4.0L V8 vs 4.0L V6

[alfa147x]

Both engines being the same size but one has larger pistons the other had more pistons. Which produces more power?

Which would react better to FI?

 

[Howard]

The V8. Better top-end breathing, mostly due to higher ratio of valve area to combustion chamber volume, all else considered (and optimized).

 

[alkemyst]

In reality there is no way to tell, if everything was equal the V8 would just be smoother, but they'd be comparable powerwise.

 

[exdeath]

The V8. Same displacement, but more total valve area to feed that same displacement. Air flow through the heads/valves is everything.

 

[chorb]

The V8. Same displacement, but more total valve area to feed that same displacement. Air flow through the heads/valves is everything.

pardon my engine noobness...

if air flow is only reason, then why dont the designers of V6 engines incorporate better air flow engineering?

[I'll attempt to answer my own question, correct me if I'm wrong]
better air flow would increase the size of the valves/heads and engine block, and if you're going to increase the size anyway, just make a v8?

 

[Howard]

pardon my engine noobness...

if air flow is only reason, then why dont the designers of V6 engines incorporate better air flow engineering?

[I'll attempt to answer my own question, correct me if I'm wrong]
better air flow would increase the size of the valves/heads and engine block, and if you're going to increase the size anyway, just make a v8?

Valve area is limited by the bore of the cylinder. You can't increase it willy-nilly. Other factors you can optimize for airflow, but not that.

A V8 costs up to 33% more than a V6 to produce.

 

[desy]

Also, aren't there more mechanical loses due to more moving parts within a V8?

 

[chorb]

Valve area is limited by the bore of the cylinder. You can't increase it willy-nilly. Other factors you can optimize for airflow, but not that.

A V8 costs up to 33% more than a V6 to produce.

and the reason you can't change bore size is?

bigger piston heads (weight)?
more volume in the cylinder = less powerful explosion per area?
thinner cylinder walls?

 

[alkemyst]

This same exact thread comes up many times. In the end until the race is raced, there is no winner.

 

[Howard]

and the reason you can't change bore size is?

bigger piston heads (weight)?
more volume in the cylinder = less powerful explosion per area?
thinner cylinder walls?

The valve area is going to be limited to a circle (minus bordering areas and whatnot) whose dimensions are limited to the bore because of interference with an adjacent combustion chamber. The bore can change, but the fact remains that you have more valve area, proportionally speaking, than you do displacement as you increase the number of cylinders (and/or reduce the displacement).

If you fix the bore/stroke ratio (example will be set to 1:1), what you have is this:

500cc per chamber in the V8 vs 667cc per chamber in the V6
bore of V8 = stroke of V8 = 8.6cm
bore of V6 = stroke of V6 = 9.46cm
area/volume ratio = pi*r^2/(pi*r^2*stroke) = 1/stroke

A/V for V8 = 1/8.6
A/V for V6 = 1/9.46

Therefore the V8 has up to 10% better breathing [(1/8.6-1/9.46)/(1/9.46)]

head space has not been factored into the calculations, obviously

 

[chorb]

The valve area is going to be limited to a circle (minus bordering areas and whatnot) whose dimensions are limited to the bore because of interference with an adjacent combustion chamber. The bore can change, but the fact remains that you have more valve area, proportionally speaking, than you do displacement as you increase the number of cylinders (and/or reduce the displacement).

If you fix the bore/stroke ratio (example will be set to 1:1), what you have is this:

500cc per chamber in the V8 vs 667cc per chamber in the V6
bore of V8 = stroke of V8 = 8.6cm
bore of V6 = stroke of V6 = 9.46cm
area/volume ratio = pi*r^2/(pi*r^2*stroke) = 1/stroke

A/V for V8 = 1/8.6
A/V for V6 = 1/9.46

Therefore the V8 has up to 10% better breathing [(1/8.6-1/9.46)/(1/9.46)]

head space has not been factored into the calculations, obviously

so why not change the bore:stroke (shorter stroke, larger bore) on the V6 to improve that A/V ratio? You'll keep the same Volume, but gain A:V ratio. Torque might take a big hit?

 

[Howard]

so why not change the bore:stroke (shorter stroke, larger bore) on the V6 to improve that A/V ratio? You'll keep the same Volume, but gain A:V ratio. Torque might take a big hit?

At what bore/stroke ratio will the V6 breathe better?

Torque won't necessarily drop as even though your lever arm decreases, the force exerted on the lever arm increases due to the additional surface area (F = P*A).

 

[EightySix Four]

so why not change the bore:stroke (shorter stroke, larger bore) on the V6 to improve that A/V ratio? You'll keep the same Volume, but gain A:V ratio. Torque might take a big hit?

At no point will you be able to improve that ratio beyond what doing the same thing to a V8 will let you.

 

[Throckmorton]

The valve area is going to be limited to a circle (minus bordering areas and whatnot) whose dimensions are limited to the bore because of interference with an adjacent combustion chamber. The bore can change, but the fact remains that you have more valve area, proportionally speaking, than you do displacement as you increase the number of cylinders (and/or reduce the displacement).

If you fix the bore/stroke ratio (example will be set to 1:1), what you have is this:

500cc per chamber in the V8 vs 667cc per chamber in the V6
bore of V8 = stroke of V8 = 8.6cm
bore of V6 = stroke of V6 = 9.46cm
area/volume ratio = pi*r^2/(pi*r^2*stroke) = 1/stroke

A/V for V8 = 1/8.6
A/V for V6 = 1/9.46

Therefore the V8 has up to 10% better breathing [(1/8.6-1/9.46)/(1/9.46)]

head space has not been factored into the calculations, obviously

You're assuming the bore and stroke have to be the same. Make the stroke the same on the V6 as the V8 and the valve area should come out the same.

 

[Throckmorton]

At no point will you be able to improve that ratio beyond what doing the same thing to a V8 will let you.

I don't see how the ratio matters. What matters is the stroke. You have to compare the same stroke between engines, because that is what affects piston speed and torque.


If you could somehow take a single cylinder and divide it into 4 quarters, the ratio would change drastically but the torque and rpm characteristics would be identical.

 

[Zenmervolt]

All else being equal, the V8 will breathe better (for the reasons previously enumerated), and it will rev higher because the individual reciprocating pieces will be lighter (less mass per piston and valve to control). The V8 will have slightly more internal friction, but the better breathing and the ability to rev higher will overcome the frictional disadvantage.

Again, this is all else being equal. There are, of course, many ways to make the situation unequal and give either engine the advantage.

ZV

 

[punjabiplaya]

bigger boom = more power

 

[speedy2]

Just look at all past/present 4.0L V8's and V6's out there. lol

 

[Howard]

You're assuming the bore and stroke have to be the same. Make the stroke the same on the V6 as the V8 and the valve area should come out the same.

I never assumed that the bore/stroke ratio had to be 1:1, I just assumed that they were so the math would be more elegant.

As for making the stroke the same, why would anybody in their right mind want to do that? To win this argument? May as well say that the V6 has only 2 valves while the V8 has 4.

 

[Howard]

I don't see how the ratio matters. What matters is the stroke.

Please don't be cryptic.

You have to compare the same stroke between engines, because that is what affects piston speed

Piston speed, yes.

and torque.

Maybe. Any changes in stroke, for a given displacement, will result in a corresponding opposite effect in piston area i.e. force applied to the crankshaft. This results in a partial or full compensation in torque reduction/increase due to changes in the length of the lever arm. What I'm not convinced of is that the one factor is weighted more heavily than the other in terms of effect on torque - and I'm not about to start integrating to find out the answer. But this is entirely irrelevant. OP is asking about power, not torque.

If you could somehow take a single cylinder and divide it into 4 quarters, the ratio would change drastically but the torque and rpm characteristics would be identical.

???

 

[RocksteadyDotNet]

If you said V8 vs straight 6 it would be a different story.

The 6 would be smoother as they have less natural harmonics than a v8, which as the same harmonic problems as a straight 4.

 

[RedRooster]

A 4L V8 that revs to 9500? Mmm mmm.
C'mon Toyota, get on that!

 

[zerocool84]

A 4L V8 that revs to 9500? Mmm mmm.
C'mon Toyota, get on that!

Toyota??? The most boring car company on the planet??? Yea not gonna happen.

You need to look at Ferrari. They have high revving V8's.

http://en.wikipedia.org/wiki/Ferrari_458_Italia

http://en.wikipedia.org/wiki/Ferrari_F430

http://en.wikipedia.org/wiki/Ferrari_360

 

[punjabiplaya]

458's engine

/nut

 

[Zenmervolt]

If you said V8 vs straight 6 it would be a different story.

The 6 would be smoother as they have less natural harmonics than a v8, which as the same harmonic problems as a straight 4.

Only if it's a flat-plane V8. A cross-plane V8 can be completely balanced and has no harmonic issues. In any case, the harmonics really don't come into play too much, especially given that a flat-plane V8 is easier to optimize from an exhaust perspective as compared to a cross-place V8, which results in better high-RPM breathing and overall generally more power (which is why racing V8 engines are flat plane and sound much different than the traditional "American" cross-plane V8 engines in muscle cars).

ZV