4.0L V8 vs 4.0L V6

[Howard]

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

Which is funny, because people claim to love the sound of Ferrari V8s and hate 4-bangers when in fact a flat-plane V8 is the same as two 4-bangers.

 

[MotF Bane]

Which is funny, because people claim to love the sound of Ferrari V8s and hate 4-bangers when in fact a flat-plane V8 is the same as two 4-bangers.

But they don't sound the same.

 

[zerocool84]

Which is funny, because people claim to love the sound of Ferrari V8s and hate 4-bangers when in fact a flat-plane V8 is the same as two 4-bangers.

The exhaust makes them sound different.

 

[exdeath]

Depends on the 4 banger. Flat 4 boxers sound killer. It's all about the firing order and exhaust pulse interactions. Inline 4s just don't sound exciting because it's a steady even drone with no exhaust wave interference. A flat plane V8 has two banks; two inline 4s running slightly out of phase from one another would sound way better than a single inline 4 droning on.

 

[alkalinetaupehat]

While we're on that, I love the sound of a Dodge 5.9L V8 and a Subaru EJ25. What causes these two engines to have such a similar sound?

 

[Throckmorton]

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.

Your argument that the V8 has more valve area is based on the premise that bore=stroke. Of course the V8 with its displacement split among more cylinders will have a shorter stroke IF both engines are square. But if the stroke is the same as the V6, the cylinder diameters shrink, and the valve area becomes the same as the V6.

 

[JCH13]

To throw my $0.02 in...

It depends on too many things for there to be a truly clear answer. But holding all else equal: both are DOHC (for example), square bore/stroke, similar head designs, similar tuning, etc the V8 can probably produce more power.

However, the V8 will weigh more, lose proportionally more power through the valve train and engine accessories. The associated systems like intake, exhaust, radiator, etc will all also weigh more. I don't know that the added weight is made up for by the added power in terms of straight-line acceleration, but for handling I'd prefer the lighter engine. Everything is a compromise...

Quite often the primary configuration (v8, v6, I6, I4, etc) of the engine is mostly irrelevant, what matters is its execution (bore, stroke, valve size & lift, head design, compression ratio, fuel/spark/exhaust/intake tuning, etc).

 

[Zenmervolt]

The exhaust makes them sound different.

The way the exhaust on a flat-plane V8 is handled is to treat the engine as two inline 4-cylinder engines. There is no crossover and no difference from how an inline 4's exhaust would be optimized. The differences in sound are either manufactured by intentionally creating a slightly sub-optimal exhaust to "shape" the sound, or simply because the higher rev limit on the supercar V8 lets it "sing".

ZV

 

[ElFenix]

Which is funny, because people claim to love the sound of Ferrari V8s and hate 4-bangers when in fact a flat-plane V8 is the same as two 4-bangers.

i pretty much don't give a crap about how ferraris sound. sound like big I4s. because that's basically what they are.

give me a blown ford V8.

Your argument that the V8 has more valve area is based on the premise that bore=stroke. Of course the V8 with its displacement split among more cylinders will have a shorter stroke IF both engines are square. But if the stroke is the same as the V6, the cylinder diameters shrink, and the valve area becomes the same as the V6.

as long as the ratio is the same the V8 will breathe better. doesn't have to be 1:1.
the question basically asked, all else equal, which will make more power. the answer is the V8. if you go full on F1 optimize everything for power, the V8 will be a better layout than the V6.

 

[exdeath]

Your argument that the V8 has more valve area is based on the premise that bore=stroke. Of course the V8 with its displacement split among more cylinders will have a shorter stroke IF both engines are square. But if the stroke is the same as the V6, the cylinder diameters shrink, and the valve area becomes the same as the V6.

Two whole cylinders worth of extra valves will more than make up for slightly smaller valves all around, while the smaller valves are also feeding smaller cylinders anyway. No matter how you slice it, geometrically, the v8 will have more valve area and air flow capacity than the v6.

