indoorff

My Feedback: (2)

Hey everyone -

I am setting into modding a surestart to up the performance, and I want to lighten the piston, as well as relieve material from the big-end side of the crank like the killer bee, and take some weight off the counter weight on the other side. But - how the heck do you go about re-balancing the crank when all this has been done?

Thanks
Chris

combatpigg

My Feedback: (3)

Chris, first of all, there are never any problems until a problem proves that it exists. In other words, lighten the piston, bolt on the appropriate high speed prop, and run it in your hand. If the plane tends to make your hands buzz, then take a drill bit or ball end rotary file and put a divot in the counter weight of the crank. Then run it again. Go easy on the amount of counterweight material that you remove, it's easy to take more, hard to put back. There is an egghead way of doing the balance job, but it requires a microgram scale.

indoorff

My Feedback: (2)

Sounds reasonable CP - I do have a scale that measures accurately to about 2-3mg, but only weighs objects up to 1.4g max. You are probably right, I will just run it and tweek it as needed. But for the record, how would you do it properly?

EDIT - GEEZ these cranks are tough steel. Are they hardened? I better find out before I go and use my dads good tools on it

Chris

combatpigg

My Feedback: (3)

I don't recall ever having to rebalance a COX engine that got the piston lightening treatment. The target rpm for these engines have all been over 20,000 with a prop that would allow such revvage. Make sure you do a good ball socket reset before doing the piston work, then recheck it after the work is done. You are shooting for zero slop with no binds.

indoorff

My Feedback: (2)

OK so what am I doing wrong? I have done the killer bee mod to the crank, lightened the piston to 2.8g, added a galbraith head, filed away the double slit exhaust to one big one like the older engines, shortened the piston skirt for sub-piston-induction, and reset the socket for zero slop, and I have not even gained 100 RPM! I tried switching from 15% to 35% nitro and STILL NO CHANGE!! constant 18000 RPM, exactly the same as before I started the mods! I am not impressed.[:@] Maybe my tachometer is to blame (wishful thinking I think). Any ideas?

Chris

ptulmer

My Feedback: (5)

Is your tach limited to 18k? Are you outside or inside? Using a flashlight for a light source? There's more things that can give you a bogus reading than will help give a good reading.

indoorff

My Feedback: (2)

PT - I am outside, inside you just read the frequency of the AC lights. I am using sunlight, but not direct sunlight, because that seems to give very erratic readings. I do not beleive the tach is limited to 18000, although maybe that is a possibility. It is an older tach somebody gave me so I dont know the specs. Its a Skyward model 70349002. I will see if I can find any details on it.

ptulmer

My Feedback: (5)

Iff, I assumed you had looked at the obvious, but you did say 18k even... I've never ran an engine that turned the same RPM run after run without deviation. Sounds like the tach to me, but if you figured out how to get super-steady runs, that's worth something too!

indoorff

My Feedback: (2)

I will have to try a new tach. This one is also starting to lose segments in the display, it is a real antique. I cant find any info about it online at all.
I also thought it was really strange when I got no change in RPM at all switching from 15% to 35% nitro.

Chris

combatpigg

My Feedback: (3)

A tach is very helpful, but you don't need one to tell if a mod has paid off or not. If after doing a mod, you are forced to OPEN the needle to keep the engine from running too lean, you have gained horsepower, if you have to close the needle to keep it running, you have done some harm. It's that simple. I have never messed with the twin exhaust slit of the SureStart, can't say I endorse that kind of work, I think Cox Sure Start engines have enough exhaust port area already. If you are trying to do hop up mods with a 6 inch prop, you are beating a dead horse, either run a 5x3 or APC 4.2x4. An engine that is ready to be tweaked has almost no cylinder leakage, the prop flips smartly with a crisp pop. The lower end fit is loose enough to allow the prop to bounce back and forth wildly between forward and backward compression strokes, but not so loose that there is axial play. It's a fine line. Inspecting the piston and liner surfaces under magnification can reveal a lot also. lightening the piston will allow rpm, but not guarantee rpm if the above conditions aren't met.

indoorff

My Feedback: (2)

Thanks for the tips CP. I have a ton of different props, and have run them all on a unmodified surestart to check RPM's and pitch speed. I am using the absolute best of the bunch, it is a 5.25" x 5" one blade carbon prop, it revved the best (by far) of any of them, even better than a 4.2x4 two blader.

