We just knew it wasn't gonna work. Over the past few years, nearly every cam manufacturer has introduced 4/7-swap cams for big and small Chevys, all claiming that the change in firing order makes power. Lunati was the only cam grinder to actually ask us to test a pair of cams to learn for ourselves what the big deal was. So we did.
Firing Order Basics
First, let's give you a clue what we're talking about. A 4/7-swap cam changes the engine's firing order, moving the power stroke for cylinder 4 to where 7 used to be, and vice versa. Therefore, a stock Chevy firing order of 1-8-4-3-6-5-7-2 becomes 1-8-7-3-6-5-4-2. To understand how this is done, you need to know about companion cylinders, which are the pairs of cylinders that reach top dead center (TDC) and bottom dead center (BDC) at the same time. On a four-stroke V-8 that fires every 90 degrees (that's any production V-8 you're working on), you can find the companion cylinders by comparing the first half of the firing order to the second half like this:
1-8-4-3
6-5-7-2
Cylinders 1 and 6, 8 and 5, 4 and 7, and 3 and 2 are companions: One in the pair is at TDC of the compression stroke at the exact same time that its companion is at TDC on the exhaust stroke. (When an engine such as the new 5.7L/6.1L Hemi has one coil feeding spark to two cylinders simultaneously, the cylinders that share the same coil are always companions.) Because the piston position in each pair of companion cylinders is identical, it's only the camshaft's timing of the valve opening and closing events that determines which cylinder is on the compression stroke and which one is on the exhaust stroke. In other words, the cam decides the firing order. Therefore, any pair of companion cylinders can swap places in the firing order; all it takes is an altered camshaft, plus moving the location of the spark-plug wires on the distributor cap.
Something You Didn't Know About Fords And Gen IIIsFord figured this out a long time ago. The original firing order for small V-8s, FEs, and 385-series big-blocks is 1-5-4-2-6-3-7-8, so the companion cylinders look like this:
1-5-4-2
6-3-7-8
All 351s and later 5.0L V-8s use 1-3-7-2-6-5-4-8, with these cylinder pairings:
1-3-7-2
6-5-4-8
As you can see, all Ford did was swap the order of cylinders 5 and 3 and cylinders 4 and 7 to get the new firing order. Also, don't forget that Ford numbers its cylinders differently than Chevy. A Ford has 1-2-3-4 on the right (passenger) bank and 5-6-7-8 on the left (driver) side. Chevy is 1-3-5-7 on the left and 2-4-6-8 on the right (see the diagram on the next page). If you simply renumber cylinder locations of the older Ford engines as if they were Chevys, you'll discover that the firing order is, in fact, identical to GM style. Further, if you take the 351 and late-5.0L Ford firing order and renumber the cylinders like a Chevy, the firing order is 1-8-7-2-6-5-4-3. That's the same as the Chevy Gen III and IV (LS-series) V-8s.
In the realm of OE production engines, the altered firing order is said to increase engine smoothness. In fact, when we questioned Gen III valvetrain engineer Steve Pass, he said the firing order was dictated by the crank guy. That was Bill Compton, who had this to say via e-mail: "Since the G3 engine was a clean-slate project, each group looked at things that they could change to optimize the performance of the small-block. Although we did not have an issue with crankshaft loading on the older V-8s, there was room for improvement in the area of distributing the peak firing loads among the five crankshaft journals. Analysis showed that main 4 had peak loads significantly higher than main 2. By changing the firing order, the peak loading on main 4 was reduced and the peak loading on main 2 went up. Overall, the loading through the mains was much better balanced. By improving the load balance across the crank, we created a better balanced oil film interface across the crank. The valvetrain group simply changed the cam lobe timing to work with the new crank firing order."
Which Brings Us To Our Test
So the OEs may have aspirations of smooth reliability, but what about the power benefits? James Humphreys of Lunati thinks it was probably NHRA Pro Stock racer Steve Schmidt who first had success making added horsepower with the firing-order switcharoo, resulting in today's aftermarket offerings of 4/7 swaps. It's also common to see juggling of cylinders 3 and 2, usually in conjunction with the 4 and 7 switch, and that results in an order often called Corvette style, or just "C," and it's the same as a new LS1 (1-8-7-2-6-5-4-3). However, our test was of two Lunati solid-roller cams that were identical other than the 4/7 swap.
The engine is the same 492ci big-block Chevy long-block you last saw in our story, "The Great Bore vs. Stroke Shootout," (June '05). It uses a bore and stroke of 4.560x3.766 with a compression ratio of 10.87:1. Airflow Research 335 CNC heads are sealed with Cometic gaskets, and the induction this time is a Holley 1,050-cfm Dominator atop a port-matched Edelbrock 454-R manifold. We ran MSD ignition and Hooker 2-inch headers with 18-inch extensions and Dynomax 311/42-inch Bullet mufflers. All our testing was done with Lucas synthetic 10W30 oil and Rockett Brand 91-octane gasoline.
