ENGINE BALANCE: Inline 6 vs. Odd fire V6 vs. 90 degree V6 vs. 60 degree V6

Secondary engine balance step by step explanation: https://youtu.be/82rxavW0A3c

It's time for another video on engine balance and in this one we're doing a detailed comparison of the inline six with different versions of the V6 engine. The odd firing v6, the 90 degree v6 and the 60 degree v6. To see why the inline six is well balanced we're going to start by figuring out it's firing interval. We do this with a very simple formula. 720 divided by 6. The result is 120, this tells us that the inline six has an even firing interval of 120 degrees. If we deduct 120 from 180 (one stroke) we get -60, which tells us that there's 60 degrees of power stroke overlap in an inline six engine. The inline six has an even number of cylinders which means every pistons is canceled out by another piston, so perfect primary balance. It also has no rocking moment front to back like inline three and inline five cylinders. What about secondary balance. In an inline six cylinder engine the pistons move in pairs, and no two pairs are ever in the same part of the stroke. Which means that the forces associated with different piston speeds at different parts of the stroke cancel each other out. But there's a problem with the inline six. It's very long making it very difficult to package under the hood. So what do you do? Of course you split the inline six in two and separate the pairs of three cylinders into two banks and then you angle the banks in a V shape and voila the V6 is born. But because we separated the cylinders into two banks a question arises – what's the best angle between the two banks of the V? To figure that out we need to again employ our useful little formula for figuring out the firing interval, and because we again have six cylinders, we have the same formula and the same result. 120 degrees. The firing interval is also the perfect angle between the banks for a V-shaped engine. But there's a problem here. 120 degrees V6 engines are useless. Instead of being compact they are bulky! The whole reason a V6 was conceived is to make it more compact but you neuter all this compactness with 120 degrees between cylinder banks. This is why 120 degree V6 engines don't really exist in mass production, and can only be found in LeMans or F1. Although back in 2020 Aston Martin showed pics of it's new TM01 V6 engine, and although I couldn't find verifiable info from the photos it seems like it has a pretty wide angle between the banks, although it doesn't seem to be quite as a wide as a 120?

Ok so a 120 degree V6 is impractical. What do we do then? Of course we reduce the angle between the banks. How about 90 degrees like in a V8? You could even make it on the same production line as a v8, just chop off a bit of block and you got a v6? Well, that's exactly what buick did in 1961 for the very first generation of their Fireball V6. They derived it from their 215 cu in (3.5 L) V8 so it had the same 90 degree bank angle the only difference was the crankshaft. It Had shared crankpins just like a v8, but because it had only three, they were 120 degrees apart instead of 90 like in the v8. The result? A very rough engine with an odd firing interval. So why did it have an odd firing interval? Because it's bank angle is different from the natural even firing interval of the v6 and the crank pin separation angle the engine must adopt an odd firing interval.

And they were able to do create an even firing v6 while still keeping the 90 degree? How you ask? Well, let's think about it, what other part of the engine can you use to make up for the „incorrect“ bank angle? The crank pin of course. Split the pin in half and offset the pins from each other. This is called a splay angle, and your splay angle is going to equal whatever your bank angle is missing. But wait! Can we make a 60 degree V6 now? Sure you can. Just adapt the splay angle. 60 degree banks means you need 60 more to „fake“ 120, so you just make a 60 degree splay angle crankshaft. Done! But doesn't this small cross section make the crankshaft weak? Not necessarily, you can do what the Alfa Romeo Busso V6 did, and put flying arms between the offset pins.

What about primary and secondary balance of a V6 engine? Well a V6 is two inline threes in opposing banks and as such it has the same engine balance DNA, an imperfect primary and a perfect secondary engine balance. What about block rigidity and torsion or twisting forces of the crank? Well the v6 should be more rigid because it's more compact but we have plenty of modern proof (2jz, barra, s54, rb26, etc.) that with the right engineering block flexing or crank twisting really isn't an issue on inline six engines.

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