049 Black Widow tuning

Really I am surprised NV would let that go with no undercut clearance.

aspeed wrote:Really I am surprised Davis Diesel would let that go with no undercut clearance.

LOL, it wasn't even DDD, These parts came from NV.

--Oz-- wrote:

gkamysz wrote:That's a thread interference problem.

It could be both. Looking closer at the pictures, the before has quite a taper where it seats, the modded version, mostly does not. Without seeing the cylinder, I am just guessing. Was it a mismatch of the seats, or threads chewing into the gasket or both? What did the chewed up gaskets look like?

I saw one of the NV diesel heads first hand. It was a simple error. The screw in head that holds the glow insert is threaded right up to the end. "Let's make the diesel head the same." Oops, forgot about the 1.5mm thick insert that prevents thread interference. Whatever was going on with the sanding on the sealing face could have been an attempt to rework them.

Regarding dropping the cylinder. Lowering the cylinder is counter-intuitive. It makes for less exhaust and transfer duration. I don't think I've ever seen a Cox spec for exhaust duration. KK talks about the top of the piston being level with the head land in the cylinder at TDC as "per spec", but that has a tolerance stackup. When the dimensions are several hundred thousandths of an inch, stackups of several thousandths is a large range, compared to a larger engine.

I'd really like to see Cox prints for parts and do the stackups. The other question is what was actually produced vs. designed.

I think duration in some cases might be easier to calculate than measure directly. We know that the crank turns 180 degrees between the TDC and BDC of the piston, and by measuring the piston movement or the heights of the ports and comparing those to the stroke length we could get the duration numbers in degrees

EDIT: the degrees to piston position is not a linear relation, so there needs to be some more math included. Maybe I’ll try to figure out a formula some day

I suppose that at least determining the SPI duration is easier by measuring the gap with a feeler gauge, than using a degree wheel. At least with my eyesight and fumbly fingers

Yes, tiny engines need precise measurements and tolerances when modifying timing. I remember reading an old article about modifying 2-stroke Saab engines, a literally more than 1000x bigger unit than a Black Widow (3 cylinders so one cylinder is ”only” about 350 times bigger than a BW), and the difference between ”street hot” and ”track” engine was something like 1 or 2 millimetres in bypass height.

gkamysz wrote:

aspeed wrote:Really I am surprised Davis Diesel would let that go with no undercut clearance.

LOL, it wasn't even DDD, These parts came from NV.

Oh ya, sorry about that. I'll edit it.

gkamysz wrote:Regarding dropping the cylinder. Lowering the cylinder is counter-intuitive.

Before I tried lowering the cylinder, I mentioned that I asked my buddy that races 2 stroke motorcycles (and builds the motors), should I raise or lower the cylinder, he said raise, I did, it was a total dud. Like in all my tuning of almost anything, test, modify, retest, results tell you what direction to go. So Lowering the cylinder is counter-intuitive, but its all relative to the starting point. From the initial deck height, 078, 078 and the dud was 084, it was obvious what direction I needed to go, but it was not hard to make a gasket to raise the cylinder, so i tried that first, failure, but it was good information that I was going the wrong way.

I tried this on a brand new black widow dud with 10% nitro and 5x3 thimble drome prop, maybe it dont help as much with higher nitro, different props/etc, I don't know.

So, who is going to try lowering the cylinder on some spare old motor that is just sitting in a box? 

OZ you can post from home screen scroll down to Non Cox engines ..

Cox engines are a little special in the sense that lowering the cylinder gives you more SPI and a higher compression ratio (if the head and gaskets are kept the same). So it is no real surprise that you see an improvement. On the other hand the highest power available might be to not change the cylinder position at all, and instead play with the SPI and compression ratio separately.

The SPI can be altered by removing material from the piston skirt and the compression ratio can be increased by removing material from the head, or simply switching to a Galbreath head.

I guess raising the cylinder helps to an extent, until you lose too much SPI, or get too much port duration that eats up the compression and work duration. The SPI can be remedied by shortening the piston skirt as Kris said. The increasing port duration gives higher rpm at the expense of low end torque.