This is essentially the same argument as 2 valves vs 4 (DOHC) where its a known geometric fact that given a particular bore, more smaller valves give more valve surface are a than fewer but bigger valves. Circles within a circle, lots of wasted space, smaller but more = packed tighter and more utilization of the area. This is why a 4.6L engine can produce the same or more power as a 6L engine. So given two identical displacements, but one with more cylinders, thus more valves, it's not hard to understand why the v8 would be the better configuration.

Not to mention more head casting to work with to shape the ports more optimally. The V8 will also run smoother and rev better due to better balance, be more tuneable at extremes due to having 8 smaller (more precise) injectors instead of 6 bigger ones, more stable flame propagation across the smaller bores, more material for the given displacement to dissapate heat in the combustion chambers to raise the detonation threshold, etc. Ever tuned a big power 4 cyl with 1000 cc injectors? LOL

 

[JCH13]

Two whole cylinders worth of extra valves will more than make up for slightly smaller valves all around, while the smaller valves are also feeding smaller cylinders anyway. No matter how you slice it, geometrically, the v8 will have more valve area and air flow capacity than the v6.

I've been playing with the geometry and valve area a bit, and with a square bore/stroke the V6 usually comes out slightly on top, by up to 2% depending on how you play the numbers, though you can manipulate them so that the v8 has slightly more area in a few extreme cases.

Things I noticed:

Square bore/stroke dimensions are: 94.7mm for the V6 and 86mm for the v8, leading the v8 to have about 10% more "piston head area" if you will, that is total area available to put valves.

The smaller valves on the v8 can't quite take advantage of the larger available area if the valves have the same allowance for a valve seat (I chose 2mm).

Edit: this doesn't take into account head geometry, the fact that less air needs to be pulled into each cyl on a v8, and the fact that flow rate isn't always proportional to flow area and the bajillion other factors in an engine.

 

[Howard]

Your argument that the V8 has more valve area is based on the premise that bore=stroke. Of course the V8 with its displacement split among more cylinders will have a shorter stroke IF both engines are square. But if the stroke is the same as the V6, the cylinder diameters shrink, and the valve area becomes the same as the V6.

The valve area would grow with the square of the bore, which is higher for the V6 if you make the stroke the same (and would therefore result in an equivalent A/V ratio for both engines).

However, by implementing such an artificial limitation, you skew power output towards the V6 by decreasing the bore/stroke ratio of the V8. Again I ask you, for the purpose of determining which engine could produce the most power, why you would fix the stroke on both engines.

 

[vlad1977]

There is an interesting situation about this topic (same size engines, one is V8 and one is V6).
For the famous Le mans race in 2011, new regs. specified that for turbo diesel engines, maximum size is 3.7 litres. There are two major manufacturers involved in the International Le mans Cup 2011 (which includes the Le mans race): Audi and Peugeot.
The interesting thing is that Peugeot chose to build a 3.7L V8 bi-turbo diesel while Audi chose to build a 3.7L V6 turbo (or possibly bi-turbo) diesel.
How could 2 big auto makers which won Le mans in the last years (Peugeot in 2009 and Audi in 2010, to mention just the last 2 years) and who invest many millions into research each year, choose 2 different engine types?
It is clear that both were trying to obtain maximum power and torque from their race-engines. So, if all the other parameters are the same, could a 3,7L turbo diesel V6 obtain the same power as a 3.7L turbo diesel V8? Would there be significant differences betwen the engines regarding fuel consumption?
Or did Audi chose V6 only because it could be lighter and the power deficit could be compensated by better center of gravity and better handling?

 

[PlasmaBomb]

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.

Just to point out to you that this is wrong...

BMW single cylinder ~650cc 50.0 hp @ 6500 rpm, 44.0 lbf·ft @ 5,000 rpm
Suzuki twin cylinder ~650cc 73.4 hp @ 8800 rpm, 47.2 lbf.ft @ 7,000 rpm
Suzuki four cylinder ~650cc 85.0 hp @ 10,500 rpm, 45.7 lbf.ft @ 8,900 rpm

 

[KIAman]

We need to see some real world examples, not just theorycraft. The only 4.0 V8 I've experienced is the E9X M3. The only 4.0 V6 I've experienced is a Ford Ranger and Toyota Tacoma.

