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What is SPI?

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What is SPI?

Post  Dave P. on Tue Sep 11, 2018 8:56 pm

I know it is Sub Piston Induction, but what exactly does that mean and how does it work? I've assumed that it's the tapered grind at the bottom of the cylinder, allowing more air/fuel to enter the ports more quickly.

Another question, is there anything to be gained by polishing the ports?

TIA.
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Re: What is SPI?

Post  GallopingGhostler on Tue Sep 11, 2018 9:12 pm

No, it is the air gaps that exists when the piston is at full top dead center. This air gap is where the piston skirt bottom clears the bottom edge of the exhaust port, allowing a brief puff of fresh air into the crankcase to mix with the air fuel mixture from the engine's venturi, whether reed valve or front rotary valve.

This colder denser air mixing with the carb/venturi's fuel-air mixture helps to boost horsepower.
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Re: What is SPI?

Post  Dave P. on Tue Sep 11, 2018 9:31 pm

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?
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Re: What is SPI?

Post  GallopingGhostler on Tue Sep 11, 2018 10:26 pm

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?

Cox International created SPI cylinders by removing existing Sure Start cylinder / piston sets from new (actually NOS - new old stock) production engines, and having a machinist modify them by removing the material between the two slits and some material at the bottom of the exhaust port. He calls them, Cox .049 Tee Dee II cylinders, because these are not original, although they function as well as the originals.

Reason for this is the original stash of SPI high performance cylinder / piston sets were sold out.

It used to be that all cylinders were SPI. Then around 1966, Cox created the forerunner of the modern non-SPI cylinder with the creation of the .049 QZ (quiet zone) engine. Here's what May 1967 Aeromodeller Magazine engine review has to say about the QZ:

Sceptre Flight Model Engine Tests, Cox 049 QZ

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.

Mufflers cause the SPI cylinder to recirculate the spent exhaust gases, further reducing power, which gave birth to an acceptable performance non-SPI cylinder for muffled engines. I found that back in the late 1970's, my muffled Cox .049 R/C Bee with this set up was more powerful than a stock Babe Bee with SPI and flew quite well even though muffled.
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What is SPI

Post  Eddy on Tue Sep 11, 2018 11:14 pm

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
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Re: What is SPI?

Post  FlipStart on Tue Sep 11, 2018 11:41 pm

Great thread!.. Never heard of SPI. I always wondered why some Cox engines would open the crankcase under the piston skirt at TDC. Huh...

Jeff
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Re: What is SPI?

Post  GallopingGhostler on Tue Sep 11, 2018 11:57 pm

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

Although lightweight, others have expressed that the OK Cub .19 because of its exhaust tri-porting around the circumference of the cylinder made it very loud compared with other engines of similar displacement.

About the only drawback to such an engine is the relative lack of control line planes able to take advantage of such power. The .35 cross scavenge (also .20 - .28 Schneurle as substitute) and larger engines seem to dominate CL circles.

And getting back to Dave P.'s question about whether polishing the piston's bypass ports would be of any advantage, that I don't know. Someone who does speed models would probably be the best ones to chime in.
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Re: What is SPI?

Post  Oldenginerod on Wed Sep 12, 2018 5:03 am

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

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.
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Re: What is SPI?

Post  GallopingGhostler on Wed Sep 12, 2018 6:13 am

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.

Recently I bought a Cox .049 OEM throttle muffler, piston and cylinder set from Cox International. This is the same type setup on my R/C Bee. Peter Chinn mentioned that the Medallions suffered HP loss by recirculating exhaust gases with the earlier throttle muffler setup on the then only available original SPI cylinder. Thus he surmised Cox's recommendation for use racing fuel to make up for the loss.

I was going to try this with my .049 Medallion. Although some have an aversion for such, I think it may help the Medallion breath a little freer and gain back some HP lost without having to resort to higher nitro racing fuel. The Medallion has the single bypass SPI cylinder akin to the Babe Bee. This set is twin bypass.
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Re: What is SPI?

Post  ticomareado on Wed Sep 12, 2018 8:19 pm

I have a very nice no/low time Cub .19 that I have cleaned inside and out and mounted on a restored Ringmaster Jr. All the literature still suggests that for its weight it is still quite a powerhouse. 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.
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Re: What is SPI?

Post  GallopingGhostler on Wed Sep 12, 2018 8:30 pm

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.
Probably not hard to understand, there is very little metal webbing supporting the cylinder between the 3 exhaust ports. Apparently the engine wasn't that popular compared with other .19's - .20's as one hardly ever sees them nowadays. Your Ring Jr. probably performs well with that engine as it's lighter weight would compliment it.
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Re: What is SPI?

Post  Dave P. on Wed Sep 12, 2018 8:53 pm

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.  The glow heads are different too, they have a circular groove in the top around the post.  The elements are much heavier and shaped like the letter "M" rather than being fine coils.  The glow is much weaker than coiled heads.  Other than that, they appear to be the same shape combustion chamber.

I'm continually amazed by the engineering that went into these little engines.  I took them so much for granted when I was a kid.  

Thanks for telling me (us) about them you guys.
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Re: What is SPI?

Post  ticomareado on Wed Sep 12, 2018 9:17 pm

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.
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Re: What is SPI?

