A firing order swap is one of those modifications that has become popular based on the potential of an often-corroborated performance upgrade; however, its true value to the engine builder may be obscured by those power promises.
I knew the driver and I wasn’t going to see him get killed.–Billy Godbold, Comp Cams
The topic of firing-order swap is rather narrow in the high-performance and racing industry. It’s usually limited to 90-degree V8 engines, both gas and diesel. You rarely hear Nissan GT-R or Buick Grand National owners asking engine builders if there’s a better firing order for their V6 engines. And if they did ask, the answer is likely to be “no.” Same for straight-6s, straight-8s and I4s.
Just what is a firing order swap?
First, we know that a conventional V8 engine is designed to generate a power stroke every 90 degrees of crankshaft rotation. Remember, it’s a 4-stroke engine, so, a full cycle of eight firings requires two complete crankshaft rotations, or 720 degrees (8 x 90 = 720).
Eight possible firing orders
Most V8 engines have a cross-plane crankshaft, which phases the connecting rod pins at 90-degree intervals. This compares to a flat-plane crankshaft where the pins are phased at 180 degrees. The flat-plane crankshaft style will be discussed later since they’re used only in high-end racing and exotic production vehicles.
Based on an engine configuration of a 90-degree V8 block spinning a cross-plane crankshaft, there are only eight possible firing-order combinations. Actually, there are 16 because GM and Chrysler engines are left-bank forward and the automakers number their cylinders differently than Ford, which is right-bank forward. With a little math and comparison mapping, we can find the common ground for the eight firing orders (see chart below) available to the manufacturers.
Four of these possible combinations are generally thought to be ineffective in performance applications, as they fire four cylinders in various sequence orders on one bank before the four cylinders on the opposing bank are fired. Such a bank-to-bank combustion cycles would create considerable engine shake and therefore is rarely used by engine builders. That doesn’t mean they haven’t been tried. Godbold knows of former a Pro Stock Truck racer and a NASCAR team experimenting with such an order–most likely 1-8-4-2-6-5-7-3. And a Top Fuel team once asked Godbold for a bank-to-bank camshaft, but he was understandably hesitant to turn a 25-foot-long digger into a 10,000-horsepower, 300-mph vibrator.
“Think of all the downforce the exhaust produces on a Top Fuel engine,” reminds Godbold. “Four massive pulses from one side, then four from the other. I knew the driver and I wasn’t going to see him get killed.”
Who discovered the swap?
The other four possible combinations alternate individual cylinder firings between the banks except when adjacent cylinders fire. As you can see from the chart, three firing orders are used today by automakers and custom engine builders, and one has yet to earn favor with the industry.
Firing orders are manipulated by changing the lobe orientation on the camshaft and wiring the spark plugs or recalibrating the spark controls in the engine’s ECU accordingly. For a diesel, of course, you change the cam and reconfigure the fuel delivery pattern.
Once familiar with the three different firing orders, as well as the cylinder numbering system for different applications, engine builders can analyze the pros and cons of each. For the purpose of this discussion, we’ll focus on the GM platforms, since that’s the segment where most of the aftermarket cam swaps are targeted—but the theories will apply to other applications.
4-7 Swap Inspiration
Just what inspired Comp Cams to team with Steve Schmidt and experiment with the 4-7 swap on the GM platform?
“I believe Jack Roush and John Lingenfelter were both playing with different firing orders during that same time frame,” remembers Billy Godbold of Comp Cams. “Clearly, Ford was heavily involved in firing order testing in the ’60s, but both John McWhirter (an engineer who helped found RHS, Cam Dynamics, and then Comp Cams) and Scooter Brothers (head of Comp Cams R&D at that time) were always quick to spot OEM technology, try to grasp what their engineers were thinking, and then apply those principles toward racing. That mindset played a huge role in bringing Competition Cams up from a tiny shop 40 years ago to our place in the market today.”
In the mid-‘90s, word drifted through the Pro Stock garages that camshafts swapping the #4 and #7 cylinders in the firing order were being used in some of the quicker cars. Steve Schmidt, who once supplied engines to eight of 16 qualifiers in a Pro Stock race, was one of the first engine builders to adopt the concept.
“I wish I could take credit for it, but it came from testing I was doing for Comp Cams (see sidebar),” says Schmidt. “It was worth about eight horsepower across the board on a 2-carb engine.”
Did Ford know best?
Actually, the 4-7 swap wasn’t new to the industry, since it translates identically to the old Ford Flathead firing order (and also the current Ford Coyote platform) when adjusting for cylinder numbering. But for GM racers, the 4-7 swap continued to gain followers in Pro Stock and other racing classes. Some had noticeable power gains, while others observed no difference. Through continued testing and analysis, however, the logic behind the swap slowly revealed benefits of heat management and torsional load control in addition to helping with the induction tuning.
Since firing orders depend on crankshaft design, no story would be complete without mentioning the flat-plane option. With a flat-plane crankshaft, there is even firing between the banks, so the induction and exhaust systems are much easier to tune for consistent results. However, there can be severe torsional vibration issues with a flat-plane crank.
Warren Johnson tried a flat-plane crank in a 500ci Pro Stock engine, and one person close to the project noted, “He didn’t see a Christmas tree on the starting line, he saw a forest!”
Jon Kaase also installed a flat-plane crank in a 400ci Boss-9 big-block for an Engine Masters competition.
“We actually got it where it didn’t shake, even with a 4.7-inch stroke,” remembers Kaase. “But it wasn’t worth anything on the power. Last minute I changed back to a 90-degree crank. But it sure did sound cool.”
