My first LS4/F40 swap was built just for me and I went all out with a lot of performance parts and custom touches to make it truly unique (only thing left LS4 is the long block). So a couple of weeks ago, I purchased an 06 LS4 from an Impala SS to do another LS4/F40 swap. With this swap, I have several different goals I would like to pursue:

1. Do the swap in a manner in which others could copy.
2. Explore the benefit of modifying the stock LS4 intake and rear exhaust manifold vs. replacement.
3. Learn about the Gen 3 E40 ecm (compared to the E67) and see if DoD/AFM will work w/o the BCM on a Gen 3 engine.
4. Have quick access to a LS4/F40 for parts development (like the flywheel and starter bracket) - allows me to keep driving my LS4/F40 Fiero.

A few ground rules for the swap...

1. Only LS4 components will be used. I would like to dyno the car with it 100% stock, then start swapping out the stock parts with modified ones one-by-one to see how much the LS4 can be improved by modifying its own parts.
2. Keep things simple so others can copy.
3. Custom parts will be kept to a minimum.

This engine was a complete drop out, but I tore it down to just the engine so I could clean it. After its first trip to the car wash (still needs more cleaning):




This engine also came with a GM replacement transmission. I need to power up the ECM to get to the VIN # and find out when it was replaced. The engine supposedly has 92K on it, so the replacement trans should have much less than that. I still haven't decided if I will sell this off to recoupe part of the purchase price of the engine, or if I will keep it and use it in the LS4/4T65e-hd swap I am doing as well. Until I make the final decision, I will use this transmission to make a better (more precise) starter location fixture.






The areas I consider a restriction on the LS4 is the Intake and Rear Exhaust Manifold.

The intake had significant notch in it to clear the OPSU. This significantly reduces the diameter of the intake neck opening and likely costs hp:


Here is my proposed mod... enlarge the neck so it has a uniform diameter for its entire length.


The LS4 front manifold is quite good with each port dumping into the main log section. This main log is 2.3" OD and many people use this manifold for turbo LS applications.


The rear manifold is where the significant restriction is. In this portion there are 5 cylinders of exhaust flowing through a 1.8 x 2.7 OD ellipse. Which is about 14% smaller than the main log on the front manifold & there is 1 more cylinder pushing through it.


The main log for the other 2 cylinders on the rear manifold is also about 16% smaller than the front one after 2 cylinders.

The proposed mod is to cut open the rear manifold and add some sections of pipe to enlarge the exhaust passages for both sides of the rear manifold:


One of the first things I wanted to verify was if the F40 would clear the stock exhaust crossover. So while the engine was in the back of the truck, I installed the F40 and the crossover to see. In 1st (can't even get to Rev) there is interference between the counter weight on the shift arm and the heat shield.



Removing a section of the heat shield might help with 1st gear, but I doubt it would fix Reverse. So I will likely modify the counter weight. Right now I am thinking about scoring it on the top side and bending it down to it will clear, then weld up the scored opening. This would allow it to keeps its general shape/weigh, but fix the clearance issue as well.

I don't have a chassis for this swap at the moment, so the LS4/4T65e-hd swap will take priority. This swap will be progressing as the same time, but likely at a slower pace.

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Website: fieroguruperformance.com
Products: 88 13" Brake Kit, 88 12" Brake Kit, 88 Lateral Link Relocation, 84-87 Machined Front Hubs, Custom Machining
Engine Swaps:
LS4/F40, HSR/SBC/F23, Pro-Flo/383/Getrag, 4.3CPI/4T60, Ramjet SBC/Getrag, 4.9/Isuzu, Carb SBC/Isuzu, 4.5/Isuzu

[This message has been edited by fieroguru (edited 06-25-2016).]

quote Originally posted by lateFormula: Guru, here's another possible solution for the small diameter region on the rear manifold:

That's and option, but I am pretty sure mine would flow better. Having two opposing exhaust streams is rarely good for flow. With mine they remain angled toward the exit which helps both sides flow into the exit.

quote Originally posted by dobey: Can you give a measurement from the top of the counter weight, to the bottom of the LS2 throttle body (since I'll be using the LS2 as well, and I'm not sure if the bottom is lower than the LS4 TB or not) on your finished car?

Stock LS4 crossover OD is 2.1", but the point of interference is more on the heat shield around the expansion joint section which is wider.
With the shifter in R (highest possible position) there is only 3" of vertical clearance between the counterweight and the bottom of the LS2 TB. They are offset from each other some so the maximum clearance is 4 1/8" if the pipe passed through them at an angle.

Just an FYI, most guys with intake swaps have to angle their TB's for clearance to the Doug Thourley Headers (and probably the eBay knockoffs).

About that counter weight mod... there is only about 1/4" between the counter weight and the transmission case in 5th gear, so not much room to push it down. I might just have to trim the point enough so it will clear and weld back on the part on the outer edge (or just leave it off and see what happens).

[This message has been edited by fieroguru (edited 08-03-2014).]