180 Degree Exhaust Headers

Little help needed. I’m trying to understand this article. First problem, I don’t understand the first sentence.

To achieve optimum exhaust scavenging on a four-stroke V8 engine, each cylinder should fire every 180 degrees of engine rotation”;

Why would that promote exhaust scavenging?

What is the meaning of the term " exhaust scavenging"?

How could each cylinder fire every 180 degrees of crankshaft rotation? They all fire every 720 degrees, right? So on an 8 cylinder engine one would fire every 90 degrees. On a four cylinder engine one would fire every 180 degrees. Are they trying to say that every 180 degrees a single cylinder fires? If so, wouldn’t that only apply to 4 cylinder engines?

Also having a problem understanding the last paragraph.

In theory, the ultimate solution to the V8 firing-order conundrum is the flat-plane crank (aka, a “180-degree crank”). Essentially, this makes the V8 act like two four-cylinder engines, allowing the optimized collector arrangement of a four-cylinder engine without the need to cross tubes. But flat-plane cranks cause such severe vibration they can only be used on a pure race car”.

Why would that arrangement result in a severe vibration?

What that means is that one of the cylinders fires every 180 degrees of rotation of the crankshaft. You are correct that each cylinder in a 4-cycle engine fires every 720 degrees, but every 180 degrees is where in the crankshaft rotation one of the cylinders will fire. Note also that what may be optimum for exhaust scavenging is not necessarily optimum for torque, and what’s optimum for low-end power is not what’s optimum for high RPMs. Everything is a tradeoff. That’s why the engines you’ll see at the dragstrip are so dramatically different in design from those you’ll see at the 24 hours at LeMans.

A flat plane V8 has the same impulse-generated vibration frequency as a 4-cylinder, but with basically twice the amplitude. Well, at least that’s the idea. In truth, cylinder size, stroke, reciprocating masses, valve size placement and timing, and some other factors (like counter rotating balance shafts and different harmonic balancers) all have great effect on the smoothness of any individual engine. These “counter” sciences have gotten refined to where a lot of four cylinder engines are as smooth as eight cylinder engines used to be known for, and some eight cylinder engines designed to impress the neighbors (look through a Dodge brochure for evidence) are rougher and noisier than four cylinder engines were known to be 50 years ago. The Bugatti Veyron engine has 16 cylinders, and one auto writer compared it to “a mountain falling over”.

If you’d really like to learn about this stuff, there are a few great textbooks on the subject that I can recommend. Just ask and I’ll be happy to post their reference information. You know, author, publisher, ISBN number, stuff like that.


Air has mass. Once moving, it has inertia. If you time pulses properly, you get most effect from it. This helps to evacuate exhaust gases more completly and draw in next charge. The same effect can be exploited on intake side. Term used is pulse charging. Done right, you can achieve greater than 100% volumetric efficiency on a naturally aspirated engine. I prefer the term mass efficiency myself…


Does “flat plane crank” mean you could lay it on a table and it would lay pretty-much flat? i.e. The centers of all the rod journals and the centers of the crankshaft main journals are in the same plane? But half of the rod journals are pointing in opposite direction than the other half? Wouldn’t all 4 cylinder in-line engines have that arrangement?

In a cross plane crank, the second harmonic vibration of the middle cylinders cancel the second harmonic vibration of the outer cylinders. On a flat plane crank, the second harmonic vibrations of the center and outer cylinders are in phase and add.
The second harmonic is caused by the connecting rod being kicked out sideways while the piston is in the center of the stroke and vertical when the piston is at top or bottom of the stroke. This adds a 2X crank speed vibration component to the piston’s motion.
On inline sixes, the second harmonics are naturally out of phase and cancel and thus inline sixes an order of magnitude more smooth than inline fours. Many inline fours have double engine speed balance shafts in the engine to cancel that design"s strong second harmonic vibration, AKA, the “four cylinder buzz”.
On cross plane V-8 engines, the second harmonic vibration forces are also self canceling, and so this engine is also very smooth.
The shorter the connecting rods, the more severe the second harmonic vibration is and the low hood lines of '70s cars demanded both short strokes and short connecting rods in order for the engine to be compact enough to fit under the hood.

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In less technical terms, most V8s fire I cylinder every 90 degrees. A flat plane or 180 degree V8 fires two cylinders at the same time every 180 degrees. This makes it sound and vibrate like two 4 cylinder engines intead of an 8 cylinder engine,

In 1932 Ford and Pontiac both came out with flathead V8 engines. The Ford was a 90 degree and was very successful and produced until 1953 in the US and through most of the 60s in some other markets. (see Simca V8) The Pontiac was a 180 degree engine and vibrated so badly it only lasted one year.

