which would you choose, and why?

[PaulK]

Assume you have a few acres of lightly hilly, roughly cleared, property.

Would you rather:

1) a 6.5 HP Predator 212 yard kart / buggy with 15" tires and a Comet 30 torque converter

2) a 13 HP Predator 212 yard kart / buggy with 15" tires, a Hilliard clutch, and 8:1 gear ratio


Whichever you choose, why?

(I know I'm minimizing my assumptions so you can add some where meaningful)

 

[mckutzy]

Why not a stock 212, with light mods(the free ones), Hillard clutch and 8:1 gearing...

 

[PaulK]

Seems like you'd have more low-end with a torque converter.

(In reality, the one I have came with a TC. I built the engine up and found that a TC can't handle the higher RPMs. Now I'm going to a clutch and wondering if I'm worse off than when I started. Still a good project for the kids, but disappointing if there are no advantages at all.)

 

[KingCobra]

#1 for obvious reasons.......

 

[OzFab]

2) a 13 HP Predator 212

Umm, a what? 13hp Predator is 420, as in, CCs...

I, too, would choose option 1, unless you have deep pockets...

Option 2 makes very little sense; why would you not use a TC to get the most out of the big engine?

 

[bob58o]

It is very possible to have a modified 212cc engine making 13HP.
It is also very possible you meant a 13 HP Predator 420cc.

I'll assume you meant a 420cc engine.

The 420cc makes ~20 lb ft TQ
Say you gear it 80T : 10T with a clutch. (8:1 like you said)
At clutch engagement, the axle will get 20lb ft x (8.0) = 160 lb ft
20 mph at 3600 RPM
28 mph at 5000 RPM

Now lets gear a 212 with TC to give the same torque at engagement.
A 212cc makes 10 lb ft.
Assume you gear 60:8 with 30 series TC and 6" Driven Pulley.
That is 20.1 :1 in low gear and 6.75:1 in high gear.
After a 20% loss due to the belt system, the 212/TC set up gives you, (you guessed it!)
The same 160 lb ft at the axle at engagement.
The TC makes up for the smaller engine's less torque.
The TC's over drive also allows it to go faster at lower RPMs
24mph at 3600 RPM
28mph at 4200 RPM.
33mph at 5000 RPM

Moral of the story is you probably don't need to gear the buggy 8:1 if using a 420cc and 15" Tires.

I don't like my choices.

I'd say 420cc with 6:1 and a clutch is a good choice for 15" Tires on a Buggy.
192 linear force pounds at engagement
37mph at 5k RPM.

Or a 212cc with 6:1 and a TC is a good choice for 15" Tires on a Buggy.
205 linear force pounds at engagement.
37 mph at 4500 RPM.

And my choice depends on where you drive, how you drive, how much you weigh, how much the kart weighs, do you carry passengers?

---------- Post added at 11:22 PM ---------- Previous post was at 11:05 PM ----------

#1 for obvious reasons.......

Obviously these obvious reasons are also obvious to the Original Poster. Obviously he knows all about this stuff. Obviously why he posed the question.

 

[Nosandwich]

Yeah, this whole thing just makes my head hurt. I'm going to take an Advil 212...

Later..

 

[PaulK]

I did mean the 13 HP 212.

The torque spec discussion was probably more or less what I hoped for.

Near as I could guess, #2 was somewhat worse off on the low end but maybe not too badly. I hoped maybe the clutch would make up the difference in better responsiveness given that it's chain driven vs belt.

My math was a lot less precise than yours but let me try this one on you, because I didn't fully understand.
Max Comet 30 differential is about 3:1 given all else is the same. Let's assume 6:1 initially.
That means 18:1 at the low end. But if you take off 20% for being belt driven that would make effective ratio closer to 14:1 (??). The 13 HP 212 has twice the HP of the stock version but what's the torque difference (assuming Black Mamba Jr cam and 9:1 compression)? Enough to offset much of the difference between 8:1 and 14:1?

It'll probably usually run 2 kids at 100 lbs each.

For the money and the trouble I certainly should have just bought a bigger engine if it were all about results, but my kids did learn a bit about mechanics. Not sure if it was worth the price but it was better than playing video games.

All that said, I have to get it running right and then I'll let you know how it feels. But it's interesting thinking about it so I figured I'd ask.

---------- Post added at 07:05 AM ---------- Previous post was at 06:59 AM ----------

why would you not use a TC to get the most out of the big engine?

Because it's not a big engine and the TC can't handle the higher RPMs.
I just melted a Comet 40.

 

[landuse]

I did mean the 13 HP 212.

I think what the guys are saying, is that a 13HP engine cannot be called a 212. The 212 stands for the cc value. A 13HP is more like 420

 

[PaulK]

If you build a Predator 212 to Stage 3 the result is greater HP (and more torque).
Does that change the cc?

 

[itsid]

you misunderstood..
it's 20% power loss not ratio adaptation.

No non-toothed belt driven system is more than 80% effective as of yet.
(in fact the comet should be worse even.. but there's no data about it specifically ...so...80%)

And since it looses roughly 20% of power and we hope for a properly transmitted rpm, that means 20% less torque is transmitted. (the rest gets eaten by the belt and sheaves)
In reality it's more likely a mix
(you loose a few rpms [say 0.6-1%] and a bit of torque [due to the belt slippage] which add up to a total of said 20% powerloss)

the series 30 has a low end of 2.7:1 (modern day series 30s with 6" driven that is)
with 6:1 chain ratio that's 16.2:1

engine torque of your modeed 212 is easy once you know at which rpm you have the most power
it's (power * 5252)/rpm = torque
so say you have 13hp @ 5000 rpms that'd calculate to about 13.66 ftlbs at 5000 rpms
(18.5 Nm in real world values)
since peak torque is shifted from peak power somewhat all values are "give or take" of course.. to know for sure one needs a dyno I'm afraid
even exactly identically built engines do not have the exact same performance, neither torque nor hp..
in fact you cannot even measure the engine twice and get the exact same reading...
it's more an average over a number of dyno runs that you'd be interested in

'sid

 

[PaulK]

I understand that the 20% loss doesn't translate literally into a lower gear ratio. I was trying to estimate the overall impact (or feel) but I guess it may not really work that way.

If I understand you correctly, by doubling the HP you also double the torque. Somehow I thought you got less of a torque increase for any given HP increase.

How about this one - would a 12:1 gear ratio get you roughly twice the initial acceleration as a 6:1 gear ratio or is the relationship not a straight line? In other words, all else equal, would you go 0-10mph in half the time?

I'm probably opening a can of worms and really showing my ignorance with that one.
If it's too much to bite off that's fine.

I'm just hoping this engine, once completed, won't have much slower acceleration. That would just really **** me off. I'm aiming for a top speed in the 30-35 mph range so I might nudge the gear ratio up a little. Not sure how many RPMs this thing will pull.

 

[mckutzy]

So what mods are you doing/done to the motor so far???

 

[bob58o]

Use the Black Mamba Jr 212cc Engine with the 30 series TC.
Best of both worlds.
Better Acceleration.
Better Top Speed.
If you are concerned about the modified engine losing some low end, you could always buy stiffer TC Driver Unit springs to raise the clutch stall RPM.

Stamp this one Done! Next. LOL

As far as max RPM, that is going to depend on weight and gearing.
On my buggy with 16" Tires, TC and 7.5:1 Gearing, I think I topped out ~ 5k RPM. 37mph
But that was with a stockish engine IIRC (air filter, exhaust, jettting,...)

The modified BM Jr engine on my mini with 13" Tires, TC, and 5.4:1 gearing spun over 7k RPM.

 

[PaulK]

Intake and exhaust
.650 Bored , Ru*ing Gas Carb
22 lb springs
Black Mamba Jr cam
Billet flywheel
Billet connecting Rod +.040"
Flat-top piston (.550 compression height)
Fuel pump

Tried a Comet 40 but fried it from high RPMs. Now I ordered a Hilliard Inferno Fury (13T sprocket). Axle sprocket has 60 teeth, jackshaft in will have 21, jackshaft out will have 12 (or possibly 11).

It's assembled, and even runs, but there's a fuel problem. I have to play with the choke while driving to get it to really hook up, and even then it doesn't feel like it should (probably). Mixture screw doesn't make a noticeable impact. I see that there's a bubble in the fuel line but I've been told that's not a problem. I'll probably try to get rid of that next anyway. Not sure what else to try.

I know it may not run 13 HP but I'm estimating ball-park numbers.

 

[Nosandwich]

Rejet the carb?

 

[itsid]

If I understand you correctly, by doubling the HP you also double the torque. Somehow I thought you got less of a torque increase for any given HP increase.

How about this one - would a 12:1 gear ratio get you roughly twice the initial acceleration as a 6:1 gear ratio or is the relationship not a straight line? In other words, all else equal, would you go 0-10mph in half the time?

I'm probably opening a can of worms and really showing my ignorance with that one.
If it's too much to bite off that's fine.

I'm just hoping this engine, once completed, won't have much slower acceleration. That would just really **** me off. I'm aiming for a top speed in the 30-35 mph range so I might nudge the gear ratio up a little. Not sure how many RPMs this thing will pull.

No, no and no
with some yes spread in.
We talk strictly industrial engines here "in general" not specifically
a certain mod or engine...
NO you do not double the torque if you double your hp
since most of that hp is coming from increased rpms mostly (governor removal passing valve float and such.

since power is torque times rpm (well speed actually but for an engine that IS rpm!)
the higher the rpms the less torque you need to get hp.

with JUST increasing rpms (remove governor, allow to breathe freely no other mods)
you increase rpms but you do NOT increase or decrease max torque
since torque values describe a bell curve it drops at higher rpms again
which means although your hp at 5000 rpms is bigger than at 3600
chances are you still have LESS torque at the higher rpms.

shifting timing moves the torque curve slightly, different cams do that as well (some even increase the torque slightly)
a higher compression ratio also increases torque..
a combination can do all sort of things in between and beyond

No, doubling the ratio would NOT automatically double the acceleration.
it's FAAAR more complicated than that, as I said torque is NOT linear, it's a bell curve.
so acceleration itself is not linear (far from it tbh)

But here's the thing, if you mod an engine properly (you can mess things up of course... but let's assume you do it right, shall we?)
you increase either hp or torque or maybe even both.
no matter what the performance of the engine will be BETTER!
and with a bit of adjusting you can make use of that increased performance.
So much slower acceleration is just not going to happen.
(once gearing is properly dialed in to your new power curves it'll certainly be a quicker acceleration!)

'sid

 

[bob58o]

I haven't pushed a 40 series much beyond 5k RPM.
In my head, I have 5k RPM as the limit for a 40 series TC.

On the other hand. I have pushed a 30 series over 7k RPM very often. And it was on a modified 212cc making 13HP or more (11:1 CR, gx390 Carb, BM Jr Cam, 26lb springs, Milled n Ported Head,...)

Maybe a 30 series would work better than a 40 series????


And since that engine will spin 6500 - 7000 RPM, you should gear it to use all of those RPMs.
You want quick acceleration and around 35 mph top speed?

C'mere, listen close, this is what you do....
Use the modified 212.
Use a 30 series TC with a 7" Driven Pulley.
Keep the 60T on the Axle.
Put an 8T on the TC Jackshaft.

This will give you an instantaneous gear ratio of 23:1 at engagement for an instantaneous linear force of 300 lbs to start you moving. And you will go 35mph at 6500 RPM.

Now if you want to calculate instantaneous linear acceleration, you can. But since that acceleration is only obtainable for an instant, it is not very useful to calc 0- x mph times.

For example...
300 force lbs = 1334 Newtons
1,334 N = mass (kg) * acceleration (m/s^2)

Assuming 660 lbs fully loaded that is around 300 kg.

So...
1,334 N = 300 kg x acceleration
Acceleration (m/s^2)= 1,334/300
Instantaneous acceleration when TC engages will be ~ 4.44 m/s^2
4.4 meters per second = 9.8 mph
So I guess I would expect 0-10mph in ~1 second.
Instantaneous acceleration is all I would even try to calculate. Torque drops as RPMs increase. and Gear Ratios change with torque converters as you accelerate.

So there you have it, my final answer.

Modified 212
30 Series TC
7" Diven Pulley
60T Rear Sprocket
8T TC Sprocket

 

[PaulK]

Ugh.
Well, my hope was that I could explain the answer to my kids and that's not going to happen.
I get some of it but not well enough to communicate it to a 12 yr old.
Mostly just affirms that I'm glad I didn't try do this kind of thing professionally.

Thanks though. It is interesting, even if only half absorbed.

 

[itsid]

LOOOL
no worries PaulK..
acceleration is "change in speed over time"
we took it a bit further and got "change in acceleration over time"
which is indeed nasty in itself
since we come close to Heisenbergs uncertainty principle
while you cannot EVER calculate acceleration for any one point in time (you will always need a timespan!)
and thus words like instantaneous are surely misleading,
change in acceleration requires a whole new level of abstraction.
to not necessarily consecutive timespans of not necessarily the same length normalized and differentiated is a level of math not many adults are willing to deal with,
12yo kids... I'd say I know ONE who would kinda enjoy that task.(she's not twelve yet so who knows)
Anyways..

the basics are fairly easy to explain to kids in basic terms.

Here's my go at it:
you (your legs) are the "motor" of your bike.
when you get going All you have is torque, no speed and no power yet.
as you pedal your leg's torque is translated to power since you start moving.
the crank turns at your leg's rpms.

Once you move it get's easier to accelerate;
at the rpms where it's easiest you reached your peak torque..
beyond said rpm speed it gets harder and harder to accelerate further again,
and just when you hit top speed you cannot accelerate any further and
that's the highest power you're outputting since you're fastest.

it's blurry alright.. but it explains the correlations in rather simple terms, not?
And since it's "close enough" to the truth... it'd be my kid-friendly explanation I guess.

'sid