Flat Plane Vs Cross Plane V8 engines










Like all high performance and race V8s (e.g. Cosworth DFV, Coventry Climax, Ferrari F40, TVR Cerbera AJP V8), the Lotus type 918 V8 uses a flat plane crankshaft configuration - as the name suggests, all the cranks pins are in the same plane, unlike the cross plane V8 crank, where 2 of the crank pins are set at 90% to the other 2 (nearly all V8s use a 90% V between each bank of cylinders and a piston/rod in one bank shares a crank pin with a piston/rod in the opposite bank)


A flat plane V8  (animated on the left) is very much like two inline-4 engines mated together in a V configuration. Therefore end-to-end balance is inherent, as the first and last piston of a bank is always exactly in the same position, as are the center two pistons on each bank.

Now, the exhaust note of an engine is related to the manifold design, which is related to the firing order, which is related to the cylinder layout and crankshaft design - therefore the sound of flat-plane V8 is usually somewhat like a pair of four-pot engines screaming simultaneously, unlike the rumble of cross-plane V8s. There have been comments like "It doesn't sound like a V8" aimed at the V8 Esprit... but this should really be "It sounds like a Racing V8!" for the reasons explained here.

While it may be true that flatplane V8s are less refined than crossplane (The flat-plane crank pins produce vibrations, being 180° opposed and thus lacking rotational balance, unless balance shafts are used - which is actually rare), refinement is not a priority for sports/race cars: light weight pistons and conrods and a short stroke "over square" configuration greatly reduce this "second-order vibration". More importantly, as a flatplane crankshaft does not require counterweights, it  has less mass and a lower moment of inertia, providing higher rpm and more rapid acceleration.

The firing order with a flatplane V8 allows for perfect balance between banks - each subsequent firing cylinder being on alternating left and right banks (there are a number of possible firing orders e.g. 1-6-3-2-7-4-5-8 or 1-4-5-2-7-6-3-8 but all follow a R-L-R-L-R-L-R-L...bank pattern). This produces even exhaust gas pressure pulses from each bank - allowing manifold design to be fairly simple, with good scavenging and no need for cross over piping from side to side.


A cross plane V8 (animated on the right) is the configuration commonly used in American road cars, plus the Rover V8 (originally General Motors/ Buick/ Oldsmobile), of course, with the crank pins set at 90°, 180°, 90° to the preceding pin(appearing in the form of a cross from crankshaft end). The cross-plane design can achieve very good balance but requires heavy counterweights on the crankshaft to achieve it. It therefore has the major inherent disadvantages of larger crankshaft mass and higher rotational inertia - making it less responsive and lower revving - the larger crankcase needed to accommodate the counter weights also makes the engine taller and raises it's centre of gravity, which adversely affects car handling





Heavy counter weights are used with cross plane cranks - normally (on non V8 configurations) counter weights with sufficient mass to balance the crank throw, con-rod and pistons cause further imbalance with rotation - as the diagram on the left shows,  when the counter weight has rotated to the right, the piston and con-rod do not move left as far as the crank pin. However, as shown on the right, with a 90° V8 the piston from the opposite bank will contribute to balancing the counter weight entirely. This endows the cross-plane V8 with near perfect smoothness, albeit at the expense of compactness, revvability and the exhaust shortcomings below.

The crossplane V8 crank makes firing order and exhaust design more problematic - the way the crank pins bring the pistons to TDC dictates that firing a cylinder on the same side as the previous cylinder must happen at least twice (once on the left bank and once on the right bank) - in fact this includes 2 adjacent cylinders on one bank, which is far from ideal in terms of mechanical stress and head soak (again there are a number of possible firing orders - but some are never used in practise as they have all 4 cylinders in a bank firing sequentially! The four usable orders are1-8-4-3-6-5-7-2, 1-8-7-3-6-5-4-2, 1-8-7-2-6-5-4-3 and 1-5-4-3-6-8-7-2 which have bank patterns of R-L-L-R-L-R-R-L..., R-L-R-R-L-R-L-L..., R-L-R-L-L-R-L-R... and R-R-L-R-L-L-R-L...

This firing order anomaly is what creates the typical American V8 burble (so it's a sign of inefficiency!) - when two cylinders fire sequentially on the same bank, the two exhaust pulses combine to form a higher pressure and associated note from the tailpipe, which alternates from side to side.

Exhaust manifold design is not easy with the cross plane crank - cross over piping from side to side is required to improve scavenging - making this configuration unsuitable for racing/single seater applications. For road applications, balance pipes are often used to equalise side to side exhaust pressure, which helps scavenging somewhat at low RPM.