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Mike Mavrigian Special Firing Order Camshafts Allow the Performance

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Mike Mavrigian Special Firing Order Camshafts Allow the Performance Performance Perspectives Special firing order camshafts allow the performance engine builder to create a smoother-running engine, with improved fuel distribution and enhanced crankshaft and main bearing durability. his article falls under the category for the same reasons...to smooth out the harmon- of “thinking outside the box.” It’s a ics in the pursuit of greater engine durability and trite phrase, I know, but the point to potentially generate more power. is that we don’t need to always Power gain potential aside, the primary reason think of an OE carmaker’s engine to address (and alter) cylinder firing order is to Mike firing order specification as gospel. achieve a smoother-running engine (a smoother, Mavrigian In certain racing applications, a special firing or- more lineal acceleration ramp), with less harmonic Tder (SFO) camshaft can be used as a tuning aid, effect (and crank deflection) on the crankshaft and [email protected] allowing the competition engine builder to fur- its main bearings. In theory, virtually all engines can ther address combustion heat and crankshaft dis- benefit from this. The engines that can most bene- turbance (harmonic) issues. fit include those that run at or near peak rpm levels In essence, the goal in changing the firing or- for long periods of time, such as oval track, road der is to create a smoother-running engine and race and marine engine applications, as well as en- more even fuel distribution, with enhanced gines that are called upon to produce maximum crankshaft and main bearing durability. In the power in a very short period of time (drag race). In process, horsepower gains may be achieved as a V8 configuration, a 4/7 swap, for example, re- well (no guarantees here, but in most cases, a duces crankshaft torsional vibration for smoother slight power increase does result). dampening and (potentially) more power. Playing with special firing orders isn’t limited By swapping firing order positions, a goal is to to the advanced race engine builder. GM adopt- reduce crankshaft harmonic effects caused by ed a special firing order in its LS engine series two adjacent cylinders firing in succession (let’s (Gen 3 and 4), which feature a 4/7 and 2/3 swap call these “companion” cylinders). By strategical- ly relocating these companions, it’s possible for the engine to idle and run smoother, 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- blocks and big-blocks 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 fir- ing order that will reach TDC at the same time as its counterpart—one on the power stroke and one on the exhaust stroke. These cylinder pairs (1/6, 2/3, 4/7 and 5/8) can be interchanged in the firing order without altering the crankshaft. While some engine builders report seeing no power improve- ment, others claim to have achieved power gains Photo courtesy Comp Cams courtesy Photo by switching cylinders 4 and 7 (to create a new fir- The cam shown on the left is for a standard firing order big-block ing order of 1-8-7-3-6-5-4-2). This can enhance Chevy. The cam on the right is a 4/7 swap cam. Notice that the sec- fuel distribution, especially in open-plenum type ond and fourth lobes from the rear on the 4/7 are opposed vs. the intake manifolds, and reportedly can result in an standard cam. This is an easy way to identify such a cam. additional 5 to 10 hp. 18 March 2009 Photo: Mike Mavrigian Mike Photo: This photo shows three big-block Chevy cams from Erson Cams. They are (l. to r.) standard firing order, 4/7 swap and LS firing order. Note that both the 4/7 swap and LS cams feature the second and fourth lobes from the rear in an opposed position, as compared to the stock firing order cam. According to engine builders at Re- be designed and fabricated in much her-Morrison, Pro Stock drag engines shorter time frames, which also helps to typically take advantage of a 4/7 swap. reduce cost. Swapping cylinders 4 and 7 in the firing Naturally, if a special firing order order eliminates the fuel distribution camshaft is installed in a customer’s en- and heat problems caused by cylinders gine, the customer must be alerted to 5 and 7 firing in succession. With the this, to avoid any confusion regarding revised firing order, the two end cylin- spark plug wire routing during future ders don’t have to fight for fuel from the service. It’s a good idea to provide the manifold plenum. The result, in