Why is 60 degrees the preferred bank angle for a V6 and 90 for a V8?

[carwhisperer]

My understanding is that 90 degrees is the best bank angle for a V8 while 60 degrees is best for a V6. I have studied the math that suggests that a bank angle of 90 degrees permits equating the effects of two pistons/rods sharing one crank pin to a rotating mass, which can then be counteracted with a counterweight. However, this analysis doesn't have anything to do with the total number of cylinders, just two pistons sharing one crank pin in a V configuration.

I would assume it has something to do with crank pin spacing. Two full revolutions per cycle=720 degrees of rotation per cycle. 720/8 cylinders = 90 crank pin spacing (not true for a flat plane crank V engine, which is another matter). 720/6 =120 degree crank pin spacing for a V6. I have read that a bank angle of 120 degrees is ideal for a V6 but that few are made that way because of packaging issues, and that 60 degrees, the supplement of 180 is therefor used. I realize there are many exceptions. For example, Ferrari made a 65 degree angle V6, the VW VR6 bank angle is very narrow, and Lancia, I believe, made a 14 degree bank angle V8. But I believe I am correct in saying that 60 is most common for a V6 and 90 for a V8. What is the math that leads to these two choices?

Brian

 

[MikeHalloran]

The 60 degree V6 has even firing intervals, sounds nice, and doesn't require a lot of space.

For economic reasons, V6 engines have been made on V8 machining lines, with a 90 degree angle.
With simple crankpins, they have uneven firing intervals, and sound a little 'off'.
With split/offset crankpins, they can have even firing intervals, and sound ok.

The math basically revolves around what you are tooled up to build, and how many you can sell.




Mike Halloran
Pembroke Pines, FL, USA

 

[benta]

What Mike said.

"For economic reasons, V6 engines have been made on V8 machining lines, with a 90 degree angle.
With simple crankpins, they have uneven firing intervals, and sound a little 'off'."

Yep, the most ubiquitous V6 in Europe is the "Euro-V6", also known as the PRV-V6 (Peugeot/Renault/Volvo). The block is also used in present Formula 1, and the sound is really 'off'.
It started out as a 90 degrees V8, but due to oil crisis, fuel costs etc. was cut down to a V6 with the 90 degrees kept unchanged and balancers, dampers and stuff added to keep it somewhat civilized.

The Dino 65 degrees V6 has an interesting background. For best performance (which Ferrari always goes for ), you need a cross-flow cylinder head. It is preferable to place the exhaust on the outside of the Vee (for thermal reasons), and the intake on the inside, providing short induction distance.
However, there is not a lot of space between the cylinder banks on a 60 degrees V6, so flow is restricted. The idea of the Dino engine was to open the banks a little, using 65 degrees, and hope that balance wouldn't suffer too much.
It worked. The Dino V6 is a powerful (for its time) engine, very well balanced with a wonderful sound. Like a 60 degrees V6, but with a touch of "overtone snarl". When you've heard it once, you'll recognize it anywhere.

You probably recognize that I like Dinos

Cheers,

Benta.

 

[BrianPetersen]

60-degree V6 engines have 60-degree-offset crankpins within each bank. This allows primary mechanical balance to be achieved, without a balance shaft, plus gives an even firing order.

90-degree V8 engines have no offset in the crankpins, which is easier to machine, and it allows primary mechanical balance to be achieved without a balance shaft and it has an even firing order.

Even if the "overall" engine has primary mechanical balance, having "each V-twin" somewhere near being balanced (via crank counterweights) reduces the loads on the main bearings. Not a problem with a 90-degree V8, not a problem with a 60-degree V6 with 60-degree offset crank pins.

A 120-degree V6 with no offset crankpins would have an even firing order but an unbalanced end-to-end rocking/twisting couple (like an inline three has; having the banks 120 degrees apart wouldn't cancel them completely out).

A 60-degree V6 is easier to package in most engine compartments than a 120-degree V6 would be, particularly if the engine is transverse.

A 60-degree V6 is easier to machine and assemble. Sorry Subaru fans but your flat-horizontal opposed engines are a pain in the tail. 120 degrees would start verging on having to split the crankcase vertically (like Subaru, old air-cooled VW, etc) and that's a nuisance.

 

[GregLocock]

Brian, always be careful about balance nomenclature. Primary balance sounds like first order (it isn't what you mean I think) and that can be achieved with any configuration, with balancing weights, using a 2 plane balance.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[BrianPetersen]

I fixed a mistake in my post; if you can explain it with better terminology (or if the original poster will chime in with anything they don't understand) go ahead.

Other configurations have certainly been done. There have been V8s built with bank angles other than 90 degrees, and plenty of 90-degree V6 engines with offset crankpins or not, and with first-order balance shafts or not. The history of the Buick and Chevrolet V6 engines is a long one. Both 90-degree V6 engines were basically the corresponding division's small block V8 engines with two cylinders chopped off. They started out with single crankpins and uneven-firing, then Chevrolet had a wierd "semi-even-firing" version with offset crankpins but not enough to fully achieve an even firing order (IIRC the offset was 18 degrees; why they chose to do this is beyond me), then they went even-firing, then they added a balance shaft. In parallel with all this, GM came out with an entirely unrelated 60 degree pushrod V6 design with 60-degree offset crank pins, and if memory is correct, this was for the front-drive X-cars, Chevrolet Citation et al, in order for it to fit transversely in that size of car.

 

[GregLocock]

I had the misfortune to work in NVH when all these 90 degree V6s became popular. At least one major OEM walked away from an entire program because they were so awful. However, there never was a problem with first order balance.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[BrianPetersen]

Oh, no doubt! It seems that the "odd-firing" Buick V6 persisted from 1962 through 1977. I don't think it was a popular option before the first oil crisis (apparently GM sold the tooling to Jeep in the late sixties - before American Motors bought Jeep) but bought it back in 1974 and the "odd-fire" was then used from 1975 - 1977 and I'm sure it was a lot more popular in those three years. It also appears that the Chevrolet "semi-even-firing" configuration was around from 1978 through 1984. The 4.3 litre was even-firing from the outset in 1985 but didn't get a balance shaft until 1992.

 

[Lou Scannon]

Some years ago, Deutz North America was testing one of our engine controls on their 90 deg. V6 engine. The control was not meant for engine mounting, but for expediency in their test cell, that is what they did, using soft rubber isolators at each corner between the control chassis feet and the bracket they had attached to the engine.
I still remember, when the engine was brought to its 1800rpm rated speed, how the control became a blur, shaking in 3 planes at 30Hz, while dancing artfully in a superimposed rotating orbit at about 1Hz!! Upon seeing that phenomenon, I did mention to them, that they shouldn't be surprised if the control experienced an early failure.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz