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SPI or Free Porting

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SPI or Free Porting

Post  wmazz on Mon Dec 31, 2018 9:11 pm

This is part of an old thread.

Cox International;10451625 wrote:
The SPI piston was very hard to start and didn’t compare with the stock piston BECAUSE it was cut too short.
The SPI gap with the modified piston in either the stock or modified cylinder was almost “three times” greater than
that of the stock TD piston and cylinder. Too much air was allowed under the cylinder. I also think the large gap
affected the engine’s timing.

I am not sure SPI actually does much based on my testing.

When the "SPI" is too large, the manifold vacuum that was built up inside the crankcase is released,
and airflow through the venturi significantly decreases.

If the engine uses a venturi to draw the fuel into the engine, there is an insufficient amount air being pushed
through the venturi to create the low pressure used to draw fuel inside the venturi, unless an electric starter
is used. (In some cases an electric starter will not help).

If the engine uses a bladder, too much fuel will be delivered to the venturi, and not enough air will be pushed
through the venturi, unless an electric starter is used.

SPI or "Free Porting" works on all 2-stroke engines. It works because of outside pressure being exposed
to the crankcase. That outside pressure pushes inert gas into the crankcase and prevents the loss of fuel-
air mixture out the exhaust port, that 2-stroke engines are famous for. With SPI more fuel is burnt and converted
to power.

Without a muffler it works with atmospheric pressure, and requires the piston to momentarily expose the crankcase.
With a muffler, the pressure is greater, and so it may require the piston to overlap the bottom of the exhaust port.
But just because the exhaust port is closed, doesn't mean that the bottom of the piston is sealing the crankcase
from the pressurized exhaust.


Bill M.
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Re: SPI or Free Porting

Post  GallopingGhostler on Mon Dec 31, 2018 10:21 pm

wmazz wrote:SPI or "Free Porting" works on all 2-stroke engines. It works because of outside pressure being exposed to the crankcase. That outside pressure pushes inert gas into the crankcase and prevents the loss of fuel-air mixture out the exhaust port, that 2-stroke engines are famous for. With SPI more fuel is burnt and converted to power.

Without a muffler it works with atmospheric pressure, and requires the piston to momentarily expose the crankcase. With a muffler, the pressure is greater, and so it may require the piston to overlap the bottom of the exhaust port. But just because the exhaust port is closed, doesn't mean that the bottom of the piston is sealing the crankcase from the pressurized exhaust.
Actually, what SPI does is allow a fresh puff of air with oxygen in it to equalize the slight vacuum that occurs when the piston moves upward to draw in the fuel air mixture from the venturi into the crankcase. This increases the fuel air charge density that increases power. Adjusting the needle allows more fuel to be drawn in to compensate for the additional puff of air added by the SPI.

If the SPI gap is too great, more air enters in, but by the same token the amount of fuel air drawn through the venturi is prematurely terminated, limiting the potential amount of fuel in the air. In essence will run too lean.

SPI is not good when muffled, because the inert (by inert I mean combusted products do not support ignition) exhaust gases are drawn into the crankcase, diluting the quantity of air-fuel for combustion, reducing power. A non-muffled exhaust restrictor ring for throttle will work in idle, because it will increase back pressure, but works when opened because SPI is still allowed to do its job bringing in a fresh puff of air.

The earlier Medallion .049's, .09's and .15's had a single bypass port with SPI. The muffled RC version (and earlier slide exhaust restrictor) as done by Cox had reduced power by recirculating exhaust gases from SPI. Thus Cox recommended that racing fuel with a higher nitro content be used to help make up for the loss of power.
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Re: SPI or Free Porting

Post  Mark Boesen on Mon Dec 31, 2018 11:00 pm

I do think people put way too much importance into SPI, that's about the last thing i look at...
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Re: SPI or Free Porting

Post  wmazz on Tue Jan 01, 2019 1:45 am

GallopingGhostler wrote:
Actually,

My first post here, and someone thinks he knows it all Sad
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Re: SPI or Free Porting

Post  Oldenginerod on Tue Jan 01, 2019 4:11 am

wmazz wrote:
GallopingGhostler wrote:
Actually,

My first post here, and someone thinks he knows it all Sad

Actually, there are people here who know plenty, including George. The good thing about this forum is that everyone gets a chance to express an opinion, or share what they know. The more knowledge which is shared, the more we all learn. The trick is to be understanding of each other and tolerant of differing opinions.

We welcome anyone, including yourself, who wishes to contribute to our discussions. For your "first post", it's not a good start to our relationship for you to criticise a fellow member for sharing some of his knowledge. We try to keep it pretty friendly here.

Rod.
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Re: SPI or Free Porting

Post  getback on Tue Jan 01, 2019 6:57 am

What the heck are you all talking about ? Does your engine not run or you just don't get the performance you want? About time for a tach race !! Small Cox Logo Babe Bee .049 Small Cox Logo
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Re: SPI or Free Porting

Post  fredvon4 on Tue Jan 01, 2019 11:49 am

wmazz

Sorry but George does NOT know it all....OTOH I do....grin

and I say....hello sir welcome to CEF ! The friendliest web forum there is

Over the years we have held many Tach races...we all send our entries to one poor sap who has to catalog inventory mount to a test stand, attach the official prop, add the official fuel and try to start the dinky buggers and record and video the engine run

many of us got to great lengths to play with compression, piston fit and adjust SPI in search of the Holy Grail....max RPM with out grenade-ing..... one of mine grenade-ed BTW

This Site Rocks! Welcome Welcome Welcome What I Meant To Say Thank You for being a new member
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Re: SPI or Free Porting

Post  GallopingGhostler on Wed Jan 02, 2019 1:28 am

wmazz wrote:
GallopingGhostler wrote:Actually,
My first post here, and someone thinks he knows it all Sad
I'm sorry you feel that way, Bill. This is a discussion forum, we all come from differing experience levels and background levels. We even have a retired Cox engineer, Larry Renger occasionally contribute to discussions.

You were doing fine in your explanation, then your use of the word "inert" didn't fit in the discussion.

SPI admits the ambient atmospheric air, which is reactive because it has oxygen necessary for combustion. In Chemistry, "inert" means "having little or no ability to react, as nitrogen that occurs uncombined in the atmosphere." This is why I clarified. Exhaust gases fit the definition of "inert", because CO (carbon monoxide) and CO2 (carbon dioxide), the resultant products of ignition can no longer support combustion.

It is not that we know more or less. We all learn from each other's experiences. Here's what one model engine authority, Peter Chinn, had to say about SPI.

Sceptre Flight Model Engine Tests: Cox Medallion 15 RC

Peter Chinn, Dec 1964 Model Airplane News wrote:On the Medallion 15, as with other Cox engines, the exhaust outlet consists of two diametrically opposed ports in the cylinder wall, there being no exhaust stack to which a coupled exhaust restrictor could be fitted. The Throttle Control conversion has, therefore, necessitated the fitting of a special collector housing around the cylinder, into which spent gas is exhausted and its escape then controlled by a sliding bar type restrictor. Inevitably, this must cause some back pressure even with the restrictor in the full open-throttle position, but, more significant, it also means that, as the standard Medallion features sub-piston supplementary air induction [SPI], gas remaining in the exhaust collector during the 46 degrees of shaft rotation for which the piston skirt is clear of the bottom edge of the exhaust port, is liable to be drawn into the crankcase thereby diluting the fresh charge.

This is a problem facing all manufacturers of engines using sub-piston supplementary air induction when such engines are converted to R/C use. Doing away with the coupled exhaust valve is not the answer, since an intake throttle, on its own, is very much less effective when, with the throttle closed, an engine can still leak air into its crankcase via the exhaust port. One solution is to use a longer piston skirt which will prevent the primary compression chamber being exposed to atmospheric pressure. This, obviously, is only practical where a special R/C model is produced.

Having made the point that converting an engine of this type to throttle-control it not so easy as with an orthodox loop-scavenged, single-stack engine without sub-piston induction, we must now remark that, within the limitations imposed, we feel that Cox have done a good job on the Medallion 15.

