hp vs displacement

T

The motor guy

Joined
Nov 29, 2009
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So lets compare my side by side to my ztr mower engine. Fairly close in size so this is why I chose these two. Only difference the side by side is dual overhead cam and fuel injection while the mower is a standard pushrod engine, and carbureted. The mower engine is 852cc's with 25.5hp and the side by side is 1000cc's with 110hp. What do they do to the side by side engine to squeeze out so much hp? Why can't I get 110hp out of my mower? Both are two cylinders btw. Also, the one with 110hp uses way less fuel.
 
First is fuel injection as it's way more efficient than carburetors. I always thought overhead cams were more efficient as well. Different applications. The mower is geared and designed to run high rpms for a significant amount of time vs the side by side. Maybe a newer engine design as well.
 
OPEs are traditionally made to run around 3600 rpm; low stress on the parts, etc. This allows those parts (crank, con rods, pistons, valve-train, cooling fan) all to be made less expensive because they don't have to survive at 10,000 rpm. If the goal was to make high rpm HP, the OE (Kawasaki; Kohler; B&S; Honda) could certainly make more, but the engines would cost more.

Plus, high rpm means more noise; a detriment when mowing your lawn. No one wants to hear a 10,000 banshee scream past at 9am on a Saturday morning while sipping their coffee as their neighbor cuts his grass.

Ironically, it's not about the fuel delivery method; I disagree with that concept. Fuel injection doesn't really make significantly more HP; it allows engines to run cleaner, because they are easier to "tune" throughout the whole rpm band. There are plenty of examples where engines are making very high HP running carbs in many forms of racing. NASCAR engines and drag-race engines make PLENTY of power at their high target operating rpm, but they often won't run as clean at much lower rpm.
 
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Jetronic said:
One way to run more hp, is to run the same torque at higher rpm. I'm sure the side by side has a higher max rpm, or at least the max power is at a higher rpm.
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Yeah I forgot about that. The mower runs at 3600rpm and the side by side will do like 8800rpm. You can't do that with a push rod engine, can you?
 
The motor guy said:
Yeah I forgot about that. The mower runs at 3600rpm and the side by side will do like 8800rpm. You can't do that with a push rod engine, can you?
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Sure. The Mercedes Indy engines from the 90's were running over 10,000 RPM with pushrod. Was specifically designed for it, and only had to last an afternoon. In a daily driver, likely not.

Power is mostly about how much fuel you can get an engine to burn and how fast. RPM makes it burn fuel faster (more cyles per period of time). Higher compression makes the explosion more powerful and liquid cooled keeps things from melting down.

I'll add to what @dnewton3 said. OPE engines are designed to be cheap, quiet (ish) and last a long time.
 
SC Maintenance said:
Sure. The Mercedes Indy engines from the 90's were running over 10,000 RPM with pushrod. Was specifically designed for it, and only had to last an afternoon. In a daily driver, likely not.

Power is mostly about how much fuel you can get an engine to burn and how fast. RPM makes it burn fuel faster (more cyles per period of time). Higher compression makes the explosion more powerful and liquid cooled keeps things from melting down.

I'll add to what @dnewton3 said. OPE engines are designed to be cheap, quiet (ish) and last a long time.
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Well I think last a long time is a relative term. I think they have different grades of quality on engines. A honda gx is designed to last longer than say the old briggs flathead with an aluminum bore for example. My 25.5hp fx801v kawasaki is also a $2500 engine, so I would hope it would last a while.
 
The motor guy said:
Yeah I forgot about that. The mower runs at 3600rpm and the side by side will do like 8800rpm. You can't do that with a push rod engine, can you?
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While pushrod engines are generally considered lower RPM configurations, the Honda CX500 and CX650/turbo were exceptions. They were pushrod 4 valve per cylinder, 10,000 RPM engines. 50HP for the 500cc version and 100HP for the 650cc turbo.
CX650_001.jpg

s-l400.jpg

honda-cx500-engine-dismantle-journey-part-01-6.jpg
 
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The motor guy said:
Well I think last a long time is a relative term. I think they have different grades of quality on engines. A honda gx is designed to last longer than say the old briggs flathead with an aluminum bore for example. My 25.5hp fx801v kawasaki is also a $2500 engine, so I would hope it would last a while.
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$2500 at retail is comparatively inexpensive for a 850CC, 25 HP engine.

But I am sure it will last a long time, because they didn't try to get 50HP out of it.
 
The motor guy said:
So lets compare my side by side to my ztr mower engine. Fairly close in size so this is why I chose these two. Only difference the side by side is dual overhead cam and fuel injection while the mower is a standard pushrod engine, and carbureted. The mower engine is 852cc's with 25.5hp and the side by side is 1000cc's with 110hp. What do they do to the side by side engine to squeeze out so much hp? Why can't I get 110hp out of my mower? Both are two cylinders btw. Also, the one with 110hp uses way less fuel.
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HP does not correlate to displacement. Engine design correlates to HP.

How do you know the 110 HP uses less fuel? You dont bog it down with grass right? You are comparing apples to oranges.
 
SC Maintenance said:
$2500 at retail is comparatively inexpensive for a 850CC, 25 HP engine.

But I am sure it will last a long time, because they didn't try to get 50HP out of it.
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Well I mean you can buy a whole mower engine and all at home depot for $2500. The mower this thing is in is like $10k
 
burbguy82 said:
HP does not correlate to displacement. Engine design correlates to HP.

How do you know the 110 HP uses less fuel? You dont bog it down with grass right? You are comparing apples to oranges.
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Correct. In the early 1980's BMW supplied 1.5 liter (~92ci) turbocharged engines to the Brabham team. The cast iron blocks were from salvage yards around Europe. Especially prized were blocks with between 40,000 to 60,000km of use. The blocks underwent some major "refinements," and the heads were of a proprietary BMW design. In qualifying trim BMW was able to get in excess of 1000hp. These engine were labeled "popcorn engines" as they'd blow up after four qualifying laps. In the lab the BMW engineers were able to squeeze in excess of 1300hp from these engine. I write "in excess of 1300hp" as the lab's dyno had a max rating of 1300hp!
 
dnewton3 said:
Ironically, it's not about the fuel delivery method; I disagree with that concept. Fuel injection doesn't really make significantly more HP; i
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Engine Masters showed that carbed engines make more power due to the fuel charge cooling the intake air due to evaporation of fuel droplets into vapor.
 
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