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Old 05-02-2019, 10:34 AM
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Default ARC Flywheel Question

When I think and dream about a billet ARC flywheel, I notice something on all of them except the adjustable timing ones:
The ignition timing is always set to 30 degrees Before Top Dead Center.
I would imagine that would really screw up a mostly stock engine performance-wise.
Is there some way to retard the timing to a somewhat normal timing?
Or is there something else I'm missing?
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Old 05-02-2019, 11:48 AM
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There are offset flywheel keys that let you change the flywheel/crankshaft slightly.
You could look at the ignition pickup and possibly move that.
The GY6 world has adjustable cdi's, some simple and some pretty extravagate ones that you can hook a computer to and do some real time monitoring and adjustments. Maybe you could rig up something like that.
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Old 05-02-2019, 11:49 AM
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You can get offset flywheel keys. I dont know what engine this is for, or what the stock timing is, but id assume if its a fixed flywheel they would set it to work with a stock ungoverned engine.
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Old 05-02-2019, 11:54 AM
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Since im building something similar thats a interesting question.
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Old 05-02-2019, 02:24 PM
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Most ARC Flywheels have 8 degree of advance built in. You have two options. You can get an 8 degree offset key and install the key backwards. Or you can get a degree wheel, lap the flywheel to the crank, set timing where you want it, and tighten the nut without using a key.
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Old 05-02-2019, 03:48 PM
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Nope.
At least not with this one: https://www.arcracing.com/6620-arc-b...inned-non-adj/
If I want to get around that, I have to pay ~$50 more for the adjustable timing one.
And explanation for that?
And also, I can't imagine how 30 degrees Before Top Dead Center could work with a stock engine.
If you please, GAB(Go All Bob) on me, I just want to figure this out, because it has confused me for a while.
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Old 05-02-2019, 07:10 PM
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bob's correct most non adjustable ARC flywheels have indeed 8 of advance built in.

And you are correct in saying the 5hp flathead wheel doesn't (it's closer to 6.. stock timing being ~24-24.5)

And as bob also correctly said.. no need to install the flywheel key at all..
lap the flywheel to the shaft and torque it down at any desired angle (it'll stay put!)
without the woodruff key installed.

And to answer the only relevant question..
how would it work with a stock engine..

it wouldn't (or at least not well that is)

ignition timing and 'ignition' are not exactly what you think they are ...
The ignition timing is set BTDC in order to have the actual gas expansion right at or very shortly after top dead center.
That propells the piston down again, in the proper rotation direction, speeding your engine up..

ignition timing on industrial engines is mostly fixed
(very few exceptions, none I know for the flathead)
means the ignition is set in a way that ensures the spark plug firing and igniting the gases
perfectly at around 2800 rpm ('ish) or more precisely
at the rpm your particular engine produces the most torque

Since the electric signal takes a fixed time
from magnet passing the coil coil to spark plug firing
and it also takes a fixed time for a certain amount
of compressed combustible gas to ignite and actually produce expansion pressure.
the timing is only perfect at exactly that rpm...

Since the faster the engine spins,
the faster the piston travels up and down the cylinder of course.
and it thus takes less time to get to TDC
So on lower rpms the timing will be a hair early (slow piston not yet reached TDC), and
-now is where the flywheels inbuilt advance timing comes in-
at higher rpms a tad late (quick piston passed TDC)

Since a modded engine is mainly modified in order to reach higher rpms,
you want to shift the torque curve UP a bit too,
in order to have enough power at higher rpms
so that it can still push some loads up there.

An 8'ish advance is essentially the threshhold..
the engine still idles okay
(ignition is still late enough for the engine not to try to stop the piston at low rpms)
but the ideal rpm are shifted upwards (IIRC 4k'ish)
So the engine produces the most torque around 4000 rpm
(maybe 3800, maybe 4200.. but thereabouts).

again at lower rpms and it fires a bit early
and at more rpms it'll be a hair late again.

With an UNMODDED engine the governor still kicks in at 3600 and
all you do is essentially remove power from the engine.
since your plug fires always too early
thus .. no.. an advanced timing doesn't make any sense on an otherwise unmodded engine.

removing the governor (not installing heavier springs nor a conrod)
the stock timing still works okay..
a 4 key can shift the torque up a few rpms (maybe 3200-3500 rpms or so)
for you to have a bit more top end power though
(that's what Briggs raptors have IIRC)

So still decent power down low
for easier rev up and quicker acceleration
and just enough power higher up the rpm band to make use over all the 5500 rpm you are about to rev.

And yes, there's even more to it than just ignition timing..
like compression ratio [piston geometry, head geometry etc...]
or valve timing for that matter...
but that's a fluid dynamics topic, far beyond what we can cover in a forum like this

Just be aware that so called torque cams are nicer for lower rpms and so called speed or "performance" cams are ideal for high rpms.
And they work best with a matching ignition timing of course.
A bit of compression improvement and maybe easier in/exhale... and we quickly move through all stages of modification.

And that's the point where adjustable flywheels come in..
(or the flywheel getting lapped to the crank [that's what ARC does to their engines btw])

no two engines have the exact stock timing for some odd reason
much less you find two engines with the exact same modifications being down
AND having identical stock timing.
they all fall within a range but really perfectly identical twins are rarely ever seen.

An adjustable flywheel now not only gives you the opportunity to set the timing to match your cam,
it also gives you the chance of adjusting that tiny bit of misalignment away,
to tweak it until your singular engines revs best.
So you get the peak torque at exactly the point
the engine is able to produce the absolute maximum amount of torque.

In all cases the engine is still the dominating part of the equation..
if it's not modified, a timing key (or flywheel offset) will do you no favour at all.
you need a higher than stock revver to make use of the advantage.

For more than maybe 4 of advance you really need to aim for 6500-7k rpms IMHO
otherwise it's not worth it since you'd lose torque down low.
you might also have issues with higher idle speed needed and followed by a higher engagement sped on the clutch etc...
all for very little benefit.

'sid

[/BOBMODE]
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Old 05-02-2019, 10:44 PM
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Call Jody at ARC. Tell him you are interested in using an ARC flywheel. Ask him what timing he recommends for your application.
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Old 05-03-2019, 12:25 AM
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Side note:
Jody has a video where he says he sets timing on box stock clones at a minimum of 32 BTDC.
Granted, these are on race karts that turn 6k+ rpm the whole race. Plus these use “stock” camshaft that are timed differently to make more power up top. The Intake center line on these cams might be around 112 -114. He sets 34 or 36 BTDC on box stock or up to 38 BTDC on restrictor plate engines.

On modified engines that turn 7k+ RPMs, he doesn’t go more than 32 because the carbs start to lean out and the engine will run hot and make less power. He uses more timing on engines that turn less rpm because the carbs don’t lean out unless the engine turns faster.

At right around the 50 minute mark
https://youtu.be/C1kQM1tzjnM

This could be a fun experiment for Grant and his Dyno. Stock engine, governor removed, and the only variable being ignition timing. 24, 28, 32, 36.
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Old 05-03-2019, 08:47 AM
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I might try to contact Jody.
And it might not be a bad idea to play around with this idea, and test it with Grant on his dyno, because either way, I live within driving distance of him.
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