Shane's SLC Build

[Ken Roberts]

We call the vent plenum the "anchor". I only used the section of the anchor that mounts under the defrost vent slots on the dash. I mounted this piece on threaded stilts/standoffs as well. Usually there is a significant gap between the anchor and the dash vent slots. Most builders fill this gap with foam tape. I can adjust the height of my anchor so that it almost touches the dash. Only downside is the fact that a bottom has to be made for the anchor since it is no longer in contact with the top of the tub.

I used threaded standoffs for maximum adjustability.

 

[Ken Roberts]

While you are working in the dash area, a good tip is to mount a cross brace between each pair of steering column mounting bolts (as shown in the following picture). Weld the bolt heads to the plate. That way if you ever need to remove your steering column in the future, you won't have to remove the dash as well to get a wrench on the top of those steering column mounting bolts.

This picture also shows the anchor mounted on standoffs and how I routed the defrost air flowing towards it. A choke type of cable pulls or pushes the door on the 3-way valve.

 

[Shane]

While you are working in the dash area, a good tip is to mount a cross brace between each pair of steering column mounting bolts (as shown in the following picture). Weld the bolt heads to the plate. That way if you ever need to remove your steering column in the future, you won't have to remove the dash as well to get a wrench on the top of those steering column mounting bolts.

This picture also shows the anchor mounted on standoffs and how I routed the defrost air flowing towards it. A choke type of cable pulls or pushes the door on the 3-way valve.

View attachment 150378

Dang! That is literally a priceless suggestion. I wouldn't have thought of that but I will do that ASAP! Great idea, thanks!

 

[Joel K]

Here are some random pics of the anchor install and vents…

 

[Joel K]

Some more..

 

[Shane]

Some more..

Thanks!! Great to see all the different ideas.

 

[Joel K]

I’ll probably do the front of the anchor similar to Stephan and used a left over HCV to shut off/on the defroster…

 

[Shane]

I’ll probably do the front of the anchor similar to Stephan and used a left over HCV to shut off/on the defroster…

I like that setup as well. What HCV are you going to use? I'm going to start looking for one.

 

[Joel K]

I like that setup as well. What HCV are you going to use? I'm going to start looking for one.

That HCV valve is from the Vintage Air kit. I wound up using an Old Air Products one. Nothing special about that one other than it never shuts the flow back to the engine which is important in an LS or LT. Many ways to approach that problem like using an H pipe and the supplied HCV from Vintage Air. Cam T took that approach.

Also, Resto Mod makes a nice variable vent you can mold into the anchor. Pricey, but probably a very clean solution.

 

[Shane]

That HCV valve is from the Vintage Air kit. I wound up using an Old Air Products one. Nothing special about that one other than it never shuts the flow back to the engine which is important in an LS or LT. Many ways to approach that problem like using an H pipe and the supplied HCV from Vintage Air. Cam T took that approach.

Also, Resto Mod makes a nice variable vent you can mold into the anchor. Pricey, but probably a very clean solution.

Man that was a rabbit hole, thanks @Jkviper!! So the never ending saga of do you need to recirculate coolant through the heater ports or not. That almost felt like a political battle. I even called Vintage Air today and the tech said that I did not need one and that on the LS engines you are just fine to use the shut off valve.

This is what I am going to buy. I think this will solve the problem and it doesn't seem like that big of a deal to put in as the hoses go to the front. Cheap insurance. Any thoughts?

Heater Hose H-pipe Crossover

 

[Ken Roberts]

The shutoff valve will accomplish two functions. It stops hot coolant from running up the side of the chassis and into the core. It also allows the coolant to be diverted back to the LS engine. Almost all builders with LS engines install them. I wouldn't listen to what one tech rep from Vintage Air tells you.
If you do an internet search on this subject you'll see that once in a while someone will comment that not returning the coolant resulted in no issues. In your case, you are providing a loop for the coolant flow but why route hot coolant alongside your chassis and passenger footwell then seep through the closed core. Your A/C will perform more efficiently. The hoses may last longer.

Most builders used the Old Air Products electronically controlled valve but I chose a OEM vacuum controlled valve. A solenoid valve controls the vacuum. Allows vacuum when energized then bleeds off/closes when turned off. Same type of solenoids used for EGR valves and emission canisters.

Many builders repurpose the stock supplied Vintage Air HCV to control the air that reaches the defrost vents (as Joel said).

 

[Shane]

Brake Light Switch
I have removed the hydraulic brake light switch and replaced it with a mechanical unit. I choose this one from Amazon.

I also designed and 3D printed the bracket.

I don't like the spring very much but it works and I have other things I need to get done. Maybe I'll come back to it later.

 

[Joel K]

Straight from ChatGPT…

Should a heater control valve always circulate the coolant back to the engine on a GM Gen 4 LS engine?


Short answer: Yes — on GM Gen IV LS engines, the heater control valve (if present) should always allow some coolant circulation back to the engine. It should not completely shut off flow.

Here’s why:

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​

GM LS engines (especially Gen III and Gen IV) rely on continuous coolant bypass flow to maintain stable engine temperature and prevent hot-spots in the heads. Even if the HVAC system is set to “cold,” the engine still needs that bypass loop.

