{"id":6794,"date":"2025-01-26T11:42:43","date_gmt":"2025-01-26T16:42:43","guid":{"rendered":"https:\/\/columbuscorvette.com\/wp\/?page_id=6794"},"modified":"2025-06-28T17:46:24","modified_gmt":"2025-06-28T21:46:24","slug":"z06-and-zr1-c8-corvettes-flat-plane-crankshaft-what-why","status":"publish","type":"page","link":"https:\/\/columbuscorvette.com\/wp\/?page_id=6794","title":{"rendered":"Z06 and ZR1 C8 Corvette’s Flat-Plane Crankshaft – What & Why?"},"content":{"rendered":"\n

Lots of people are talking about the \u201cFlat-Plane Crankshaft\u201d used in the high-end Z06 and ZR1 C8 Corvettes and exotic cars like Ferraris and McLarens. What\u2019s the big deal?
For years, American V-8s used a two-plane or, a.k.a., cross-plane crankshaft. If you look at the crankshaft from either end, the crank throws form a \u2018cross\u2019, with 90 degrees between the throws (or crank arms). The YouTube video at https:\/\/www.youtube.com\/watch?v=ua1zJIKqKO8 <\/a>does a pretty good job explaining this, though there are a few flaws in what is said.
And you can read more about it here: https:\/\/www.edtengineers.com\/blog-post\/gearhead-quandary-be-flat-or-be-crossed<\/a>
In GM V-8s, either flat-plane or cross-plane, the cylinders on the driver\u2019s side (and starting at the front) are numbered 1, 3, 5, and 7. The cylinders on the passenger\u2019s side are 2, 4, 6, and 8. And, in both flat and cross-plane V-8s, a cylinder fires every 90 degrees of crank rotation. So, in both cases, flat or cross-plane, firing impulses are evenly spaced.
Old Chevy small-blocks and big-blocks: With the cross-plane crankshaft, achieving this 90 degree firing interval results in a problem. An old small-block or big block Chevy had the firing order 1-8-4-3-6-5-7-2. Notice that the cylinders fire odd-even-even-odd-even-odd-odd-even. So, the exhaust pulses on both left and right exhaust manifolds are not evenly spaced. We have two pulses on the same side occurring one immediately after the other. But, for best exhaust system tuning, the pulses should enter each side\u2019s header evenly spaced. And worse, cylinders 5 and 7 are right next to each other, which on the intake side, to some degree starves cylinder 7 if you have a common-plenum intake manifold.
New small-blocks: The updated LS and LT engines also used a cross-plane crankshaft, but the firing order was revised to 1-8-7-2-6-5-4-3. This revised firing order has similar problems. The firing order is odd-even-odd-even-even-odd-even-odd-odd. So the exhaust pulses, again, are not evenly spaced on each side. And on the intake side, cylinder 1 and 3 are right next to each other, so again, with a common plenum intake manifold, cylinder 1, pulling air after right after adjacent-cylinder tends to be starved a bit.
With a flat-plane crank, the Chevy C8 Z06 and ZR1 firing order is 1-4-3-8-7-6-5-2: odd-even-odd-even-odd-even-odd-even. So, all exhaust pulses on the odd (driver\u2019s side) bank are evenly spaced and all exhaust pulses are also evenly spaced on the even (passenger-side) cylinder bank. This allows optimum exhaust header pressure-wave tuning for higher peak power. And you no longer have two adjacent cylinders in a bank firing one right after the other, which is better for intake tuning.
What are primary and secondary shaking forces?: Both forces are caused by the pistons going up and down. If the piston had a cross-slotted wrist pin hole and was connected directly to a crank throw, you\u2019d have pure primary shaking forces, but building a useful engine that way would be complicated. Instead, we use a connecting rod between the crank throw and the piston. As the piston goes up and down, it tries to shake the engine up and down once per rev. These are called primary shaking forces. The up and down force from the reciprocating piston and rod can be cancelled by counterweights on the crankshaft placed opposite the crank throw. But, in turn, the counterweights cause a primary shaking force to the sides. Those can be counterbalanced by another bank of cylinders set 90 degrees to the first bank \u2013 a V8 \u2013 with either cross-plane or flat crank format. But the counterweights on the crank of a cross-plane are equal to 100% of the piston and rod weight \u2013 a relatively heavy counterweight that makes the crankshaft assembly heavier and slower to rev.
A flat crank engine has smaller counterweights. Why? Because the primary forces on each bank of a flat crank V8 (or in-line four cylinder) automatically cancel each other out. With smaller counterweights, the flat crank assembly is lighter and can rev quicker.
It gets more complicated: The angle of the rod to the crank throw goes through a right angle during the upper portion of its stroke which makes the piston move a bit faster in the upper portion of its stroke compared to the lower portion of its stroke, where the rod is never at a right angle to the crank throw. This geometry causes an unequal speed phenomenon in the upper and lower portions of the stroke. This causes a secondary shaking force which cycles twice per rev.
A V8 crankshaft with throws spaced at 90 degrees allows the secondary shaking forces to cancel out. For example one piston will be going up in the faster, upper portion of stroke, while another piston will be going down during the faster, upper portion of the stroke. This provides perfect secondary shaking force cancellation.
But with a flat crank, the throws are spaced at 180 degrees, so, for example, two pistons are going up in the faster, upper portion of the stroke while two pistons are going down in the slower, lower portion of the stroke. The secondary shaking forces are not cancelled. While secondary shaking forces are substantially smaller than primary, they are still significant and noticeable.<\/p>\n\n\n\n