First, one must remember that, although a drop of 30 to 40 percent in power sounds rather a lot, The Medallion 15 starts off with quite a good power output for a sport type engine (our test on the standard Medallion 15 indicated a maximum bhp of .255 at 16,000 rpm on Cox glow fuel). Secondly, the drop in power is mostly due to reduced top end power (the bhp peaking speed being reduced by more than 4000 rpm) so that the actual loss of output on the props likely to be used for R/C models is not quite so noticeable.

Personally, due to SPI losses, the Medallion .15 R/C is comparable to say, the cross scavenged .09's and .10's like the Enya .09-III TV and OS Max .10R/C. Since they have a true RC carb that is more linear, I'd prefer them as the Medallion 15's HP is reduced to that level and they are better throttling as well as lighter in weight.

I am a bit surprised that as your first post, you picked a very technical topic to discuss without introducing yourself. We are like a family here, and like I said earlier, we come from all different backgrounds, some hobby and some professional. We don't think of one as more highly than the other, we share and share alike. We benefit from each other's experience, even have a laugh a time or two.

So, I hope you don't feel offended by my post. We aren't a vindictive or domineering forum, but quite the opposite.

- George
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Re: SPI or Free Porting

Post  Cribbs74 on Wed Jan 02, 2019 2:05 am

Bill,

Welcome to the forum, it’s good to have you. If you have the time post an introduction so we can all get to know you a little better.

This forum rarely has any problems that other forums may have. Mostly because we are pretty tolerant of everyones opinions. That’s not to say we don’t debate from time to time. Differing opinions are good as well as the healthy debate that follows.

Again, thanks for signing up and we all hope to hear more from you!

Ron
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Re: SPI or Free Porting

Post  wmazz on Wed Jan 02, 2019 2:33 am

I know how to be respectful among a forum, especially with tech topics.
When I disagree with someone, I say so: "I disagree with Name." I don't
begin with a word like "actually" that suggests there is only one answer.

Perhaps I should have used the title of "SPI or Free Porting as a modification"
I came across a thread where someone from Cox Int. was using SPI as a
modification, but clearly he didn't realize how sensitive the modification can
be. I thought this forum was part of Cox Int.

I use SPI or Free Porting currently on a 800cc racing engine. I have used
(or tried it) it on many engines, but I used it the most on small to large
Zenoah Hobby engines. I used to manufacture pipes for the small Zenoah
engines (and clones) from  23 to 45cc.

Zenoah uses a version of SPI on a few of their "Strato Charged" reduced
emissions engines for use in California. But Zenoah uses a 2 barrel carb
that routes clean air instead of exhaust underneath the piston.

The 2 stroke engine builders called it "Free Porting" because of the gains
in power they got after they accidentally installed a piston backwards with
the intake skirt facing the exhaust port.


Bill M.

Everyone here probably knows more about Cox engines than me. But I have
been aware of tuning problems related to vacuum leaks for a very long time.

I need help with understanding crankshaft rotation directions, and hopefully
the cylinder port specs to my 73' & 74' .049 TD cylinders, and find some old
engine and carb parts for a F/F.
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Re: SPI or Free Porting

Post  Cribbs74 on Wed Jan 02, 2019 12:42 pm

Hey Bill, no worries.

So as to the crankshaft rotation. Cox only provided two different style cranks for the TD.One that allows counterclockwise rotation and one that allows clockwise. Counterclockwise being the most common.

Of course a reed calve can run either direction if you have any of those.

What cylinder specs do you need exactly?
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Re: SPI or Free Porting

Post  fredvon4 on Wed Jan 02, 2019 1:15 pm

Some back ground cuz I read all 5 forums

WMAZZ necroed a long old thread on RC Universe where Bernie AKA Cox International was discussing how and why he was reproducing Black Widow and TeeDee cylinder piston sets--- by machining open the twin exhaust slits to make back to one port each side...

