propjobbill

I made a modification to my Cox .049 surestart engine, It now turns an APC 5.7 x3 prop at 15,500 through 16,000 rpm is this much better than stock performance? All I did was to enlarge the intake venturi

combatpigg

My Feedback: (3)

That is enough power to have some real fun. I think .140" is about as large as I dared to hog out a SS venturi to. A KillerBee has more like .180" diameter.

propjobbill

Well maybe I made it too large it was enlarged to .1875 But so far it seem to run more even than when it was stock.

skaliwag

My Feedback: (1)

Now how about a tad more SPI by taking a smidge off the botom of the piston. This would also make the piston lighter.

combatpigg

My Feedback: (3)

Yes, the piston mod really puts these engines over the top. A big venturi like .1875" wont be the limiting factor in your quest to build a 30,000 rpm Surestart! Should work well with a 4.2x4 prop.

KidEpoxy

Is this gonna need frequent Socket Reset Tool lovin when it gets up in screamer RPMs?

skaliwag

My Feedback: (1)

Yep... They need their "Cod piece" whacked on a regular basis when they get into those higher plane #'s.

propjobbill

At this point I am not trying to get top rpm. When I started it was around 13,000 rpm. With the new rpm that gives me an extra 3 to 4 oz of thrust and 8 = 10 mph faster. I wouldn't know what to do with 30,000 rpm. I am not familiar with what you are talking about on the piston. The engine seems to be screaming now. I wouldn't mind a couple thousand more rpm. But I don't want to have to take the engine off every month to reset the piston. As it is now I will check it after about one or two hours of flight, which won't happen for me until spring. The extra rpm that I am getting now will realy help on climb rate.

Please don't misunderstand me, I am all for getting more out of the engine if there is a easy way to do so. However I am not at this point into racing or combat. But more power for sport and areobatic flying would be great. What I don't want to do is get it going so fast it snaps the crank or something like that.

combatpigg

My Feedback: (3)

The Estes generation engines seem to loosen up easier than the earlier Coxes did.

DeviousDave

[scribbles notes furiously]

ProBroJoe

Hal Howard wrote a nice article on improving reed valve performance - I picked it up and several other "hop up sheets" over at the Yahoo Group [link=http://groups.yahoo.com/group/049Collectors/]049Collectors[/link] - anyone serious about 1/2A engines should check it out. (There's also a few links to Leo Squires' and Bob Beecroft's modded cox engines that'll make you drool.)

OH, BTW, that kind of RPM outta a SureStart on a 5.7x3 ain't too shabby...[8D]

combatpigg

My Feedback: (3)

There was a one panel cartoon years ago that showed a scrawny 90 pound weakling standing next to a perfect physical specimen type gym coach. The punch line has the weakling saying very sternly to the coach, "I'll go along with this program and do what you say, but I don't want to become muscle bound. "

That said....I don't think there is any level of performance with a good running Surestart where you can set the rod play initially but not check it regularly and expect any good to come from that.

I was mildly joking about trying for 30,000 rpm.

The piston mod is to thin the skirt down to paper thin and to shorten the skirt enough to net about .008" of clearance between the lower edge of the exhaust ports and the bottom of the skirt. Lightening the piston will also take some pressure off the ball socket, but still expect to do regular P.M.

propjobbill

I was not even aware that setting the piston was needed or how to do it until joining and reading this fourm a few years ago. This fourm has been a wealth of information for me. Now the first thing I do before running a new cox engine is to set the piston, and I do check them from time to time. With these extra rpm I will check more often.

Years ago if I would have known about piston reset some of the engines that broke for me would still be running. One post that is now on this forum talks about a Sure Start that ran great until. I can relate to that because a few years ago I bought a diesel head for cox .049, and after a couple of very powerful flights mine ended up the same way.
It was from reading that post that I learned that you can buy (I guess) a beafed up crank from Davis.

