Firing Order

In Top Fuel , TAD engines,  engine builders have set blowers further rearward on the engine as a way to address fuel distribution. In addition, these guys are experimenting with both 4/7 firing order swaps, and the LS 4/7 and 2/3 swaps. So be careful with used camshafts
Stock HEMI (and most Mopar and GM engines) 1-8-4-3-6-5-7-2
4&7 swap 1-8-7-3-6-5-4-2
LS firing order (the 4&7 swap plus an additional 2&3 swap) 1-8-7-2-6-5-4-3

There are three basic issues that engine builders or developers try to address with firing order as follows:

A) Hot Spots in the Head and Block – With a common pin V8 crankshaft, it is impossible to eliminate having adjacent cylinders fire subsequent to one another in the sequence. While this is unavoidable on a four throw crank, you do have some choice as to which pair(s) fire together. The 4&7 firing order moves that hot spot from the 5&7 cylinders up to the 4&2 cylinders. Clearly, the front pair is easier to keep cool with a front mounted water pump than the back pair.

B) Main Bearing Issues – Most engine builders see better bearing life and less indications of scuffing when going to the 4&7 swap. The LS firing order (the 4&7 swap plus an additional 2&3 swap) may be even better for bearing life. Again, firing pairs is what we are trying to change. Here I think the focus is on pairs that fire on the same pin, thereby focusing the force on the adjacent crank main bearings. Moving this to the back of the engine seems to be a good idea because, even with a crank dampener, the drive train is much better at dampening these forces than a small mass on the end of the crank. When we consider the power that can be gained with lighter weight, lower viscosity oils, bearing life becomes not only a durability, but also a power concern.

C) Fuel Distribution – Changing the firing order is one of the best ways to change the cylinder to cylinder fuel and air distribution in the engine. Just like throws on the crank and cylinders next to each other in the block or head, we have to deal with ports next to each other and across from each other asking the manifold for air right after one another. This creates a very dynamic system in the manifold plenum where pressure waves of air (and also fuel in a wet manifold) are moving from front to back and side to side. Changing the firing order definitely changes how the ports interact. Depending on the configuration, this can be the main plus or minus for an engine builder when they consider changing firing order.

On any even fire, common pin, 90 degree V8 application, a cam can be made to swap any of the cylinders that are four apart in the firing order. What may be the third most common order is called a “Bank-Bank” where you fire all the cylinders on one side and then the other. Simply swapping 2&3 (w/o 4&7) will give 1-8-4-2-6-5-7-3 with the each bank’s cylinders firing all four before jumping to the other side. Some engine builders believe this helps traction limited and/or restricted applications. It may be that a particularly clever NASCAR engine builder used something like this at Daytona one year and this led to NASCAR not allowing ANY firing order changes…