There are companies today that make flat plane crankshafts for your Ford or Chevy V8s. I think the Chevy V8s that ran in the Indy 500 many years ago when they allowed more displacement for stock block engines used flat plane cranks.

Buick used a very odd firing crank to smooth out the vibration created by building a V6 with 90 degree separation cylinder banks. All previous sixes were either inline, 60, 120 or 180 degree separation engines.

A 180 degree crank is stronger and is always forged. It is not counter-weighted and is lighter. The cylinders don’t fire in a smooth sequence. That is one reason all-out fuel drag cars idle roughly. Scavenging is a more complete evacuation of exhaust gases.

Actually. a flat crank V-8 fires every 90 degrees, it just has a different firing order than a crossplane crank V-8. In order to fire two cylinders together every 180 degrees, four cylinders would have to come to top dead center at the same time, two of them firing and two of them reloading. With a 90 degree V angle of the cylinders, when either the two middle pistons or the two outer pistons of the left bank are at TDC, all the pistons on the right bank are at mid stroke.

The actual source of the infamous “4 cylinder buzz” is the fact that piston motion is not sinusoidal. The piston acceleration at top dead center is much higher than the piston acceleration at bottom dead center. The motion from mid stroke to tdc is not a mirror image of the piston motion from mid stroke to bdc.
The asymmetrical motion of the pistons prevents the inner cylinders from perfectly balancing the inertial forces of the outer cylinders. Boxer fours are a different story, they lack the “four cylinder buzz” even though they also have a 180 degree firing order just like inline fours because the inertia of a piston at bottom dead center is balanced by another piston at bdc an the inertia of a piston at tdc is balanced by another piston at tdc.
This picture I drew illustrates what connecting rod length does to piston motion.

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These are the kind of threads I really enjoy. The collective knowledge assembled on this site is impressive!

While rare in street cars, there is a flat plane crank in the Mustang GT350 5.2 L.

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I stand corrected. I learn something new all the time here.

The arrangement of plenums on intake manifolds of carburated V-8s is to take advantage of sequential spacing of cylinder intake events. Each side of the carburetor feeds 4 cylinders at 180* intervals. If the intake sourced one bank from each side of the carburetor cylinders 7 and 4 (GM firing order) would run awfully lean.

Ahh praise be to sequential fuel injection to solve that problem! :star_struck:

And the LS series V8’s that swapped those cylinders and changed the 5-pulse/3 pulse sound of the original small block Chevy. Runs smoother, sounds smoother but the exhaust note is definitely different.

You folks are going over my head. I’m going to have to study what’s you’ve said here.

I think I understand this part. Let me re-state it. With a flat-plane crank v8, if you looked at each bank, when two cylinders were up, the other two would be down, just like an inline 4. But when the cylinders in the first bank were either up or down, the cylinders in the other bank would be neither up nor down, but halfway in between, 2 half way on the upstroke, and 2 half way on the down stroke. Is that sort of correct?

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Yes, that is correct.

I was in the Lone Star Museum of Flight at Scholes Field Airport in Galveston and saw on display a Curtis OX-5 V-8 aircraft engine, most famously used in the Curtis JN-4 (nicknamed the Curtis Jenny) airplane.
Studying the nameplate with the cylinder numbering convention and the firing order written on it, I came to realize that it had to have a flat crank in order to have such a firing order.

Per Wikipedia, Cadillac introduced the first cross-plane V8 in 1923.

I had to look up the Pontiac flathead V8 from an earlier comment. Actually a 1930 to 1932 Oakland (later Pontiac) that pre-dated Ford’s flathead. It was also more powerful at 85 hp. The Oakland engine had a flat plane crank because they were cheaper to make. The engine mounts were really elaborate (and expensive?) to smooth the vibration. Both the intake and the exhaust were on top of the engine.

And even that was pre-dated by the Viking, a car built by Oldsmobile, that used a similar engine in 1929 and 1930 when the brand was killed.

Rod ratio is the ratio of connecting rod length to stroke, if your engine has a 4 inch stroke and connecting rods that are 8 inches long, the rod ratio is 2:1 or simply “2”. Most auto engines have rod ratios of around 1.6 to 1.8. Formula 1 engines have rod ratios of around 2.4 to 2.6, which is one factor in why a flat crank might work ok in those engine. The shorter the connecting rods are, the more severe the secondary vibration is.

Here is a link to an article that goes in depth about the effects rod ratio on piston speed and acceleration.

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doesn’t “180* header” mean that the collectors have 4 cylinders attached that are 180* apart in the firing order? The headers are connected to the same combination of cylinders as the V-8 carbureted intake, On a GM engine that would be 2-8-3-5 on one header and 1-7-4-6 on the other.

Have you ever seen headers built that way?