Joel’s SL-C Build Thread

Joel K

Time for the next update. I recently fabricated the Supercharger Plenum Intake extension and it is ready for welding.

The LT4 requires you to relocate the throttle body since in its stock location it butts right up against the firewall and there is no room for an intake tube. Most builders cut out a section of the rear brace/cross member and modify the seatbelt bar attached to the roll cage, but I prefer to leave the frame as-is.

By sliding the engine back 1.5” I now have enough room behind the firewall for a custom supercharger intake extension and think I’ll also be able to change the supercharger belt and water pump without having to remove the engine. The compromise being a little more rear weight bias. Since I am not going to track the SL-C I don’t think this will materially affect the handling of the car.

Originally I was going to use a 4” silicone Cobra-head intake tube but after getting the engine in the car their is just not enough room. After thinking about the approach a bit, I would have preferred to do what the other builders do and use a 120 degree 4” intake elbow but there is still not enough room for that without modifying the chassis due to the placement of the supercharger plenum opening.

To make it all work I decided to fab up a cobra-head style aluminum intake extension thinking it will provide enough airflow to run a stock tune and be compact enough to fit behind the firewall without modifying the chassis.

Here are pics of the actual cobra-head elbow and my protype made out of plywood, duct tape, and aluminum duct material...


Here is a detailed discussion on the approach....

Here are the raw materials and parts needed to fab up the elbow. Two 2.25” 120 degree elbows, 4” aluminum tube and Holley throttle body flanges...

Scribed and cut the elbows in half...


Made the cobra-head shape...

Fabricated the front and back panels...

Milled out the throttle body flanges to fit the 4” tubes...
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Joel K

Here is a discussion of the fabrication process, this was more work that I thought it would be, but at least it’s ready for the welder.

Here are more pics of the fabrication...

Making the first pattern for the longer throttle body tube out of A/C ducting and painters tape to get a rough form...

Then due to complexity of the shape decided to fabricate the back half of the tube separate from the front...

Adding the front

Now made a pattern out of the two pieces...



And the final product, hopefully it flows enough air so the engine runs without throwing codes, if not I’ll re think this in 18-24 months when I am in go-cart stage...



Nice and compact and squeezes in between the supercharger plenum and firewall. There should be enough room even with some sound deadening and heat shield in place....

Not planning to weld this up for a while since I want to be 100% sure if I will run the throttle body over or under the frame.

Next step will be to fabricate rear engine mounts to accommodate the slightly shifted location of the LT4 in the chassis.
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