rcnats

My Feedback: (56)

I have a Cox TD 051 with a Tarno R/C carb. The instructions say it's best to use a muffler for better idiling (sp?). Anyone using this setup? Also, what type of starters are you using? Finally, GLAD TO SEE THIS FORUM ON RCU! KEEP UP THE GOOD WORK!!

*rcnats*

robind

mabe? safety first.

David Ingham

At least the two TDs I have have "subinduction" which means they take in fresh air from around the cylinder through the bottoms of their exhaust ports. One can see light under the piston when the piston is at the top (TDC). Replacing this fresh air with exhaust gas naturally reduces power. Unless the carburettor supplies fresh air to the exhaust ports, it does not change this. On the other hand, TDs have too much power for many purposes; maybe the idle is more important for this application.

fritzke

My Feedback: (1)

I have a TeeDee .051 with the Tarno and the standard COX annular muffler. It ran very well this way.
(At least as good or better than my best Norvel Big Mig anyway.)
Don't forget, it does not take nearly as much "idle" on a 1/2a plane to bring it down.
At 5000 rpm you are probably landing!
Dave
http://www.rcuniverse.com/forum/m_28...tm.htm#2874106

lukesp

I may well be wrong but I always thought it was the pressure of the incoming charge that made the difference with SPI, not if it was fresh air or not. Anyone know?

ptulmer

My Feedback: (5)

Whether you have SPI or not, too much backpressure will kill the brute power of a Cox engine.

Yuu

luke...someone wrote on these threads that SPI lets in a 'little' more clean air...thus 'leans' the mixture for a little more RPM. Putting a muffler or exhaust throttle on the engine lets the engine 'inhale' a little more exhaust from inside the muffler..... now, let's calculate the length of 'time' that the SPI is actually 'OPEN' at 14,000 RPM....stroke about 0.400"......SPI listed on these threads at 0.010" to 0.015"...we'll use 0.012"......so 14,000 RPM / 60 sec. = is 233 strokes per SECOND....or 0.004286 seconds per stroke...but the SPI is only 0.012" of the 0.400" stroke, say 0.03 or 3% of the stroke... so 0.03 of 0.004286 seconds...0.0001858 seconds... guess 2 / 10,000 ths of a second. WOW !! Not much time at all !! So I had to try it...and it's true in my shop...the muffler w/ the SPI slows the engine and makes it run poorly. Then when you slow down the RPM with the throttle, things get really worse. Granda Pa said, "If you want something done RIGHT, ....do it yourself."

lukesp

I'm glad you did the maths Yuu, I'm terrible at it. Putting a muffler on any Cox 049 slows it down regardless of SPI or not. They don't seem to appreciate back pressure. What I heard was that the incoming charge of pressure (albeit a small one judging by the time you mentioned) punched the charge of mixture up into the combustion chamber with greater velocity and allows a greater charge of fuel/air. Interesting to think about but difficult to prove. I think your explanation will suffice

Yuu

But is it an "incoming charge of pressure"?? What pressure? The piston going 'up' causes Suction... so SPI should 'suck IN' air...just at the TOP of the piston travel...until the piston starts going Down...then it's pressure to push the fuel up into the intake ports and to the cylinder. The little engine is really a "pump" type of action, right? Suck, push...suck, push...etc.

lukesp

The piston rising causes a vacumm which is broken by sub piston induction , hence the brief inrush of air. The downward force of the piston after combustion compressess the charge until the cylinder transfer port is uncovered. If the engine takes a gulp at the point of SPI then the charge in the crankcase would be at a higher pressure than if there was no SPI, no?

I am not claiming this to be fact, far from it, I am trying to see the purpose of SPI and wether it matters if fresh air or not is a requirement.
I admit I used the term "pressure" in a lazy fashion in my last post.

lukesp

OK, after some thought I agree, fresh air introduced via SPI would be more beneficial than mixed exhaust fumes due to the fact that it adds to the overall combustible charge. It would only make the intake charge lean if the needle wasn't adjusted to compensate for the extra air. Man, I needed to get my head around that.

So, by my way of thinking there is a dual effect to SPI in the form of a small addition of air to the mixture thus allowing a greater overall charge and a small amount of crankcase filling which would increase the pump effect. Fitting a muffler would reduce the effect of SPI, but not necessarily negate it. Well, fitting mufflers certainly causes a power loss regardless of SPI, I installed one on my Black Widow and on a Babe Bee and they both suffered power loss. I hope that makes sense.

combatpigg

My Feedback: (3)

Not only is the SPI port open for such a brief moment in time, but there is also a prevailing outward going blast of high pressure exhaust gas getting blown away from the engine at 300-400 times per second.

It is like saying that working out an hour a day will make you lose weight because that is one less hour while you are awake that you could be eating instead. I use this analogy because I'm not sure if the real reason why SPI seems to work with TDs is understood.

I think SPI exposes the crankcase to more negative pressure than what the engine can generate without SPI, which causes a slight supercharging effect for the incoming mixture. This effect has a prime rpm range where it enhances performance. It also must not work so well with mufflers because the pressure differential between the case and atmosphere is lost, then the SPI port acts more like a crankcase pressure leak.

combatpigg

My Feedback: (3)

I would like to see a non SPI TD fitted with a clamp on exhaust header that closes off one of the exhaust ports. Below that closed off port, grind a bypass port into the liner. Now take a Norvel muffler bowl and attach it to the home made exhaust adapter and see what you come up with.
I'll bet this idea [Norvelizing a Cox] would take some of the bark out of the exhaust tone and not hurt performance as much as the stock cox muffler.
Sounds like a good project for Patrick.

ptulmer

My Feedback: (5)

CP, I've got a header on the drawing board, fer sure. Looking at the numbers for making a pipe show an exhaust port area more than twice as large as a normal engine, so you have a very good idea there. I was just going to close one off, but adding another bypass... Now, how in the heck did they grind those in there? That is a mystery to me.

Yuu

I don't see that working. Take a Medalion cylinder and piston...no SPI...and put it on a TD...like the control-line stunt guys do for smoother runs and easier starting; then block off one of the exhaust ports...okay so far; then grind a little slot below the blocked port and attach a Novvverl muffler bowl....ya lost me here! The piston only comes down so far...the top of the piston is just a little higher than the bottom of the exhaust port we just closed... so the piston skirt is blocking the new little slot we just made, so NO EXHAUST...then the piston goes UP...suction into the case from the front venturi...and as the piston goes UP...BEFORE the piston gets to the top... the suction SUCKS IN from the new little slot at the Norvvvel muffler.. then the piston goes over the top, and starts back down...pumping the intake charge in the crank-case OUT the Norvvvel muffler slot.... then the intake ports open and the Norvvvel slot closes and what's left of the fuel charge goes UP into the combustion chamber...?? Something's not right. But, YES...I'd like to see a Norrvvll type muffler for the Cox engines with a tube to get the slop away from the plane. It would be worth a thousand RPM for us sport flyers for closing off one exhaust port and fashioning a Norvvlle muffler and tube for the other exhaust slot to get the slop away. Let's keep thinking on this... I see another experiment coming my way. Now where's that aluminum cigar tube...

drayegon

There is a guy by the name of Wild Canyon or id or something like that on Ebay that was making Mufflers for several versions of both reedys and TDs and medallions I do know him have nothing to do with him but he has the mufflers you might want. eith that or take one from a Queen bee or an RC09 which is a TD with a carb and muffler so which ever but part of what you are doing is just back wards give it some more thoguth you almost have it there. dray

David Ingham

I have a Flight Streak, modified to take a beam mount, with a TD with a Black Widow cylinder and piston. The cylinder has no subinduction, and it works fine with a muffler. It runs very differently from a stock TD: the needle valve works as a governor. It flies at a steady rate, up and down, instead of always full out like a fully fledged TD.
I notice that my Pee Wees have a bit of subinduction, but I am looking for a muffler anyway.