What determines hp?

[John2189 453]

I have specs for an18 hp Kohler and a 20 b&s. 
The kohler has bigger bore a little less struck and more cu in  than the b&s

sorry the pic is sideway

[ebinmaine 67,497]

This is a question that has literally dozens of variables.

Horsepower is a calculated figure based off measured torque. 

Many things influence HP. 

Engine configuration. (Flat, inline, V etc)

Cam shape and timing. 

Intake and exhaust port shape and smoothness. 

Size of all the different components. 

 

Also very important is the fact that manufacturers will advertise HP that may not be exactly what the engine produces. 

 

Sooooo many answers...

 

 

[pfrederi 17,739]

Partly marketing maybe???

 

Consider KT-17 and Magnum 18  identical engines except for ignition system...

[John2189 453]

18 minutes ago, ebinmaine said:

This is a question that has literally dozens of variables.

Horsepower is a calculated figure based off measured torque. 

Many things influence HP. 

Engine configuration. (Flat, inline, V etc)

Cam shape and timing. 

Intake and exhaust port shape and smoothness. 

Size of all the different components. 

 

Also very important is the fact that manufacturers will advertise HP that may not be exactly what the engine produces. 

 

Sooooo many answers...

 

 

Ok so I get the cam size and timing. So let’s say if it’s put on a dyno and levels out at let’s say 18.75 hp and it spikes at 20. They could say it puts out 20 hp?

[lynnmor 7,306]

5 minutes ago, John2189 said:

Ok so I get the cam size and timing. So let’s say if it’s put on a dyno and levels out at let’s say 18.75 hp and it spikes at 20. They could say it puts out 20 hp?

 

You could say that is a 20 hp engine since it has a peak of 20.  Manufacturers might use the figure that is produced at the engines rated rpm, usually 3600 on lawn and garden equipment.

[John2189 453]

Ok thanks 

[8ntruck 7,004]

In the automotive industry, the Society of Automotive Enngineers (SAE) have set a standard way to test an engine for horsepower.  This came into existence during the 'horsepower wars' during the early '60's.

 

There probably is a standard testing procedure for the small engine industry.

 

 

[Skipper 1,788]

What is also very much interesting is how the torque is made, because as Eric so well pointed out, the hp number is just a straight forward calculation, based on torque, rpm and a constant. That doesn't tell you the full story though, of the engines temper. The way it delivers that power. Is it a long stroked engine that delivers oodles of torque down low, or is a shorter stroke that bases it's power more on rpm. Therefor it is important to find an engine that delivers enough power, in a suitable manner, for the task at hand.

[kpinnc 12,047]

10 hours ago, 8ntruck said:

There probably is a standard testing procedure for the small engine industry.

 

I heard (and I'm not making a joke) that there was actually a class action lawsuit against many mower manufacturers some years ago because adverised power grossly exceeded actual horsepower- or something like that. I think it was more against big box store brands than commercial grade stuff. 

[lynnmor 7,306]

2 hours ago, kpinnc said:

 

I heard (and I'm not making a joke) that there was actually a class action lawsuit against many mower manufacturers some years ago because adverised power grossly exceeded actual horsepower- or something like that. I think it was more against big box store brands than commercial grade stuff. 

 

Here is information on the lawsuit.

 

Here is information on the standards for measuring torque and horsepower in small engines.

 

Briggs & Stratton has been giving torque ratings instead of horsepower in recent years, I believe that is just a reaction from the lawsuit and a way to sidestep more legal action.  No matter how you slice it, horsepower is the amount of work that can be done and a torque number is of little value since it is already included in the horsepower calculation. 

[kpinnc 12,047]

7 hours ago, lynnmor said:

 

Here is information on the lawsuit.

 

Thank you! 

[Handy Don 12,227]

8 hours ago, lynnmor said:

 

Here is information on the lawsuit.

 

Here is information on the standards for measuring torque and horsepower in small engines.

 

Briggs & Stratton has been giving torque ratings instead of horsepower in recent years, I believe that is just a reaction from the lawsuit and a way to sidestep more legal action.  No matter how you slice it, horsepower is the amount of work that can be done and a torque number is of little value since it is already included in the horsepower calculation. 

This is nice to have, thanks!

As a bit of a data nerd, I always appreciate when the manufacturers provide horsepower and torque vs. RPM graphs in their engine manuals. Rare now except for commercial grade stuff.

[Snoopy11 5,714]

RPM, RPM, RPM!!!

 

Keep in mind, a couple hundred more or less RPM can make a HUGE difference in power. We see this widely on the dyno.  

 

Don

[Maxwell-8 4,275]

Torque is what we use in lawn equipment engines. and why a 10hp diesel is stronger then a 10hp gas engine. (in our application)

 

HP= TQ x RPM / 5252

TQ is in foot pounds

 

engine's make more torque below 5252 rpm then HP, above that RPM they make more HP then torque.

That's also why Lawn equipment enines make so little Horsepower for their displacement. They just don't rev to 6-7k RPM like a car or bike. (and they don't need to)

 

Edited September 28, 2021 by Maxwell-8

[Snoopy11 5,714]

24 minutes ago, Maxwell-8 said:

They just don't rev to 6-7k RPM like

 

Like my Tillotson and Duromax engines... LOL

 

Don

 

 

[oilwell1415 563]

In simplest terms, horsepower is determined by how much air and fuel you can move through the engine per minute.  On the flip side of that, torque is determined by how much air and fuel you can move through the engine every revolution.  As was stated above, the relationship between the two is purely mathematical.  It's a simple matter of converting a linear equation for work and power into a circular one.