Even with a flat-plane crank, there are numerous firing-order options. Kaase says he adopted one from the Cosworth. For the record, the Cosworth 2.4-liter V8 engine that became famous for hitting 20,000 rpm on the Formula 1 circuit featured a right-bank forward block that was numbered similar to a Ford V8. Cosworth started out using 1-8-3-6-4-5-2-7 with the original 3.0-liter DFV (shown above) that Ford helped pioneer in the ‘60s. In the ‘90s Cosworth changed to 1-5-2-6-4-8-3-7 to enhance performance, but also at the cost of increased torsional vibration. “That’s why we have lots of dampers,” an engineer was once quoted.
Results will obviously depend on intake manifold design, cam timing and header selection. Conventional wisdom says that when there is a pair of adjacent cylinders firing in sequence, they will compete for the same air in the plenum.
“There’s always a lot of mixture robbing in a plenum,” adds noted West Coast engine builder Kenny Duttweiler. “Even on a Cup motor with fuel injection, they can still go through reversion cycles where they blow fuel up the runner and it gets picked up in another runner from the plenum. It’s not unlikely that with dual four barrels cocked sideways over the runners that you might find some power by swapping the firing order, if only because what you’re doing is addressing what’s going on internally in the intake.”
Managing heat is definitely one benefit of a 4-7 swap. With the standard GM order, the left rear cylinders #5 and #7 fire consecutively. Depending on the engine bay layout and header construction, a considerable amount of heat can get trapped in that corner. With a 4-7 swap, the adjacent-firing cylinders are moved from the left rear corner to the right front corner where cylinders #2 and 4 will fire consecutively.
“Do you want to stick that at the back of the engine bay as far away from the water pump as possible?” questions Godbold. “Or do you want to stick that at the front of the car near the water pump and where it’s inherently cooler in the race vehicle?”
Heat management with a 4-7 swap
Godbold has seen situations, especially in critical tight-lash applications, where the heat buildup in the back of the engine consistently leads to some type of mechanical failure, even in short-term runs on a dragstrip.
I followed suit when the NASCAR guys designed their latest Cup engines.–Kenny Duttweiler, engine builder
Heat is also an issue with sealing combustion pressure.
“You ask someone at Total Seal, most important thing is keeping the ring conformed to the bore,” adds Godbold. “If you get heat in the back corner of the block, it’s hard to keep that cylinder from having blow-by.”
Calming down the crankshaft
The final benefit for a 4-7 swap is addressing torsional vibration issues with the crankshaft and main bearings. Again, it’s a matter of transferring the loads but there may be a benefit to keeping standard GM firing order in certain applications, like NASCAR.
“You’re going to have to hit twice on one of the outside pins, either 7-8 or 1-2 on a GM style firing order,” says Godbold. “The benefit of hitting 7-8 is that the car weighs more than the harmonic balancer. In the front there’s a balancer. In the rear there’s a clutch, transmission, driveshaft, rearend and tires—basically hooked up to a 3,000-plus pound car. Which has more inertia? If you can move that piston hit to the back, instead of the front, you’ll get a little less crank whip. The whip on the front of the crank can change timing of the piston, so the valve timing may need to be tweaked.”
Reduced crankshaft torsional vibration was the primary driving force behind GM adapting the 4-7/2-3 swap for the LS engine series. One engineer was quoted as saying the reduced vibrations were necessary to ensure an accurate reluctor-wheel reading, and computer modeling indicated that 1-8-7-2-6-5-4-3 provided the needed stability. By the way, the LS firing order is the same as the Ford 351W and 5.0-liter engines.
NASCAR wisdom helped lead Duttweiler to finalize a firing order for his stable of high-boost small-blocks that run over 400 mph at Bonneville. He had tried all three firing orders, but finally had to pick one because it was time consuming to re-pin the wiring harness following every engine swap. While reading a tech article on the development of the Ford FR9 he learned that Cup teams have tried every possible crank design and firing order.
“On a boosted engine you’re pressurizing the plenum, so it doesn’t make any difference what sequence they draw from. You can move them around, but at the end of the day and two revolutions you’re still going to fill all eight cylinders,” says Duttweiler. “I followed suit when the NASCAR guys designed their latest Cup engines.”
Swap a no-no on a V6
Adjusting the firing order never entered Duttweiler’s mind when he built some of the most powerful Buick V6 turbo engines ever to run on the dragstrip.
“It would be particularly troubling on an odd-fire V6, especially a 90-degree V6,” says Duttweiler. “The 1-3-5 bank is on a 90-degree phase and the other 2-4-6 bank is on a 150-degree phase, or a 60-degree offset. I think a guy could dig himself a pretty deep hole playing around with that. Our solutions, sometimes, are just plenty of fuel and plenty of water flow.”
In some situations, changing the firing order can set the engine performance back. For example, Tri-Y headers are designed around a specific firing order to keep adjacent-firing cylinders from merging in the first Y collector. Changing to another order will likely void any benefits built into the design.
Following extensive testing by NASCAR teams and the automakers, there appears to be some harmony in the industry as to the pros and cons of the three different firing orders. The stock GM order has its inherent drawbacks, but the 4-7 swap simply moves some of the problems to the front of the engine—which may result in smoother, cooler operation—while helping induction dynamics slightly in certain applications. The 4-7/2-3 swap is a definite advantage for the LS platform.
“You’ll hear all sorts of goofy things, but most of the smart guys will tell that there might be one percent difference if you get everything right,” sums up Godbold. “I’m not trying to discredit the claims. If you’re racing Pro Stock, you cannot give up one percent. But most guys are never going to know if they’re one percent higher or lower.”