SPECIAL FIRING ORDER CAMSHAFTS

SPECIAL FIRING ORDER CAMSHAFTS

Tricks for smoother-running, longer crank bearing life, and maybe a few extra horses in the process.

by Mike Mavrigian

photos by author

The cam on the left is a standard firing order bigblock Chevy. The cam on the right is a 4/7 swap cam. Notice that the second and fourth lobes from the front are opposed. This is an easy way to identify a 4/7 swap cam. (photo courtesy Comp Cams)

A special firing order (SFO) camshaft is a tuning aid that allows the engine builder to further address combustion heat and crankshaft disturbance issues. The goal is to create a smoother-running engine, and more-even fuel distribution with enhanced crankshaft and main bearing durability. In the process, horsepower gains may be achieved as well.

Power gain potential aside, the primary reason to address and alter firing order is to achieve a smoother-running engine, a smoother, more lineal acceleration ramp with less harmonic effect and crank deflection on the crankshaft and its main bearings. Which engines can benefit from this? Virtually all. Which engines can most benefit? Engines that run for long periods of time, such as oval track, road race and marine engine applications, as well as engines that are called upon to produce maximum power in a very short period of time (e.g., drag race). A 4/7 swap, for example, reduces crankshaft torsional vibration for smoother dampening and, potentially, more power.

By swapping firing order positions, a goal is to reduce crankshaft harmonic effects caused by two adjacent cylinders firing in succession (let’s call these companion cylinders). By strategically relocating these companions, it’s possible for the engine to idle and run smoother, to reduce isolated hot spots (e.g., cylinder-to-adjacent-cylinder walls), and to even out fuel distribution, primarily in applications that feature a single plane intake manifold and even more noticeably in tunnel-ram intake manifold applications.

The traditional firing order for Chevy small and bigblocks in automotive applications has always been 1-8-4-3-6-5-7-2, which is determined by the crankshaft rod pin layout. Each cylinder has a companion in the firing order. This companion cylinder will reach TDC at the same time as its counterpart, one on the power stroke and one on the exhaust stroke. These cylinders (paired as 1 & 6, 2 & 3, 4 & 7, and 5 & can be interchanged in the firing order without altering the crankshaft. While some builders claim to have seen no power improvement, other builders claim to have achieved power gains by switching cylinders 4 & 7 to create a new firing order of 1-8-7-3-6-5-4-2. This can enhance fuel distribution, especially in open-plenum-type intake manifolds, and can reportedly result in an added 5-10 horsepower.

Pro Stock engines typically take advantage of a 4/7 swap, according to Reher-Morrison Racing Engines. By swapping No. 7 and No. 4 cylinders in the firing order, this eliminates the fuel distribution and heat problems caused by cylinders No. 5 and No. 7 firing in succession. With the revised firing order, the two end cylinders do not have to fight for fuel from the manifold plenum. The result is a measurable power increase (typically 8-10 horsepower) and a smoother, cooler running engine, according to Reher-Morrison.

However, achieving this special firing order requires a specially designed camshaft, not simply switching plug wires. The camshaft lobes must be repositioned to accomplish this power stroke/exhaust switch. Years ago, this required making a special firing order camshaft from scratch, which was time-consuming and, naturally, more expensive.

By taking advantage of today’s CAD programs and CNC machining, special cams can be designed and fabricated in much shorter time frames, also helping to reduce cost.

Naturally, if a special firing order camshaft is installed in a customer’s engine, the customer must be alerted to this to avoid any confusion during spark plug wire routing during future service. It’s a good idea to provide the customer with a decal that displays the correct firing order, which can be installed on the race vehicle (e.g., underside of the hood, in the engine bay, etc.).

CAMSHAFT MANUFACTURER COMMENTS

We spoke with a few cam makers to obtain their views on this subject. Steve Tanzi at Erson Cams noted that a firing order change accomplishes the task of alleviating the problem of cylinders 5 and 7 firing next to each other in a Chevy V8. However, Tanzi said that this “moves the problem” to the cylinders 2 and 4 area. No matter what the firing order, you’ll always have a pair firing in succession, either in adjacent cylinders or cylinders that share the same rod pin. However, the front of the block will generally cool more efficiently, so the “problem” isn’t much of an issue.

Over the years, GM performed quite a bit of research in this area, beginning back in the days of the early Cadillac and Oldsmobile engines and, most recently, the LS engine family. All along, the goal was the same-to smooth out engine pulsations and to create a smoother accelerating engine. Smoothness of operation was always the target. If additional power resulted in the process, so much the better.

