4/7 Swap

[Paulster2]

I posted this question on a different board a long time ago, but received no "intelligent" answers, so thought this would be a great place to bring it back up:

Has anyone (you or someone you know) done the 4/7 firing order swap on an LT1? If so, results?

The reason I ask is that, as a simple exchange of spark plug wires and camshaft (all else stays the same), this swap was supposed to net you in the neighborhood of 6-8hp ... at least that was the story back in the day with the SBC. I would assume you'd see much the same with the LT1, but was wondering what the results might be. For those who don't know, this swap in firing order allowed for better harmonics in the engine, which in turn allowed the engine to produce a bit more HP. While 6-8hp is not a huge amount, it is a gain for a setup which really shouldn't cost you any more than a regular build ... it's just you have to plan for it with the purchase of your cam. I'm also curious if there is greater effect on an engine which is made to produce more HP ... lots of questions, hopefully it will spur some conversation.

EDIT: It also might be a PITB with cross-swapping the wires on the Opti as well ... another thought for consideration.

[Mystery Bird]

Also, the injector harness would need reconfiguring because I don't know of any way to reprogram that.

[94Blackbird]

I posted this question on a different board a long time ago, but received no "intelligent" answers, so thought this would be a great place to bring it back up:

Has anyone (you or someone you know) done the 4/7 firing order swap on an LT1? If so, results?

The reason I ask is that, as a simple exchange of spark plug wires and camshaft (all else stays the same), this swap was supposed to net you in the neighborhood of 6-8hp ... at least that was the story back in the day with the SBC. I would assume you'd see much the same with the LT1, but was wondering what the results might be. For those who don't know, this swap in firing order allowed for better harmonics in the engine, which in turn allowed the engine to produce a bit more HP. While 6-8hp is not a huge amount, it is a gain for a setup which really shouldn't cost you any more than a regular build ... it's just you have to plan for it with the purchase of your cam. I'm also curious if there is greater effect on an engine which is made to produce more HP ... lots of questions, hopefully it will spur some conversation.

EDIT: It also might be a PITB with cross-swapping the wires on the Opti as well ... another thought for consideration.

Basically what you are doing with the 4/7 swap is evening out the exhaust pulses between the two sides and getting better exhaust scavenging. With the standard firing order, 5 and 7 fire sequentially and in effect act like a much larger piston due to the exhaust pulses being so close together. With the exhaust pulses being evened out between the halves of the engine, it will then run smoother, and with better scavenging, show the increase in output.

As far as the spark plug wires, I don't imagine it would be too much trouble, just a couple of custom wires and careful routing to the appropriate spark plugs

Mystery Bird is correct in that you would have to swap the 4/7 injector connectors as well.

[DMBlack]

I would also like to hear the pros/cons of this swap. It is something I have been thinking of for my next build. I do know that Gregrob did the swap when he did the 24x conversion. That may make it a little easier with the plug wires and injectors.

[popo8]

This is interesting, is this not the same system that BLK95z has on his B4C?

[zmm95]

Also, the injector harness would need reconfiguring because I don't know of any way to reprogram that.

You could easily avoid this with simply switching the two pins with eachother at the pcm connector.

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[noice]

There is really no point unless you are running some kind of all out rebuild after every quarter mile engine. I have seen an engine with the 4-7 swap and the 2-3 swap. Everyone has an opinion on what it does, some say it spaces out the firing on the crank throws so that pairs of rods aren't firing back to back. Others say it prevents adjacent cylinders from firing in order. Anyone who takes out a pencil and maps it out can see it gets rid of the 5-7 firing but what about the 4-2 firing it creates. I bet there is something to this swap with high rpm engines, like 8000rpm plus but that isn't for an LT1.

[94Blackbird]

There is really no point unless you are running some kind of all out rebuild after every quarter mile engine. I have seen an engine with the 4-7 swap and the 2-3 swap. Everyone has an opinion on what it does, some say it spaces out the firing on the crank throws so that pairs of rods aren't firing back to back. Others say it prevents adjacent cylinders from firing in order. Anyone who takes out a pencil and maps it out can see it gets rid of the 5-7 firing but what about the 4-2 firing it creates. I bet there is something to this swap with high rpm engines, like 8000rpm plus but that isn't for an LT1.

Granted it moves the adjacent cylinder firing to the 4-2, but think about this. cylinders 4 and 2 are closer to the water pump and the incoming coolant. This swap could be a way to control temperature spikes in the left rear cylinders and equalize the combustion temps across the engine by moving the adjacent firing cylinders in the sequence closer to the incoming coolant and spacing the firing of the rearward cylinders out so as to keep temps down.

I am sure that there are other ways to accomplish this, but, that could be a natural outcome of the swap aside from changing the exhaust pulses and engine harmonics

[shownomercy]

What about your 02 sensors, if they wanted to command less fuel, it would cause the one cylinder on the other bank to drop fuel, and then cause that bank to add more fuel?

[joelster]

I have the 4/7 swap done. I typed a good response to this very question a while back. I will dig for it and post it up.