cc an hp, help

[kraz0r]

Hello. Im sort of a noob when it comes to engines.
Im going to build my self a go-ped, and found an engine with 39cc and 6,7hp.
What has cc to do with speed and power?
Will this engine be less powerfull than a 80cc with 5hp?

Thanks :)

[airshifter]

This is a much more in depth question than can be answered in a short post but......

The smaller engine in your example has more stated power (6.7 I think) and therefore is the more powerful engine, regardless of displacement. That is not to say however, that it will be the faster engine on your go-ped.

There are a great deal of factors in engine design that lead to differences in total HP, as well as peak torque. The demands on an engine are many and varied depending on application. If the engine with the higher HP has a rev range too high to reach under the loads, it may never reach that power level.

If the engines were of the exact same design with scaling of every component to increase size, the larger engine would almost always be favorable. But that is very seldom true.

An F1 engine makes around 800 or so HP. My truck makes only about 260. Many would think that the F1 engine would make my truck very fast. In reality probably not. The engine in my truck makes peak torque at somewhere near 2700 RPM. With the heavy weight and fairly tall gearing this engine moves the truck just fine. But the high torque peak of the F1 engine would struggle to get it moving, and the tall gear ratios would mean it would likely never near the peak power output levels.

[kraz0r]

Okey, thanks. So its a combination of rpm, cc and hp?
If thats so i think i can understand it.
Thanks for such answer airshifter :)

[Mark]

An F1 engine makes around 800 or so HP. My truck makes only about 260. Many would think that the F1 engine would make my truck very fast. In reality probably not. The engine in my truck makes peak torque at somewhere near 2700 RPM. With the heavy weight and fairly tall gearing this engine moves the truck just fine. But the high torque peak of the F1 engine would struggle to get it moving, and the tall gear ratios would mean it would likely never near the peak power output levels.

Indeed, I've often heard that F1 engines don't have very much torque in fact.

[schmenke]

I’m no mechanical engineer, but afaik horsepower and torque are two distinctly different units of measure.
Horsepower is the measure of power, which is the rate of work, i.e. X units of work measured over Y units of time, e.g. 1 N-m / 1 second = 1 W (Watt) (I think).

In reference to automotive engines, HP and torque are related through engine speed: RPM. Power (HP) = Torque x RPM / (some constant figure that I can never remember).

The reason that high performance race cars produce so much horsepower with very little torque is largely because of the short piston stroke, which allows for high engine speed. High RPM allows the engine to remain at peak HP longer.

Horsepower can alternatively be maximized by altering the bore/stroke ratio but this generally results in lower RPM and peak HP is compromised.

(Gawd, I’m awful at explaining stuff like this... :mark: )

[Mark]

I've read explanations dozens of times but I still can't grasp the difference between power and torque :crazy:

[schmenke]

Torque is merely the measure of turning force. An example is removing the lug nuts of a wheel. A force is applied to the end of a wrench to apply a level action. A stuck nut can be removed by either applying more force (pushing down harder on the end of the wrench), or using the same force but increasing the length of the wrench handle. Either way, more torque is being applied.

Power is the measure of time spent removing the nut for a given torque setting.
The wheel nut can be removed completely by spinning the wrench around quickly, say in 5 seconds or slowly in say 10 seconds.
Either way the nut is removed, but twice as much power is required to remove the nut in 5 seconds.

(I know, probably a simplistic example, but like I said, I'm pants at this stuff...)

[airshifter]

Torque is merely the measure of turning force. An example is removing the lug nuts of a wheel. A force is applied to the end of a wrench to apply a level action. A stuck nut can be removed by either applying more force (pushing down harder on the end of the wrench), or using the same force but increasing the length of the wrench handle. Either way, more torque is being applied.

Power is the measure of time spent removing the nut for a given torque setting.
The wheel nut can be removed completely by spinning the wrench around quickly, say in 5 seconds or slowly in say 10 seconds.
Either way the nut is removed, but twice as much power is required to remove the nut in 5 seconds.

(I know, probably a simplistic example, but like I said, I'm pants at this stuff...)

I thought it was a very good example. Torque is a measure of force, HP is a measure of how fast that force is applied.

As for the formula HP = torque x RPM/5252

HP and torque will always be equal at 5252 RPM for this reason. HP will always be less than torque below 5252 RPM, and HP will always be greater than torque above 5252 RPM. This is not to say that peak torque must happen below 5252 RPM, simply that HP will be greater above that point.

In simple terms that is why cars with low rev torque seem quick, they make greater HP at lower RPM. On contrast high revving cars often make very little torque. In my example of an F1 car, if the car makes 800 peak HP right at the 18,000 rpm redline then it is only producing 233 lb ft of torque. It's just doing it very quickly. :)

[kraz0r]

Okey, thanks everyone :)
The engine im looking for has around 18000 rpm.
Will this 39cc, 6.7hp with 18000 rpm works fine to get a nice top speed?

[Mark]

233 lb ft of torque.


What's than in English?

[schmenke]

Okey, thanks everyone :)
The engine im looking for has around 18000 rpm.
Will this 39cc, 6.7hp with 18000 rpm works fine to get a nice top speed?

2-stroke :?:

[Roamy]

Hello. Im sort of a noob when it comes to engines.
Im going to build my self a go-ped, and found an engine with 39cc and 6,7hp.
What has cc to do with speed and power?
Will this engine be less powerfull than a 80cc with 5hp?

Thanks :)

The cc = how many time you are going to have to pedal in one mile for building that POS

[kraz0r]

2-stroke :?:

yes its 2-stroke

[Zico]

Put your known figures of each engine into this online calculator.. Horsepower Design Equations Formulas Calculator - Torque Rotating (http://www.ajdesigner.com/phphorsepower/horsepower_equation_rotating_torque.php) and compare.

:)

Then find out the engine weight of both units and use the power to weight ratios calculator (at the bottom of that page) to see which offers the best compromise

[airshifter]

What's than in English?

316 Nm

[airshifter]

Okey, thanks everyone :)
The engine im looking for has around 18000 rpm.
Will this 39cc, 6.7hp with 18000 rpm works fine to get a nice top speed?

In theory the smaller engine with the higher horsepower output would be capable of a higher top speed if it was geared correctly. My guess is that in actuality it would not be the better engine to use. The larger displacement engine is likely a four stroke, or a two stroke designed with a wider power curve. Unless you have a gearbox often high revving engines are not suitable for many tasks.

To top that off, an engine with a high RPM power curve isn't usually well suited to long term use. They are rotating much faster, and thus will wear faster.

If you are building your go-ped for top speed only, on flat ground, and don't care about slow acceleration, then the smaller engine might work just fine. If you are building it for all around use and can sacrifice some top speed for more ability to pull up hills, accelerate quicker, and last longer the larger engine would most likely be a better pick.

Of course this is based on the assumption that the larger engine is a four stroke engine without any crazy specifications of build. If you have specifics or links to the engines in question we could probably give more solid input. The two stroke engine you are speaking of would have a very, very low torque output, and would not be suited to getting a person moving very quickly.

[Mark]

316 Nm

Interesting, considering my car has something like 215 torques, that's not much more. But then I've got 90bhp instead of 800!

[airshifter]

Interesting, considering my car has something like 215 torques, that's not much more. But then I've got 90bhp instead of 800!

Small difference. Good driver skill should be able to overcome the power deficit. :)

Due to the relative low torque output of an F1 car, the final drive ratio is very low to allow the engine revs to come up to where the power is made. Take note of your cars speed in 1st gear at 6000 RPM, then triple that speed. That tripled speed is where your redline would be in a formula 1 car geared the same way.