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Flywheel for ej25

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slociviccoupe

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looking at what others think on this topic. Hoping airtrikes will comment and others as well.

Topic is chromolly, aluminum, or stamped steel flexplate for subaru ej25 with a reduction drive. And what weight?

So airtrikes said in emails any flywheel will work with their spg-5 gearbox and bellhousing for subaru. But for torsional vibration reasons what is better chromolly flywheel 6-18 pounds, or aluminum flywheel 6-8 pounds. And lastly an auto flex plate with ring gear atatched .


Marcote uses an aluminum flywheel they provide so asuming their gearbox has bedn tested with the weight of their flywheel.
 

akwrencher

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I know little about it, but my understanding is you generally want high mass moment of inertia with low weight. Hopefully I got my terms right. Check out rv6ejguy, he has some pics and talks about his ej257 flywheel somewhere on this site.
 

slociviccoupe

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I understand that the ineeta to keep things swinging smoothly. But you also dont want 25#'s up there if its not necessary. There has to be happy medium.
 

wsimpso1

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Look up rv6ejguy's commentary on his EJ22 with a Marcotte.

System reliability is predicated upon the lowest natural frequency being sufficiently below idle firing frequency to keep transmitted vibration down and avoid operation in resonance. This is done by having high enough mass moment of inertia flywheel and low enough spring rate between flywheel and the input gears. This is called a "soft system". Put a low Inertia flywheel in, and you raise the natural frequency towards the firing frequency, which, at best, increases the firing pulse sizes passed through, and at worst, goes near resonance and amplifies firing pulses.

The other big inertia in the system is the prop, much larger than everything else. A low Inertia prop will require either softer springs or higher inertia flywheel or both. If you know the mass moment of inertia or make and model of your prop, the maker of your redrive should be able to specify flywheel and isolators. If they can not tell you what is needed to make their gadget run, maybe re-think whose gadget you use.

Mass moment of inertia is the sum of dm*r^2 where dm is mass at each radius and r is the radius. You can minimize mass at any given MMOI by preferentially putting mass toward the largest radius you can. Limits are placed by the radius and length of the housing the flywheel and isolator operate within.

Ross DI find he had quite a bit of vibration with his standard Marcotte system and his rubber isolators appeared to be life limited. He did some careful modeling, then added inertia to his flywheel with a substantial decrease in vibration and presumably increased isolator life. You can look up all of his posts with the Advanced Search tool. He has a LOT of input on how to do EJ engines and PSRU correctly.

Billski
 

slociviccoupe

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Marcote just got back with me that he is still producing drives but with him being retired im thinking it might not be long till drives or parts not available.
Which has me strongly considering the air trikes spg-5 psru. They have many subaru installations flying. They dont include a flywheel with their unit and when asked said any flywheel would work. In that manner i could use automotomive chromolly flywheel or the automotove aluminum flywheel. Then it just came down to weight. Air trikes uses the bmw driveshaft coupler with a bolt on drive from the crank. Also noted spg still manufacturing gearboxes all the way through 2019.
 

slociviccoupe

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Rv6ejguys page only shows him using the supplied aluminum flywheel from marcote and changing out bronze pilot bushing out for a ball bearing. Boring flywheel and crank spacer as a whole.
 
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wsimpso1

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Wow. SPG must have a a plenty low spring rate to not specify flywheel inertia. Unless they say that the auto trans style flexplate is OK, I would not use it - much lower inertia (and thus higher natural frequency) than either of the clutch type flywheels.

There is a danger in having too low a natural frequency in the system - if the natural frequency is not at least half an octave above firing rate at cranking speed, the system can hang there and not accelerate through to idle. Best is to split idle speed and cranking speed octave-wise. Usually you aim for a 1-1/2 octaves above cranking and 1-1/2 octaves below min idle speed. Neither one will drive any amplification, with the engine accelerating cleanly through resonance in just a few firing strokes. In some of these systems, (AeroMomentum for one) min idle ends up being set based upon adequate frequency headroom at idle.

Billski
 

slociviccoupe

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Still been doing lot of research and still have questions. Obviously there will be a resonance difference between aluminum, stock grey or ductile iron stock flywheel and the sfi certified automotive chromolly flywheels.

Rv6ejguy had an aluminum flywheel supplied from marcote that was 8 pounds and added 8 more pounds of weight to it to get vibration under opperating range. Would a different material flywheel behave differently? And it seems want to keep most of the mass towards the outside of the flywheel.

Since the spg drive uses a standard flywheel with a coupler bolted to the crank bolts id assume i coukd start with a stock steel flywheel and see if get any bad vibrations. Then add or subtract weight and test again. Then move on to a chromolly flywheel and test again. Bolting on metal for weight to the friction surface area of flywhedl using pressure plate dowel pins and bolt locations.

And lastly try an aluminum flywhedl. Fidanza has bolt on friction plate and could just add to it or bolt on a thicker one.

Can these tests be done on a run stand on the ground or does it need to be done on a plane in the air?
 

wsimpso1

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Tough to say. Ross was using the isolators and flywheel and gearbox that Marcotte supplied, and he found that he needed more inertia at the crank. You are running a different isolator and gearbox, and we do not know what the margin (in octaves) is between natural frequency and firing at cranking and firing at idle. This is vibration of the rotating parts, and so tough to sense without instrumentation designed specifically for this stuff.

As to material behaviour, these are just supplying mass moment of inertia on the engine side of the isolator. Aluminum at same MMOI will be thicker than one in iron. MMOI is calculated as Sum of dm*r^2. My inclination is to pick a manual tranny flywheel of steel or iron (similar density) that has the most mass towards the OD. This will put resonance lowest and give the best shot at staying out of trouble. Risk is that if this much inertia drives resonance too low, during engine start, it can get hung at the resonance rpm and not tend to accelerate up to idle right away. If it does this down around 200-300 rpm, you need a lighter flywheel. If you go with too little MMOI, natural frequency could be high enough that you will amplify in the gearbox. You might hear this clattering gears at idle or it might just quietly kill the gearbox.

Billski
 
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