7.4 L29 swap into a 1997 4x4 lwb 5.7 truck

Performance modifications, tips & tricks

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Real82it
Posts: 11
Joined: Wed Sep 24, 2014 9:08 pm
Location: Albuquerque, nm

7.4 L29 swap into a 1997 4x4 lwb 5.7 truck

Post by Real82it »

Finished the swap and started the truck today! Yeeee Haw! No codes, only one small fuel leak and a exhaust leak, both easily fixed this weekend.

My 1997 truck had a 204,000 mile 5.7. It is a LWB 3500 4x4 crew cab. Always woefully underpowered with the 5.7. I even put a whipple supercharger on it for a few years. Truck caught on fire pulling a trailer back from Reno Hot August anights this summer. I didn't like the fact the motor was always in boost and still struggled pulling the trailer. I decided to sell the supercharger and pick up a low mile 7.4 L29 motor.

In case anyone is interested in doing this swap, here is some info on it.


Here is what I started with:
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Here is the motor cleaned up with new gaskets, injectors, etc

Almost ready to install.

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This is what a stock 5.7 engine bay looks like (not mine, stock photo)
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7.4 Installed in truck:
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The swap was not too bad.

Wiring:
I used my original 5.7 computer and harness because it was in better shape than the donee 7.4 harness. I had planned to use the 7.4 computer, but it was from a 2000. The 97 computer has an extra connection on the computer (5 plugs vs 4 on the 2000). Rather than modify to use the 2000 computer I decided to just use the 97. It fired right up, but I do plan on having the 97 5.7 computer reprogrammed to optimize the tune for the 7.4. (97 computer on top)

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I had to lengthen the egr/emissions, air conditioner, temp sensor, and throttle body wire length so the connectors would reach their new locations.

I cut the fuel injection connector off the 7.4 harness and wired it into the 5.7 harness. Very easy to do. Just matched up the wire colors. Used 2 of the existing common 12v pink wires for the new connection and terminated all the others.

The 5.7 only has one knock sensor, the 7.4 has 2. I had to cut the second knock sensor off of the old 7.4 harness and tap into the single wire of the 5.7 sensor wire. It would probably be fine if you just continued to run the one if keeping the 5.7 harness in place.

I used the larger alternator from the 7.4. It has a different plug than the smaller 5.7 alt that was on there, so I cut the 5.7 one off and installed the correct one from the 7.4 harness.


Motor Mounts:
Motor mounts, upper and lower, swapped over and bolted up without modification.

Exhaust:
The exhaust took a little work. The 5.7 had 2 1/4 downpipe flanges that then expand into 3 inch pipes?? The 7.4 has 3 inch flanges at the exhaust header flange. I cut off both down pipes, cut off the reducers on the pipes, and then bought and welded in some 3 inch flanges/reducers on the pipes.

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Cooling System and AC:
I had to modify the heater core hose to connect to the radiator rather than the water pump. I used a 90 degree section of the old hardpipe from the 5.7 and added a new short section of hose.

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The fan and fan shroud were weird. I pulled them from a 1999 3500 7.4 truck. When I installed on my 97, the fan hit the lower shroud....and not by just a little. I had to reduce the blade length by 1/2 inch to clear. (So fan diameter was reduced 1 total inch, from 20 inches to 19). I will see how the modified fan performs. I can't figure out why the fan/shroud did not fit in my truck, unless there was some type of change to the radiator support from 97 to 99?? Or, the 454 factory lower engine mounts are actually different compared to a 5.7 truck....the new motor in my truck sits about 1.5 inches lower than the doner 7.4 truck I compared it to...hence the fan hits the lower shroud. Motor mounts were in good condition, so not sagging...this is a head scratcher, but I got it all to work.

The 7.4 has a taller radiator than the 5.7. (18 vs 20 inches). The radiator hold downs are unique to the 7.4. (7.4 laid on top for reference). I think the 5.7 radiator will work, and that is what is pictured below (5.7 radiator with 7.4 fan shroud) but keep this in mind if doing a complete swap....grab the 7.4 radiator and hold downs if you want the taller radiator. My 5.7 radiator is actually a larger capacity 3 inch core vs the 2.25 on the 7.4 doner. I am sticking with my original 5.7 radiator.

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The upper radiator hose from the 7.4 fits under the 5.7 AC manifold hose assembly. You can clear the oil filler neck by bending the line some. If doing this swap, you may want to grab the 7.4 AC hose manifold set, it would make it easier to install that way. Lower radiator hose swaps over 5.7 to 7.4.

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Fuel lines:
You will need to get a set of 7.4 fuel lines. You may be able to bend the 5.7 lines to fit, but it is a pain to work in that area. Rock Auto sells a nice set of stainless lines for about $90.http://www.rockauto.com/en/moreinfo.php ... 454&jpid=1

Real82it
Posts: 11
Joined: Wed Sep 24, 2014 9:08 pm
Location: Albuquerque, nm

Post by Real82it »

I am wondering about the timing or fuel table differences in the 5.7 vs the 7.4. I wonder how significant they are?

Also, anyone know of or can provide a summary of computer tuning changes typically done by one of the mail order companies for the L29 7.4? I have an old Jet performance programmer I can use to update some parameters.

Real82it
Posts: 11
Joined: Wed Sep 24, 2014 9:08 pm
Location: Albuquerque, nm

Post by Real82it »

Had a fuel leak I had to fix today. Fuel leak turned out to be a leaky #3 injector. Sort of a pain to fix with the injectors being under the upper manifold, but got it fixed today. (Replaced upper o ring). Glad it wasn't the fuel pressure regulator....that would have been worse to fix...hard to access.

Took the truck for a 20 mile test drive today. Throughly pleased with the results. Power is noticeable better. Engine runs very smooth and quiet. Oil pressure is good: 25lbs at 700rpm idle, 65 lbs going down the road. Dropping down into passing gear at 70 mph produces nice acceleration without the engine laboring too much (the 5.7 would fall over flat trying to doing this).

I will be interested in gas mileage and will post an update after my first tank. The 5.7 only averaged 9mpg in town, 12 on the highway. My guess is that the 7.4 will be close to that.

I will get the new tune on the PCM in January and post any info on power improvements.

silver97
Posts: 8
Joined: Sun Jul 22, 2012 4:39 pm
Location: Virginia

Post by silver97 »

Thanks for posting your swap. I am doing something similar. I have a 97 suburban 2500 5.7 that has a nv4500 I swapped in 5 years ago. I used the harness and ecm from the donor truck. Now I am swapping the 5.7 for a 7.4 that i got from another 97 suburban with a 4l80e. Changing the injector plug wiring is going to be easier on my existing harness than using the 7.4 harness. I want to keep the nv4500.

Real82it
Posts: 11
Joined: Wed Sep 24, 2014 9:08 pm
Location: Albuquerque, nm

Post by Real82it »

Got the tune back from blackbear performance. All seems good. With the stock 5.7 computer the truck ran rich at WOT in the 3-5k range. (About 10.5:1 AF). Now, after the tune, it runs perfect at 13:1 in the same range. Small noticeable increase in power also while driving.

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