Today we'll be taking a deep dive into the engine balance of three different kinds of V8 engines:
1. The cross-plane V8 as found in the previous generation of the corvette c7 z06 and in pretty much all other corvettes of the past and almost every single american muscle car ever made.
2. The flat-plane v8 as found in the brand new c8 z06 corvette and in exotics like Ferrari or the Porsche 918 Spyder
And finally we're talking about an oddball flatplane v8 in the form of the Ford Voodoo engine that graces the engine bay of the Mustang Shelby GT350.
The first thing we need to do is obviously to explain what crossplane and flatplane V8 actually means. Now I'm sure most of you already know this so I'll be very quick about it.
In a cross-plane v8 the crankshaft pins where the conrods attach are arranged in two planes which are perpendicular two each other. Hence the name, cross-plane, two planes crossing each other. On a flat-plane the first and fourth pin have 180 degrees of separation between the second and third pins resulting in all the pins resting together in a single plane. This difference in the distribution of crankshaft pins changes the engine's behavior and character influencing almost all the other elements of the engine's design.
Now the easiest way to understand the V8 engine is to observe it as two inline four engines, because that's what it essentially is. two inline fours at 90 degrees to each other. Now the flat-plane V8 consists of two flat-plane inline fours and the cross-plane v8 consists of two cross-plane inline fours. In fact the v8 crankshafts look the same the inline four crankshafts, only stretched out so that they can accommodate two connecting rods on each pin.
Both the Corvette C8 Z06 and the Shelby GT350 alternate between banks for each firing. Their firing order is different and they number their cylinders differently but neither of these engines fire two cylinders on one bank in succession. This is good not just for engine balance but also for performance because it gives the exhaust pulses in one bank enough time to clear the exhaust manifold before another pulse arrives.
Now the crossplane inline four does not have an even firing interval. This unevenness also gets replicated on the cross-plane so the cross-plane does not alternate between the banks in the same way as a flat plane. In the cross-plane both banks experience times when they have two firings in quick succession. By having two cylinders on the same bank fire one after the other we're sending two exhaust pulses in very quick succession down the exhaust manifold. This of course means they can easily stack up which increases back-pressure, reduces performance and complicates the design of the exhaust system on cross-plane v8 engines.
So here we can see the flat-plane inline four and here we can see the cross-plane inline four their engine balance forces. The flat-plane has perfect primary and poor secondary balance while the cross-plane has perfect secondary and poor primary balance. But there's good news for the cross plane and that's that the 90 degree V engine configuration is unique in that it can use crankshaft counterweights to balance out pistons. So by adding mass to the crankshaft the crossplane v8 can restore perfect primary balance. The result is incredible smoothness but the price is high weight. The heavy crankshaft is the reason why cross plane v8 engines can never rev nearly as high as flat plane v8 ones. The LT4 engine inside the corvette C7 Z06 manages "only" 6500 rpm, while the C8 Z06 corvette reaches a stratospheric 8600 rpm. The unusual voodoo engine inside the Mustang Shelby GT3500 comes in a close second with 8200 rpm.
Speaking of the voodoo the weirdness comes from the fact that this engine uses an up down up down crankshaft configurattion instead of a typical flat plane v8 up down down up configuration. This means that the voodoo engine essentially sacrifices perfect primary balance in the interest of... nothing. The voodoo engine gains a primary rocking couple thanks to its peculiar configuration. But the reason behind it is the incredible soundtrack of the Mustang GT350. The voodoo engine is essentially a flat plane v8 that sounds like a cross plane v8 that lost its mind and went berserk. This was achieved by combining the weird crankshaft arrangement together with a crossplane type intake manifold and unorthodox exhaust headers.
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#d4a #v8 #enginebalance
00:00 Two planes vs One
02:06 Firing orders
04:23 The sound
06:00 Primary and secondary balance
09:12 Crankshaft counterweights and redlines
12:17 How crossplanes make more Torque
14:11 Tolerating secondary imbalance
15:20 Do the voodoo that you do