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





RE: Why is 60 degrees the preferred bank angle for a V6 and 90 for a V8?
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
RE: Why is 60 degrees the preferred bank angle for a V6 and 90 for a V8?
"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
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.
RE: Why is 60 degrees the preferred bank angle for a V6 and 90 for a V8?
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.
RE: Why is 60 degrees the preferred bank angle for a V6 and 90 for a V8?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Why is 60 degrees the preferred bank angle for a V6 and 90 for a V8?
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.
RE: Why is 60 degrees the preferred bank angle for a V6 and 90 for a V8?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Why is 60 degrees the preferred bank angle for a V6 and 90 for a V8?
RE: Why is 60 degrees the preferred bank angle for a V6 and 90 for a V8?
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!!
"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz