Somebody explain.

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.

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...

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).

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

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. :)

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

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.

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

Thanks Hudson- That really clears it up for me.

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

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.

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

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

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.

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)

Originally posted by GTStang


Also when i speak of stroke i'm not talking bout the 4 stages of a Carnot cylce.


Carnot cycle?!! What do you drive, a refrigerator?

:D

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

Its the other way around (as Texan said). Stroking the 302 to a 347 provides for a great increase in leverage on the crank (ie torque), but it does not lend itself to high RPMs because of the rod ratio. The 351 design lends itself much better to high RPM use, and higher hp for torque.

There are so many factors that come into play for power. Why does a Ferrari 360 makes 400HP with 3.6 liters yet the Viper only makes about 460 with 8 liters?
Ferrari: Dual intake manifold, flat-plane crankshaft, titanium connecting rods, variable back pressure exhaust (i think), Variable valve timing, 5 valves per cylinder, aluminum block and pistons, dry-sump oild lubrication the list goes on...

The Viper has a simple 2 valve per cylinder V10. So despite its massive size, it only makes 60 more HP than the Ferrari engine. However, the sheer size means torque up the ass, unlike the Ferrari engine.

You must remember that HP is work over time. A smaller engine "works" faster, so it can make a lot of HP relative to it's size. A big engine has large moving parts, so it doesn't "work" as fast, but can make big torque.

Hope this helps...:)

The Viper used duel intake manifolds also. But anyway...

HP = RPM * Torque/5252

So 500 ft*lbs of torque at 2626 RPMs and 250 ft*lbs of torque at 5252 RPMs both equate to 250 hp.

Anyway, spin a 427 SOHC up over 7000 RPMs if you want to watch a big engine "work fast". :)

Well true... but a smaller engine CAN (not always does) spin faster. Spin a 3.6 V8 to 8500....:)

While a Ferrari 3.6L V8 might put out more specific power than a Chrysler 8.0L V10, the LARGER engine actually gets better gas mileage. You may not care about this fact, but it does show that the Viper isn't necessarily tuned for all-out power.

That's true. The gas milage for the LS1/LS6s are amazingly good on the highway with that deep 6th, while the smaller, higher specific displacement, Italian 3.6 is just plain horrible. We'll leave sex appeal out of this. ;)

Anyway, the 427 SOHC is 35 years old but still made more power and more torque. 658 hp @ 7500 RPMs and 575 ft*lbs @ 4,200 RPMs even with a 7 ft long timing chain. :)

The SOHC motor wasn't a standard production engine.

Yeah it was, just very hard to get. It was intended for NASCAR so they had to sell a certain number of street cars equiped with them to make it legal. With a small carb and less wild cam they were crammed into Galaxies.

Some other teams cried "no fair" and so the OHCs were weighted down with mandatory ballast. Eventually OHCs were made illegal for NASCAR, but a rellatively small number of them were sold as the stock motor in cars.

Its like saying the GNX isn't a production car.

Or the Chrysler Hemi for that matter, someguy. And it was at least as impressive as the cammer in terms of high RPM output.

Originally posted by Someguy

HP = RPM * Torque/5252
So 500 ft*lbs of torque at 2626 RPMs and 250 ft*lbs of torque at 5252 RPMs both equate to 250 hp.


I always feel compelled to speak up when I see the aforementioned formula for shaft horsepower without mention of the (implied) units... In this case the only unit that wasn't stated in the formula was torque, which would need to be input in ft*lbf for the equation to work.

Of course ft*lbf was stated in the example calculation, just not in the formula...

Originally posted by texan
Or the Chrysler Hemi for that matter, someguy. And it was at least as impressive as the cammer in terms of high RPM output.

Yeah, the whole point of it was to go head to head with the Hemi. Just goes to show that even NASCAR at one time had a use. :)

I always feel compelled to speak up when I see the aforementioned formula for shaft horsepower without mention of the (implied) units... In this case the only unit that wasn't stated in the formula was torque, which would need to be input in ft*lbs for the equation to work.

Of course ft*lbf was stated in the example calculation, just not in the formula...

Okay, we can go though the whole sheeml:

Power = Work/time

Horse power is some what arbitrarily defined as:

1 hp = 550 ft*lb/sec

Not 550 ft*lbs of torque per second, but the application of 550 ft*lbs of work in one second, or the application of 550 lbs of force over a distance of one foot in one second.

But since our engine rotates the force it applies is a in the form of a moment or torque and its speed is relative to the unit circle, we need to convert RPMs to radians/second. So we multiply by 1 minute/60 seconds * 2Pi radians/revolution which gives us:

hp = torque(in ft*lbs) * Revolutions/minute * 1/550 * 1 minute/60 seconds * 2Pi radians/revolution

For convenience we want to keep power in terms of RPM and ft*lbs of torque, so we do some house keeping, so we end up with:

hp = torque(in ft*lbs) * revs/minute * 2Pi minutes /(550*60 seconds)

Collect our units and we end up with:

hp = torque(in ft*lbs) * RPM * 1/(5252.11) (minutes*radians)/(seconds*revolutions)

Radians are dimensionless and the minutes and revs cancel, apply our deffinition of hp, and our final solution is:

hp = torque * RPM * 1/5252 (hp)

Are ya happy now? ;)

yep.


I'm sure you would believe the number of people I've run into online who think that the "5252" is somehow fundamentally linked to calculating power for a shaft (other units? what other units?)...