Cooling With A Smaller Flywheel

[JTSpeedDemon]

Ok so I just got this really clean 3 HP flywheel from my buddy Darrell(the guy who sold me the rototiller) along with some other parts for $1.

I want to put it on the Hothead for performance reasons, but I just have one inkling concern:

Will the smaller flywheel move enough air to effectively cool the engine? I don't want it to overheat, but at the same time, I would love a little faster revving mill.....

 

[mckutzy]

Off hand... With the smaller diameter.. the coil pickup bosses won't be in the same place..
Also...the diameter of the wheel dictates the firing position... Hence why we have offset keys...
Being smaller, it won't fire in the right position...

 

[JTSpeedDemon]

Yes, I understand I need an adapter bracket.

I do have some offset keys, but we'll see how much it's actually affected. Has anyone on here done this mod? If so, what needs to be done besides the adapter bracket?

 

[itsid]

timing (with the matching adapter bracket) shouldn't be too much of an issue IMHO,
especially not if you can further adjust timing with keys.

BUT I'd suggest you get the timing card template downloaded and printed on cadboard or such,
to read your exact timing as it is atm.
and replicate that as good as possible for a first starting point with the new flywheel.
(just to rule out different timing affects how the enigne runs with that yet unknown flywheel)

then go from there.
the overall moved air should be fairly identical, so cooling shouldn't be too different either.
it's the missing inertial mass that should have you worried.
the engine might be more willing to rev up, but also less smooth in idle.
it's a double edged sword really.
slightly less torque for slightly quicker revving ...

especially if you up the compression a bit, that small flywheel might not be the best idea..
but you can always reinstall the original one, sooo I don't see too big of a problem at all.

Just be sure the taper fits.. ideally lap the flywheel on and make sure the ignition coil and the adapter plate is tightly bolted down.

'sid

 

[JTSpeedDemon]

Got a link to that timing card?

As someone said in another one of my threads, someone noted that some drag cars accelerate better with heavier or lighter flywheels. I also don't know what retarding the timing will do yet(I don't want to have to replace the key every single time, as I will be doing timed test runs for the forum, I'd much rather just undo 2 bolts and re gap the coil).

I don't think timing would be affected much more than a degree or so at maximum, but to be sure, I will definitely check with the existing flywheel and the 3 HP 'wheel.

So either the timing card, or the ol' "in the hole" trick.

 

[itsid]

no link, no just google for a "printing timing degree wheel"
and get one you like.
Frankly since for now all that matters is that the timing is identical,
you can also just take a blank piece of round cardboard and mark it by hand.

if you intent to skip the flywheel key, keep in mind, that lapping the flywheel on the crankshaft is no longer optional.

'sid

 

[JTSpeedDemon]

My reasoning is that the smaller flywheel would retard the timing a tad..... Does anyone know?

On the plus side, I can use some offset keys to retard the timing a bunch, and then use the coil bracket to compensate.

The only problem I can foresee is possibly spark plug wire length.

 

[Snaker]

The diameter difference wont affect the timing, as long as what ever adapter you come up with reposions in line with the shaft center. It will be a issue with signal strength so you have to keep the proper gap. You need to keep the positioning between the keyway and the ignition pad of the flywheel and between the keyway and the pickup to keep the same timing.

 

[JTSpeedDemon]

Alright, awesome. Thanks for the input guys! The hardest thing will be removing the old stuck flywheel.......

 

[txluke]

I lap all of my flywheels and hardly use keys.

 

[JTSpeedDemon]

I'm planning to lap my flywheel at least a little bit. I don't want it to be so perfectly taper locked that I'll never get it off!!
It's kind of irritating how the small block Briggs flywheels don't have puller holes.......
It's just going to take good ol' elbow grease and a few hammers.

 

[jmaack]

Go rent a pulley puller and get a cheap heat gun from harbor freight. That will help for now and many projects to come.

 

[Flyinhillbilly]

My Briggs open has a 3.5 flywheel on it.

 

[itsid]

The diameter difference wont affect the timing, as long as what ever adapter you come up with reposions in line with the shaft center.

And the exact opposite would be correct..
the addapter would have to shift over slightly.
(easier to just turn the flywheel a few degrees though)

let me quickly draw you a pic...


