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.

[joelster]

Ok, I found it, I had to do some digging:


There are a few theories as to why this works out but they are hard to prove exactly. On extreme-rpm small blocks, altering the firing order tends to produce a little more power way up high. People claim that it is because of the cylinder scavenging is different. On a regular firing order you have 1-8-4-3-6-5-7-2. Notice how 5 and 7 are right next to each other. They claim that cylinder 5 robs some of the intake charge from cylinder 7 and that is why moving those firing cylinders away picks up some power. I say that's crap because when you alter the firing order to the 4/7 swap it becomes 1-8-7-3-6-5-4-2. Now you have 4 and 2 right next to each other so technically 4 should be robbing from 2, yet the car picks up power?

The best theory that I have heard comes straight from the GM LSx design team. There were groups of engineers working on different aspects of the motor. There were teams working on the intake, the crankshaft, and valvetrain. The valvetrain and intake guys said that the firing order was dictated by the crankshaft guys. On the traditional firing order they measured the bearing load on the main caps (1 through 5). Then they took an average. Main #2 was lower and main #4 was higher than the average. By moving the firing order they incresed the bearing load on #2 and decreased the load on #4 bringing both closer to the middle. Basically they smoothed out the load accross the entire crank ie: lowered the deflection. Anytime you smooth out a motor or bring it into balance better it will pick up power. The 4/7 swap gets you closer to this balance.

I have never heard of a negative thing assiciated with the 4/7 swap. I have heard of guys swapping and not picking up power, but i've never heard of anyone losing power. BTW, motors with the 4/7 swap sound incredible!

[noice]

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

If you are truly worried about cooling run an external pump and put AN lines to the middle of the heads like the pros.

[Paulster2]

The way I see it, if there is the possibility of picking up some ponies, it's a win. Like I said, the gains are small, but every little bit counts. Harmonics count for a lot (if this is really where we are picking up the extra ponies), especially when you are counting for longevity (hence one of the reasons for balancing the internals, right?).


If you are truly worried about cooling run an external pump and put AN lines to the middle of the heads like the pros.

Unlike a 4/7 swap, this costs money (machining/parts). A 4/7 swap, if planned for during a build, is a zero cost option (you'd probably be getting a custom cam anyway ... all else is moot). I'm not suggesting the 4/7 swap would have anything to do with cooling, it's just your suggestion costs money (nothing wrong with your suggestion, either, I might add ;) )

I am not seeing the down side to this. And if what joelster said is right, the added bennies of sound is a good one. It's one of the reasons why I love the BBC is because of loping sound it makes (besides the low end torque) ... very distinct. If nothing else, the discussion here adds to forum and gets people thinking :)

[joelster]

There's a few things you have to change to do the swap. Obviously the camshaft is the biggie. Then you have to re-wire the 2 injectors if you want to. Notice that you don't have to do this. 2 cylinders can fire in batch mode while the rest will be in sequential mode. I don't recommend that, but i'd bet you'd never notice the difference. Then you have to swap the wires on the opti, and it's quite cramped up front.

The one downfall for doing this and running a stock-type pcm is this:

If you run in closed loop and 1 side sees a lean spike it will dump fuel to that bank, but in reality it will be dumping into 3 cylinders on that bank, and 1 cylinder on the opposite bank. With an aftermarket pcm you can change the firing order in the box itself.

[z_speedfreak]

I've always been told this swap is for harmonics & mains and that you wont gain any power from better scavenging unless you pair it with headers to match(tri-y's). T? F? what about equal length headers in conjunction w/ the swap? how would they work out?

[Paulster2]

... The one downfall for doing this and running a stock-type pcm is this:

If you run in closed loop and 1 side sees a lean spike it will dump fuel to that bank, but in reality it will be dumping into 3 cylinders on that bank, and 1 cylinder on the opposite bank. With an aftermarket pcm you can change the firing order in the box itself.
Definitely something I hadn't thought about, but seems plausable.

[Paulster2]

I've always been told this swap is for harmonics & mains and that you wont gain any power from better scavenging unless you pair it with headers to match(tri-y's). T? F? what about equal length headers in conjunction w/ the swap? how would they work out?I understood (as stated in my OP), with no other changes, the 4/7 would net about 6-8hp on a SBC. This came from one of the car rags ... er ... mags, back 10-15 years ago. Back then it wasn't near as feasable to do the swap as custom cams were not as easy (or cheap) to come by. I believe you can order cams off the shelf with the swap already built into them now, but don't know this for sure.

[joelster]

Reher-Morrison were the first guys to experiment with altered firing orders. They built tons of Pro-Stock engines back in the 80's. The gains are small, but they are GAINS.

[Camaro96]

The 4/7 swap looks like it could increase longevity of high horsepower engine by improving harmonics and balance. With that being said the way. The SFI is set up for the second generation LT 1 that were produced from 1992-1997 is not a simple fix. The fuel injection and computer along with the cam will need replacing with either after market stand alone computer or go back to the carb and a single /dual plane intake with electrictronic/ Magneto ignition.


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[1968NovaSSLT4]

This is an older thread but thought I would just mention I have done the 4/7 swap in my car. Custom ground cam from Elliot. I am running the Torqhead conversion so I just swapped the pins for the correct coils and injectors at the PCM. It was a really easy to do being I am using the LS1 computer system now. My machinists is an old Italian that always preached I had to do it to one of my motors as for the HP gains. Do I believe it makes that much difference, not really. I do feel that the motor is smoother Climbing to top end 6800 RPM's but I also have a balanced rotating assembly so I would expect it to be smooth. Does it sound different? I cant really tell since it was a complete new stroker build. I don't have numbers for the motor yet as I am trying to locate a dyno and tuner near me that has the time now to do it.

Here is a photo of the motor.

40021

[harner]

Interesting thread and follow-up. I like the intake/coil cover you have there!