful Tee Dee and Conquest engines and worked as a consultant up until his retirement in 1975. He was also a noted model flying competitor and is a member of the AMA Hall of Fame.

Mark Boesen wrote:The "W" filament was a cost saving measure for product engines only, as the coil was still the standard for high compression heads. Designed by Bill Atwood to reduce rare metals, it was used from '69 to about '73. According to Mud there was no 'cease and desist' order as the savings vs. performance just wasn't there.

The 'w' filament is easy to spot.