Well...DUH!!!

[Eric H.]

Hey,

I liked reading the responses to the post "Old Trick I Just Made Up?" Great! It really got me thinking about the V8 engine geometry. With the cylinder banks oriented at 90 degrees to each other (they are at 90 degrees, right?), you can get these nice 90 degree firing intervals. 720 degrees/8 = 90 degrees. Perfect, and only 4 crank journals required for the rods. Really a neat design.

MY QUESTION (finally). What is the firing interval on a V6? I assume these cylinder banks are also oriented at 90 degrees to each other and that there are only 3 crankshaft journals for the 6 rods. How can this possibly work out to nice even numbers? I assume the straight six (The Blue Flame Six) has 3 crank journals for the six rods oriented at 120 degrees to each other.

What about the firing geometry of the V10?????

Ric

[Eric H.]

O.K. I remember.. but what about...

O.K.... I guess straight sixes have individual crank journals for all six cylinders. I remember the crank from my old '56 Ford straight six. A rather long crank, I believe it had individual journals for each cylinder. Still, what about V6's and V'10 with the "compressed" design. Do opposing cylinders share crankshaft journals on these engines? and if so, how can the firing intervals be a nice even number?

Ric

[Joe L.]

Re: O.K. I remember.. but what about...

Rick-----

Most, but not all, V-8 engines are of 90 degree configuration. For a V-8 engine, the 90 degree configuration is the "inherently balanced" configuration for exactly the reasons that you described. This is one of the reasons that 90 degree configuration V-8 engines are generally so smooth (and the primary reason that they were so popular in US built cars for so many years before fuel economy and emissions became a critical issue). A V-4 (if any were made) would also be inherently balanced as would a V-16 (and Cadillac may build one in the near future based upon "siamesed" Northstars.

For a V-6 engine (or a V-12), 60 degrees is the "inherently balanced" configuration. The GM 2.8/3.1/3.4 liter engine family is an example of a 60 degree V-6 engine. The 60 degree architecture of this engine is what makes it so smooth (and popular) in many GM cars. It also makes "packaging" it in FWD cars much easier, since a 60 degree configuration is much more compact than a 90 degree configuration.

GM (and other manufacturers) also make 90 degree V-6 engines. These engines are "inherently unbalanced". Some examples of 90 degree GM V-6 engines are the 3.8 liter (the old Buick V-6 which dates from 1964 and is GM's "premium V-6" to this day and the 4.3 liter (basically, a 350 V-8 with 2 cylinders "lopped off" and used primarily in trucks). To compensate for the inherent inbalance in the 3.8 liter engine, GM installed balance shafts to help smooth the engine out (so that they could "update" this engine, make it smooth enough to compete with foreign engines which are much later designs, and allow this relic to "soldier on" into the 21st century). The 4.3 engine has no balance shafts and, although powerful, is a rough engine. I wouldn't be caught dead owning one.

As I recall the Dodge V-10 used in the Vipers and trucks has an "oddball" cylinder angle configuration. I think that this was done in order to achieve, as close as possible, the "inherently balanced" architecture for a V-10 engine configuration. John H. may be able to add more on this as I'm not very familiar with the Dodge V-10 engine.

[Clem Z.]

Re: Well...DUH!!!

there are 2 types of V-6s in the GM family of engines. 1 is odd fire and 1 is even fire where the even fire has each rod journals made with a offset on each journal. the odd fire has conventional straight rod journal just like your std V-8. the even fire is a smoother runnung engine

[John H.]

Re: O.K. I remember.. but what about...

The 90-degree GM 3.8L V-6 started out years ago as an odd-firing design with conventional shared rod journals, and shook something awful - the initial production design had enormous rubber donut motormounts, and they just "let it shake". This was soon changed to "offset journals", where each pair of rods still rode on a common journal, but the two halves of the journal were ground with their axial centerlines offset to one another in an attempt to smooth out the firing impulses; they're still that way today (offset journals), but they've also added the balance shafts as Joe indicated as a further refinement to meet today's market expectations for freedom from NVH (noise, vibration, and harshness).

The Viper V-10 is also a 90-degree engine, with conventional common-centerline shared rod journals, and an "uneven" sequence of firing impulses; the uneven firing impulses are not as noticeable as they were on the original Buick V-6 due to having more cylinders, which tends to mask the effect. The link below shows a current Viper forged 4340 crank, which is fully-machined on both the O.D. and cheeks of the counterweights (very rare thing today in a production engine); also rare is the fact that they're internally balanced by welding Mallory-Metal slugs into drilled holes in the counterweights rather than using external counterweighting on the harmonic damper and flywheel like 400 SB's and 454 BB's use. Expensive, but effective.