Key points:​

• Most LS engines do not use a heater control valve at all — the heater core is always in the coolant loop, and temperature is controlled with a blend door inside the cabin.
• When a heater control valve is added (commonly by aftermarket swaps), it must be a bypass-style valve, not a shutoff-style valve.
• If coolant flow is completely blocked from the heater hoses, the engine can develop:
  
  • Hot spots in the heads
  • Temperature fluctuations
  • Overheating at idle
  • Cooling system pressure spikes

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What happens if you use a “shut-off” valve?​

A shut-off valve that stops heater core flow completely can lead to:

• Erratic temperature gauge behavior
• Overheating at low RPM
• Damage to the water pump (due to pressure imbalance)

This is because LS engines expect the heater circuit to act as a secondary bypass.

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What type of valve is correct?​

If you must use a valve, it should be:

✔​

When “heat = off,” the valve redirects coolant back to the engine, not to the heater core.

When “heat = on,” flow passes through the heater core normally.

GM used bypass-style valves only on a few trucks/SUVs, but the heater loop was never fully blocked.

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Bottom line​

Yes — the coolant must always circulate back to the engine.​

A Gen IV LS should never have a heater control valve that completely closes off heater-hose coolant flow.

If you're doing a swap or correcting a problem, using a bypass-style valve or simply deleting the valve entirely (as GM did) is the correct approach.

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If you want, I can also:

• Provide a diagram of proper LS heater hose routing
• Recommend correct aftermarket bypass valves
• Help diagnose a specific overheating or temperature fluctuation issue

 

[Shane]

Different Heater Control Methods
So here are the two ways to accomplish the circulation on the heater control circuit on an LS engine. Maybe there are others but these are the two main ones. I spent far too much time researching this and thought I might share to help others that didn't know.

H-Pipe and a shutoff valve
This system works fine but you are allowing heat transfer of the coolant fluid to possibly heat up the coolant in the heater core. Basically the HCV shuts off flow completely so there is no flow through the heater core, but the coolant can still be warmed because of the heat transfer at the H-Pipe. I don't know how real this is. It would depend on length of pipes, type of pipes etc to dissipate the heat at the same rate as the heat transfer. But this system works.

H-Bypass Valve
This system uses an H-Bypass Valve that allows the coolant to be circulated back to the engine and completely isolates the coolant to the heater core. There is no flow and no heat transfer so as both @Jkviper and @KENS80V stated the system is more efficient and allows less heat in the cabin (depending on where you put the H-Valve).

So here are links to the different system components. Obviously you could choose to have a mechanical or vacuum valve, I have just listed the servo valves.
H-Pipe
Shutoff Valve
Servo H-Bypass Valve

Here is a good diagram of the Heater Bypass Valve from Old Air Products

 

[Joel K]

One thing I was concerned about with the H pipe is because the heater core does not sit low with the SLC, the water(or a good % of it) would still circulate through the H due to head pressure so I went with the Bypass valve. Not to say I’ll drive it in the winter, but that was a consideration.

 

[Shane]

One thing I was concerned about with the H pipe is because the heater core does not sit low with the SLC, the water(or a good % of it) would still circulate through the H due to head pressure so I went with the Bypass valve. Not to say I’ll drive it in the winter, but that was a consideration.

After thinking about this a lot, I really believe that the H Pipe would or could work, but why chance it when you can get the H bypass valve and be sure of it. That is what I am going to do.

 

[Hector]

As far as the A/C air flow. I followed Cam's build and did not use the anchor. Nice to have options. Choose what works best for you. I think this is a simpler way. I added one vent at the very front of the dashboard , in the middle, facing up, to serve as a defroster. I also added another hose and vent into the driver footwell to keep your feet cool. That works very well. I STRONGLY recommend it . Hope this is helpful , I used a PVC tube, insted of metal like cam did. I used PVC joints from Lowes like Cam did . I used the plastic , flexible A/C hose that came with the kit as well as the vent covers. All open and close manually. Joel K came up with actuators to open and close sections, super cool.

 

[Shane]

We set the body on to check a couple of things and I noticed the engine looked like it is setting up too high.

So we set the back on and got it all lined up and sure enough it hits the window area.

Talked to Bill and he said to turn my motor mounts around so the engine will sit farther forward. I think that might work but I'm looking for others who have the LS3. Does this look correct.

 

[Joel K]

First. I would make sure the body is not too far forward. The rear cup holder looking things should be about 1/4” behind the frame cross member. Just about every SLC I’ve seen including mine, if you mount the front clam over the front wheels without adjusting the forks back a bit the body sits too far forward.

IMO your rear engine mounts look correct.

 

[Shane]

First. I would make sure the body is not too far forward. The rear cup holder looking things should be about 1/4” behind the frame cross member. Just about every SLC I’ve seen including mine, if you mount the front clam over the front wheels without adjusting the forks back a bit the body sits too far forward.

IMO your rear engine mounts look correct.

I'm on vacation and can't look at the body. But I can't stop thinking, dreaming and planning things for this car!

I know that the body is flush or pushed back against the front here.

This is the best picture I have, I did notice the little recess in the body in front of the cup holder isn't close to being centered on the cross member. It is about half on and in front of the cross member.

Does the engine seem like it is sitting too high?