For our past few dyno tests we've been using Cometic multilayer steel head gaskets, which can be custom ordered to fit the bore size and are usually reusable. The ARP stud kit for the AFR heads on a Mark V block is PN 235-4715 (with undercut-style studs).
The Lunati solid-roller cams both had the same lobes: 272/276 duration at 0.050 tappet lift, 0.712/0.712 lift with 1.7:1 rocker arms, and a lobe separation angle of 110 degrees. Once the engine was tuned up with the standard cam, nothing else was changed when we installed the 4/7 swap. We tried total ignition timing settings of 38 , 40 , and 42 degrees with both cams, though 40 degrees proved optimal either way. During testing, we made certain to execute the dyno pulls at exactly the same water and oil temperature.
Guess what? It worked. The 4/7 cam swap gave our pump-gas Rat a little extra midrange power by as much as 12 numbers and added a repeatable 2-3 hp to the top end. Lunati tells us that, cam for cam, the 4/7-swap versions cost $30 more than a standard cam. If we were building a street/strip engine from scratch, it seems like that's $30 we'd choose to spend.
But why?
"No one knows why." That was Reher-Morrison Racing Engines' Darrin Morgan when we asked him why 4/7 swaps make power, and sure enough, we ran into a lot of differing opinions from the experts we spoke with. Morgan told us that every Reher-Morrison crate engine above 555 ci uses a 4/7 cam, and that he's used them to prove gains of 4-6 hp on single-four-barrel engines and 6-8 on tunnel-rams, and the gains are everywhere in the curve. The bigger the displacement, the greater the gain of up to 10 hp. He says he has no hard-core evidence as to why the firing order makes power, as he's seen conflicting information from test to test, but he did comment that, "The Corvette firing order has been proven to be more dynamically stable in the crank, but it's not what made the most power for us. The 4/7 is the only one that makes power on every engine we tested, and it's even better with a properly tuned intake."
On the other end of the opinion scale is Tony Bischoff of BES Racing Engines, a winning NMCA and street-car-racing builder. He told us of three separate back-to-back tests with regular cams and 4/7 cams in small-blocks operating above 6,000 rpm. "You're not going to like the answer," he told us, "because I didn't find any power anywhere." After we bench-raced all the theories, he allowed that, "Some people say the power is due to reduced crank bending, so maybe I don't see anything because I always use the best billet cranks. Others say it's in the intake, but any off-the-shelf intake is symmetrical on both ends."
That comment leads to James Humphreys at Lunati, who's adamant that the power gains are in the intake manifold. Many believe that the point of the 4/7 swap is to prevent cylinders 5 and 7 from firing next to each other and drawing too much from the same area of the intake and carb. However, a 4/7 swap puts cylinders 4 and 2 firing next to each other at the right front of the engine instead of 5 and 7 at the left rear, so what's the difference? Further, if you swap the order of cylinders 3 and 2 in addition to 4 and 7 to get the LS1 firing order, then it's 1 and 3 that fire next to each other. There's no way around it with an even-fire V-8. So the issue is not consecutive firing. According to Humphreys, it's the sequence and location of the pulses in the intake that change locations when you alter the firing order, and which cuts down on reversion.
Finally, Charles Jenckes, in engine development for Dale Earnhardt Inc., puts merit on the theories that the added power is through both intake and crankshaft dynamics, but stresses that overall engine harmonics, and especially those in the valvetrain, also add up in the effect of firing orders. Don't forget this is a guy who's a hero if he can make 1 hp here and there. He was also cautionary about how firing order can affect intake manifold design, stating that, the more highly specialized the intake-including tuned EFI intakes-the more trouble you might cause with cylinder starving if you change the firing order without altering the intake to match. He didn't even want us to make the statement that a 4/7 swap is probably beneficial with nearly any off-the-shelf single-plane, telling us that in our test, "You just got lucky that more cylinders seemed happier for more of the time."
Is it right for you?
Clearly, the 4/7 swap has benefits, but whether you'll see them or not in your engine depends on its purpose. Lunati's Humphreys explains, "It really has to do with good heads and intake. With stock castings, I wouldn't expect to see any difference." In fact, he went so far as to say he'd actually lost a little power once when testing the 4/7 swap on a stock 350 Chevy.
Some claim that the firing-order swap should not be used with a dual-plane intake, as the split-plenum arrangement is tuned for the conventional firing order. And while we have not tested that theory, consider this: Ford did not change the intake design when it changed the firing order. Also, Edelbrock's carbureted dual-plane for GM Gen III engines uses the same exact runner configuration as the old small-blocks.
Weighing all the evidence and even the conflicting opinions, here's our conclusion. If you've got a stock daily driver under 5,000 rpm, the 4/7 swap probably isn't worth messing with. If you're into technology and love messing with EFI and want a new toy to play with to justify individual cylinder tuning, by all means get a 4/7 swap and you might find some power.And if you're building a street/strip or maximum effort engine with a single-plane or a tunnel-ram, spend the extra $30 and find yourself a little power and smoothness.
We knew it would work.
CAM-SWAP POWER STANDARD