Every engine needs an intake system. The factory intake manifold, for most engines, is the foundation for an intake, but other, different designs can be implemented as well.
For example, a couple of builders have created engines with individual intake runners, as from suppliers like Jenvey and Harrop. These have the advantage of looking great, offering tremendous throttle response, and greater power. It's sometimes harder to build a filter system for these intake manifolds, though, and the filters do sap some power.
LS engines have a wide aftermarket range of different intake manifolds, from fabricated sheet metal intakes from suppliers like Beck, to the traditional-looking polymer intakes by FAST. You should research the intake manifold choices available for your engine.
Once you choose an intake manifold, though, the greater problem becomes how to get air into the intake manifold. That's the focus of most of this page.
Cold Air Intakes (we hope!)
A key problem in designing Cold Air Intake (CAI) is finding a source of cold air. The easy way to get air into the engine tends to use the engine compartment itself as the source for air. But this is problematic as that air tends to be heated, which hurts power production. How much does hot air kill power? The rule of thumb is that there is a 1% loss of power for every 10 degrees of temp rise over ambient. It should be clear then, that just putting the end of the intake tube where it's convenient, is probably not optimal for making power. Ever seen an aftermarket CAI and wondered why the filter was wrapped up in an insulated box, sealed from the rest of the engine compartment? That's why.
So a key strategy is to find a source of outside air. Related to that is the idea that intake tubes should be insulated, to keep radiant heat from the engine compartment from heating up the air already in the tube. The shiny, polished aluminum tubes on some cars provide increased bling at the cost of reduced zing. You choose what's more important!
Approaches for building a CAI for the SL-C
Intake Tube Size
The factory throttle body, and most all other aftermarket variants all support a maximum of a 4" intake tube. Much experience has shown this size to be more than adequate for power levels up to 1000 HP, and possibly more. Virtually all SL-Cs with conventional intake manifolds use this size. Bigger tubes actually tend to lose power as they reduce airspeed, and smaller tubes tend to add a little more restriction that costs power at the top end.
Go with the 4" intake tubes unless your application is very unusual.
Exactly how and where to route the intake tube is the source of considerable discussion for some builders. There are really several options, and a couple of them will be explored here.
The first, and most common solution is to run a 4" tube with a 15 or 22 degree bend on the filter end, dropping down over the end of the chassis brace. This is very easy to make, inexpensive, and allows a wide range of filters (as from K&N). Performance is very good, and if executed properly, these intakes can duct cool air (instead of hot engine compartment air) to the intake manifold.
Here's one example of how a builder created one:
Note the modifications to the body of this race-tail equipped car to allow air to flow into the top of the filter from a scoop that will be fitted on top of the opening. The method used by this builder was to carefully open up the body, and tape up the tube with clear packing tape in the critical area. The cream-colored epoxy mixed with thickener was then applied to the body and tube. While wet, the rear clam was closed, forcing the wet epoxy mixture to take the shape needed. As it cured, the body was opened, and the epoxy trimmed to fit. It could later be painted or undercoated to match. See other pictures below for more details.
Another solution is to make a much more complex tube which curves down to exit at a filter near the tire. This has the advantage of not requiring body modifications with the race tail, and also allows a presumably cooler source of air. The disadvantages include greater fabrication complexity (including welding, typically), somewhat greater cost, and a greater length which has the potential to reduce top end power. Also, that area will likely have turbulent air from tire/wheel pumping, and so ducting around the filter will have to be made as well, to ensure smooth airflow to the filter.
Here's an example of that kind of approach.
If you do choose this approach, the roll bar stay may need to be shortened and a new bracket made to anchor it to the chassis, adding to the complexity and cost of this approach. Note that this car has already had that done- the rear stay no longer connects to the billet upright as the other side does. There is insufficient clearance to run a 4" tube in that area without relocating the roll cage stay.
Depending on the details of your filter location, your choice of the race or street tail, you may find that a scoop is in order to better direct air to the filter. This is especially pertinent for the race-tailed cars, as their is relatively less room in these cars compared to those with a street tail. Several builders have used this scoop as shown here to open up the rear area, and duct cold air from the laminar flow over the rear of the race tail.
These scoops can be fastened by epoxy, screws, other other trim fasteners. Scoops can be home made, or the scoop shown can be purchased from one of our fellow SL-C builders, Jack Moeller, who can be contacted here for pricing and availability details.
Fitting the intake tube in the body
Most builders will find that, especially with the race tail, there is insufficient clearance to fit a 4" tube from the throttle body if made long enough to get over the chassis brace. The solution is to trim the body, and usually the rear of the back window to allow for clearance for the tube. See the pictures below for details.
MAF Sensors and Their Attachment
If you are using a Mass Air Flow (MAF) sensor, the placement is important. The GMPP controller kit includes specific instructions for their location, including distance from the throttle body, and the amount of straight tube before and after the sensor. Failing to follow these instructions won't make the engine not start or run, but doing it wrong may cause reduced power, driveability, and durability issues. However, some tuners experienced in the GM or other operating systems in the ECU may be able to tune for a non-standard length. Be sure before you fabricate your intake and MAF sensor in a different way than is specified in the instructions you received with the engine controller kit.
Rest of System
The rest of the intake system consists of filters, flexible hose adapters (to adapt the tube to the throttle body, for example), hose clamps, hose barbs and vent hose. Be sure you plan for all of these when you order parts for your car, to save time when you begin working.
You'll also probably need to fabricate a box around the filter to keep out hot air, and seal the filter to whatever source of cool air you have selected. Typical boxes include ones made of thin aluminum, or composite (fiberglass or carbon fiber).
Sources of silicone hose connectors, clamps, etc are all over the internet, but some the author has used come from silicone hoses and others.
Most people use filters from K&N because they are ubiquitous, and inexpensive. Be sure when you order to understand sizing, and in general, select the biggest filter that will physically fit in the area you have, consistent with tube size, etc. It's typical to go through a couple different filters before you find one that fits perfectly, so order these from a source that has a generous return policy.
Be sure you account for insulation, as mentioned above. The popular gold foil, or the DEI reflective coatings are all effective. When you apply them, be sure you leave the areas where a hose connector and clamps go uncovered, so perfect sealing is maintained. A leak after the MAF will definitely cause your engine to run badly.
If you have extremely tight limits, consider using 4" oval tubing. This is expensive, but allows much more flexibility in designing an intake tube over obstacles. The overall area is the same as a round tube, so performance is almost identical. You will need to plan for transitions from round to oval, and these require more welding and cost. Here's a link for such tubing for a start.