Lowering the cylinder, you will get a longer compression and work duration, but gradually increase SPI and at some point may get too much of it, and it is difficult to fix. Only way seems to be using a non-SPI cylinder and maybe expand the exh port upwards. The longer work duration will ”in theory” give more low end torque at the expense of max rpms.

When you add the manufacturing tolerances to the equation, it gets really interesting.

To match the timing, individual parts and their combinations to the propeller used, to the ambient temperature, humidity, air pressure, fuel and airframe used... Lots of variables!

Yes, lots of variables. The low hanging fruit is the modding the case deck height, why, its soft ALU and only sandpaper/wd40/smooth surface is needed. Where as working on the piston, not so easy or specially the cylinder. The glow plug is a consumable part, so if its modded, everytime it dies = more work.

What I found on my new dud motor was 0.063" deck height was optimal (testing with 0.005" increments, not very fine). Might be quicker to take it down farther to something like 0.053", then test and add 0.005" spacer (now 0.053 + 0.005) and retest, then keep adding 0.005" till it is optimum (of course adding the correct head gaskets along the way). This should cut the tuning time down quite a bit as you only need to disassemble the motor once to lower the deck height, then its just simply removing head and cylinder and adjusting with spacers.

There is so many variables, it would take time to blueprint one of these motors, would be fun if you had the time and tools.

Wow, I just watched the link in your signature.
https://youtu.be/uHoFfpLSb2g


I don't speak the esoteric language of you multi-rotor guys(reading the comments), but I can say Dayum! That's some badass piloting. Great work. You gotta be grinning when you set it back on the ground at your feet after a flight like that. I enjoyed the video, thanks.
Rusty

Be careful when sanding the top of the case. It is best to machine it, or it may not be square to the crank. Always keep the same piston to head space when comparing. Also use the high compression head unless you are using a huge prop. A timing wheel is good to check the exhaust timing. 160 degrees is what I would consider max for most engines without a pipe. I would think the Cox is similar.

RknRusty wrote:Wow, I just watched the link in your signature.

I don't speak the esoteric language of you multi-rotor guys(reading the comments), but I can say Dayum! That's some badass piloting. Great work. You gotta be grinning when you set it back on the ground at your feet after a flight like that. I enjoyed the video, thanks.
Rusty

Thanks! I did a lot of gliders from 1976, then rc onroad cars for 2 decades (sponsored for 10yrs), did planes and gliders at that time too. Planes are fun, relaxing, 3d (loved slope combat), but cars were 2d and competition (I lived for that ), then I tried heli's, about a half dozen crashes later i found quads. Once you build a quality $200 quad, they are indestructible, yep 80mph into the ground, change some 37 cent props and fly again. they are more difficult than cars or planes (always need to be on top of 4 channels at once), but compared to heli's with the massive cost after a crash and the huge time it takes to repair the huge mechanical tuned craft (several hours), quads, zero mechanical tuning. This robustness promotes taking way more risks and zero worry, unlike the heli. Also, the response of the quad (stick to movement) is second to none (not fpv glass to glass latency, as I only fly LOS). My max rate is 7.6 rolls per sec, basically any direction at any time, limited by your imagination. My current quad (the one in the video I have flown for almost 4 years, the power train is about 2 years old and accelerates at 9 G's (new quads are upto 14:1 thrust to weight). When a plane is doing a simple u-turn, I have done several tricks, it keeps me interested.

aspeed wrote:Be careful when sanding the top of the case. It is best to machine it, or it may not be square to the crank.  Always keep the same piston to head space when comparing.  Also use the high compression head unless you are using a huge prop.  A timing wheel is good to check the exhaust timing. 160 degrees is what I would consider max for most engines without a pipe.  I would think the Cox is similar.