So why did Chevy and others use the cross-plane crankshaft all these years if it had breathing shortcomings? Because the cross plane-crank allows all the primary and secondary shaking forces to cancel each other out, providing a very smooth engine. I remember showing my dad 40 years ago how I could balance a nickel on top of the air cleaner of my \u201969 Camaro with a 350 LT-1 small block — even though it loped a little at idle.
With a flat-plane crankshaft, primary shaking force can be inherently cancelled, but secondary balance cannot. The math to show this is kind of cumbersome. But what it means to us is that it\u2019s hard to make a car with an engine with a flat-plane crankshaft feel smooth to the driver. Things like shorter piston stroke, lighter pistons and connecting rods, longer connecting rods, special engine mounts and chassis tuning help a lot, but the engine itself will be inherently buzzy. The press reported that a Chevy engineer admitted that Z06\u2019s oil filter tended to vibrate off during development! Really bad for warranty claims!
So far, so complicated, right?
But the two major shortcomings mentioned for the smooth, inherently balanced cross-plane crank V8 have solutions! And I have to tell you, I think Ford led the way:
Exhaust: If you\u2019re as old as me, you\u2019ll remember the \u201cbundle of snakes\u201d exhaust systems on 60\u2019s Ford GTs and Indy cars. https:\/\/burnsstainless.com\/blogs\/articles-1\/bundle-of-snakes-180-degree-headers-1?srsltid=AfmBOoouR7vvZmwH5AVMXiWkTSJbpr4O65E0LL6zIDGWNbb08X5v_ydU<\/a>
Ford re-plumbed the exhaust headers around the sides and behind the top of the engine so that each header collector received evenly-spaced exhaust pulses. I think there\u2019s plenty of room on top of the C8 engine to accommodate this very hot, bulky plumbing, but it would cover up a lot of the otherwise attractive top view of the C8 LT1 engine. And, the header tubes become rather long, which is great for a torquey street engine, but not optimum for a high-redline engine like the C8 Z06. And the long pipes would dissipate some exhaust heat energy from the turbo ZR1. But, with a twin-turbo engine, does the back-pressure that turbos cause in exchange for boost – typically about two psi back-pressure for every one psi of boost, affect the exhaust resonant tuning as much? I\u2019m honestly not sure. Maybe it matters more — because separating those adjacent cylinder pulses helps even out flow through each turbo for steadier turbine wheel speed.
Intake: Ford installed four, two-barrel Weber carbs on some of these \u201860s race engines, and also installed individual-runner injector systems on others, so there was no common-plenum intake manifold to starve back cylinder number seven (OK, to add to complication, Ford numbered their V8 cylinders differently than Chevy, of course). Anyway, with modern fuel injection systems, the intake plumbing handles dry air, i.e., no liquid fuel, which minimizes fuel distribution concerns, and the air passageway plumbing can be large enough to minimize the \u2018starvation\u2019 of cylinder 7 that demanding lots of air immediately after adjacent cylinder 5.
So, why did Chevrolet develop a very special, expensive, but inherently buzzy, engine used only in the most expensive, limited production Corvettes?
Because they could.
Because the new flat crank engines can rev to the moon.
Because they wanted their best Corvette to sound like a Ferrari. And perform like a Ferrari, at a fraction of the price.
Because they wanted the maximum feasible performance from an optimized V8 engine, while accepting and overcoming some drawbacks.
Chevrolet, and GM, have provided worthy treasures for us to aspire to!!<\/p>\n\n\n\n

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