Based on my experiences, the V8 produces significantly more power but the V6 produces lower RPM torque.

Based on simple logic, rather than technical tidbits, we can conclude some of the following.

1. V8 breathes easier
2. V6 has less rotational mass
3. V8 has better vibrational characteristics (aka can rev higher)
4. V6 has better exterior packaging
5. V8 has more head design options
6. V6 has less points of failure

 

[EightySix Four]

http://www.youtube.com/watch?v=MSZTbR6C6Lw

That is a corvette with 180* headers. The headers swap the cylinders across the bottom to make the exhaust scavenging far more like a flat-plane v8, like that of a Ferrari. I've only ever seen one modern corvette with those headers and they never mass produced them. Rumor was the headers made almost 25whp more on the dyno than normal long tubes, but packaging them properly was nearly impossible and they'd hit the ground.

 

[PlasmaBomb]

There is an interesting situation about this topic (same size engines, one is V8 and one is V6). For the famous Le mans race in 2011, new regs. specified that for turbo diesel engines, maximum size is 3.7 litres. There are two major manufacturers involved in the International Le mans Cup 2011 (which includes the Le mans race): Audi and Peugeot.
The interesting thing is that Peugeot chose to build a 3.7L V8 bi-turbo diesel while Audi chose to build a 3.7L V6 turbo (or possibly bi-turbo) diesel.


How could 2 big auto makers which won Le mans in the last years (Peugeot in 2009 and Audi in 2010, to mention just the last 2 years) and who invest many millions into research each year, choose 2 different engine types?

It is clear that both were trying to obtain maximum power and torque from their race-engines. So, if all the other parameters are the same, could a 3,7L turbo diesel V6 obtain the same power as a 3.7L turbo diesel V8? Would there be significant differences betwen the engines regarding fuel consumption?

Or did Audi chose V6 only because it could be lighter and the power deficit could be compensated by better center of gravity and better handling?

There are numerous factors in play -

The LM racers are running mandated ACO restrictor plates so it becomes an issue of how best to use the available air - if you are limited to say 400 cfm - there is little point designing an engine to work with 500 cfm.

Fuel consumption is also important in LM, due to the length of the event - if you can be competitive and use less fuel it will pay dividends. A V6 should have slightly better fuel economy than a V8 due to less internal friction.

Finally you have packaging issues - aerodynamics will be very important, weight distribution is important, and Peugot are developing a hybrid LM diesel.

 

[Throckmorton]

Just to point out to you that this is wrong...

BMW single cylinder ~650cc 50.0 hp @ 6500 rpm, 44.0 lbf·ft @ 5,000 rpm
Suzuki twin cylinder ~650cc 73.4 hp @ 8800 rpm, 47.2 lbf.ft @ 7,000 rpm
Suzuki four cylinder ~650cc 85.0 hp @ 10,500 rpm, 45.7 lbf.ft @ 8,900 rpm

Those numbers are because of different RPMs and probably efficiency of the design. Of course more balanced 4 cylinder would make more power and spin faster than a 1 cylinder

 

[Throckmorton]

The valve area would grow with the square of the bore, which is higher for the V6 if you make the stroke the same (and would therefore result in an equivalent A/V ratio for both engines).

However, by implementing such an artificial limitation, you skew power output towards the V6 by decreasing the bore/stroke ratio of the V8. Again I ask you, for the purpose of determining which engine could produce the most power, why you would fix the stroke on both engines.

Because you're talking about valve area. There's no reason the valve area would be inherently better on the V8. And if you want similar torque and rpm you'd make the strokes the same on both the V8 and V6.