Viper V-10 Forged/Machined/Weighted Crankshaft

[Duke W.]

Engine firing and balance basics

There are two basic 90 degree V-8 layouts. The first as used by early 90 degree V-8s and current Cosworth and Ferrari V-8s have a "flat crankshaft" - like a four cylinder. Engine firing intervals are even - 90 degees - and even on each bank- 180 degrees - but the engine has a second order unbalanced horizontal shaking force, so it vibrates like an inline four, but in the horizonatal rather than vertical plane.

Modern large V-8s have a "cruciform" crankshaft with throws at 90 degree intervals. By balancing each throw relative to rotating weight the shakiing force is eliminated, but a first order rocking couple is present. It can be eliminated with additonal weight at both ends of the crankshaft. The balance of a cruciform crank V-8 is very complicated and the math wasn't worked out until the 1930s, and this lead to the modern high speed, high compression OHV V-8 developed by GM in the late forties. The tradeoff for balance is uneven firing intervals on each bank, which compromises exhaust tuning since two cylinders fire at 90 degree intervals.

BTW, modern V-8 crankshafts have five main bearings (but the old Ford flathead only had three), four on a V-6, and seven on a I-6. I-6 layouts are inherently balanced - there are no shaking forces or rocking couples below fourth order, but ALL V-6 configurations are unbalanced. The 60 degree V-6 is probably the best as it only has a second order rocking couple. The 90 degree V-6 has both a first order and second order rocking couples, and the first order couple can be eliminated by a counter-rotating shaft running at crankshaft speed.

A 90 degree V-6 has uneven firing intervals as evidenced by the old Buick V-6, but in the seventies, the crankpins were offset 15 degrees to create even firing, and some versions have a counter rotating balance shaft to eliminate the first order couple, but the second order couple remains. The current Mercedes V-6 engines also have a counter rotating balance shaft to eliminate the first order couple.

V-10s typically have a 72 degree bank angle to give even firing, and a V-10 can be viewed as two inline fives tied to a common crank. An inline five has no unbalanced shaking forces, but has a rotating couple for which there is no practical solution, so a V-10 has two rotating couples 72 degrees out of phase.

V-12s can be viewed as two inline sixes on a common crank, so they are free of any unbalanced forces/couples below fourth order. A sixty degree bank angle yields even firing intervals, but interestingly, the V-12 in the 550/575 Maranello has a 65 degree bank angle. Because of their excellent balance and six power strokes per revolution, V-12s are very smooth - the ultimate engine configuration IMO, but they can be big and heavy relative to a V-8 of the same displacement.

Charles Fayette Taylor's "The Internal Combustion Engines in Theory and Practice" Volume 2 has a very good chapter on engine balance. The two volume set is available from SAE.

Modern mounting technology can usually deal with unbalanced shaking forces and rocking couples, but big inline fours - two liters or more - represent the biggest challenge. An Englishmen named Lanchester invented and patented the dual counter-rotating balance shafts running at double crankshaft speed back in 1914, but there were no takers because back then, with the crude machining technology, it was cheaper to just build a I-6. Mitsubishi somehow was able to repatent the system back in the seventies and Porsche bought a license for the big 944 inline four. GM didn't add balance shafts to the Quad Four until 1996 after the Mitsubishi patent expired, and their new 2.2 liter Ecotech engine has balance shafts.

At idle, balanced shaft four-cylinders betray their four-cylinder origin because of only two firing cycles per revolution, but above idle they are as smooth as an average inline six. A balance shaft four is an excellent configuration for a small car.

Duke.

P.S. The "order" of an unbalanced force or couple represents the number of cycles it completes in one crankshaft revolution. First order is once per revolution, second order is twice, fourth order four times, as so on and so forth. Magnitude drops with increasing order and depends on some internal design dimensions including crank throw/rod length ratio, but assuming we assign 100 percent to first order unbalance magnitude, second order is typically about 27 percent, and fourth order is about 4 percent. Higher (even numbered) vibration orders also exist, but from a practical standpoint fourth order and above are not considered significant, and are isolated by normal soft engine mounts.

[Duke W.]

Re: O.K. I remember.. but what about...

V-4s of any bank angle including flat fours have couples.

The heavy metal auxillary balance weights on the Viper V-10 crankshaft are probably there to minimize the effect of the out of phase rotating couples originating from each bank, but here is bound to be a residual.

Ninety degree V-8s are not "inherently balanced" as is the I-6. Both the flat crank V-8 and cruciform crank V-8 are inherently unbalanced, but with a cruciform crank, proper counterweighting can eliminate the rocking couples without resorting to auxillary balancing devices.

An inherently balanced engine can be built without balancing each throw with respect to rotating mass, however, by balancing each throw with respect to rotating mass, crankshaft bending moments and their associated additional loading on main bearings and the block are eliminated, so inline sixes are typically one hundred percent balanced for rotating mass on each throw. The same applies to inline fours. The residual second order unbalanced vertical shaking force is the same regardless of whether each throw is balanced for rotating mass, but they are typically 100 percent balanced for rotating mass at each throw to eliminate the crankshaft bending forces, and the stuctural vibrations that would be excited by these internal bending stesses.

Duke

[Craig S.]

Re: Engine firing and balance basics

Duke...thx for the excellent post. It's a keeper in my cut and paste file!...Craig

[Clem Z.]

Re: Engine firing and balance basics

we tried flat crank setup in small block chevy race cars but we had so much vibration it would crack the welds in the oil pan but it ran like hell!! back in the good old days they used a flat crank in flat head fords where they fired 2 cly at the same time. that was before my racing days as i started out working with straight 6 270 GMC truck engines in race cars. the V-8 small block chevy put the end to the GMCs but you could not beat the sound a straight 6 with open headers made.

[Clem Z.]

Re: Engine firing and balance basics

that was smokeys indy engine and flat cranks seem to be used in indy car racing as the chevy IRL engine was changed to a flat crank from a 90 degree last year. the cranks are a lot lighter which may be the reason

[Duke W.]

Re: Engine firing and balance basics

I think some 305 CID F5000 engines were built with flat cranks, but I'm not sure if they

were successful. Also, didn't Smokey Yunick use a flat crank in one of his

turbocharged 209 CID Indy small blocks.

Duke

[Duke W.]

Re: Engine firing and balance basics

When the IRL dropped the displacement from four liters to 3.5 it appears that most of the engines switched to flat cranks. At least they sounded a lot better. Cruciform crank engines don't sound very good at 10K revs, but with a flat crank they sound like a Cosworth DFV.

Duke

[Eric H.]

I have to ask...

All -

Thanks for the great responses. Really a lot to be learned. Now I know why I always wanted a V12 Ferrari!

I think that there may be a lot of us who don't know the difference between a cruicform crank and a flat crank. Can this be explained simply?

If my SBC has a cruicform crankshaft, than I know what that looks like. Is a flat crankshaft one where all of the rod journals are oriented at 180 degrees to each other?

Ric

[Clem Z.]

Re: I have to ask...

correct the throws are 180 degrees apart.

[Duke W.]

Re: I have to ask...

Yes. A "flat crank" has the rod journals phased at 180 degree intervals. A V-8 flat crankshaft looks just like a four cylinder crankshaft except for wider journals to fit two rods.

"Cruciform" referred to a cross or "X", which distinguishes 90 degree journal phasing from the 180 degree phasing of the flat crank.

Next time you look at a Chevy V-8 crankshaft, study the distribution of balance mass. As stated previously, a cruciform crankshaft MUST be balanced with repsect to rotating mass and additional mass must be placed at each end to eliminate the first order couple. From a practical standpoint the rotating mass counterweights can be redistributed toward each end to reduce the amount of additional end mass requried to balance the first order couple. This is why the two center webs do not have any balance mass.

Even though a flat crank does not have to be balanced for rotating mass,they usually are to reduce crankshaft bending loads, which reduces main bearing loads and block stress. Even if a 180 degree crankshaft is fully balanced for rotating mass, the resulting end item is both lighter and has less rotational inertia than a cruciform crank. This and the 180 degree firing interval on each bank is why the flat crank is the choice for small V-8 racing engines, but the tradeoff is the second order horizontal shaking force. As long as the engine is less than about 3.5 liters the shaking force can usually be absorbed by the vehicle structure, but when you get up to the size of a Chevrolet V-8 the shaking force is of greater magnitude and may be exessive for both chassis and driver, which is why you rarely see a flat crankshaft in a Chevy V-8.

Duke