270 Offy

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dsage

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Terry et.al.
If you have a scope you might want to try one of these. They are low volume, you can measure the peak pressure pulses and no need for a shrader valve. You can put it in place of a sparkplug with the engine running. (short the sparkplug wire to ground to protect your coil).
With a bit of training (Youtube search pressure pulse transducers) you can also diagnose valve sealing issues, valve timing, blocked exhaust systems etc, etc. They are used in the automotive industry all the time now.


You'll get best resolution if the sensor is just over the maximum expected in the application. For most work the 200lb version would be suitable.
 

Vietti

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Dsage,

Great idea but beyond my capabilities. What would be great is for someone to build one of these and then calibrate the common gauges like Terry recommended. Then anyone could build one as Terry described and know the correction factor.

Terry, the magneto article and subsequent improvement articles by Don Grimm were kind of a disappointment to me. I've been to many shows since the articles were published and seen very few, 2-3, magnetos in use. Too bad, they are relatively simple and cheap to build, and they are within the capability of anyone who can build an engine. Also more prototypical than a CDI and no batteries or Hall effect sensors etc. Don Grimm's Red Wing with a low tension magneto is extremely reliable and runs for two days without drama at the shows. He wrote a great build article in MEB.

The hardest part is winding the coil but that is unnecessary as commercial coils exist and low tension coils are easily wound by hand. I've continued to work on magneto design such as the oscillating mag on my Red Wing. I also have a new version of a magneto that uses commercial points and an old Stihl chainsaw coil, very easy to build. Continue to look for small coils however, all the newer small coils since about the early 70s are made for CDI and don't work in a magneto. Gittig coils are still avaliable as are the Stihl coils, even some cheap ones from China. If you have a suggestion of a small coil that could be used in a mag I"d love to hear about it.

John
 

Art K

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Terry,
I have been following along but haven't said much. I just wanted to say that in talking to guys that have built the Offy they know its time to shut it down when they see bubbles in the top coolant line. That is the sign its getting hot. I'm glad that you were able to do a compression test and remove that possibility from the list of potential problems. When you get right down to it the cooling is just a few holes in a block of aluminum, not nearly enough. Great work by the way.
Art
 

mayhugh1

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Terry,

I have just about collected all the material needed to make a 1/2 scale model of this engine. In your opinion what would you change if you were able to do so due to making it on a larger scale? Any suggestions would be appreciated

Jimmy.
Half scale is going to be a beast! I hope you do a build thread - it will be really interesting. Cooling would become an even bigger concern, I would think and so Ron's idea of a constant displacement water pump might be worth considering. You'll have some really deep oil and water passages to drill at half scale. If you choose to not split the crankcase but instead assemble through the side covers you'll at least have more room to work. I'm not sure how the gears will work out inside the gear tower - I'can't visualize how they will scale and keep the same ratios, but that's due to my current brain cloud. I can't wait to watch progress. Best of luck. - Terry
 

The_reach

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Half scale is going to be a beast! I hope you do a build thread - it will be really interesting. Cooling would become an even bigger concern, I would think and so Ron's idea of a constant displacement water pump might be worth considering. You'll have some really deep oil and water passages to drill at half scale. If you choose to not split the crankcase but instead assemble through the side covers you'll at least have more room to work. I'm not sure how the gears will work out inside the gear tower - I'can't visualize how they will scale and keep the same ratios, but that's due to my current brain cloud. I can't wait to watch progress. Best of luck. - Terry
I'm hoping that along with the other issues that being scaled up brings that it may be possible to go for the 1 piece head and block construction too though the jury is still out on that one. I've watched many of the build threads on this forum with great interest and looked at several that I would love to attempt "scaled up" but always come back to the offy.
100% there'll be a thread once I get my backside In gear and make a start. It's been a close run thing between the offy and a German designed 3 cylinder inline aero engine
 

tornitore45

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all the newer small coils since about the early 70s are made for CDI and don't work in a magneto.
Let me clarify that. When a coil is not a coil.
The classic Kettering ignition coil is what a purist would call a 2 winding INDUCTOR. What characterize an inductor is the storage of magnetic energy. It also act as a transformer but that is not as fundamental as the fact that the energy for the spark is stored in the magnetic field. When the contact open much of the energy is coupled to the secondary and goes into the spark, a small amount goes into the point capacitor.

CDI ignition "Coill" are actually a high ratio pulse transformer, a completely different animal. The energy is stored in the Capacitor at about 200V. 1/2 uF @ 200V = 10 mJ Incidentally, 200V is the same order of magnitude of the primary voltage on a Kettering Coil after the points open and the capacitor across the points charge to its peak.

Transformers change the voltage via the turn ratio, they do not store magnetic energy. They use a small amount of the primary current to create the magnetic field that is at the base of the transfer of energy. The primary current is the sum of the load current reflected at the primary plus the usually much smaller magnetizing current. The energy passes through the transformer. The magnetizing current is something that may be undesirable sometime but is an unavoidable necessity, since is normally designed to be minimized by assuring a high primary inductance the energy involved is minimal and is never stored to be reused later is just wasted.

Inductors have gapped cores, because the gap is where the energy is stored. Except rare cases, transformers do not have gapped cores, the lamination are alternated to minimize the gap.

What happens when one try using a CDI coil as a Kettering coil? The high primary inductance will ramp the current at a much slower rate, that current is technically the magnetizing current of the transformer, is not going to store much energy LxI^2 Big L small I. When the point open, the small stored energy is barely enough to charge the capacitor across the points.

The reason that CDI are easier to miniaturize is that Capacitors have a higher volumetric storage than Inductors.
Magnetic storage is limited by saturation, typically a hard limit of about 1 Tesla. On the other hand we have available very good insulators capable to withstand high voltage at extremely small thickness. On top of that the permeability of the gap is set by nature to about 1, while the dielectric constant of the insulator allow for some manipulation by the material engineer, one can chose paper, Mylar, Teflon, Polypropylene etch.
 

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