This really got me thinking, so I decided to look at the 5.0 firing order vs. the old 289/302 order. This is how they look when you consider all 4 strokes for each cylinder over a full camshaft rotation, or one full rotation of the distributor:
5.0 Firing Order
#1 - power, exhaust, intake, compression, power, exhaust, intake, compression
#3 - compression, power, exhaust, intake, compression, power, exhaust, intake
#7 - intake, compression, power, exhaust, intake, compression, power, exhaust
#2 - exhaust, intake, compression, power, exhaust, intake, compression, power
#6 - power, exhaust, intake, compression, power, exhaust, intake, compression
#5 - compression, power, exhaust, intake, compression, power, exhaust, intake
#4 - intake, compression, power, exhaust, intake, compression, power, exhaust
#8 - exhaust, intake, compression, power, exhaust, intake, compression, power
289/302 Firing Order
#1 - power, exhaust, intake, compression, power, exhaust, intake, compression
#5 - compression, power, exhaust, intake, compression, power, exhaust, intake
#4 - intake, compression, power, exhaust, intake, compression, power, exhaust
#2 - exhaust, intake, compression, power, exhaust, intake, compression, power
#6 - power, exhaust, intake, compression, power, exhaust, intake, compression
#3 - compression, power, exhaust, intake, compression, power, exhaust, intake
#7 - intake, compression, power, exhaust, intake, compression, power, exhaust
#8 - exhaust, intake, compression, power, exhaust, intake, compression, power
This illustrates a couple things. First, since the cam spins at 1/2 engine speed a V8 is really two groups of 4 cylinders. For each 360 degree rotation of the crank 4 cylinders will undergo the 4 cycles, and then the other 4 undergo it in the next 360 degree rotation. Since there's only one distributor, which is geared to the camshaft and spins at camshaft speed, the second group of cylinders is basically along for the ride during the first crank rotation. Its strange to think of that, because it means that every cylinder goes through a full revolution without an ignition event every other crank rotation.
Second, if you look at the two different firing orders what becomes immediately apparent is that cylinders 1, 2, 6, and 8 are in the same order regardless which firing order is used, and really all that was done was to swap 5 & 4 with 3 & 7 going from the 289/302 order to the 5.0 order. If you light cylinder 3 followed by cylinder 7 instead of cylinder 5 followed by cylinder 4 there's nothing inherently wrong with that, because the pistons are on the correct stroke to fire. You wouldn't be creating any issue like say trying to fire a cylinder that was on the intake stroke (backfire), the exhaust stroke (no ignition), or the compression stroke (preignition; this one could be very harmful).
Now here's where it gets interesting: fueling. With a carb the air and fuel is just mixed by the carb and hangs out in the manifold/ports until its needed and then gets sucked into the cylinders and burned.
Batch fire EFI does much the same thing. A batch fire system fires one bank of injectors during the intake stroke of #1 cylinder and then the other bank of injectors during the intake stroke of the #6 cylinder. Meaning that on the first batch fire the fuel may have to hang around for up to 3 strokes before its used, and some cylinders will need to fire while not being supplied any fuel. That seems impossible, but in actuality things are happening so fast that it doesn't matter. Even at an idle of 500 RPM each group of 4 cylinders is doing the full 4 stroke cycle 250 times a minute, which is over 4 times a SECOND. These engines are always started with some kind of start up enrichment, so the only time it would really matter would be on initial start up and that's been circumvented.
What's it all mean for a SEFI system like the 5.0? I think much the same as how it works in batch fire. Just because the injectors are not spraying exactly when they should likely makes no real difference since things are happening so fast. The fuel hangs around until its needed. There would be some loss of atomization, and its not going to have maximum efficiency, but none of that is probably enough to make a difference in how the engine runs, and probably even has a negligible effect on power.
Okay, I know that was really long, but its sort of a show your work thing I do. This can now bring us back to the O2s and the potential issue. The theory is that the way the cylinders are grouped on the RH vs. LH banks could effect the O2 readings. Using our example, on the first revolution the RH O2 is supposed to read 3 cylinders vs. the LH O2 reading 1, and this flip flops on revolution 2. The thing is, if you change to the old firing order it still does the same thing. Honestly I don't see it having an effect.
My conclusion is: I don't think there's any harm in trying it without changing the firing order in the ECU, so long as that plug wires are routed correctly its not going to hurt the engine. If you encounter an issue related to the O2s the injector firing order can be changed, and that should get around it.
Now where you may have an issue is if you run the EECIV on an engine that has a firing order very different from either the 5.0 or 289/302. Some engines number the cylinders even on one bank and odd of the other for example. But again, I would try it and see.
Last edited by TKOPerformance (12/30/2020 4:50 PM)
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Nos681 wrote: