which would you choose, and why?

[ezcome-ezgo]

Get a big blackboard/whiteboard (whatever) and set your kids down in chairs in front of it (strap them down if necessary). Go through all this with them at great length using math, pictures, examples, etc. Do not stop until these concepts are thoroughly understood. Result: smart kids.

 

[KartFab]

NO you do not double the torque if you double your hp
since most of that hp is coming from increased rpms mostly
'sid

this is what so many people don't understand.

e.g. well i can nodify my predator 212 to get 11-13 hp for $200 so why should i waste my money on a predator 420?

I dunno, more than double the torque?

predator 212 has 8.1 ft lbs of torque
predator 420 has 18.4 ft lbs of torque

This is why manco geared their large go karts with 10-13 hp engines for 40 mph, and their small go karts with 5-6.5 hp engines for 25-35 mph.

 

[KingCobra]

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

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

OBVIOUSLY

 

[OzFab]

I did mean the 13 HP 212.

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

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

Understood

 

[KingCobra]

I don't believe a STAGE 3 Preditor 212cc 7 HP engine would even come close to a 13 HP.

 

[landuse]

One thing people also do not understand is "Why they do not go any faster when they have hopped up their engine." Unless you change your gearing to match the new found power in your engine, you are still going to get the same top speed.......you will just get there a lot faster

 

[Nosandwich]

I don't believe a STAGE 3 Preditor 212cc 7 HP engine would even come close to a 13 HP.

How would you know?COLOR="Silver"]

---------- Post added at 12:38 AM ---------- Previous post was at 12:37 AM ----------

[/COLOR]

One thing people also do not understand is "Why they do not go any faster when they have hopped up their engine." Unless you change your gearing to match the new found power in your engine, you are still going to get the same top speed.......you will just get there a lot faster

True that.

 

[PaulK]

That's all I'm looking for.

Saw this video of a guy that had built his kart engine. He put his daughter on it, probably weighing in at 75 lbs or something, and she pulled a wheelie to probably 45 degrees. It was hysterical. I thought right then - gotta try that!!

My kids are pretty nuts. Have no interest in having them push 50 mph.
But a little whip lash could be fun.

 

[landuse]

My kids are pretty nuts. Have no interest in having them push 50 mph.
But a little whip lash could be fun.

Nothing wrong with some whip lash

 

[PaulK]

Reread this thread, trying to absorb more.
Yesterday I asked a question about gear ratio and it's impact on acceleration.
I think the answers given, though interesting and helpful, may have related more to the impact engine mods have on acceleration. (easy to happen with so much going on in these threads)

Minimizing the technical formulas where possible (Itsid's analogies were great), could you tell me whether the impact on low end acceleration between 6:1 and 9:1 is linear or curved or exponential?

 

[Nosandwich]

Linear ,dummy.

 

[itsid]

Within limits you can safely assume it's a linear change
(it is NOT! it's inverse exponential if I'm not mistaking)

but for an engine like yours and an ordinary kart and your ratio area from 6:1 to 9:1
the difference will be not noteworthy (I think we talk 0.1m/s² difference from a linear change)

saying it's linear is just stupid tbh,
since it's not at all, and when you come to the limits of feasibility of gear ratios and such
it'll simply kick your behind, if you assume it's linear

just friction is a terrible thing and want's to be accounted for, but let's not go too deep into that rabbit hole

On the other hand, within the realms of "decent values" it simply doesn't make much of a difference
if you calculate it precisely (jolt from torque at rpm per friction and so on)
or
just take the effective torque at engagement speed and multiply it with your gear ratio
and just wing it.

Within the small rpm band and the narrow gear ratio window, the difference will not be noteworthy
(we talk 0.1m/s² or so over a 50% ratio increase)

So as long as you are aware that your assumption of it being linear is WRONG,
you can safely assume it is (within these limits)

'sid

 

[bob58o]

First Lecture of College level Calculus-based Physics was started off like this....

"Physics is learned through a series of approximations, bs, and lies."

 

[bob58o]

One thing people also do not understand is "Why they do not go any faster when they have hopped up their engine." Unless you change your gearing to match the new found power in your engine, you are still going to get the same top speed.......you will just get there a lot faster

I disagree.

Remove Governor - Allows more RPMS - Higher top speed
Upgrade Valve Springs - Allows more RPMs - Higher top speed
Advance Ignition Timing - Shifts power to higher RPMs - Higher top Speed (assuming gov and springs already done)
Bigger Carb - More power at higher RPMs - Higher Top Speed (assuming gov and springs already done)
Camshaft with more duration - More Power at Higher RPMs- Higher Top Speed (assuming gov and springs already done)

Assume 10" tires and 6:1 gearing.
The tire size and gearing will remain constant for the following example.

Lets say you can go 18mph at max of 3600 RPM with a stock engine.
Remove the gov and now you can go 26mph at max of 5200 RPM.
Upgrade the valve springs and now you can go 30mph at max of 6000 RPM.
Advance the ignition timing and now you can go 32mph at max of 6400 RPM.
Upgrade the carb and now you can go 34mph at max of 6800 RPM.
Upgrade the cam and now you can 40mph at max of 8000 RPM.

These numbers are just here to demonstrate the point. Not representative of actual data.

Modifications that increase max rpm WILL increase max speed.
What I wouldn't do is modify an engine and assume the increase in torque is enough to try to change the gearing. Like stated above, not very much TQ is gained by modification.
So don't take a 6:1 kart with stock 212cc powerplant, throw a stage 3 or 4 kit on it, and think you can go to 4:1 There is No Replacement for Displacement!!!!!

Some people think that swapping a governed 420 in place of a governed 212 will increase top speed. IN THAT CASE the top speed does not increase unless you change the gearing( Unless the gearing was severely jacked up to begin with and you weren't reaching 3600 RPM).

 

[landuse]

I defer to your superior knowledge. Thanks for clearing that up

 

[itsid]

I disagree.
...

And with all modifications that provide higher rpms
(achievable WITH the given gear ratio)
that is perfectly correct.

BUT there are ways to add power to an engine that does NOT necessarily lead to higher reached rpms.
At which point Paul was correct all along.

Say you do not add heavier springs after removing the governor,
and your valve float sticks at ~5500 rpms.

a compression enhancement or cam upgrade or a combination of the two will still add power to the engine... but no speed to the kart.

Say you do add 18lbs springs and your engine could go 6800 -7000 rpms
but you don't have a cam or haven't increased the compression yet,
instead you just advanced timing.
your engine might not have the torque to rev up all the way to 6800
at all with the gear ratio you have.
while technically you still gained a horse perhaps,
but speed sticks still at around the same value
and gearing a hair lower will allow you to reach the rpms needed for more speed..
Yes.. that's an unlikely case but it's still a possibility that is not unseen.
And still Paul would be correct.

You modify your engine (no matter how)
you should make sure your gearing is adjusted so you can make use of the additional power in the best way possible.

I bet there are several modded engines around mounted with a less than optimal gearing ratio
that have 5 to 10 mph of speed unused
(some intentionally for better take off, some simply because the owner doesn't know)

So yeah, you have a point there.. but I still disagree with your disagreement (generally speaking)
Since if you mod your engine, it's still a good idea to check your ratio, chances are you can do better now

'sid

 

[bob58o]

Since if you mod your engine, it's still a good idea to check your ratio, chances are you can do better now

'sid

I have a question. Maybe you could shine some light on it.
I have this idea that engines will reach a max RPM for one of two reasons.

A.) The load on the engine is sufficient where the engine cannot spin any faster.
Say you are geared 3:1 with 12" tires, burning up clutches doing 43mph at 3600 RPM and cannot reach any higher RPMs even after governor delete and valve spring upgrade. Switch to 4:1 and max rpms will increase. Maybe now you top out at 40 mph and 4500 RPM.

B.) The engine runs out of air.
Say you are geared 10:1 with 12" tires and a built engine with camshaft that should make peak HP around 7.5k RPM. Now this engine might reach 8k RPM, but the engine might run out of air before the kart reaches 30 mph. Not enough cam duration, Not enough lift, valves too small, carb too small, ...
Would it be possible to gear faster and faster up to a point without seeing a large drop off in max RPMs????
Maybe you switch to 6:1 and can still come close to 8k RPM, now at 48mph. The max rpm remains the same (or just slightly less) because it was the air flow limiting the RPMs not the load on the engine.

Does this make sense? Or am I just describing the same thing two different ways?


And as far as gearing "better". That's tough unless you are building for a specific race. As you mentioned, people aren't always gearing for top speed.

If my fun kart can take corners at 25mph and my engine makes peak torque at 4k RPM and peak HP at 7k RPM, I might gear so 4k RPM is 25 mph. That way I'm coming out of turns and my engine is making peak torque. It might only do 45 mph tops, but possibly better gearing for a fun kart than if geared to top out at 55mph. I want my gearing so I'm spending as much time possible between peak TQ and peak HP. I was talking CC's here. But isn't that what TCs are supposed to do. Keep the engine close to peak HP for as much time as possible.

Or if I'm racing from stop sign to stop sign, I might want to gear so 7500 RPM is around 35mph.

I wish I could intern on go kart pit crew for a summer.
They would hate me so much. "Wait, not yet! Let me get my calculator!"

 

[itsid]

interesting question.

In some cases there should be a setup where you can increase speed without seeing a large drop in rpms
In fact I'd say in most cases you can.
none of yours though (8k..really.. ?!)
Anyways..

I don't see how an engine can rev 3600 on 3:1 on 12" wheels but should fail on revving 5500 on 4:1 really (which would translate to ~49mph) after governor removal.
don't forget you add 40% of torque (and thus 5k rpms should be easily doable [45mph]) I'd say even the 'full' 5500 but since that's mostly a question of the stock springs..
and ignition timing (yes chances are it's not always the same on stock chinese engines)

So in your described case... lower gearing would still lead to higher top speed.

Anywhoo, that's not what you were asking,

Now to your question:
Say you shift your torque to the upper revband (sporty cam, ignition advanced 8° or more etc...)
With say 22lbs springs you'd gain a solid 7500 rpms (let's assume)
if you're geared low (7:1) on the 12" wheeled kart from the example above
you'd hit a solid IDK 40mph or something.. easily no effort whatsoever.
You can easily gear 6:1 and reach 45mph
or 5:1 and reach and reach 54 mph
and maybe you still have the torque to gear even higher
4:1 and reach 67mph

All without loosing any rpm from a lack of torque.

it all depends what your starting values are in the first place.
So if you start with 4:1 and your engine is already at it's limits,
the answer would be no.
you could still find a more exact limit by gearing higher and higher until your engine drops rpms
but the steps should be smaller

And then you'll run into difficulties as soon as it get's warmer (less oxygen per volume of air...)
and the engine might need a bigger carb to perform identically
(or you just gear down again )

I hate it..
but get my app on your phone do some testruns with the tach (saved akrt) and see how it does.
(it's the closest a website can come to a dyno)
repeat on the same track with different gearing and compare the two results.
with a good enough GPS accuracy (5m is great..10m should suffice)
and a constant driver the results should be fairly accurate too.

'sid

 

[Denny]

Bob and Sid it sounds like you guys are arguing about horsepower over torque. If you build for torque the horsepower will usually follow. At higher RPMs torque will never exceed horsepower (above 5250 rpm).
Something observing engines on dynos has taught me.


Denny

 

[bob58o]

Bob and Sid it sounds like you guys are arguing about horsepower over torque. If you build for torque the horsepower will usually follow. At higher RPMs torque will never exceed horsepower (above 5250 rpm).
Something observing engines on dynos has taught me.

Denny

Not sure we're arguing at all. More of a nerdy conceptual discussion.
If you want to start a TQ vs HP argument, then I give up already. My head hurts when I start thinking about Force, Work, and Power. I get confused when I start pushing boxes around. I wonder how I can do work at a quicker rate without applying more force. Then I just say scrw HP. Torque is Real, RPMs are real, HP can be calculated. If designing an engine, I'm not sure what is most important. The combustion pressure= Force? The crankshaft stroke=lever length? Or the ability to make more burns per minute (even if that means less force or less lever length)?

And if there is no replacement for displacement, I guess Torque is more important than HP.
For single geared fun/yard karts I'd prefer a broader curve, opposed to a peaky curve that might make more HP at a specific RPM.

https://www.carthrottle.com/post/wmeo99r/

"So, as you can see, we’ve calculated the acceleration of a car, without any horsepower figure whatsoever. This might be very confusing for some of you but this is what it is. Torque is what does all of the work, and Horsepower is just a byproduct “fabricated” from torque and means almost nothing."
<Not sure if this is true, but I like it!

"Horsepower sells cars, torque wins races"
< Like this too!

Apparently below is the secret key to making more power. MAGIC SLIDERS!!!!!