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it was on that 400hp race motor thread. some one replied and another person confirmed it. i was thinkin its the opposite.
Those guys are idiots.
Displacement does not determine power, it caps the theoretical ceiling of power to be made all motor, but it has nothing to do with hp/liter.
 

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If all you change is displacement by expanding bore, stroke, or both, without reconsidering other engine parameters, you might lose some Hp/L on a highly tuned motor. Things like bore size will affect Hp/L as it can affect how the combustion happens - flame front has to travel farther, so high rpm performance will be affected.

Now if you were given a displacement spec and allowed to setup the bore/stroke any way you wanted, you could preserve Hp/L. This is why often times in a racing series where displacement is limited, most engines are very similar in terms of bore/stroke.
 

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As a blanket statement this isnt true. A more valid statement would be
"More stroke=less hp/liter"
That doesn't work though, if the stroke goes up so can the bore and you will have the same hp/liter. It is a proportion.
 

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That doesn't work though, if the stroke goes up so can the bore and you will have the same hp/liter. It is a proportion.
Bore doesnt really effect hp/L that much I think. Increase the bore and your trq/L remains fairly constant but since horsepower is torque @ high revs and a longer stroke moves the torque curve down, all things being equal, you can conclude that increasing stroke decreases the ratio of HP/L. In motorsports which engines have the highest HP/L? I would say F1 and MotorGP, its not a coincidence that a 2.4L V8 modern F1 motor has a 39mm stroke.
 

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so why is it when you increase the stroke, your hp/L drops?
When you go bigger, the parts are heavier. In an all motor engine, the heavier the part, the less efficient. To make a large, light part, it costs a lot of money.
How big is a F1 V-8 compared to Nascar V-8? Why does one spin 7000 more RPM than the other? What is the formula for HP?
Where does the size of the motor fit in that formula?
 

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When you increase stroke in most cases, you will reduce the topend WHP, but you increase the torque at lower rpm's, and gearing is used to take advantage of the changes. This also depends on engine component construction, design and materials used, so it isnt a strict measure untill you are at the upper end of the engines performance envelope. It is all a compromise that needs to take into account the intended use of the engine when it is designed/built.
 

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Comparing a pushrod NASCAR engine (that has rpm governed by flat-tappet cams, pushrods, and NASCAR rules governing gearing) with a F1 engine is like comparing apples and oranges.
In the Sport-Compact arena, the main reason large engines can't make the power per cubic inch their smaller displacement counterparts make is primarily breathing related. Head casting wall thickness prevents ever-larger ports and valve size is limited by guide centerlines, and valve to valve clearance at overlap.
 

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In a nutshell, yes. More displacement demands larger valves and larger volume ports...all of which equates to more airflow. Every aspect of the head and associated manifolding must be designed for the engine's displacement and operating (rpm) range.
Engines are complex systems, and must be treated as such.
 

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I would like to look at the ports on an 8000HP funny car...

of course their are a lot of factors that come into place
I am going to tell you right now there is not a single thing you can learn from an 8000hp funny/top fuel car that will be applicable to any road going motor.

The only thing you can learn is how these guys can disassemble and reassemble a motor in 20mins.
 
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