6

Based on some resources, I was able generate a list of all theoretical firing orders for an LS engine with a cross-plane crankshaft. Here they are (L,R denote which bank each cylinder in the firing order belongs to):

1-5-4-2-6-8-7-3 LLRRRRLL
1-5-4-3-6-8-7-2 LLRLRRLR
1-5-7-2-6-8-4-3 LLLRRRRL
1-5-7-3-6-8-4-2 LLLLRRRR
1-8-4-2-6-5-7-3 LRRRRLLL
1-8-4-3-6-5-7-2 LRRLRLLR
1-8-7-2-6-5-4-3 LRLRRLRL
1-8-7-3-6-5-4-2 LRLLRLRR

The standard LS firing order is 1-8-7-2-6-5-4-3, which does a decent job of alternating firing between banks as much as possible.

A popular modification that recently came to my attention is known as the "4/7 swap", which switches the firing order of cylinders 4 and 7. What's interesting to note here is that it causes all 4 cylinders on one bank to fire in succession, followed by the other bank's 4.

I would have imagined that this firing order should be avoided because it places more load on the crankshaft and rod bearings when all of the cylinders on one bank are fired one after another.

So is there any benefit to be reaped from performing this modification?

Some forums appear to indicate a minor improvement in power/torque output, but I fail to see how this would be the case.


Here is the cylinder numbering convention on an LS engine. All cylinders on Bank 1 (L) are odd, Bank 2 (R) are even:

LS Engine Cylinder Numbering

5
  • there's a mechanical grip component to this too IMO. It's rumored Corvette team that just won the 24 Hours of Daytona had this firing order. Feb 11 '16 at 21:12
  • Cooling the head as well. Might even serve to help smooth out the vibrations too, allowing for a little extra in the rpm range. A cam and computer adjustment later...
    – Tobin S
    Feb 12 '16 at 1:08
  • I can't remember where I saw it recently, but, the new Mustang 5.0 and rumored GT40 have gone back to the flat plane crank because of its ability to alternate LR firing order. Another question to add, if a 12 cyl crank can have the same characteristics as an inline, will it remain as smooth and consistant as an inline 6?
    – Tobin S
    Feb 12 '16 at 1:11
  • I know the 4/7 swap was a consideration on the Gen I/II SBC, but had not heard of it on the LSx motors. I'll have to take a looksee around and do some research. For the Gen I/II engines with a standard 1-8-4-3-6-5-7-2 firing order, it was said to make the harmonics of the engine better and gain a few HP in the process. When you are shooting for max effort, every little bit helps. Feb 12 '16 at 3:12
  • Please realize the link you posted, they are talking about a Gen I SBC, not an LSx engine. Feb 12 '16 at 3:17
4

No.

This modification is intended for Gen I/II Small-Block Chevy engines only.


What I failed to realize was that while the cylinder numbering convention stays the same across Gen I/II SBC's, the firing order is quite different: 1-8-4-3-6-5-7-2

With this firing order, there is no concern about bank imbalance when performing the 4/7 swap:

1-8-4-3-6-5-7-2 | 1-8-7-3-6-5-4-2
----------------+----------------
L-R-R-L-R-L-L-R | L-R-L-L-R-L-R-R

SBC Numbering Convention

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  • Something else to be aware of is the 4/7 swap also applies to Big Block Chevy (BBC) engines as well. They share the same firing order. Also know, I upvoted you ... counteracted the downvote. Feb 14 '16 at 1:43
  • Something I just found out is the LS series engine firing order already encompases the 4/7 swap. GM engineers figured out the swap's benefits and incorporated them into the Gen III small block when they designed it. I believe the firing order carries through to the Gen V SBCs (or LT1 based) engines. Feb 29 '20 at 20:18

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