 

[JCH13]

There is an interesting situation about this topic (same size engines, one is V8 and one is V6).
For the famous Le mans race in 2011, new regs. specified that for turbo diesel engines, maximum size is 3.7 litres. There are two major manufacturers involved in the International Le mans Cup 2011 (which includes the Le mans race): Audi and Peugeot.
The interesting thing is that Peugeot chose to build a 3.7L V8 bi-turbo diesel while Audi chose to build a 3.7L V6 turbo (or possibly bi-turbo) diesel.
How could 2 big auto makers which won Le mans in the last years (Peugeot in 2009 and Audi in 2010, to mention just the last 2 years) and who invest many millions into research each year, choose 2 different engine types?
It is clear that both were trying to obtain maximum power and torque from their race-engines. So, if all the other parameters are the same, could a 3,7L turbo diesel V6 obtain the same power as a 3.7L turbo diesel V8? Would there be significant differences betwen the engines regarding fuel consumption?
Or did Audi chose V6 only because it could be lighter and the power deficit could be compensated by better center of gravity and better handling?

The actual engine configuration or technology is second to it's execution, i.e. it doesn't matter precisely what engine one uses, but how that engine fits into the overall design of the vehicle. There is never "the best solution" to such a complex problem like building a race car.

For example, in Formula SAE there have been winning cars in virtually every configuration you can imagine: N/A 4, 2, and 1 cyl, turbo 4 and 2 cyl (maybe even a 1, but I'm not sure) opposed cylinders, v cylinders, and inline cylinders, and even single-rotor engines before they were banned.

My point being that neither engine is automatically better than the other, a winning car will result from the optimal integration of whatever engine configuration is chosen.

 

[DivideBYZero]

While we're on that, I love the sound of a Dodge 5.9L V8 and a Subaru EJ25. What causes these two engines to have such a similar sound?

Many Subaru engines are fitted with unequal length headers, so pots 1&2:
BANG===EXHAUST!

Pots 3&4:
BANG=====EXHAUST!

and both these meet in the same pipe, so you get an interesting sound as the exhaust(and therefore sound) pulses interrupt each other.

It's not as efficient as equal length, so much so that if you're getting the true performance oriented Subaru models they have equal length headers from the factory to smooth the flow, but it sounds amazing.

 

[PlasmaBomb]

Those numbers are because of different RPMs and probably efficiency of the design. Of course more balanced 4 cylinder would make more power and spin faster than a 1 cylinder

Funny I thought you said...

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.

If you now accept this, then why are you having such problems with the V6 and V8 question?

PS. Their efficiencies are similar, depending of course on the metric for measuring efficiency...

 

[Throckmorton]

Funny I thought you said...



If you now accept this, then why are you having such problems with the V6 and V8 question?

PS. Their efficiencies are similar, depending of course on the metric for measuring efficiency...

I meant separate into 4 separate cylinders so that each is 1/4 the capacity, with 1/4 the valve area and the same stroke.... I didn't say arrange them in a straight config so the balance is much better and the engine can spin faster.

A V8 isn't that much more balanced than a V6 so it makes no sense to compare a 1 cyl to a 4 cyl as an analogy.

 

[Howard]

Because you're talking about valve area. There's no reason the valve area would be inherently better on the V8. And if you want similar torque and rpm you'd make the strokes the same on both the V8 and V6.

I'm not talking about valve area. I'm talking about making maximum power. You are talking arbitrary restrictions.

 

[PlasmaBomb]

I meant separate into 4 separate cylinders so that each is 1/4 the capacity, with 1/4 the valve area and the same stroke.... I didn't say arrange them in a straight config so the balance is much better and the engine can spin faster.

A V8 isn't that much more balanced than a V6 so it makes no sense to compare a 1 cyl to a 4 cyl as an analogy.

My point was that each had 1/4 the capacity, so that is as near as you'll get - no one would give each cylinder 1/4 the valve area because it's beyond stupid.

If you are aiming for power both engines are going to be over-square anyway and there is a limit to how over-square you can go.