Advanced engine performance tuning via camshaft designIn certain racing applications, a special firing order camshaft (SFO) can be used as a tuning aid, allowing the competition engine builder to further address combustion heat and crankshaft disturbance (harmonic) issues. In essence, the goal in changing firing order 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 (no guarantees here, but in most cases, a slight power increase does result).
Experimenting with special firing orders (based on camshaft design) isn’t limited to the advanced race engine builder. GM adopted a special firing order in their LS engine series (Gen 3 & 4), which feature a 4/7 and 2/3 swap for the same reasons…to smooth out the harmonics in the pursuit of greater engine durability and to potentially generate more power.
By swapping firing order positions, one of the primary goals 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, and to reduce isolated hot spots (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 commonly been 1-8-4-3-6-5-7-2. 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 & 8) can be interchanged in the firing order without altering the crankshaft. While some builders report seeing 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 reportedly can result in an added 5 to 10 horsepower.
According to engine builders at Reher-Morrison, Pro Stock drag engines typically take advantage of a 4/7 swap. By swapping #4 and #7 cylinders in the firing order, this eliminates the fuel distribution and heat problems caused by cylinders #5 and #7 firing in succession. With the revised firing order, the two end cylinders don’t have to fight for fuel from the manifold plenum. The result, in many cases, is a measurable power increase (typically 8-10 HP, according to R-M) and a smoother, cooler running engine.
However, achieving this special firing order requires a specially-designed camshaft (not achieved by 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, however, special cams can be designed and fabricated in much shorter timeframes (also helping to reduce cost).
Note: 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 (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 that task of alleviating the problem of cylinders 5 and 7 firing next to each other (in a Chevy V8). However, Steve noted 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 Oldsmobile and Cadillac 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.
Tanzi also noted that “to yield the greatest gains requires header design work…matching primary tubes with regard to the intake manifold and cylinder head ports is also necessary. 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. 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.”
“The greatest benefits,” said Tanzi, “are seen in oval track and road racing, because those engines run for longer periods of time. 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 HP), it’s worth experimenting with firing order changes. It’s common practice for many Top Fuel, Pro Stock and Comp Eliminator builders to take advantage of firing order modifications. 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,” Tanzi continued, 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. 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. However, the LS firing order takes advantage of a 4/7 and 2/3 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.”
Crane Cams’ Chase Knight notes that Crane offered SFO cams at no extra charge. Order time for a custom SFO was about four days.
Applications included smallblock and bigblock Chevy, 429-460 Ford, Chrysler LA smallblock, Chrysler B blocks and Chrysler 426 Hemi.
Chase noted that “Regardless of what approach you take, you’ll always have two cylinders firing next to each other (or two opposing cylinders that share he same rod pin). 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.”
The 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 & answer format.
1. A typical camshaft firing order swap of 4/7 seems to be fairly common today. Why is this firing order beneficial?
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 pair(s) 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 less 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 drive train 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.
2. In what type of motorsports application is a firing order swap most helpful? (drag racing, oval track, road race, etc.)
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, OEM’s 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.
3. It’s commonly said that a firing order swap (4/7 or the LS swap of 4/7 and 2/3) results in reduced crankshaft and main bearing deflection. Is this true?
See part B of #1 above, but it simply has to do with neighboring cylinders firing subsequent and the subsequent force pulses due to the firing sequence.
4. 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 make of engines?
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 (w/o 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…
5. Are there any concerns or issues with regard to valve operation with the use of a special firing order camshaft?
We have not measured a change in valve duration that we can trace to a firing order change.
6. How does an SFO cam benefit engine horsepower and/or torque?
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% are actually quite common (much to my personal surprise). If you see changes greater than 3%, 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).
7. Is it correct to assume that all SFO cams require the use of steel billet stock?
Actually COMP Cams does have cast iron flat tappet cores to make flat tappet 4&7 swap camshafts for Small Block and Big Block Chevys. Also, most cam companies have two firing orders available for small block Fords due to the factory 302 versus the later 351W (5.0 HO) firing order. That 351 firing order is basically Ford’s version of the Caddy/LS1 4&7 & 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.
Steve Fox has over 16 years experience in the engine building industry with eight of those years spent working in the machine shop. Steve is an ASE-certified Master Machinist, as well as a longtime member of the drag racing circuit. Dave Hagen, our Senior Technician, has over 32 years of experience in our industry. An ASE-certified Master Machinist, Dave specialized in cylinder head work and complete engine assembly for the first 17 years of his career. Mike Caruso brings over 35 years of rebuilding and high-performance experience to AERA. An ASE-certified Master Machinist, Mike came to us from FEL-PRO’s high-performance R&D and tech line, where he worked for 11 years.
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