Somebody explain.

[tigerirons]

Can someone explain how displacement increases engine performance. For example, why does the viper have an 8.0L V10. Would performance be much different if it was a 4.0L V10? Also, how would the performance be different if it was a V8 or a V12. Thanks.

[CraigFL]

This is a very difficult question to explain exactly but I'll try to give you some basic concepts that should help you understand better. All things being equal:

1. If you have an engine of 1/2 the displacement (your 4L-V10 vs. 8LV10 example), you will have less power because there will be less fuel/air mixture to be burned and less piston area for that force to act on.

2. If you have an 8L-V8 vs. an 8L-V10-- At any given RPM, the V10 has more power pulses than the V8 which allows it to generate higher average torque and horsepower. (In 80 revolutions of the engine, the V8 fires 5 times and the V10 fires 8 times -- 1.6 times more often)

I said "all things being equal" above because inspite of the different number of cylinders and displacement, there are many other things that can be modified to change these factors -- increasing the available RPM, increasing the available mixture(turbocharging), stroke change, etc...

[Hudson]

Originally posted by CraigFL
This is a very difficult question to explain exactly but I'll try to give you some basic concepts that should help you understand better. All things being equal:

1. If you have an engine of 1/2 the displacement (your 4L-V10 vs. 8LV10 example), you will have less power because there will be less fuel/air mixture to be burned and less piston area for that force to act on.

2. If you have an 8L-V8 vs. an 8L-V10-- At any given RPM, the V10 has more power pulses than the V8 which allows it to generate higher average torque and horsepower. (In 80 revolutions of the engine, the V8 fires 5 times and the V10 fires 8 times -- 1.6 times more often)

I said "all things being equal" above because inspite of the different number of cylinders and displacement, there are many other things that can be modified to change these factors -- increasing the available RPM, increasing the available mixture(turbocharging), stroke change, etc...

To your first point...a larger displacement allows for a larger amount of fuel and air. Kinda like a stick of dynomite will make a larger bang than a firework.

To your second point...huh? In 80 revolutions of an engine, a V8 will fire 320 times (four per revolution) and a V10 will fire 400 times (five per revolution)...only 25% more on the V10 than the V8. An engine fires each cylinder once every other revolution (4-stroke engines take two revolutions to fire once).

[tigerirons]

Forgive me-
what does it mean for an engine to "fire"

[Polygon]

As the piston comes up gas and air are let into the cylinder. The piston compresses the gas and air, then the spark plug sparks the gas and air causeing it to explode and force the piston back down.

That is why they say fire.

[CraigFL]

Thanks for the correction Hudson... I guess it was too early in the AM for me on that one ("senior moment").:o :o :o

[Hudson]

Originally posted by Polygon
As the piston comes up gas and air are let into the cylinder. The piston compresses the gas and air, then the spark plug sparks the gas and air causeing it to explode and force the piston back down.

That is why they say fire.

Do I have to go through the 4-strokes in the Otto cycle engine?

The piston moves down with the intake valve(s) open: this is INTAKE. Gas and air are drawn into the combustion chamber.

As the piston moves back up, the valves close: this is COMPRESSION. Gas and air are compressed which lowers the combustion point of the mixture.

The piston nears the top and the spark plug fires, igniting the mixture: this leads to the POWER stroke. The piston is driven downward by the force of the explosion.

The piston moves back up and the exhaust valve(s) open: this is EXHAUST. The burnt gases escape through the exhaust valve and, eventually, out the tailpipe.

The cycle is repeated.

[Polygon]

I knew the air and gas came in on the downstroke. However much better explanation than mine.

[tigerirons]

Thanks Hudson- That really clears it up for me.

[GTStang]

One big thing is that if you have a car with the Same Displacement and one has 8 cyclinders and another has 12. The 12 will have power gains but also a very noticable difference in the smoothness of the engine. Notice the diiference between being in a Ferrari v-12 and a Chevy with a 454. The Chevy will vibrate you out of your seat! All this comes from the stroke of the engine and I don't wanna explian what stroke is so ask some1 else and also the stroke controls where the torque curve will lie and a million other things you will understand with about a million wasted hours reading and building up engines

[texan]

Originally posted by GTStang
One big thing is that if you have a car with the Same Displacement and one has 8 cyclinders and another has 12. The 12 will have power gains but also a very noticable difference in the smoothness of the engine. Notice the diiference between being in a Ferrari v-12 and a Chevy with a 454. The Chevy will vibrate you out of your seat! All this comes from the stroke of the engine and I don't wanna explian what stroke is so ask some1 else and also the stroke controls where the torque curve will lie and a million other things you will understand with about a million wasted hours reading and building up engines

This is true, but only to a certain extent. The V12 may have power gains over the V8, but it depends more upon cylinder head design and other factors than simple cylinder count. Remember, the V12 has more reciprocating mass and also has more friction, which reduce it's acceleration advantage. Balance also has something to do with it, but a 90 degree V8 with a crossplane crank is just about perfectly balanced too. Which is why a Chevy 454 motor is actually a smooth running engine (for it's per cylinder displacement), try building the same motor with a flat plane crank (180 degree crank) and see how smooth it is.



Also, fuel is combusted in the cylinder during the power stroke, not exploded. Explosion (uncontrolled expansion) is what happens during detonation, combustion (controlled expansion) is what happens during normal engine operation.

[GTStang]

Originally posted by texan


This is true, but only to a certain extent. The V12 may have power gains over the V8, but it depends more upon cylinder head design and other factors than simple cylinder count. Remember, the V12 has more reciprocating mass and also has more friction, which reduce it's acceleration advantage. Balance also has something to do with it, but a 90 degree V8 with a crossplane crank is just about perfectly balanced too. Which is why a Chevy 454 motor is actually a smooth running engine (for it's per cylinder displacement), try building the same motor with a flat plane crank (180 degree crank) and see how smooth it is.



Also, fuel is combusted in the cylinder during the power stroke, not exploded. Explosion (uncontrolled expansion) is what happens during detonation, combustion (controlled expansion) is what happens during normal engine operation.

But you put smooth running for it's cylinder dispacement and that's what I'm talking about. It doesn't matter how well balanced an engine. If u take to engines perfectly balanced and spread that displacement over move cylinders the one with more cylinders will always be a smoother running engine because the stroke of the engine is smaller.

Also when i speak of stroke i'm not talking bout the 4 stages of a Carnot cylce. I'm talking bout the actual stroke each connecting rod has. BoreXStrokeX#of Cylinders=displacement.
EX. 347(Stroked 302) will create more hp but will not create as much torque as hp but will move the torque curve more to the end of the curve.
Where as a 351 will have equal hp as the 347 but will have more torque that is in the beginning of the torque curve to the end. The

[GTStang]

Originally posted by texan


This is true, but only to a certain extent. The V12 may have power gains over the V8, but it depends more upon cylinder head design and other factors than simple cylinder count. Remember, the V12 has more reciprocating mass and also has more friction, which reduce it's acceleration advantage. Balance also has something to do with it, but a 90 degree V8 with a crossplane crank is just about perfectly balanced too. Which is why a Chevy 454 motor is actually a smooth running engine (for it's per cylinder displacement), try building the same motor with a flat plane crank (180 degree crank) and see how smooth it is.



Also, fuel is combusted in the cylinder during the power stroke, not exploded. Explosion (uncontrolled expansion) is what happens during detonation, combustion (controlled expansion) is what happens during normal engine operation.

But you put smooth running for it's cylinder dispacement and that's what I'm talking about. It doesn't matter how well balanced an engine. If u take to engines perfectly balanced and spread that displacement over move cylinders the one with more cylinders will always be a smoother running engine because the stroke of the engine is smaller.

Also when i speak of stroke i'm not talking bout the 4 stages of a Carnot cylce. I'm talking bout the actual stroke each connecting rod has. BoreXStrokeX#of Cylinders=displacement.
EX. 347(Stroked 302) will create more hp but will not create as much torque as hp but will move the torque curve more to the end of the curve.
Where as a 351 will have equal hp as the 347 but will have more torque that is in the beginning of the torque curve to the end.
This lil example does not even begin to explian everything that is involved in this but I hope it clears up things I was talking bout

[texan]

Originally posted by GTStang


But you put smooth running for it's cylinder dispacement and that's what I'm talking about. It doesn't matter how well balanced an engine. If u take to engines perfectly balanced and spread that displacement over move cylinders the one with more cylinders will always be a smoother running engine because the stroke of the engine is smaller.

Also when i speak of stroke i'm not talking bout the 4 stages of a Carnot cylce. I'm talking bout the actual stroke each connecting rod has. BoreXStrokeX#of Cylinders=displacement.
EX. 347(Stroked 302) will create more hp but will not create as much torque as hp but will move the torque curve more to the end of the curve.
Where as a 351 will have equal hp as the 347 but will have more torque that is in the beginning of the torque curve to the end.
This lil example does not even begin to explian everything that is involved in this but I hope it clears up things I was talking bout

It does matter how well balanced an engine is, a poorly balanced engine or a bad engine configuration (such as an inline 3) will not run smoothly regardless of displacement. That's one of the basic tennents of engine design, and why there's really only 7 popular engine formats 95% of manufacturers use (inline 4, flat 4, inline 6, flat 6, V6, V8 and V12).

While I haven't heard of the Carnot cycle in awhile, it's the Otto cycle we are talking about anyway. Also, it's the rod journal offset from crank centerline that determines stroke length, not connecting rod length. Additionally, engine displacement is found using this equation, not the one you listed...
Engine displacement = bore x bore x stroke x # of cylinders x 0.7854

Beyond that, my point is that just because one engine has more cylinders than another but similar displacement does not mean it has a longer stroke than said engine. One engine could be moderately oversquare (such is the case with most domestic V8's) and the other could be square or undersqaure (as is the case with many 4 valve import motors), so even a higher cylinder count could still result in different stroke sizes than you claim. Which I why I said I agree, but only to an extent.

Lastly, a 347ci Ford V8 using a 302 small block (8.2" deck height) will naturally want to produce significantly more torque than the stock displacement 302, and do so at significantly lower RPM. It will also run out of breath (assuming equal head and manifold design) sooner, meaning it's power curve is shifted lower in the RPM band, not higher. A 351ci Ford V8 using the 351 small block (9.2" deck height) will actually outperform the 347 at moderate to high RPM, but not at low RPM. The reason is not only in stroke length but also the rod/stroke ratio.

At any rate, I partially agreed with your original statement, I just wanted to point out to the readers it is actually more complex than just comparing cylinder count and displacement.

[Hudson]

While the above equation works for finding cylinder displacement, it is better explained with the full equation.

It is the volume swept by the movement of all pistons. Assuming that all pistons are the same size (bore and stroke), the equation is:

Pi*((Bore/2)^2)*Stroke*(number of cylinders)

..otherwise known as...

(volume of a geometric cylinder)
Pi *(R^2) * H

(where R is the radius of the cylinder and H is the height)