4/7 swap cam question

It’s been a while since I’ve been on here. I got derailed by another project. But now that’s done and I’m back to finishing my engine rebuild.

Ok, so I’m building a ‘96 Vortec engine as we speak. I’m using factory Vortec fuel injection but upgrading to the MPFI spider vice the factory CSFI spider.

So when I ordered everything from my machine shop guy it was early on in COVID and everything was sold out and backlogged. And I wanted a custom grind camshaft to meet my needs. Well, the only camshaft cores that they have available, at the time, were 4/7 swap cams.

For those that don’t know, cylinders 4 and 7 are on opposite sides of the engine (companion cylinders), second from the back and literally the grind on the cam that determines when the intake and exhaust valves open and close are literally just swapped. They both travel to TDC and BDC at the same time on the crankshaft, but while one is TDC on the compression stroke the other is TDC on the exhaust stroke.

In a carbureted engine the only thing that you have to change with this cam would be to swap, on the distributor, the 4 and 7 plug wires. Sounds easy enough, right??? WRONG!!

This is fuel injected. Soooo I’m wondering if there will be enough slack in my spider legs to swap the 4 and 7 legs. This is based on the assumption that the only fire/pulse fuel on the compression stroke. Which I have to assume is the case or else it seems like it would be horribly inefficient and a waste of fuel.

Can anyone confirm one way or the other if this is the case as far as when they inject the fuel? Luckily I thought about this ahead of time or else I probably would have had an expensive 6 cylinder since I would have had a miss on cylinders 4 and 7.

The only other option I can think of, if the spider legs won’t reach across to the opposite sides, is to dissect the spider injector harness and find the correct wires (2 each) for 4 and 7, and cut them and swap them in it.

Sorry, I know that this is much more in depth and convoluted than most questions or issues. But I am where I am, and I already own this cam, so I need to figure out how to make it work correctly. I get that the computer will always think that cylinders 4 is 7; and that 7 is 4 from now on, but I can live with that. Thanks in advance for the help.

Also, if anyone has a good wiring diagram with what color wires goes to what fuel injector that would also be helpful!!

I think that I might have figured it out for myself. But I’m fairly certain that there are 2 wires per fuel injector. I wonder what the second wires are and if they matter. I guess, worst case, I can peel back the harness protector on mine, look at the wires, and do a reverse lookup on this chart. It maybe that it’s a bank wire. Meaning all from one side of the engine, or bank, and then all of the opposite. If so, it would have to be swapped as well.

I’m sure I can figure it out, but if anyone has any other inputs or thoughts, they would be greatly appreciated.

You don't mention the year or motor but I'm presuming vortec since you mention the spider. You'll want to swap the wire to the 4/7 injectors at the ECM not the injectors themselves. You'll also need to swap on the distributor to change the firing order. Lastly you'll need to retune (not only for the cam change itself) to move the #4 injector to O2 sensor bank 1 and the #7 to O2 sensor bank 2. It won't run right without the sensor banks swaps.

clgodwin79 said:

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I think that I might have figured it out for myself. But I’m fairly certain that there are 2 wires per fuel injector. I wonder what the second wires are and if they matter. I guess, worst case, I can peel back the harness protector on mine, look at the wires, and do a reverse lookup on this chart. It maybe that it’s a bank wire. Meaning all from one side of the engine, or bank, and then all of the opposite. If so, it would have to be swapped as well.

I’m sure I can figure it out, but if anyone has any other inputs or thoughts, they would be greatly appreciated.

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he's not using a 0411 ECM at least he did not say he was. if he is using a stock black box this has no bearing on it.

Ok, so a little more background. It’s a 1996 Vortec engine from a Tahoe, going into a 1995 Z71 K1500. So putting a Vortec engine, harness and all into an older TBI style truck.

So basically you may as well think of it as a 1996 Tahoe or K1500. I have the diagram and 100% plan on doing the 0411 PCM swap immediately after getting it running on the black box. That was the recommendation from the guy over on lt1swap. He said that since there’s so much going on, that I should make sure that I can get it up and running on the black box before I convolute an already big job by throwing in the 0411 at the same time. That way I can troubleshoot issues before the PCM change. I get that it’ll probably run like **** on the black box based on a lot of the changes that I’m making. But as long as it runs, I plan on sorting those out and tuning it correctly once I’m on the 0411.

So basically it sounds like all I’ll need to do is swap the 4/7 plug wires and 4/7 fuel injectors (one way or another) for now. Then I’ll worry about what cylinders are in what bank after the 0411 swap.

Is that a correct statement? Thanks for the help!

Can you keep the actual injectors as-is, but swap the wires/connectors on 4 and 7? Perhaps there isn't enough length of wire-harness between the central control box and the injectors at the ends of the individual fuel tubes.

If you did nothing with the injector wiring, those cylinders should still run...but not optimally. They'd be like a batch-fire injection system, where the fuel spray is not timed to the intake valve opening.

The plug wires MUST be swapped.

The computer will have incorrect info for misfire detection--misfire on 4 will show up as P0307, and the reverse for misfire on 7.

I appreciate all of the help with this!

How long before I have sufficient privileges to post in the wanted section? Thanks

clgodwin79 said:

How long before I have sufficient privileges to post in the wanted section? Thanks

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Pretty sure it is 100 posts now. They just recently bumped it up

If you use o411 computer you can change firing order in the tune