(red) magnets are the same size and are inline (timed identically in regard to the keyway)

the magnet on the smaller (lightgrey) flywheel has already reached it's yellow'ish pickup,
whereas the one on the slightly larger (darker grey) flywheel has still a few degrees to go to hit his blue'ish pickup (3° for that relative size ratio)

And while it's not the leading edge of the pickup to be important
(spark is triggered by the trailing edge)
the offset remains the same though,
consider that a "start of induction" pic

'sid

 

[wenee9w]

When we ran stock appearing class we ran them for years. Ran one once on a stock governed Briggs. Acceleration was better and it was drop the coil to gap spec straight bolt on. There are brackets available to do this mod. Timing will be real close to stock,not enough to tell, with the stock key. If your looking to mod it timing is a great tool to play with. Never ran a stock appearing engine with out a small flywheel so gains on those I can't speak on. See link.

---------- Post added at 10:17 AM ---------- Previous post was at 10:12 AM ----------

https://www.viperizerracing.com/3hp-Coil-Bracket-p/vpr-26.htm#

 

[Snaker]

And the exact opposite would be correct..
the addapter would have to shift over slightly.
(easier to just turn the flywheel a few degrees though)

let me quickly draw you a pic...
View attachment 115406

(red) magnets are the same size and are inline (timed identically in regard to the keyway)

the magnet on the smaller (lightgrey) flywheel has already reached it's yellow'ish pickup,
whereas the one on the slightly larger (darker grey) flywheel has still a few degrees to go to hit his blue'ish pickup (3° for that relative size ratio)

And while it's not the leading edge of the pickup to be important
(spark is triggered by the trailing edge)
the offset remains the same though,
consider that a "start of induction" pic

'sid

Itsid that's a great diagram you made. Very straight forward. Its a bit of an exaggeration of example, which I do all the time to visualize. But it clearly makes your case. I used to do that sort of Geometry and Trig and wanted to whip up a size chart but I found that I forgot how. Must be the age and the meds. Still it seems like it shouldn't be any where near as much as 3'.

 

[itsid]

nah, it's 3° in the pic only (something about the specific ratio of the two fictive flywheels and the magneto width in the image.. I winged it.. but the proportions shouldn't be too far off)

I can't tell how much of an timing advance one would want to expect tbh..
my guts say maybe somewhere between JT's suspected 1° and 2° or such *shrugs*
nothing to write home about.

still it'd be good to know if it's the weight/size or the minisule timing advance that makes the difference.
Since if it's just the slight timing difference, you can have that with the larger flywheel aswell
PLUS have the slightly better torque of a heavier moment of inertia

'sid

 

[JTSpeedDemon]

Thanks for your input, wenee9w!

I'm planning to either use this coil bracket: https://www.viperizerracing.com/Coil-Bracket-3-Degree-Adjustable-p/zeadjust coil.htm
Or this one: https://www.dynocams.com/item/jr-6733/

I'm thinking that since the 3 HP 'wheel will advance the timing ever so slightly, and I can just use one of the above linked coil brackets to allow for easy adjustment without having to remove the flywheel every single time!

Since the coil brackets mostly advance timing, I can use an offset key to retard timing about 3 degrees, so that way when the coil is in the fully advanced position, it will have at least close to stock timing, and then when the coil is in the stock position, timing will be retarded 3 degrees. I checked, and I think the 5 HP coil spark plug wire will be long enough, so that's good.

And also itsid, I will be pulling control times with the same engine/kart setup, with the only variable being ignition timing. So it won't affect the test results.

 

[itsid]

you said you wanted to do some testing,
otherwise I wouldn't even have mentioned the tiny timing offset

Also.. no it will not be the only thing affecting the results.
Just the only thing you can influence more or less.

Even if you test on a dyno,
two consecutive runs of the same engine unlikely ever produce the exact same outcome.
(I'd even claim they'll NEVER do!)
and doing a buttdyno run is certainly even less precise

tiny hiccup in the carb, a little bit less eager on the throttle, a gust of cold wind,
a speck of carbon deposit on the spark plug and so on and so forth
all result in a measurable difference (on a dyno)

you simply cannot control every atom in your vicinity
Anyways.. do some testing, it's far from pointless having a good idea of how it affects the performance.
a buttmeasured setup is more important albeit less precise anyways
(hence racing drivers testdrive a setup to dial in what the technicians couldn't with all their fancy lab tools )

it's just good to know what knob to turn.
(flywheel weight or timing)

'sid

 

[JTSpeedDemon]

I am aware that my numbers will be far from ideal. I plan to have my dad with a stopwatch to time some approximately 1/16 mile drags at a parking lot I have permission to use. That's about as precise as I can get, unless Grant wants to do a dyno day.

What I meant is: every test will be with the same flywheel, not "take one number, change the flywheel, then do it again".
Obviously that^^ will produce very different results, but the way I will do it, the only controlled variable will be ignition timing.
The 1/16th mile drag is about the best I can do, and that will be enough room to get the clutch into full lock and see how the timing affects other areas of the rev range.