V8 & V12, which is better?

[hungdynasty]

If we compare two engines with the same displacement(say 6 litre), what is the advantage and disadvantage of a V8 to a V12?
I can see that in north american such a big engine usually use V8(like LS2 Corvette) but in europe mostly are V12. I would like to hear your opinions.

 

[TooSlick]

A V-12 is inherently better balanced ...it's like two in-line sixes attached at the crank.

TS

 

[cortazzo]

Would the engine with less pistons get better economy given equal displacement?

 

[JHZR2]

I think its the balance of piston surface area and thus heat loss that equates to efficiency. long stroke engines are more efficient than big bore engines. That and compression (which I guess is indirectly related).

JMH

 

[Steve S]

With all thing equal probably a v 12 cause it will have a lighter reciprocating components lighter valve train etc. But to me less parts are better.

 

[T-Keith]

I'd go with the V8, less parts, more compact.

smaller cylinders can help with total valve area, but so can bore/stroke changes. It depends on what your looking for.

 

[kenw]

quote:

---

Originally posted by TooSlick:
A V-12 is inherently better balanced ...it's like two in-line sixes attached at the crank.

TS

---

the same can be said for the v8=2x i4.....the v12 can be smoother (it's not inherent), simply because the firing is divided 12x vs 8x. Higher resolution so to speak.

As long as the # of cylnders is divisble evenly into 360, there's no reson for it NOT to be at least even firing. But, the more you divide it the smoother it SHOULD be in theory (in that resolution sense).

If v12 were so much better, they would be more popular with the industry. they aren't.

Why? A LOT more cost to make, more things to go wrong (warranty cost) and with the same displacement, no appreciable power gain to justify that higher cost to the buying public.

 

[Jimbo]

For a given displacement, and assuming an equal bore-stroke ratio, more cylinders mean more frictional and thermal losses. The upsides are lower mass reciprocating components and faster burn, lowering detonation risk for the same compression ratio. In other words, the V-12 is better for higher rpms.

 

[cestes]

kenw - You're right aobut the number of cylinders being evenly divisble into 360 degrees, but there's on additional consideration:

If the number of cylinders is evenly divisible by 4 *and* the firing order is well thought out, then it will be really smooth. Why 4? Because it's a 4-stroke engine (well, I assume we're talking about 4-stroke engines). With a properly designed crankshaft, the pistons can be evenly distributed, and can 'balance out' the behavior of the others (e.g. on in compression while another is in exhaust, one in power while one is in intake).

I remember reading something about v-6 engines being inherently unstable, because the pistons are not at TDC, BDC, etc. at the 'right' times. The crank in a v-6 typically has counterweights that help compensate for the 'unnatural' behavior of the engine. Sounds reasonable, I suppose.

Still, I had a Buick 3.8 v-6 in my first car and that was a great little engine.

Now, if you want a v-12 have a look at this:

http://www.falconerengines.com/prod04.htm

I bet that makes a lovely noise!!!

-Chris-

 

[kenw]

"Early Buick V6s were simply a "Fireball" V8, with 2 cylinders cut off and an Iron block. It ran like a V8 missing on two cylinders... Watching one idle is an interesting sight. It jumps around quite a bit in the soft motormounts that keep the vibration, felt inside the car, to a minimum."

we had one of these, not quiet inside at all. Outside it sounded like a thrashing machine. Coupled with a 2 speed auto, it's what I learned to drive in. (63 Buick Special wagon)

Many of these had an aluminum block and shared it with an Oldsmobile of the time. When our 1st died, we had a dickens of a time finding the iron block version, the aluminum were all over the place.

 

[Win]

quote:

---

Originally posted by robbobster:

Not sure if hand-assembly is absolutely required for a V12

---

Not required certainly, but that is how Jaguar built them: one at a time, one person start to finish. I would expect BMW, MB, Ferrari, did likewise, as they are low volume engines.

 

[robbobster]

I like the idea of a hand-built engine at any rate. Often, this is accompanied by carefully hand-selecting parts that are balanced to tighter tolerances.

At least with newer engines, there's often a little placard with the builder's name on it. Taking a little professional pride in your handiwork is great.

Too bad his home number isn't on there, should you run into any engine problems.

 

[michaelc80]

quote:

---

Originally posted by hungdynasty:
If we compare two engines with the same displacement(say 6 litre), what is the advantage and disadvantage of a V8 to a V12?
I can see that in north american such a big engine usually use V8(like LS2 Corvette) but in europe mostly are V12. I would like to hear your opinions.

---

On a V8 I'd only have to change 8 plugs. On a V12 I'd have to change 12!

 

[VaderSS]

quote:

---

Originally posted by michaelc80:
On a V8 I'd only have to change 8 plugs. On a V12 I'd have to change 12!

---

Some 8s have 16 plugs, some 12s have 24...