Post  Dave P. on Wed Sep 12, 2018 9:56 pm

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.    
Already replaced them with Tee Dee heads.  I figured that was the case. What was the infringement?
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Re: What is SPI?

Post  Mark Boesen on Wed Sep 12, 2018 11:23 pm

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.
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Re: What is SPI?

Post  GallopingGhostler on Thu Sep 13, 2018 9:31 am

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.
You have the earlier one. I've found my answer for legitimacy of my proposed changes to my earlier standard Medallion .049 to RC like later ones (prior to Estes era) with Bernie's later OEM Cox exhaust throttle kit (slit cylinder, no SPI, silicon rubber muffler cover over throttle ring like the early R/C Bees).

Cox Engine Forum Cylinder Identification
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)
The freer flowing front rotary valve intake system along with dual bypass helps maintain the higher performance of the Medallion, plus compensate by removing the power robbing SPI in muffled conditions.
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Re: What is SPI?

Post  Dave P. on Thu Sep 13, 2018 9:46 am

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?
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Re: What is SPI?

Post  GallopingGhostler on Thu Sep 13, 2018 12:19 pm

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?
This is true, the throttle rings are roughly 1/2 the width of the exhaust port width and as you surmised, and because of the cylinder web width between exhaust port slots.

Throttle ring slot width compared with cylinder between exhaust slits.


Throttle ring slot width compared with cylinder slits.


Throttle ring slot width other side (with idle exhaust dimple each side) compared with cylinder slits.


Cox installation instructions, Tee Dee and Bee tanked engines shown. Please note, Spacer ring "A" is not required if a starter spring is installed.

Please remember that this throttle is not linear in response (not i.e., 1/4 open = 1/4 throttle, 1/2 open = 1/2 throttle). It is more like 1/4 to 1/3 open = half throttle.

It is not as bad as you may think. This throttle from Bernie has the later Sure Start type cylinder with an additional boost port on each of the two bypass ports. My late 1970's R/C Bee has 2 bypass ports but no boost ports, yet it's power with muffler cover on was still a tad stronger than a Babe Bee. My late 1970's Golden Bee with Ace R/C (Dale Kirn) unmuffled throttle ring was also stronger.

In contrast like with yours, my earlier model .049 Medallion has only 1 bypass port with no boost port and SPI like the earlier Babe Bees and Golden Bees.

With some restrictions caused by this type of RC throttle, a hint to prop selection is given by Peter Chinn's June 1966 Model Airplane News article, Cox Medallion .049 R/C Engine Review.

Sceptre Flight Model Engine Tests, Cox Medallion 049 RC
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.

IMO, this will limit props used to those of larger diameter, lower pitched to make use of the engines good lower end torque. You won't get the ultra screaming RPM's that some others hear love to hear with the smaller diameter higher pitched props. If you want those, then go with a later .049 Tee Dee R/C with truly carburetor. But I believe that the Medallion performance will still be improved over that of the R/C Bee because of its freer flowing intake system over reed valves, and the general nature of our RC sport aircraft, which tend to have larger cross sections, greater wing area, with more aerodynamic drag that benefit from a larger diameter prop than with half-A speed models.

For me, rather than running racing fuel, I'll go with higher 25% nitro content (to save glow heads).
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Re: What is SPI?

Post  RknRusty on Thu Sep 13, 2018 5:56 pm

Lots of good answers to the original post. GallopingGhostler explained it well. Here is a wordier explanation I wrote some time ago and is posted several times:
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.
Paul G. could answer your polishing question. He is a member here, so you can PM him
I hope that adds something.
Rusty

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Re: What is SPI?

Post  ticomareado on Thu Sep 13, 2018 7:41 pm

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
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Re: What is SPI?

Post  GallopingGhostler on Thu Sep 13, 2018 10:57 pm

Thanks Rusty for the technical details on SPI in the 2 cycle combustion process.

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  
Sceptre Flight has several articles on Atwood engines dating in the mid 1950's. I.e.,

Sceptre Flight Model Engine Tests, Atwood .049 Shriek (Jun 1956 MAN)

By May 1967, Bill Atwood was working for Leroy Cox (who along with Dale Kirn developed the Cox .049 QZ that started the improved performance non-SPI .049 cylinders).

Sceptre Flight Model Engine Tests, Cox 049 QZ

Found more info on Bill Atwood's history, Craftsmanship Museum, Other Commercial Model Engine Builders
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.

Found the same info on Wikipedia: Cox Model Engine. I guess that Wikiwand and Wikipedia are somehow related.

Bill worked for Cox and without further details (and the "•#???" unknown), it could be that the Wikiwand web page contains an error. Reason why I say this is that Mark Boesen thus far has been a compendium of information on Cox engine history. Other lawsuits, such as the Cox versus Walker bellcrank one are well known.

AMA August 25, 2011: The control line bell-crank and patent # 2292416

Since Bill Atwood already had an employment relationship with Cox and was intimate with glow head design, I don't think it would be he who initiated a lawsuit. That leaves Bob Holland, but the article doesn't clarify who. That lack of definitive information on exactly who casts a shadow of doubt with the authenticity of the claim until further clarified.
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Re: What is SPI?

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