The more I read and ask question is the more I learn. I learned about increasing the venturi from reading one of these fourms. THANKS TO ALL YOU GOOD FOLKS THAT ARE WILLING TO SHARE YOUR VAST KNOWLEDGE WITH OTHERS: THANKS

Wayne C

In reference to SPI. SPI means sub-piston induction. This is the .008" clearence between the bottom of the piston skirt and the bottom of the exhaust port when the piston is at top dead center that CP mentioned. IT WORKS. Many Cox engines came from the factory with this feature. You can tell by putting the piston at top dead center and looking at the exhaust ports. If the engine has SPI you can see a gap between the piston and the lower edge of the exhaust port. In theory, as the piston travels upwards it draws fuel air mixture into the cylinder. In a sure start this means pulling the mixture through the reed. The airflow must have some restriction so that this motion also draws fuel through the needle valve and results in air fuel mixture rather than just air. Unless the venturi is perfect, there will still be some "vacum" in the cranckcase when the piston reaches the top. SPI allows atmospheric pressure to finish filling the crankcase with air thus increasing the charge available to the combustion chamber -- sort of a poor man's super charger. I don't see how this has time to happen at the RPM level of these small engines but it must because you can notice a difference between the engines with SPI and those without it. Of course the needle has to be adjusted to compensate for the extra air so the result is a greater volume of fuel and air being pushed into the combustion chamber which means more preassure, more heat, more expansion, and more power. It seems to me that as venturi size is optimised SPI may become less of an asset. On some engines with really large venturis there isn't enough suction to draw the fuel and some sort of preassuerised fuel delivery is needed -- usually mufffler preassure or a bladder. On these large venturi engines with pressurised fuel systems SPI may be no benefit at all.

propjobbill

Wayne, Thanks for the insite on SPI. I checked out the surestart in question. I didn't take it apart but when the piston is at top dead center there is no light under the piston, meaning the bottom of the piston is below the of the lower exhaust port. So I guess it doesn't have SPI. I bought several of these engines, and so far all of them seem to be alittle hard to start. up until a few years ago it had been years since I started Cox .049 engines. To my rebemberence I use to squirt a little prime into the piston port ant one snap and it was running.

Whith these engines they seem to have good power compared to most of the older Cox engines I had (mostly Baby Bees). I like the chock tube because it eliminates priming and now days everyone wants mufflers on the planes to fly at club fields. And with the cox muffle squirting in a prime is not so easy. These engines that I have now fire right up, but I was finding it easier to open the neddle wider for chocking and then having to turn it back in after the start. Both before and after making the vinturi larger they were not as easy to start as I would like. But it seams to me like once it is running now it runs much smoother . But so far it seams to start better if I slightly chock it when starting. I have thought about putting a spinner on and using an electric starter

But if I am understanding what you are saying, if the bottom of the piston is shortened to where there is a slight gap .008 it should get more rpm. I have noticed of many of my older engines with a single exhaust on each side there was a gag at the bottom of the piston when it was at TDC.

gcb

Bill,

Nothing is free. If you open up the venturi for more power, you also decrease fuel draw. That means you need to place the tank as close as possible behind the firewall. When you start the engine, use the choke tube to allow you to pull fuel into the fuel line. You need to position your tank height so the fuel does not run back into the tank after choking, but you also need to watch out for the fuel flooding into the engine.

One of the reasons the tank mounted engines start easy is that the fuel is close to the venturi, so it chokes while running off the prime.

If you increase venturi diameter, you also should make sure you use a smallish prop and allow it to rev up. This will increase venturi flow velocity which in turn will increase fuel draw.

I'm told that one of the "tricks" that Leroy Cox used was to teach the assemblers about parts fits, especially the piston to cylinder fit. The assemblers would sit there with a pile of pistons and cylinders and match them up for best fit. That's why they were sold as sets...and why they required very little break-in.

