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Old 06-20-2012, 08:43 AM
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BBC Firing Order Combinations?

Has anyone ever seen this article out of Hot Rod Mag? Anyone ever used a different sequence? If so, which one would you reccommend to try? I'm just curious. I've just never heard of it.


A typical Chevy firing order is 1-8-4-3-6-5-7-2. For the vast majority of applications, this firing order works nicely and there’s probably no need to change it. But in a race application, there are various firing orders that can effectively improve power and reliability in an engine. Typically, there are seven other firing order arrangements which can be used without changing the crankshaft layout on a Chevrolet V8. These firing orders include the following:
1-8-4-2-6-5-7-3
1-8-7-2-6-5-4-3
1-8-7-3-6-5-4-2
1-5-4-3-6-8-7-2
1-5-7-3-6-8-4-2
1-5-7-2-6-8-4-3
1-5-4-2-6-8-7-3
Reher-Morrison has tested the various combinations and found some interesting results. When the firing order is revised to 1-8-7-3-6-5-4-2, you’ll find some power and reliability. Now, if you race a Pro Stock car, this is no secret since the revised order has been in vogue for some time. With the revised firing order, fuel distribution dilemmas are resolved. The engine tends to run cooler, and perhaps just as important, the engine is actually smoother. How much extra power is there? Anywhere between 8 to 10 measurable horsepower. There was once a caveat here though: The average enthusiast couldn’t easily get a cam with the revised lobe positions, but not so today. Reher-Morrison sells such a package (called Big Stick Cams) for Chevy big block/Big Chief head combinations.

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Old 06-20-2012, 11:13 AM
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While really good information, hardly "new". We've been doing the 4-7 "swap" for MANY years in Chevys (mostly small blocks). Virtually every engine family that uses the same 1-8-4-3-6-5-7-2 arrangement can gain from the swap. A little more is found when 2 & 3 are also swapped. The Pontiacs pick up about 12 HP and 10 lb. ft. The Dodges gain, too.

Comp offers cams for any application where this can be accomplished. While not in their catelog (all there for Chevys!), neither are a lot of other options and grinds.

Jim
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Old 06-20-2012, 11:26 AM
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One last question. I just want to make sure I understand correctly. "This can be done without a new cam/crank installed or it does it require a new set-up"?

Thanks for the info.
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Old 06-20-2012, 01:59 PM
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There is no way to change the firing order without a camshaft change. Firing occurs at TDC, both valves closed.
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Old 06-20-2012, 06:30 PM
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Daniel is correct. A cam change is necessary. And it'll run like doodoocacapoopoo if you forget to swap the wires...

No changes necessary to the crank or other "bottom end" components.

Jim
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Old 06-23-2012, 05:43 AM
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I run an LT1 engine which is a newish small-block from '93-'97. I run the 4/7 firing order swap. There are a bunch of different theories as to why it picks up power. I've heard that it "fixes" the robbing of the intake charge between #5 and #7 since those 2 cylinders fire next to each other. The theory is that #5 is stealing some intake charge away from #7. I say BS on that theory because going to the 4/7 swap brings #4 and #2 next to each other so then they would theoretically be stealing from each other.

The best reasoning for the power gains that can be found came right from the GM engineers when they were developing the LSx engines.

They had a specialized machine that could measure the bearing loads placed on the main caps. By altering the firing orders around they could alter the different loads placed on the bearing caps. The traditional sbc firing order had #1, #3, and #5 caps all relatively close but #2 and #4 were high and low ( I can't remember which one was high and which one was low). By going to the 4/7 swap, they got those bearing loads closer to the 1,3 and 5, but by also swapping cylinders #2 and #3 they got the best EVEN load on the bearings. This would make sense as to why it would pick up power, because if the bearing loads are all the same then the crank is in balance better, and also longevity is improved. So basically with the sbc or bbc crank you have, there are 20-something firing orders you can use, but the LS firing order is the best according to GM.
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