Chris

lukesp

Try a white Grish Tornado 5x3, guaranteed to give 20,000 at least. You did remove the screen and bore the venturi out ? Admittedly the Grish props do not load the engine like a Cox prop does, but still, they allow the engine to rev. I am waiting for the wind to die down here along with a lot of other variables (time being one of them) before I can flight test my latest SureStart, but it is giving 22,000rpm with the Grish in static testing. It bogs down to 19500 (or thereabouts) with a rubbery Cox 5x3, so my guess is that the Cox is more efficient.

Luke S.

indoorff

My Feedback: (2)

Luke - I did not remove the screen yet or bore the venturi out. How does one go about that?

Here are some pics of before and after piston/crank mods.

Chris

lukesp

I use a T pin to hook out the screen. Remove the needle and seat, pop the reed retainer off and drill from the reed side of the backplate. Make sure the drill chuck doesn't "kiss" the reed seat when the drill breaks through. Drill size is your choice, but I would opt for the same diameter as the KB backplate. I have been using an 11/64th's drill bit but it is overkill I admit, it's simply what I had in the toolbox. The SureStart backplate is very restricted compared to a KB backplate.

Nice job on the crank web Chris. Mine was cut with a dremel , so it has straight sides, not the curved sides you have. You know, with that piston lightened like that it should really pick up some revs, to say the least. The biggest improvement I have seen with SureStart's is via raising compression, even if it has a leaky cylinder/piston fit, and the Galbreath head would guarantee that. It's bizarre that your Surestart shows no improvement. How many head shims are fitted ? Perhaps it's still a tad under compressed ?

Luke.

lukesp

My only other suggestion would be to swap in a stock piston and cylinder and see if it gives you some revs. Perhaps the exhaust port work you did is causing a problem.

The next time I mod a SureStart I am going to raise the compression first and tach it, open out the venturi and tach it, wedge the crank etc. etc. that way I will know what really does work and what doesn't. I have this terrible habit of doing a heap of things and then not knowing what was worthwhile or not. I know for sure that raising compression and opening up the venturi work, it's all the other things I am not sure of.

Bear in mind that some of the SureStarts have abysmal piston/cylinder fits. I have several that give 16500 with a Cox 5x3. I also have a single port 049 with no SPI (number 8 cylinder) that gives the same revs, and yet the Surestart is a twin port with boost ports. Pathetic ! Playing with compression and making them breathe has boosted them to around 18000 to 19000 rpm.

What Mr Pigg said is 100% correct, look for a snappy, bouncy behaviour from the prop when you flick it. And holding the prop against compression to see if it leaks away will soon tell you what sort of fit you have between the piston and cylinder. If it's a leaker then you still have a good sport engine, but it won't really give you what you are looking for in revs.

indoorff

My Feedback: (2)

Luke- great info, thanks. Do you run it with the choke tube?

I did use a dremel for my crank, but a round stone bit. I had a few of different radi and picked the best for the job. I have 1 head shim, and a standard galbraith head. Compression feels great. I was fully intending on doing one mod at a time and taching it, but got carried away! When you did the web on the crank, did you need to remove much material from the counterweight?

I will try a stock cylinder and piston and see what happens. I was trying to duplicate the TD exhaust ports, knowing that that engine was built to rev. Restrictive exhaust port are a great way to throttle your engine down.

I tried a new tach, and that was not the problem (darn)

Chris

lukesp

Yep, same as me, I got carried away as well. Modifying a Surestart is so much simpler (and cheaper) than hot rodding a car and it tends to go to your head.

I didn't remove any material from the counter balance at all, I simply wedged the crank disc to reduce weight on the piston side of things. It didn't require balancing after this either. I use them with the choke tube, no restrictions there.

