Question about flywheels

[zogthegreat]

Hi everyone!

I'm in the process of buying the parts that I need for a build with my daughter this summer and I noticed this on BMI Karts:

PVL Flywheel for Clone, GX160, or GX200 Engine

Product Specs:

Brand: PVL
Engine: Clone 196cc, Honda GX160/200
Material: Diecast Aluminum
Weight: 3.8lbs
Timing: 28 Deg.
Fins: 12

http://www.bmikarts.com/PVL-Flywheel-for-Clone-GX160-or-GX200-Engine_p_2442.html

Billet Flywheel for 196cc Honda / Clone

Product Specs:

The flywheel to which all others are measured.
SFI certified billet aluminum flywheel for Honda GX160, GX200, & 196cc Clone.
The keyway is fixed and the stock pull starter can be used.
Weighs 3.4lbs and incorporates a special low drag fan design unique to ARC flywheels.
A specially designed high-power rare earth(neodymium) magnet and carrier design creates the hottest spark possible.
Now featuring 8 degrees timing advance built into the keyway. 32 degrees BTDC.
Set coil gap to .030"
Flywheels made for the GX200 and 196cc Clones, NOT for Predator Engine (Shaft taper is not same).

http://www.bmikarts.com/Billet-Flywheel-for-196cc-Honda-Clone_p_32.html

What is the real world difference between these two flywheels? Which one would be better for a off road kart? There's only about a $40.00 difference between the two flywheels, but that's $40.00 that I could spend on other parts.

Thanks!

zog

 

[itsid]

cast flywheels should be handled with great care.
A hard impact and it can form an invisible crack that leads to failure
(hence the warning on all websites that sell them)

That's why the flywheel has been tested twice
(once in germany at pvl; once in the americas to be sure it's not cracked on the way)
If you trust in your postal services to not throw the package araound too much and in BMI to pack it accordingly...
then there's no reason to not save the 40 bucks.

If you don't trust in packing/shipping .. get the ARC flywheel instead
http://www.arcracing.com/6689-gx200-196cc-clone-speedway-billet-flywheel/
billet flywheels do not crack as easily (they too can though)

'sid

 

[zogthegreat]

Thanks for clearing that up itsid. Since this motor will be constantly removed and reinstalled on the kart when I'm traveling, I'll go with the Arc billet flywheel. Based on what you are saying, penny wise, pound foolish can be dangerous with something like this!

BTW, here is how I'm planning to have my daughter build the motor:

Clone 6.5 block, crankshaft and cam
Honda GX160 5.5 head, ported and polished
Honda GX160 5.5 flat piston
Mikuni PZ22 clone carburetor
Comet 30 clone torque converter
1 1/2" exhaust pipe

What type of valve springs and head gasket should I get and should I plan on changing the timing? If so, how much should I change it by? Also, what gear ratio should I be looking at? I want something that is more torquey than high speed, this will be a off road kart.

 

[anickode]

Thanks for clearing that up itsid. Since this motor will be constantly removed and reinstalled on the kart when I'm traveling, I'll go with the Arc billet flywheel. Based on what you are saying, penny wise, pound foolish can be dangerous with something like this!

BTW, here is how I'm planning to have my daughter build the motor:

Clone 6.5 block, crankshaft and cam
Honda GX160 5.5 head, ported and polished
Honda GX160 5.5 flat piston
Mikuni PZ22 clone carburetor
Comet 30 clone torque converter
1 1/2" exhaust pipe

What type of valve springs and head gasket should I get and should I plan on changing the timing? If so, how much should I change it by? Also, what gear ratio should I be looking at? I want something that is more torquey than high speed, this will be a off road kart.

You really won't see much gain from the timing advance alone, without doing a cam. Stock head gasket will be fine. If you're running the stock con rod, leave the valve springs alone.

 

[zogthegreat]

You really won't see much gain from the timing advance alone, without doing a cam. Stock head gasket will be fine. If you're running the stock con rod, leave the valve springs alone.

What type of cam should I go with?

 

[Whitetrashrocker]

You really won't see much gain from the timing advance alone, without doing a cam. Stock head gasket will be fine. If you're running the stock con rod, leave the valve springs alone.

I will disagree with you.
I changed only the timing on a stock engine and got about the same amount of gain as I did on another engine that I p&p and decked the head, slide carb and open pipe exhaust.

Zog go to Vegas carts and look for the adjustable timing bracket.

With all the mods your doing some timing advance will make that a beast!

I'm kinda supprised Bob hasn't GABBed on it yet!

 

[bob58o]

I will disagree with you.
I changed only the timing on a stock engine and got about the same amount of gain as I did on another engine that I p&p and decked the head, slide carb and open pipe exhaust.

Zog go to Vegas carts and look for the adjustable timing bracket.

With all the mods your doing some timing advance will make that a beast!

I'm kinda supprised Bob hasn't GABBed on it yet!

Whatever you do, I wouldn't get the NR280-0211 Ultimate Torque Camshaft with extreme Torque designed for 2000 - 5500 RPM.

I have searched the web and haven't found anybody using it.
I plan to be the only one. LOL

People mention that cam, then everybody gets the Black Mamba Jr Camshaft. I have already built a BMJr engine.

Timing, I don't really know.
Flywheel weight, I don't really know.
I don't really have the time or energy to parrot and speculate.

I would guess that 28 BTDC would be better than 32 BTDC for low end. Would stock 24 BTDC be even better? I don't know.

My offroad, 5500 RPM build will use 28 BTDC to start with.
10:1 Compression Ratio
93 Octane Gas

 

[bob58o]

Also timing depends on compression ratio. Octane rating vs burn speed is also a thing to consider that is debated on the interweb.

With a static timing, you can tune for an RPM range where the charge burns to produce peak chamber pressure around 15 degrees ATDC. This is where the pressure is most useful to turn the crankshaft. In a basic model, the time it takes for an intake charge to burn and create peak pressure depends on octane of fuel and compression ratio. Other things affect this too. You are setting the spark timing to start the burn so that at some time later (15*ATDC), the pressure in the chamber is at its peak.

Higher compression ratio - intake charge burns faster - needs less advance
Lower compression ratio - intake charge burns slower - needs more advance

Higher RPM - crank spins faster - needs more advance
Lower RPM - crank spins slower - needs less advance

 

[zogthegreat]

Also timing depends on compression ratio. Octane rating vs burn speed is also a thing to consider that is debated on the interweb.

With a static timing, you can tune for an RPM range where the charge burns to produce peak chamber pressure around 15 degrees ATDC. This is where the pressure is most useful to turn the crankshaft. In a basic model, the time it takes for an intake charge to burn and create peak pressure depends on octane of fuel and compression ratio. Other things affect this too. You are setting the spark timing to start the burn so that at some time later (15*ATDC), the pressure in the chamber is at its peak.

Higher compression ratio - intake charge burns faster - needs less advance
Lower compression ratio - intake charge burns slower - needs more advance

Higher RPM - crank spins faster - needs more advance
Lower RPM - crank spins slower - needs less advance

So using the Honda GX160 5.5 head and flat piston will increase my head pressure, therefor I need to advance the timing?

 

[bob58o]

So using the Honda GX160 5.5 head and flat piston will increase my head pressure, therefor I need to advance the timing?

Nope.
In general, if the engine is tuned properly to begin with - increasing CR means you would retard the ignition.

Higher compression ratio - intake charge burns faster - needs less advance
Lower compression ratio - intake charge burns slower - needs more advance

But I don't how tuned these engines are to begin with. Does a spark at 24 BTDC make peak pressure at 15 ATDC with 8.5 CR and 87 octane gas at 3600? To make peak power at 3600, I think it would - but is that how these come out of the box? I'm not sure. Maybe people found that even with 8.5:1 and 87 octane that 32 BTDC makes better power, even at lower RPMs. I don't know.

But getting back to your question. More compression, less timing. Less compression, more timing. That is why you should back off the timing when using a blower on a car. If timing was optimized for naturally aspirated intake, and now you up the effective CR by using forced induction- You should retard the timing.

 

[Randy H]

Also, there is more to compression ratios than calculated static compression ratios. What's more important to consider is the trapped volume or so called dynamic compression ratio. Which is dictated by the intake valve closing point. IVC ATDC. The intake valve doesn't actually close until the piston is at some point after it reaches the bottom of its stroke and is on its way back up. In other words gasses aren't being compressed until the intake valve closes. Depends on the cam profile, but the DCR will always be less than the static CR.

DCR is a nifty acronym, however I'm not sure how accurate the term is. I mean unless you have variable valve timing it isn't really dynamic. Possibly taking into account scavenging, wave tuning and higher than 100% VE (Volumetric Efficiency) IDK.

My point is that ignition timing is influenced by other factors. Without testing on a (butt)dyno or some gnarly engine simulation software it's a pig in a poke.

 

[bob58o]

Also, there is more to compression ratios than calculated static compression ratios. What's more important to consider is the trapped volume or so called dynamic compression ratio. Which is dictated by the intake valve closing point. IVC ATDC.

This is why I really like the way NR describes their camshafts.
This cam for 10:5 or higher static compression ratio.
This cam for 11:1 or lower static compression ratio.

If they are telling you NOT to use a high compression ratio with a certain camshaft, that is because the valve timing makes a lot of dynamic compression.

If they tell you NOT to use a low CR, that is because the valve timing bleeds off lots of the compression at low RPMs (think long duration cam, with narrow LSA, lots of overlap). Lower dynamic compression.

A hot cam will need a higher static compression ratio to make decent power at low RPMS. The sound of the lope at idle that everybody likes, is pretty much the sound of your engine begging for more air. You might have intake charge going out the exhaust. Exhaust gasses mixing with intake gasses. Lots of stuff I don't fully understand. At higher RPMs you get better VE due to scavenging and such.

A cam designed to maximize low end torque will cause engine knock if you have too high of a static compression ratio, (or too much timing, or too low octane). The valve timing is set so that you get all the air in, don't let any out, compress the heck out of it, and burn it to smithereens. The price you pay for this low end torque is, as expected, limited max RPMs.