I don't think sub-piston induction will help much unless you are racing. The idea is to kinda super-charge the amount of air in the crankcase to it will push more volume into the cylinder when the piston goes down. If you are using a muffler, often it is exhaust rather than fresh air that gets induced.

Good luck with your Sure Starts, I like mine.

George

tewitt1949

Where is the cheapest/best place to buy surstarts? I've seen them for as little as $6-$7 but I don't remember where. Quite often about $10. Any link/help is appricated. Tery

Andrew

[link=http://www.coxmodels.com/prodinfo.asp?number=008901]COX Models[/link]

tewitt1949

Thanks but it won't work for me. I'll try it later. Terry

propjobbill

George, Thanks for the advice, I am finding out much of what you are saying, but there is so much I don't know that I love to here from others. I got my first Cox .049 in 1966 but have not just flown .049 engines. Over the years I have learned a lot about these engines, but I still have a lot to learn. I likewise like these small engines. I never thought about it until you said something about the engines with the tank attached. Those were the ones that would start on the first flip. I'm going to take the tank thing in to consideration from now on.

Andrew

Terry --

Link is up for me -- just try www.coxmodels.com and choose Classic Cox from the menu selections on the left.

propjobbill

I just came in from starting the Surestart. I found that if I pointed the engine downward so the fuel would flow toward the engine that the plane starts much better. not always on the first flip but usualy within 3 trys or less. I put a 7x3 prop on the engine and I don't think it will run unless I was to presureize the tank. But all in all I'm willing to make a little sacrifices for the extra power I was able to get.

I fine tunes the engine and It was giving 16450 rpm with the 5.7 x3 APC prop, I don' think I would ever run it there unless I had presure on the fuel tank, because it was running leaner than I like to run, and would more than likely lean out and die in flight.

combatpigg

My Feedback: (3)

PJB, with the big intake, this engine will most likely do its' best work with a 5x3 prop and fuel with 20-25% oil. With the big intake and no fuel pressure the engine will be sensitive to the G forces on the fuel system, keeping the rpms up [with a low prop load] will help because that is where the fuel siphon action is at its' best.

propjobbill

Hi Combatpig, I have already considered trying a 5x3 prop but that will have to wait until after the holidays. I have one 5x3 prop but it is 3 blades and I thing with the third blade the 5.7x 3 prop will proably be easier than that. I do have 2 older 4" props off WenMac engines, but I don't want to use a prop that small. I have not yet tried them, but I am afraid they will give me more rpm than I want. I may be wrong, but I have had cranks snaped before and I'm thinking that with the speed I will get out of those 4" props not only will the rpm be very high, but the plane may fly faster than I want. So I will be ordering some 5x3 props after Christmas.

I can remember a few years ago when you did thrust test on a Baby Bee engine, and from what I can remember about those test with a 5x3 prop I may be getting close to 19,000 rpm, and I know a 4 " prop would have more rpm than the 5" prop.

I just went back and checked that post it was called Thrust o meter and the engine you were using was a TD .049 (I think) and on that post the APC 5.7x3 prop was the best one there. I tried those props, and I really like them from that time on they are just about the only prop I buy for .049. But because I think you may be right I am going to get some APC 5x3 props. I have tested different props but it seams like APC must make the best overall prop. What I don't like about APC is that they break too easy.

gcb

propjobbill,

To run a 7x3 with consistency you would need to decrease the intake size again or run pressure. There is also the timing factor. Engines are set with somewhat narrow parameters to run correctly. Normally a stock Sure Start should not run larger than a 6x3. To make the 7x3 work, you should play with additional head gaskets and different nitro contents. All other things being the same, switching to a 7x3 will effectively give you timing that is too advanced and cause the engine to overheat.

Also, if you are running a 5" prop you should get a piston reset tool because the high RPM will loosen the ball socket on the piston. This is considered standard care and feeding for higher RPM running.

The Sure Start will swing a larger prop than the older Babe Bee because the Babe Bees had a single bypass port. This provided less power but easier handling (trade off again).

George