I have done three motors lowering the cylinder, all three got to 25Krpm (one sometimes hit 26Krpm with new fresh balanced prop). If you lap-sand in a figure-8 pattern, with a even pressure all the way around, you should be ok. If you sharpie the surface your going to take off, some light passes with repeated check will give you a decent indication if your doing it right. Of course machining it would be best, most pilots dont have one (mill) or access to one. I really dont think its fair that it's necessary to "keep the same piston to head space" when tuning. My goal maybe different than others, max rpm, if it takes more or less compression to achieve max rpm with my setup (5x3, stock head and 10% nitro), then its game. If you look at my dud example, it started at 0.084 and one head gasket (0.005), following "keeping the same piston to head space", this motor would of never hit 25K because is was 0.006" more head space than my other normal motors (with deck height of 0.078). Have fun.

The stock head is the regular low compression one, as far as I know, and it simply has a too low compression ratio for 10% nitro. So my guess is that it is the increased compression ratio is what gives you the power gain, possibly in combination with the increase in SPI.

You can sand off material from the head too, if you want, but it is better to use a lathe (in my experience).

The compression ratio will always have to be optimised for each chosen fuel and prop load. This is very obvious to anyone who has ever used a diesel engine. Glow engine are not so different, it is just a little more time consuming to play around with different glow heads and head shims.

Surfer_kris wrote:The stock head is the regular low compression one, as far as I know, and it simply has a too low compression ratio for 10% nitro. So my guess is that it is the increased compression ratio is what gives you the power gain, possibly in combination with the increase in SPI.

Great points!

I still find it hard to believe lowering the cylinder to improve rpm, maybe it only applies to low nitro, well kept secret or?

Lowering the cylinder - till the piston bumps into the glow head -will increase the compression ratio and this will increase the mean pressure in the cylinder during the working stroke of the cycle. Providing the higher compression work and premature ignition caused by the higher compression ratio do not rob more energy from the engine during compression stroke than what is gained by the higher mean pressure in working stroke, the two effects net out positive and result in higher engine speed.

balogh wrote:Lowering the cylinder - till the piston bumps into the glow head -will increase the compression ratio  and this will increase the mean pressure in the cylinder during the working stroke of the cycle. Providing the higher compression work and  premature ignition caused by the higher compression ratio do not rob more energy from the engine during compression stroke than what is gained by the higher mean pressure in working stroke, the two effects net out positive and result in higher engine speed.

Because my dud cylinder was way to high (at least 0.006 or maybe even 0.011"), lowering the cylinder 15thou did increase the rpm (not as high at I finally got it), but it was hard to start, so I added head gaskets and easy to start again. But I kept going till it got worse, then backed up one step, then played with the amount of head gaskets to get the highest rpm, at that point I could of lowered number of head gaskets and got more compression, but it was slower rpm and harder to start, there is a balance of everything.

--Oz-- wrote:
I still find it hard to believe lowering the cylinder to improve rpm, maybe it only applies to low nitro, well kept secret or?

You seem a little fixated on the lowering of the cylinder as if it would be something counterintuitive? But it really isn't.
Lowering the cylinder by .005" will not change the timing of the engine. The effect on the position of the ports are minimal, corresponding to about 1-1.5° of total change in the exhaust port duration. The exhaust duration would go from 132° to around 130-131° only. This is nothing to speak of and it can be neglected.

Increasing both SPI and the compression ratio are far more important, and that is likely why the power goes up.

Surfer_kris wrote:

--Oz-- wrote:
I still find it hard to believe lowering the cylinder to improve rpm, maybe it only applies to low nitro, well kept secret or?

You seem a little fixated on the lowering of the cylinder as if it would be something counterintuitive? But it really isn't.
Lowering the cylinder by .005" will not change the timing of the engine. The effect on the position of the ports are minimal, corresponding to about 1-1.5° of total change in the exhaust port duration. The exhaust duration would go from 132° to around 130-131° only. This is nothing to speak of and it can be neglected.

Increasing both SPI and the compression ratio are far more important, and that is likely why the power goes up.