many customer with a decal that displays the cases, is a measurable power increase correct firing order, to be affixed to the (typically 8 to 10 hp) and a smoother, vehicle (the underside of the hood, in cooler-running engine. the engine bay, etc.). However, achieving this special firing We spoke with a few cam makers to order requires a specially designed obtain their views on the subject of camshaft (not achieved by simply changing the cylinder firing order. Steve switching plug wires). The camshaft Tanzi at Erson Cams noted that a firing lobes must be repositioned to accom- order change accomplishes that task of plish this power stroke/exhaust stroke alleviating the problem of cylinders 5 switch. Years ago, this required making and 7 firing next to each other (in a a special firing order camshaft from Chevy V8). However, he noted that this scratch, which was time-consuming “moves the problem” to cylinders 2 and and, naturally, more expensive. 4. But the front of the block will gener- By taking advantage of today’s com- ally cool more efficiently, so the “prob- puter-aided design (CAD) programs lem” isn’t much of an issue. and computer numerical control (CNC) Over the years, GM did quite a bit of machining, however, special cams can research in this area, beginning back in continued on page 20 Circle #9 March 2009 19 Performance Perspectives the days of the early Oldsmobile and controller) to eliminate detonation, so Cadillac engines and, most recently, the firing order changes may not be as ben- LS engine family. All along, the goal was eficial in an injected engine, since fuel is the same: to smooth out engine pulsa- delivered on an individual-cylinder ba- tions and create a smoother-accelerat- sis. However, the LS firing order takes ing engine. Smoothness of operation advantage of 4/7 and 2/3 swaps as an ad- was always the target; if additional pow- ditional tuning aid, to produce an even er resulted in the process, even better. smoother acceleration profile and to Tanzi also noted that “to yield the benefit crank and bearing life. greatest gains requires header design Special firing order camshafts have work....Matching primary tubes with re- become so popular that what was once gard to the intake manifold and cylinder considered a special grind has now be- head ports is also necessary. This usually come the norm, and what was once relates more directly to applications that “normal” is now considered “special.” use a single-plane manifold, and tunnel Crane Cams’ Chase Knight notes ram manifolds in particular. In terms of that his company offers SFO cams at no fuel distribution, we see greater benefits extra charge. Order time for a custom with single-plane manifolds as com- SFO is about four days. Applications in- pared to dual-plane manifolds, since clude small-block and big-block Chevys, dual-plane manifolds are less prone to 429-460 Fords, Chrysler LA small- reversion and negative pulsation.” blocks, Chrysler B blocks and Chrysler In Top Fuel engines, for example, 426 HEMIs. engine builders have set blowers further Knight noted that “regardless of what rearward on the engine as a way to ad- approach you take, you’ll always have two dress fuel distribution. In addition, cylinders firing next to each other (or two these guys are experimenting with 4/7 opposing cylinders that share the same firing order swaps, and the LS 4/7 and rod pin). But, by swapping certain cylin- 2/3 swaps. The issue of firing order is der firing orders, you can more evenly much more complicated than it appears distribute combustion pressures and re- on the surface. sulting crankshaft loads to either generate “The greatest benefits,” continued more power or extend crankshaft, main Tanzi, “are seen in oval track and road bearing and crankshaft seal life, or im- racing, because those engines run for prove both power and durability. There longer periods of time. In a quarter- are so many variables, depending on fac- mile situation, you have only a few sec- tors such as intake manifold design, en- onds to determine any gains. But when gine speed, type of use, etc., that the ben- a builder is trying to find any repeatable efits are sometimes difficult to quantify. gains (even 1 or 2 hp), it’s worth experi- There are cases where no power im- menting with firing order changes. It’s provement is evident on the dyno, yet on- common practice for many Top Fuel, track power is improved.” Pro Stock and Comp Eliminator Following are some insightful com- builders to take advantage of firing or- ments—presented in question & answer der modifications.
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