Air Intake Technology De-mystified
See Attached PDF for a Nicely formated version of this
There is so much information flying around about filters and air intake kits, I thought it time to give all of your some objective information about this subject. Most specifically about the 3.7L but most of this applies to the 4.0 as well.
I’ll also give you some tips to build a homemade intake that will perform nearly as well as the $300.00 kits. I’ll admit I bought a K&N and if you look at my posts I’ll tell you it was over priced. I decided to splurge because the K&N looks really nice and fits like a glove.
First fundamental: An engine is an air-pump from a mechanical standpoint. The more gas & air that can flow through it, the more power you can make. That means anything that gets in the way of the intake air or the exhaust gas will decrease performance and gas mileage.
Intake kits build HP for 4 reasons:
- Lower restriction filter (generally)
- Smoother, less restrictive path from air cleaner to throttle body
- Larger air intake area on the dirty side of the filter --- open element in most cases.
- Ability to suck air from a cooler part of the engine compartment.
Item 1 can be significant depending on many factors including stock vs replacement size etc. Generally K&N, Fram and other oiled cotton mesh filters flow better than pleated paper of the same surface area. The Stock Nitro filter is quite large for a 3.7L engine and although replacing it with a K&N will help, it may not be as dramatic difference as on some other cars. You need to address the other areas to make a bigger HP impact.
Item 2 This is the big one that most people don’t understand. OEM designers put all kinds of twists turns and chambers to reduce the intake noise of the engine. They put in little dead-end chambers called resonators to further reduce the noise. Also the OEM tends to make bends in the system for layout convenience rather than low restriction.
In the case of the 3.7L, the filter box has a clean air path, but the throttle body air box is quite restrictive.
If you flip this box over you can see the path the air must take from the filter see picture 1 (left side of box above) to the throttle body (top of picture).
see picture 2 -Bottom View of Throttle Body Air Box : Note the sharp 90 degree bend. The arrows indicate where the 3 dead-end resonator chambers enter the main air path. These have openings inside the tube that are positioned such that air slams into the dead ends rather then taking a smooth bent. The resulting turbulence reduces noise but also impedes smooth air flow.
see picture 3 & 4-The above is a view though each end of the 90 degree bend that goes to the throttle body. You can clearly see how the air rams into these dead end chambers rather than making a smooth turn.
Item 3 The opening to the Nitro filter box is not bad—about the same diameter as the throttle body. The problem is that there is a tube inserted into the filter box that tapers down to less than ½ this diameter. Again the purpose of this taper is to further reduce intake roar. See the pictures below of the filter box and the tube. I have placed a 9V battery in each opening to give a sense of scale.
Filter Box Opening without tube Intake tube inserted into filter box. see picture 5 & 6
Item 4 Pulling colder air is the least likely to give real gain except in the case of a very hot engine compartment and actual ducting to the outside – ram air hoods, under bumper ducts etc. Even at that the gain in practice is not that much. When a car is rolling at 70 MPH lots of air gets under the hood and it is not as hot as you might think. K&N and other CAI kits com with a sheet metal shield that closes off the back and sides of the filter to help it suck cooler air. I am sure it helps but the biggest boost is the clean air path not the heat shield.
2 solutions:
1. Buy a K&N or other CAI kit for $ 200-300
2. Build your own intake or parts of an intake
The sewer Pipe Intake Trick (my personal favorite home-brew intake):
This is my favorite one that I learned from the Impala SS forum (NAISSO). We did this to our LT-1’s when intake kits were just getting started. Follow the instructions below and you will have an intake that will produce nearly the same HP as a $ 300.00 kit for about $10.00 +the cost of a K&N replacement filter.
- Take off the nasty throttle body airbox shown in the first picture and bring it to Lowes, Home Depot or some other plumbing shop where you can pick up and play with drain pipes and fittings.
- There are 2 kinds of drain pipe in the US--- white PVC and Black (think it is ABS) either will work but the black looks better,
- Select a pipe size that closely approximates the outside diameter of the pipes on the intake air box. A little bigger is OK as the rubber will stretch.
- Once you have determined pipe size, turn the airbox upside down and select some bends and elbows that come close the tube that runs through the air box. You will most likely need a small stub of pipe to join the bends together. If you can’t quite get the length, there are some flexible rubber couplings in the sewer section that can help. You may need to take a trip or two back your Nitro to get this right, but you’ll be able to do it.
- Once you have the parts to make your sewer-pipe replacement for Throttle Body airbox, test fit them to the Nitro before gluing together. (keep your filter box and everything else on until you get this part to fit well.
- Once you have your sewer connector from above, move on to the filter box. Undo all the rubber fittings to the filter box and just pull it straight up. It will take some force but it will come off unharmed and can be put back buy pushing it back into the holes that align with the studs in the filter box.
- Remove the top of the filter box and the OEM filter. Pull out the front air tube (the one that narrows down. You can stop here, and put back the air box plus a K&N replacement filter (the one that is a direct fit for the Nitro)
- If you want even more air flow, take a 1” spade bit or a hole saw and drill a bunch of holes in the OEM air box (dirty side of the filter of course. If you are worried about hot air, just drill holes in the front and you will pretty much have the same inlet air temp as the K&N with a shield.
Ken