A "little fixated", lol, no, a lot    Lowering the cylinder and tuning the amount of head gaskets is the only thing I did to gain 7Krpm, i would think that for that gain in preformance, it would be a standard tuning item, yet no one is talking about it (that I have read/heard). I never raced with 049's and if I did, I would not be sharing this info as its a huge advantage. With 40 years of racing (mostly electric) and all the tuning, I guarantee I am not the only one that knows about lowering the cylinder. It would be simple to just have the deck height at say 0.050 (instead of the random 0.073 to 0.084") and tune with gaskets, very simple but highly effective on rpm.

I mentioned my buddy racing 2-strokes (yamaha rd350/400's), he also built/tuned them. He said I need to raise the cylinder, then I find the spi tuning shims, but nowhere do I see lowering the cylinder. It took a 14Krpm dud to 25Krpm and my normal 18K to 25K on low nitro. 25K is TD territory, not really reedy's zone (unless you prop down). Kinda see why I am a "little fixated"?

As a Kid I just bought the hottest fuel the hobby place had and rann the snot out of store bought black widows...none of us ever had a Tachometer but we all had a glow engine ear...I am amazed at how many McCoy or Fox bigger engines can be snuffed very fast at PEAK RPM---

But a dinky Cox Black Widow was happy as can be screaming lean..hell we never even knew...  other than it sounded faster.... that in flight, they were running super lean!   I bet to this day that is why Cox insisted on 25%~29% all castor   cuz us kids just did not know how to back off to a tad rich and let it peak in flight

Fast forward 40 years and I relit the Cox bug...now with a serious background in mechanics, engines, and a garage full of decent quality measuring tools

I was eventually taught how to set up high rpm Combat engines.... almost always starting point was ensuring Top Dead Center was exactly at the top of the cylinder/liner...small depth micrometers are inexpensive BUT the Model industry has a few specially adapted...they sell to folks who have no real experience with measuring tooling , quality, purpose and costs..... so I recommend you never need the Pylon Racing XXX GeeWiz $129.99 depth micrometer...the Harbor Freight one is more than adaquat for $15~$29 depending on coupon and sale

I tend to believe with the ball socket piston assy there is very little slop when running....by this I mean when measuring the TDC vs the absolute TOP of the cylinder...no math is necessary for the small amount of ball joint slop....Compression and explosion will ALWAYS have the con rod piston dimension at the shortest at TDC every revolution

so once we adjust the cylinder for perfect TDC...

then the variables are: combustion shape, squish area, compression Ratio, catalytic heat range, piston and cylinder Fit (Paul Gibault Tap to drop test) exhaust orientation (very little effect from my testing/tinkering) and SPI ....the later being closely tied into carburetor  or in our case venturi size

There is a sweet spot for venturi size and total SPI

I have long lost my notes as when I was doing this tinker with half dozen BW and dozens of Cox International good parts...and seriously got into TeeDee tinkering for combat training engines......

BUT I seem to remember I was able to get one reed valve BW into seriously high RPMs ---evidenced then with a decent electronic Tach...high enough on the bench to break a crank and not get to send to Ron Cribs For the Tach race  my reed valve entry that year was a dismal last place also ran pathetic engine...I think my TeeDee was top dog before it broke the crank pin...

one hint I will absolutely stand by...Cox Star Beryllium/Copper reeds are the best for high rpm under a circlip retainer

BTW Beryllium/Copper can be bought on line...bring a banker the stuff is safe but freaky expensive even in small 4x5 inch very thin sheets
I actually think Cox stopped the Beryllium/Copper reeds due to excessive cost of the raw stock

@ fredvon4 Thanks for the post!

"almost always starting point was ensuring Top Dead Center was exactly at the top of the cylinder"

I have read this several times now (in other web places), maybe on paper this is true, but with the three BW I tuned by lowering the cylinder, the top of the piston was well above the top of the cylinder shoulder (with 4 head gaskets). I will dig out my box and do some measurements and report back.

I believe that is the design nominal. It would be fun to see Cox prints and tolerances.