Is there an element open for discussion with regard to finding additional power? “To yield the greatest gains requires header design work … matching primary tubes with regard to the intake manifold and cylinder head ports,” Tanzi said. “This usually relates more directly to applications that use a single plane manifold and tunnel ram manifolds in particular. In terms of fuel distribution, we see greater benefits with single plane manifolds as compared to dual plane manifolds since dual plane manifolds are less prone to reversion and negative pulsation.”
“In Top Fuel engines, as an example, engine builders have set blowers further rearward on the engine as a way to address fuel distribution,” Tanzi said. “In addition, these guys are experimenting with both 4/7 firing order swaps, and the LS 4/7 and 2/3 swaps. The issue of firing order is a much more complicated issue than it appears on the surface.”

Firing order swapping isn’t a magic elixir. Some builders report that they see no benefit, while others claim either power or durability improvements, or both.
“The greatest benefits are seen in oval track and road racing because those engines run for longer periods of time,” said Tanzi. “In a quarter-mile situation, you have only a few seconds to determine any gains, but when a builder is trying to find any repeatable gains, even 1 or 2 horsepower, it’s worth experimenting with. Many Top Fuel, Pro Stock and Comp Eliminator builders are currently taking advantage of firing order changes. Keep in mind, though, that depending on the sanctioning body rules, firing order deviations may not be allowed, such as in IMCA Modifieds.”

“Generally speaking, smoothing out the engine by virtue of a firing order change serves to diminish harmonics experienced by the crankshaft, which has a direct impact on bearing life,” Tanzi continued. “When you have a lean [hot] cylinder, detonation shocks are transmitted to the crank and main bearings.”

“With a fuel injection setup, a lean cylinder can be richened via the engine controller to eliminate detonation, so firing order changes may not be as beneficial in an injected engine since fuel is delivered on an individual-cylinder basis,” Tanzi said. “However, the LS firing order takes advantage of a 7/4 and 3/2 swap as an additional tuning aid, to produce an even smoother acceleration profile and to benefit crank and bearing life.”

“Special firing order camshafts have become so popular that today, what was once considered a special grind has become the norm and what was once normal is now considered special,” Tanzi said.

Crane Cams’ Chase Knight noted that Crane offers SFO cams at no extra charge. Order time for a custom SFO is about four days.

Applications include smallblock and bigblock Chevy, 429-460 Ford, Chrysler LA smallblock, Chrysler B blocks and Chrysler 426 Hemi.

“Regardless of what approach you take, you’ll always have two cylinders firing next to each other, or two opposing cylinders that share the same rod pin,” Chase noted. “But by swapping certain cylinder firing orders, you can more evenly distribute combustion pressures and resulting crankshaft loads to either generate more power, or to extend crankshaft, main bearing and crankshaft seal life, or both power and durability. There are so many variables, depending on factors such as intake manifold design, engine speed, type of use, etc., that the benefits are sometimes difficult to quantify. There are cases where no power improvement is evident on the dyno, yet on-track power is improved.”

Following are comments provided by Billy Godbold of Comp Cams. This is very insightful info, so I urge you to read this. We’ve provided this information in a question-and-answer format.

