Tech Articles

Ask Away! with Jeff Smith: Swapping an LS Firing Order Into a Traditional BBC or SBC Engine

 
big block malibu - north shore classics

(Image/North Shore Classics)

I just read an article online that talked about a big block Malibu that was running an LS cam to change the timing. I have never heard of such a thing. I didn’t have a clue that the cams were interchangeable. 

“The engine was converted to an LS firing order with a Cam Motion LS camshaft that has 224/258 degrees duration and 0.629-/0.630-inch lift. This firing order swap is a method of smoothing the big-block’s harmonics and improve cooling,” the article said.

How exactly does that even work? What does the 1-8-4-3-6-5-7-2 change to? What bearings need to be used? Is there performance to be gained?  — M.C.

Jeff Smith: I can see how you might get confused with that statement.

What the author was referring to is a conversion of the traditional small- and big-block Chevy firing order of 1-8-4-3-6-5-7-2 to the late model LS engine firing order of 1-8-7-2-6-5-4-3.

When we compare the tow sequences, you can see that the 4-3 pairing in the original firing order has been swapped with 7-2. According to what we’ve read from GM, after performing tons of computer simulation work and engineering comparisons, they determined that this later firing order helps reduce harmonics in the crankshaft and increases durability.

As for the contention about engine cooling, that seems a bit more of a stretch.

To reduce your confusion, this plan uses the same basic big-block camshaft as always but merely reconfigures the opening and closing points of the intake and exhaust lobes on the camshaft to change the firing order. If you would decide to perform this swap, it would require your camshaft supplier to use a custom core since the standard layout of the lobes will be significantly different. This also means this revised firing order cam would be more expensive. Once the new revised firing order camshaft was installed, you would also have to change the firing order of the spark plug wires in the distributor cap on a small- or big-block Chevy to make it run properly.

This is a somewhat different approach from an earlier move perfected originally by Pro Stock racer Mike Schmidt who apparently was the first to attempt what is now called the 4/7 swap. This move merely exchanges the 4 and 7 cylinder firing order from the traditional 1-8-4-3-6-5-7-2 to 1-8-7-3-6-5-4-2.

There are several theories as to why this swap is performed but most hold to the idea that many small- and big-block engines suffer from air-fuel ratio problems related to the paired cylinders of 5 and 7 firing right next to each other.

The 4/7 swap doesn’t eliminate that issue but instead moves the paired cylinder relationship from 5 and 7 to 4 and 2 at the right front of the engine instead of the left rear.

Since most single plane manifolds tend to be symmetrical in terms of port length and layout, the contention that power improvements come from eliminating the 5-7 cylinder pairing doesn’t seem to make much sense since all this does is move the pairing instead to cylinders 4 and 2.

Hot Rod magazine did a test several years ago on a 492 c.i.d. big-block Chevy to test the results of this idea and they did find a small amount of power. It appears that this swap improves power throughout the entire rpm curve. The Hot Rod test generated a torque improvement in the middle of the curve of 10 to 11 foot-pounds with the engine improving from 615 ft.-lbs. at peak to 626 ft.-lbs. The horsepower gains were more conservative with a gain of only around 2-3 hp at the 740 hp level.

As evidence of the popularity of this firing order swap, there is an entire half-page section in the COMP Cams catalog devoted to 4/7 swap drag race mechanical roller camshafts and a smaller selection under the mechanical flat tappet versions These are all full competition mode cams starting with the smallest at 278 degrees of duration at 0.050-inch tappet lift and going all the way up to 292 degrees of duration at 0.050 on the intake side.

These are not cams you would normally choose to run on the street. Of course, you could have any camshaft, mechanical or hydraulic converted to a 4/7 swap. This idea is not limited to any style camshaft.

For a street engine, the conservative power gains might not seem to be worth the effort and expense although if you were building a brand new engine, it would certainly be an easy change to make. The biggest issue would more likely be in permanently marking the engine to identify it as a 4/7 swap engine. If it was not marked properly and the engine was subsequently sold to a new owner, imagine the tuning grief that would entail if the new owner was unaware of the firing-order change!

So from that standpoint alone, I would not recommend doing this swap.

Plus, Hot Rod’s interviews with other engine builders also contended that the milder the engine, the less effect this swap has on power. Essentially, stock engines don’t benefit from this firing-order swap.

We hope this discussion helps make this subject a little less confusing.

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5 Comments

  1. Christopher Vincent says:

    Good read. I appreciate you going into a little depth on the subject. I learned me something.

  2. HAROLD LOVETT says:

    It is almost stupid to do this swap on a street engine or a drag race car that does not run under 10 seconds. You should instead get a good Hydraulic Roller Cam or Hydraulic Flat Tappet Cam and have a kick ass car. You can get a cam to suit your need by contacting the cam manufacturer of your choice tech support. You will need to know things like the following, Compression Ratio, Piston to Deck height, Bore, Stroke of Crankshaft, Connecting Rod length, Rear Gear Ratio, Automatic Transmission or Manual, Weight of Vehicle. Be sure to ask what Torque Converter stall speed you should get to best work with the cam if running an automatic transmission.

  3. Troy Kennedy says:

    How does one contact Jeff Smith about one of his articles being plagiarized?

  4. Good article explaining the process to a first time inquiry. It is a detailed ecplination of the how and why along with the pros and cons. Five thumbs up.

  5. Howard Fry says:

    Sir,
    Im building a All Aluminum BBC for Road Racing, you can blame Bruce Mclaren for the idea.
    Im looking at putting it in a 5th gen Camaro an use the computer system and a HP Tuner to dial her in. My question is ,Im doing a 4,7 3,2 swap ,to match the LS system,am I opening up a can of worms by doing so and a thought had come to mind that I could use the computer from say a 99 truck, a thought but will more than likely use the LS system ?
    She will be a Brodix, Big Bore 4.600, Short Stroke 3.75″ 498ci fuel injected manual trans 3.91 geared 5th gen camaro.
    Any thoughts would be greatly appreciated.
    A SCCA CAM C driver.
    HMF
    Westminster Co.

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