 

[VaderSS]

Here is a good explanation;
http://www.fordscorpio.co.uk/tech2_3.htm

The I4 has primary balance, but nor secondary. The I6 has Primary and Secondary balance A V12 is two I6s Veed together while a V8 is 2 I4s Veed together.

V6s properly designed, are not too bad, but they are a pair of I3s, so they can never have perfect balance without aid such as balance shafts. Oddly enough, a 3 can be smoother than a 4 though. Most Buick V6s have balance shafts. The 3.3 is one that does not, but it's not bad. I guess the extra reciprocating weight of the 3.8 makes it more necessary.

Early Buick V6s were simply a "Fireball" V8, with 2 cylinders cut off and an Iron block. It ran like a V8 missing on two cylinders... Watching one idle is an interesting sight. It jumps around quite a bit in the soft motormounts that keep the vibration, felt inside the car, to a minimum.

The "Fireball 8" design was later sold or liscenced to Rover and production of it only ceased this year.

[ May 26, 2005, 12:17 PM: Message edited by: VaderSS ]

 

[robbobster]

quote:

---

Originally posted by Jimbo:
For a given displacement, and assuming an equal bore-stroke ratio, more cylinders mean more frictional and thermal losses. The upsides are lower mass reciprocating components and faster burn, lowering detonation risk for the same compression ratio. In other words, the V-12 is better for higher rpms.

---

Well put.

But there's a lot more to a balanced engine. firing order and the angle of the "V" in a V-type engine also contribute.

In a nutshell, assuming regularly spaced crank throws, several engine designs achieve perfect balance, courtesy of the Bosch Automotive Handbook:
Inline-6
Boxer-6
V12 (60-degree V)

A 90-degree V8 almost achieves perfect balance (it has some small 1st-order free moments), with both Boxer-4 and inline-4 engines a little further behind.

If you get into irregularly spaced crank throw, then all bets are off. But most automotive engines will use regular crank throws.
Also, a balance shaft can help an engine overcome inherent imbalance, but this saps power and will not eliminate forces (at least I don't think they can completely eliminate them).

 

[JHZR2]

I know it is not correct according to the info here, but I thought that I read somewhere that 5 cyl engines were easy to make balanced and VERY smooth.

I know this is the case with my MB diesel, the higher the RPMs, the smoother and quieter (to some extent) the engine becomes.

JMH

 

[Win]

You guys can talk theory all you want - I own a semi modern V12 car, a Jaguar XJ12 with a 6 liter 60 degree V12, (they quit making these engines in 1997), and a modern V8, the GM 5.7 liter LS1 in a 2004 Pontiac GTO.

Here are the pros/cons from a real world perspective:

V12,

pro:

fantastically smooth power delivery; small pistons and 50% more firing pulses per crankshaft revolution give a turbine like flow of power;

almost silent operation at idle and normal throttle opening;

really cool sound as the air shrieks into the engine at wide open throttle;

hand built by one person for optimum balance and fit;

people know you don't give a da** about the price of gas, fuel economy, trees, clean air, or any of that crap.

cons:

expensive, the V12 was a $22K option over the I6, there are a lot of extra parts and support systems to deal with the heat;

needs a big transmission and rear end to handle the torque, as an example the Xj12 uses a GM4L80E transmission and Dana diff, the Goat gets by with a 4L60E and Dana diff;

mass, the V12 block is big, and heavy, even for an aluminum engine, this also wears suspension components and tires faster;

lots of heat generation relative to a V8;

poor fuel economy compared to a V8;

hard to work on, because so much engine is crammed into a small space, this also compounds the heat problem;

V8 pro: well, look at the disadvantages of a V12, for the most part these are the strengths of a V8;

nice burble exhaust if it has a cross plane crank;

con:

Not a V12, so not the best.

 

[robbobster]

quote:

---

Originally posted by Win:

V12,

pro:
hand built by one person for optimum balance and fit;

---

Not sure if hand-assembly is absolutely required for a V12

 

[Kernel Potter]

quote:

---

we had one of these, not quiet inside at all. Outside it sounded like a thrashing machine. Coupled with a 2 speed auto, it's what I learned to drive in. (63 Buick Special wagon)

Many of these had an aluminum block and shared it with an Oldsmobile of the time. When our 1st died, we had a dickens of a time finding the iron block version, the aluminum were all over the place

---

As a clarification, Buick V6s were never offered with aluminum blocks or heads in production cars. The V8 that they were designed from were offered in all aluminum for about 4 years. As posted previously in this thread, it was the aluminum v8 engine that GM sold to Rover in the mid 60s.

As far as the more cylinders vs. less cylinders debate, more cylinders give you shorter strokes/bores and lighter reciprocating weight per cylinder, allowing more revs and/or more reliable revs; everything else equal. It also gives more valve area for better breathing; everything else equal.

Of course the variables of more friction, size and weight also play into this. That's why initially under the normally aspirated engine rules in F1, since the early 1990s, engines were either v12s or v8s. Now due to packaging and overall balance, weight, size and power - they're all v10s.