PE: A typical camshaft firing order swap of 4/7 seems to be fairly common today. Why is this firing order beneficial?
BG: There are three basic issues that engine builders or developers try to address with firing order as follows:
A. Hot Spots in the Head and Block – With a common pin V8 crankshaft, it is impossible to eliminate having adjacent cylinders fire subsequent to one another in the sequence. While this is unavoidable on a four throw crank, you do have some choice as to which pairs fire together. The 4 & 7 firing order moves that hot spot from the 5 & 7 cylinders up to the 4 & 2 cylinders. Clearly, the front pair is easier to keep cool with a front-mounted water pump than the back pair.
B. Main Bearing Issues – Most engine builders see better bearing life and fewer indications of scuffing when going to the 4 & 7 swap. The LS firing order-the 4 & 7 swap plus an additional 2 & 3 swap-may be even better for bearing life. Again, firing pairs is what we are trying to change. Here I think the focus is on pairs that fire on the same pin, thereby focusing the force on the adjacent crank main bearings. Moving this to the back of the engine seems to be a good idea because, even with a crank dampener, the drivetrain is much better at dampening these forces than a small mass on the end of the crank. When we consider the power that can be gained with lighter-weight, lower-viscosity oils, bearing life becomes not only a durability, but also a power concern.
C. Fuel Distribution – Changing the firing order is one of the best ways to change the cylinder to cylinder fuel and air distribution in the engine. Just like throws on the crank and cylinders next to each other in the block or head, we have to deal with ports next to each other and across from each other asking the manifold for air right after one another. This creates a very dynamic system in the manifold plenum where pressure waves of air, and also fuel in a wet manifold, are moving from front-to-back and side-to-side. Changing the firing order definitely changes how the ports interact. Depending on the configuration, this can be the main plus or minus for an engine builder when they consider changing firing order.
PE: In what type of motorsports application is a firing order swap most helpful?
BG: We have certainly seen the types of benefits engine builders look for with firing order swaps being beneficial across the board. You see them more in drag racing and road racing than in oval track, but that is likely a function of the sanctioning body rules. Certainly, OEMs competing in NASCAR try various firing orders before submitting a new engine to NASCAR because they know it will be tied down if it is accepted.
PE: It’s commonly said that a firing order swap results in reduced crankshaft and main bearing deflection. Is this true?
BG: See part B of No. 1 above, but it simply has to do with neighboring cylinders firing subsequent and the subsequent force pulses due to the firing sequence.
PE: As far as we are aware, SFO cams for small and bigblock Chevy include the 4/7 swap and the LS firing order. Are there any different firing order swaps that are applicable to other makes of engines?
BG: On any even-fire, common-pin, 90-degree V8 application, a cam can be made to swap any of the cylinders that are four apart in the firing order. What may be the third most common order is called a “Bank-Bank” where you fire all the cylinders on one side and then the other. Simply swapping 2 & 3 without 4 & 7 will give 1-8-4-2-6-5-7-3 with the each bank’s cylinders firing all four before jumping to the other side. Some engine builders believe this helps traction limited and/or restricted applications. It may be that a particularly clever NASCAR engine builder used something like this at Daytona one year and this led to NASCAR not allowing any firing order changes.
PE: Are there any concerns or issues with regard to valve operation with the use of a special firing order camshaft?
BG: We have not measured a change in valve duration that we can trace to a firing order change.
PE: How does an SFO cam benefit engine horsepower and/or torque?
BG: The power and torque changes vary depending on the intake manifold, fuel delivery exhaust system along with a host of other considerations. Improvements of 1-3 percent are actually quite common, much to my personal surprise. If you see changes greater than 3 percent, I would imagine you have either created or eliminated a major distribution problem that could have been addressed in another way, likely in the manifold.
PE: Is it correct to assume that all SFO cams require the use of steel billet stock?
BG: Actually COMP Cams does have cast iron flat tappet cores to make flat tappet 4 & 7 swap camshafts for smallblock and bigblock Chevys. Also, most cam companies have two firing orders available for smallblock Fords due to the factory 302 versus the later 351W, 5.0 HO firing order. That 351 firing order is basically Fords version of the Cadillac/LS1/4 & 7 and 2 & 3 GM firing order. There are also billet steel cores on the shelf for the common two Chevy and Ford firing orders readily available from COMP Cams. Most of the other firing orders require a very expensive custom core.

FIRING ORDER EXAMPLES: SMALLBLOCK/BIGBLOCK CHEVY
STANDARD FIRING ORDER………1-8-4-3-6-5-7-2
4/7 SWAP FIRING ORDER………1-8-7-3-6-5-4-2
LS FIRING ORDER………1-8-7-2-6-5-4-3

QUICKLY IDENTIFYING A 4/7 SWAP CAM

Referencing cam lobes from front to rear of the cam, a 4/7 swap cam will feature lobes No. 12 and 13 that oppose each other with lobe No. 12 positioned at 12 o’clock and No. 13 lobe at 6 o’clock. This is an easy way to identify a Chevy cam that features a 4/7 cylinder swap.

Also, when looking at the cam from front-to-rear, immediately past the second journal, the 2 and 4 lobes would be oriented 180 degrees apart.

Shown here are three bigblock Chevy cams: (from left to right) a standard firing order, a 4/7 swap and an LS firing order cam. (cam cores courtesy Erson Cams)

Note that both the 4/7 swap cam (center) and the LS cam (right) feature the second and fourth lobes from the front in an opposed position as compared to the stock firing order cam. (cam cores courtesy Erson Cams)

This view of the rear of the cams shows the same lobe orientation of the rear lobes on the stock cam (left) and the 4/7 cam (center), but the second and fourth lobes from the rear of the LS cam (right) are opposed as compared to the stock and 4/7 cams. These tips help to easily identify a 4/7 swap and an LS firing order camshaft. (cam cores courtesy Erson Cams)

Tags: CAMSHAFTS, COMP CAMS, ERSON, FIRING ORDER, LUNATI, SFO, SPECIAL FIRING ORDER

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