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In other words, what factors influence an engine designer's decision to go with a certain firing order?

I had some free time today to investigate what firing orders are possible on a flat-plane 90° V8 and determined that there are 8:

1-5-2-6-4-8-3-7
1-5-2-7-4-8-3-6
1-5-3-6-4-8-2-7
1-5-3-7-4-8-2-6
1-8-2-6-4-5-3-7
1-8-2-7-4-5-3-6
1-8-3-6-4-5-2-7
1-8-3-7-4-5-2-6

Of these eight possibilities, Wikipedia documents Ferrari as using the 1-5-3-7-4-8-2-6.

Since this is a V8, evenly-spaced power strokes (at 90° crank intervals) is guaranteed.

So what other factors influence the decision to choose one firing order over another?

  • Excellent question! I've wondered that too. The below link seemed to be the best discussion on selecting a firing order for a given engine. It seems to boil down strictly to balancing the engine to avoid vibration and optimize airflow in the intake and exhaust. quora.com/… – atraudes Feb 10 '16 at 16:18
  • 18436572 seems to be missing from your list.... – Solar Mike May 14 '17 at 22:29
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This is not an easy question to answer, but I will give it a shot. First off, a flat plane crankshaft means all journals are on the same plane, so if you looked at it straight on it would have all journals in a line, straight up and down. A cross plane engine, when looking from the end straight on would look more like an X, evenly spaced.

There are many reasons to use one configuration over another. A flat plane design does not require counterbalance weights on the crankshaft. That allows for less inertia, higher possible rpm's, and faster acceleration. Although it is a little less refined as far as vibrations go. This is one reason this engine design is used in most V8 race car engine designs, but not used in most production cars.

Several factors influence firing order, but in a flat plane 90 degree V8 there are only a few choices. First of all, it depends on how the cylinders are numbered by the manufacturer. Some number their engines with number 1 on the right bank (1,3,5,7), number 2 front left (2,4,6,8) and so on, one accross from the other. Some have number 1 on the left front cylinder and number straight down that side to number 4 (1,2,3,4) and the front right cylinder is number 5 going back to number 8 (5,6,7,8) on the right bank. Ferrari numbers theirs starting on the right front with number 4, and continue on that bank 4,3,2,1. The other bank would be 8,7,6,5. In a flat plane V, regardless of the numbering system, the firing order always fires one bank, then the other bank. Notice the firing orders you listed. All have one side firing, then the other. All have numbers 1 and 4, 360 degrees apart.

Think of a flat plane V8 as two 4 cylinder engines put together, one on each bank. If you look at the firing order for the Ferrari, and the numbering system they use, every other number is on the same bank. This is the normal firing pattern for a typical 4 cylinder engine (1,3,4,2). The same is true for the other bank using the other bank numbers (5,7,8,6). This is the same pattern. Put the two together and you have their firing order (1,5,3,7,4,8,2,6). Because the V is at 90 degrees, this means each cylinder fires 90 degrees apart, and directly opposite on each bank. If you want to change the firing order, change the camshaft and it changes the order, but always 1 and 4, and 5 and 8 are opposite each other, as are 2 and 3, and 6 and 7. The numbers could swap order, but only in the pairs as shown (2,3 or 3,2 etc).

Another reason for this configuration, with one cylinder firing one each bank in progression, is that it helps with exhaust scavenging, keeping the exhaust manifold very simple to make, and does not require any crossover pipes.

I hope this helps with your question. There is more information available here: http://www.projectm71.com/Cross_FlatPlane.htm

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  • Quality answer! – I have no idea what I'm doing Feb 11 '16 at 8:49
  • Some great information here. But I was hoping for more information on why someone would opt to fire Cyl #3 first and not Cyl #2, even though both options are plausible. I understand that the firing has to alternate between banks, but all of the 8 options shown are like that. Is there something related to the load on the journals and mains? What about the air flow on the intake side? Thanks – Zaid Feb 11 '16 at 12:13
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The reason for this order is that a 4 cylinder isn't perfectly balanced, dynamically. Up and down is balanced (primary) but side to side (second order) is not. Balance shafts cancel out these secondary forces. The forces aren't terrible, and engines under 2.0L haven't always used balance shafts. Ford's Voodoo V8 with a flat plane crank doesn't use balance shafts, even though the forces would call for them. Instead, they added a LOT of mass to the crankshaft...which negates all those wonderful benefits of a flat plane crank. Ford did it just for the sound!

Specifically, the cylinders can't fire 1-2-3-4 because when the cycle starts again at 1, the engine wants to teeter totter (rocking moment). This is why the firing order has the centre cylinder fire, then the adjacent outside cylinder, and mirrors on the other side. The order can fire from the inside cylinder out, or outside cylinder in. ie. 3-4-2-1 (iow 1-3-4-2) or 1-2-4-3. Whichever one you use limits the choice of firing order for the other bank (the cylinders that share a crank pin fire 360° apart). And the design of the crank limits those potential sequences to one firing order. Since Ferrari uses inside out, and a typical flat plane crank, it makes sense that it is what it is.

Ford flipped the rod journals 3/4 so you get the front two cylinders firing inside out, and the rear two firing outside in. iow they fire consecutively from back to front. This rocking motion, together with the side to side vibration, was so bad it nearly killed the project.

If they're both equally smooth, why pick one over the other? Probably the way it sounds? Although I can't imagine why they'd sound different.

I believe Ferrari does in fact use a crossover. Power-wise, it's worth a few prancing horses to have independent exhaust. Dumping all the cylinders into one pipe doubles the pitch frequency. That's why a Ferrari 458 wails instead of sounding like a Civic.

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