The discussion turned to SPI and Bernie mentioned the various pistons had different length skirts so the opening of the exhaust slits was kinda critical beause some combinations caused TOO MUCH SPI this was all back around April of 2011

Dec 31 2018 MWAZZ adds in his take on Free Porting..on the RC site ,,,,then Joins CEF to make the same observation and opinion

We all can discuss and or argue all about free porting and all manner of other ways to run 2 stroke engines including our use of different pistons and shims under the cylinder to lie up exhaust or gain or reduce SPI, even heads and gaskets to chase compression and timing....there is an amazing amount of variability in adjusting such a simple 3 moving part Engine

Seems Bill has tuned pipe experience so he would probably be fascinated with our member who does this to Cox and other dinky engines with a lot of success....

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Re: SPI or Free Porting

Post  balogh on Wed Jan 02, 2019 2:18 pm

Just to chime in here with my limited 049 TD shaft information...apart from the direction of rotation, especially the vintage TD shafts may also differ in their thickness of the web whereas later TD-s came with thicker webs, indicated by a No 2 stamped on the crankcase mounting tab..so a later production
shaft if put into an older case w/o the No 2 stamp may end up with its crankpin touching the backplate..at least expectably. For example the one on the top is a vintage pre-72 engine and the bottom is a later production with the No2 stamp on the tab.

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Re: SPI or Free Porting

Post  roddie on Wed Jan 02, 2019 6:30 pm

Cox OEM engine mufflers generally quiet-down the engine.. but don't offer much else. I designed a muffler for my Cox engines that has the capability to do a lot more.. IMHO... Smile Traditional mufflers are an SPI killer.. but my design has infinite adjustability.. and the option for a pressure-tap.

See this thread.
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Re: SPI or Free Porting

Post  Oldenginerod on Wed Jan 02, 2019 7:59 pm

wmazz wrote:
I came across a thread where someone from Cox Int. was using SPI as a
modification, but clearly he didn't realize how sensitive the modification can
be. I thought this forum was part of Cox Int.

I guess this is an easy assumption to make, but while Bernie (owner of Cox Int.) is a much-respected member and supporter of this forum, that's where the connection ends. He, along with Matt from Ex Model Engines, is our main source of new Cox parts.

Many of those parts are not original Cox and are either current production replicas, or modified original old-stock Cox parts made to try to replicate those parts that are no longer available. Cylinders and pistons are a case in point. Where the anomaly occurs, and may have been the source of Bernie's frustration, is where a piston skirt has been cut short to allow SPI in a standard non-SPI cylinder. The main source of new cylinders is SureStart cylinders, which have double exhaust slits and no SPI. The transfer ports in these cylinders are quite good and they performance-mod really well in my experience. Because of the narrow exhaust slits, the design is not optimal for using a shortened piston. Therefore, the cylinders are modified by cutting the "bar" from between the exhaust slits and machining the lower part of the port to allow SPI with a standard length piston. A real problem occurs if you try to use a combination of both the shortened piston and the lowered exhaust port. Maybe this is what Bernie was trying to use in your original reference to his post about the topic. The "double whammy" of piston and cylinder modifications means that the timing gets all messed up and the downstroke of the piston is effectively reduced in duration as the crank needs to move a number of degrees after TDC before the port is closed off and mixture transfer (or crankcase compression) begines. In effect, less air/fuel charge can be transfered due to the shortened duration of the transfer stroke.

Merely my opinion, but based on my experience building my Tach Race winning Silver Bee reedy using one of Bernie's modified cylinders, which added up to 6,000 RPM, as well as knowledge gained after many years as a mechanic and dealing with the important matter of valve timing.

Rod.
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Re: SPI or Free Porting

Post  gkamysz on Wed Jan 02, 2019 8:23 pm

I'm not sure what the "2" indicates. I have two cases without a "2" and they fit the crank with the thick web.
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Re: SPI or Free Porting

Post  944_Jim on Wed Jan 02, 2019 9:20 pm

Umm, I have no problem with "inert" in context. I re-read the paragraph referencing "inert" and noticed the reference parked neatly next to the use of a muffler.

GG said "SPI is not good when muffled, because the inert (by inert I mean combusted products do not support ignition) exhaust gases are drawn into the crankcase..."

If I have this correct, I strongly suspect there is practically no usable fresh air in the muffler to be reintroduced to the engine via SPI.

Had "inert" been used in context to open exhaust, ie no muffler, then "inert" would be incorrect...useless exhaust gas introduced needlessly.

I kind of think of SPI and mufflers being more like a leaky exhaust gas recirculation (egr) valve on a carbureted gasoline automobile engine.

Laughing
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Re: SPI or Free Porting

Post  Mark Boesen on Wed Jan 02, 2019 10:07 pm

....how 'bout them Bears!
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Re: SPI or Free Porting

Post  Dave P. on Wed Jan 02, 2019 10:29 pm

gkamysz wrote:I'm not sure what the "2" indicates. I have two cases without a "2" and they fit the crank with the thick web.

I'm not an expert by any means, but I've heard that the "2" stamp was used to differentiate the different crankcases on the assembly line as Cox converted from the Medallion and early Tee Dees to the later version of the Tee Dee with the thicker crankshaft web.
As I understand it later in production, the 2 stamp was eliminated as the earlier version crankcase was no longer in the assembly boxes.  

This is a picture of the two different crankshafts in their specific cases.  The bottom case would be a 2 but it's enough later to not have the stamp.

The earlier (top) crankshaft will work in the later crankcase with a bit of end play, but the thick web crank won't work with the early case.  The drive plate binds on the nose of the crankcase.  Harder to see is the width of the bearing face on the cases, but the difference is there, the same as the web thickness.

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Re: SPI or Free Porting

Post  Cribbs74 on Wed Jan 02, 2019 11:15 pm

Good illustration, thanks Dave.
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Re: SPI or Free Porting

Post  balogh on Thu Jan 03, 2019 1:50 am

gkamysz wrote:I'm not sure what the "2" indicates. I have two cases without a "2" and they fit the crank with the thick web.

I read it somewhere here the later production thick webs were assembled with the crankcases having the No2 stamp. Maybe they fit the cases without the stamp but the crankpin is then off of the piston centerline?
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Re: SPI or Free Porting

Post  Dave P. on Thu Jan 03, 2019 8:36 am

balogh wrote:
gkamysz wrote:I'm not sure what the "2" indicates. I have two cases without a "2" and they fit the crank with the thick web.

I read it somewhere here the later production thick webs were assembled with the crankcases having the No2 stamp. Maybe they fit the cases without the stamp but the crankpin is then off of the piston centerline?

I guess it would have been more accurate to have said "fit" instead of "work".  The only dimensional difference in the early crankcases and the later ones (earlier versions of the later design stamped 2, later ones are not stamped) is in the distance from bearing face to the nose, from my measurements.  The overall dimensions of the two different crankshafts also are identical except for the web thickness, which is added to the front of the webbing, making the forward part shorter on the thick webbed crank when measured from the front of the webbing to the nose.  So you're right, the early crankshaft will fit in the later case, but the back of the crankshaft and the crank pin will be too far forward and won't line up properly under the piston.

I hope I got this right, it's all from my own measurements of the parts I have coupled with what I have learned here.  If not, someone please correct me.
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Re: SPI or Free Porting

Post  balogh on Thu Jan 03, 2019 8:54 am

Thanks Dave for confirming my assumption with measurments. I guess with full interchangeabiliy the differentiating No2 stamp would have been unnecessary.


Last edited by balogh on Thu Jan 03, 2019 8:58 am; edited 1 time in total
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Re: SPI or Free Porting

Post  gkamysz on Thu Jan 03, 2019 8:55 am

I suppose it makes sense as a temporary marking to allow distributors to move older inventory. Did the thick web revision also occur in Medallion cranks?
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Re: SPI or Free Porting

Post  getback on Thu Jan 03, 2019 10:34 am

I just looked and i have a medallion with the 2 stamp but don't know about the CS/ and also noticed i have a medallion with a 51 piston in it Huh... , These are some i bought a long time ago. Very Happy
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