Agree with you re exhaust restriction, but if you inadvertantly move the port up a few thou when you filed it this may be causing problems for you. I still intend to open out (remove the bar) an exhaust myself to see if it improves things, but I will be careful to not enlarge the exhaust port size. It changes timing.


Luke.

indoorff

My Feedback: (2)

Luke - if you try the exhaust port enlarging, like you say be careful where you file. I found it best when I was doing it to put a thin brass shim in each of the slits, and then do some careful needle-file work between on the bar. If you slip, you just scuff the brass. Evenentually you wear through the brass, and it needs to be repaced- watch out. The hardest part is carfully de-burring after. I took an old piston and found where the tight spots were and carfully de-burred with an exacto knife. I kept doing this until it felt perfect, then I ran it once with an old piston to hopefully wear down any microscopic burrs that might be left.

I am just about to go out and run it with the venturi mod, then I will swap cylinders and try again.

EDIT- ok now getting numbers in the 20000 regime until the cylinder head came loose.

Chris

ptulmer

My Feedback: (5)

Someone here showed a method of lowering the slit for SPI. The method showed using tie wraps top and bottom, the sawing with a hacksaw blade. I see no reason that won't work here and since the tie wraps aren't in contact with an abrasive surface, they should last the whole job.
Chris, sorry to hear it wasn't the tach. That would have been the best news. But, that just means you get to play a little more. I doubt removing the slit did anything but make you feel better. If you have ever tried to throttle one of these things, you have to close it a good bit before the throttle responds. The venturi may be the same way. Looking forward to your results! Hey, if nothing else, you've made it a steady running engine.

PS, you may try more gaskets under the galbreath head. I use three with 35% to get the best results. One gasket was overcompressing and cost me a couple Nelson's.

combatpigg

My Feedback: (3)

I think a stock Sure Start has more exhaust port area than a Cyclon or Fora. I would like to try closing off one of the exhaust ports and converting it to a bypass port.

indoorff

My Feedback: (2)

So Luke says undercompressed, PT says over-compressed..... I think I will try adding a shim or two.

All I know is the engine is threatening to vibrate itself to pieces now. I need to do some work on balancing that crank.

Chris

ptulmer

My Feedback: (5)

Chris, I have have a habit of just reporting my experiences. Don't ever make the mistake of thinking I actually know what I'm talking about! The Galbreath head has less volume than a TD head, though.

lukesp

Ha! No no!, I meant that the Surestart in stock configuration is often undercompressed. I presumed you may have been running with too many shims under the head. When you said you are only using one copper shim under the Galbreath head I didn't think to reply to it, I didn't pick it up. As PT said, that doesn't sound correct at all. Try it with three or four shims and tach it, remove one shim at a time checking it each time with the tacho. In the end the prop should oscillate back and forth instead of running meaning that it's over compressed. If that happens add a shim.

Chris, I am the same as PT, I am no expert by a long shot. I simply have this fascination with trying to get more power out of the world's cheapest engine. I figure if I screw one or two in the process then it's no big deal.


That's interesting, seriously. I was thinking along the lines of the old number 1 and number 4 cylinders which had large exhaust slots, but I happen to have a Medallion with a single slit for an exhaust slot (each side). It's area is smaller than the double slit Surestart setup. Unfortunately there is no number stamped on the cylinder to identify it.

combatpigg

My Feedback: (3)

Once you get the engine back to running vibration free, then it's time for the tuning. take note of not just rpm changes, but also fuel demand and needle sensitivity when adjusting compression. Keep your eye on rod slop, or tuning efforts will be confusing and meaningless. My best running TDs only use 1 shim on 30% nitro, but blow a TD plug often. Nelson plugs will hang in there better.
I think you really need to start with a batch of SS engines to be able to pick and choose which parts make the best combos for maybe a couple of strong runners.

lukesp

Was this because of the venturi mod ?

BTW, do you know the oil content of your fuel, and wether it's a blend or not ? Curious.

Luke.