Flat-Plane Crank Q&A With Tom Leib Of SCAT Crankshafts

There’s a lot of chatter on the internet lately about the new Chevrolet C8 Z06 that is most definitely on the way to the production line. This new Corvette has a lot of hype behind it as people are talking about the possibility of a turbocharged 5.5-liter engine with the addition of a hybrid powertrain. The new model C8 is also thought to possess a flat-plane crankshaft in addition to these new features. While all of these assets are exciting, there is a lot of interest in the flat-plane crank. But what is a flat-plane crankshaft, and why’s it so special?

In 1962 Tom Lieb, the owner of SCAT, ran an auto parts business out of his parent’s garage while he was in college. Over the years, the company has evolved and is now one of the most respected manufacturing companies in the automotive aftermarket.

You might think the flat-plane crank is some wild new technology that GM built in a secret lab, but that’s hardly the case. In fact, the flat-plane design has been around for decades. Cadillac was one of the first manufacturers to use it in a V8 back in the early 1900s. And the flat-plane crank is probably more common than you realize. Just about every four-cylinder in existence utilizes this technology, including that annoying Honda Civic that keeps doing laps in your neighborhood getting into the infamous “V-Tec” mode. But don’t let that guy sway you one way or another — the flat-plane has some uniqueness to it when installed in a V8 engine.

Flat-plane crankshafts are often found in both super and hyper-cars — McLarens, Ferraris, Porsches, and Lotus’ are no strangers to this type of crank, and if you want to know what else they have in common, it’s a great exhaust note. The flat-plane crank is responsible for this phenomenon. These lightweight cranks rev higher and faster than the cross-plane crank that most American V8 enthusiasts are familiar with while making a delightful sound that the now-standard cross-plane designs can’t duplicate.

Tom Lieb, the owner of SCAT Crankshafts’, knows the ins and outs of just about every crankshaft made, including the flat-planes. And while GM does not offer any flat-plane cranks for the LS or LT platforms at this time, we knew Lieb would be an excellent asset to this article since he’s worked with big-name companies to develop flat-planes used in popular hyper-cars. We sat down with Lieb to ask to glean some of his many years of knowledge on these cranks.

Here is an image of a SCAT flat-plane crank going through an extensive machining process.

LSX Mag: When did the first V8 flat-plane crank show up in the U.S.?

Tom Lieb: Cadillac first used the flat-plane crank in 1915. Later, dozens of manufacturers started using them. Scripps-Booth, Cole, Wills Saint Claire, and Lincoln in the teens and the ‘20s all had flat-plane cranks. It wasn’t until 1924 that Cadillac moved to a 90-degree crank where the firing order was spaced evenly. Because these engines now fired from side to side every 90-degrees, some of the initial vibrations in the crankshaft were now gone. This transition to the cross-plane made these engines run much smoother.

LSX Mag: What distinguishes a flat-plane from a cross-plane crank?

Lieb: The throw on a standard four-cylinder crank, the number one pin is up, two and three are down, and four is up. With a V8 cross-plane crank, you would start with the number one pin, pin two at 90-degrees, pin three at 180-degrees, and pin four would be at 270-degrees to the number one pin. This spaces everything equally around the centerline of the crankshaft. A flat-plane crank is equally spaced around the centerline as well, but at 180-degree spacing instead of 90-degrees.

The first image is a SCAT flat-plane crankshaft, and the second is a SCAT cross-plane crank for the LS1. You can see the differences between the two designs by looking at the rod journals. The flat-plane journals are 180-degrees apart, while the cross-plane journals are 90-degrees apart.

LSX Mag: What are some of the advantages of a flat-plane crank over a cross-plane?

Lieb: We hear a lot about scavenging on a V8 engine with equal length headers, and they have absolutely no value due to the engine’s firing order. If you split the engine bank on a V8, you will see that the firing order into the header is odd, and all exhaust pulses arrive at different increments. Some tubes would need to be longer to utilize exhaust scavenging, and some would need to be shorter.

The whole purpose of a 180-degree crankshaft is a better cylinder head breathing symmetry and exhaust scavenging. Because of the firing order on a flat-plane crank, when the exhaust comes out of the combustion chamber somewhere around 1,300-1,500 feet per second, it jump-starts the intake port to move. This jump-start makes the engine more efficient. A flat-plane crank doesn’t necessarily make more horsepower. Still, the torque curve flattens out for a more extended period because it breathes better, creating a more usable powerband. Plus, if you’re getting a more efficient fill of air in the cylinder, you will generate more horsepower with less fuel.

LSX Mag: What are some of the disadvantages of a flat-plane crank?

Lieb: One of the disadvantages to a flat-plane crank is they tend to make a buzz/vibration in the engine at a particular RPM. This phenomenon will vary due to the rod, stroke, and bore of the powerplant. The buzz comes from the pistons’ reciprocating motion moving up and down and converting it into a rotating motion. If you lean on the crankshaft on one area more than another, this is what causes the problem. Engineers that build these engines run a torsion graph on the crankshaft’s nose to see where this issue is. Sometimes it’s a big deal because it happens in the powerband, and other times it’s not because the engine will never run in that particular RPM.

Another problem with these cranks is the stroke; the more stroke is added, the more issues you would have as far as balancing is concerned. Some customers want to run a 4.5-inch flat-plane crank, and I won’t make it for them. I would confine a flat-plane crank to a small-block engine with less than a 3.875-inch stroke. As you go out in stroke, at some point, you will sacrifice strength due to its design. If we look at the Ford GT350 (Voodoo) crankshaft, a flat-plane, it has a 3.78-inch stroke.

LSX Mag: Do flat-planes work well with power adders and high horsepower builds?

Lieb: A crank is a crank. The engine will respond to a power adder based on its efficiency. And the better it is, the better it’s going to react to whatever you do. The crankshaft itself is not going to change that phenomenon. However, because of the equal exhaust pulses due to a flat-plane firing order, that means your heat expansion is going to be similar as well. The efficiency of this combination is going to go way up. If you think about how a turbocharger works off exhaust gas expansion and how a flat-plane has equal exhaust pulses, the turbocharger’s efficiency will go way up.

LSX Mag: Does the flat plane have more vibration than a cross-plane?

Lieb: In most cases, when we have made a flat-plane crankshaft, we have not balanced it as you would typically do for a cross-plane V8 crank with bob weights. Instead, we will balance it as a shaft like a four-cylinder crank which is a different process.

A cross-plane crank will run smoother, but let’s face it. People aren’t building an engine with a flat-plane crank to fulfill a 100,000-mile warranty. If they want the broader power curve and all of the coolness that goes along with it, it is what it is.

At this point, we’re still not sure if the newest C8 Z06 will have a cross-plane crank, but if you listen to any of the sound clips of this car on the internet, it undoubtedly doesn’t sound like any V8 from GM before. And after talking to Lieb, we are sure that we would love to build a small cubic-inch LS with this new crankshaft. And why wouldn’t we want to? These cranks are lightweight, make the engine more efficient, and make crazy exhaust noises. It just sounds like fun to us.

 

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About the author

Brian Havins

A gearhead for life, Brian is obsessed with all things fast. Banging gears, turning wrenches, and praying while spraying are just a few of his favorite things.
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