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Cold Air Intake Air Box 2005 Ford Focus ST
I found some parts at Home Depot that may be suitable for an Airbox for my 2005 Ford Focus ST. Several uses come to mind. One is just to protect the filter from drips. A second could be to possibly build a positive pressure or ram air systm by ducting the filter the front of the car. And a third, which is my main concern, is to make a free flowing “fording kit” similar to the stock system, although I don’t plan to go through deep water. Nobody does. The parts are a 4 x 3 outlet basin and a 4 x 3 inch outlet. The parts are made of ABS plastic and seem to be strong and light. I modified the 4 x 3 inch outlet by removing the outer ring from the outlet and removing some locking tabs so it would fit the basin. It now has a 3.5 inch opening at the top in photo 1. There is a choice of 3.5 inch inside diameter outlet on the basin side or a 4.75 inch inside diameter (5 inch outside diameter) outlet. The parts are all available at Home Depot
When you drive through deep water, as everybody does in Houston, a bow wave builds up in front of the tire and the car. If this wave gets over the top of the inlet pipe, the motor ingest water and hydrolocks. Not a good think. Usually you bend connecting Rods. A small gulp might result in water in the oil and later spun bearings.
Here are some dimensions of the Focus Sport ITG JC 60/73 filter that comes with the Focus Sport Kit and the parts that I found.
Height of can- 8 inches -This can be reduced to 6.25 inch by cutting some off the open end of the part. The conical section of the filter is 140 mm or about 5-5/8 inch long.
Inside Diameter at upper (in photo 4) bell 156 mm - The ITG filter is 149 mm outside diameter at the largest part of the cone.
Outside diameter of can bottom 150 mm.
Assembly
The original outlet is shown on the lathe in photo 2. I used a parting tool on the lathe to remove the 5 inch outside ring (at the left in the photo 2). Photo 3 shows the parting tool that I used and the part with the ring removed. Then I used a brazed on carbide tip tool to remove the locking tabs shown at the right of the part in photo two until this outlet was a snug fit in the basin. I found 1/8 inch thick red sheet rubber material at the hardware store to make a gasket that is about 1/4 inch wide. I used six small bungy cords with reshaped hooks to apply pressure to the lid. I shortened the cords until they each applied about 5 lbs of force measured with a fish scale. The total clamping force is about 30 pounds which should be sufficient. All of these parts will be hidden in the car
Continued
. Photo 4 shows the setup that I used to part off 1.2 inch of the large open end of the basin. I bought the green grate just to use to help hold the part in the lathe. I drilled the grate center with a 1/8 inch hole and used a 1/8 inch shaft in the drill chuck to hold the grate. The parting went smoothly with this setup. Pressure with a center will cause binding of the cutting tool (This could be done with a Handsaw and then smoothed with a Dremmel and then sanded flat on a flat surface). I cut 1 inch off the bottom end of the bottom Focus Sport Steel Pipe to gain additional clearance. I cut the outlet bell 3/4inch long for a good fit to the filter endbell. The airbox fit well in the wheel well. Now it draws air from the very upper part of the inner fender behind the headlight as well. I drove the car it feels good. Bottom end seems better. No rattles apparent.
There was some dark staining inside the bottom steel pipe. This is an indication that the filter ingests some water from drips or mist. I got a note from another Focus owner with six months use that there was rust in his pipe. Rust on the engine side of the filter is a bad thing. Eventually the engine will draw in particles of rust.
Parts Required
4 x 3 inch basin used for Outdoor Drainage
4 x 3 Outlet
3" rubber coupling, remove wide steel band or a 4 inch rubber coupling.
3 diameter rubber long 90. or a 4 inch light PVC long 90.
6 small bungee cords.
#10 24 thread per 2 inch long bolt (cut to size) two nuts, three washers and an acorn nut.
1/8 inch thick rubber sheet to make a gasket. I found this at my hardware store.
Results
I did an acceleration test and timed the car from 5000 to 6000 RPM in third gear. Stock 3.26 second average. Focus Sport Cold Air Intake 3.01 second average. Focus Sport Cold Air Intake and New Airbox with 3 inch inlet 3.13 second average. I have lost about half the power that I gained from the focus Sport Cold Air intake. The power from 3000 to 4000 thousand RPM feels better to me. This is a range that I use more often. I have no data to support this other than a slight loss of traction in the middle of first gear that was not apparent before. Note that the airbox shrouds the big part of the ITG cone filter a bit. I might try a different K&N RF-1039 K & N filter with it that has slightly smaller dimensions.
I made a new airbox using a 4 inch diameter long PVC ninety and the large outlet of the basin which is 5 inch diameter. I connected these with a stretched 4 inch rubber coupling and hose clamps. The induction sound was more pleasant now. Performance test resulted in a time of 3.08 second average. I still probably need the K&N filter which will fit the box size with a little more clearance.
I have the K&N filter and 4 inch inlet airbox installed now. I am very happy with the improved midrange power from current form of the airbox and the sound with the airbox. I await another 60 F day for another acceleration test.
Since photo 8 above, I have changed from a ITG to a K&N cone filter 5 inch on the big end x 3.5 inch on the small end by 6 inch long. Here are the final performance numbers from 5000 to 6000 RPM. Bone stock, 3.26 seconds average, Focus Sport intake 3.01 seconds, Focus Sport Intake with my latest airbox 3.02 seconds. The top end is unchanged. Midrange is improved.
Some additional notes. The intake sound is excellent now. I shortened the upper bell of the intake on the airbox about 1.75 inches from the last photo. See photo 9. Part throttle response, the type that is used maintaining a four wheel drift in a corner is better than with the Focus Sport Intake. The slight bog when the throttle is opened an additional bit is gone.
I am better protected from another driver in my family going through water that is a little too deep here in Houston.
If you decide to try to make an air box with ram air here is a link to an interesting article. Siting a Cold Air Intake. So it seems that the inlet position just behind the front license plate seems ideal for ram air.
I notice that Mugen Air Boxes are also made of ABS plastic as is my air box. The oil on an ITG filter is not particularly compatible with ABS so contact with the filter should be avoided. The oil tends to melt the plastic a little bit.
Rain Test (new)
I drove through a heavy rain in Houston. I wanted to make sure the airbox was not collecting water. I was able to burp it since it has elastic straps and no drops of water came out.
Photo 9 shows a picture the pattern of water on the side of the air box near the inner fender. There is a steady stream of water that runs down here from above under some driving conditions. This is shown by the pattern of the water spots. The stream would be getting on the bare ITG filter. Because the filter is cleaned with water (it is probably coated with water soluble oil), this is probably not the best situation in rainy cities.
Anyway I am satisfied with the install. I especially like the return of the part throttle response. I am testing some insulation on the steel pipes to see if there are some additional performance gains available. On hot days the steel probably warms the air. I know that performance is reduced in hot air due to the lack of density, but the performance drop seems a little high in my acceleration tests in warm weather. There is a considerable surface area of the intake pipe exposed to engine bay temperatures that is in contact with the incoming air stream to the engine.
Resonant RPM
I did some web research on airboxes and found the following formula derived from basic principles and adapted it to our Focus 2.3 L. It helps explain some observations that I have made with the two airboxes and snorkel modifications that I have made. The airbox and inlet tube will resonate at a certain RPM. This will allow it to pass air better at this RPM and increase power there. In an airbox system the airbox itself acts as an air spring. The mass of air in the inlet pipes act on this air spring. The system resonates and passes air easier when the inlet pulses are timed just right (similar to pushing somebody on a swing. It is easy if you time the pushes just right).
Resonant RPM =19,100 *square root ((116.5 *Inlet Cross-Sectional Area))/(Air Box Volume)(Inlet Pipe Length) this is for a 4 cylinder 4 cycle engine.
Resonant RPM with 3 inch intake
When applied to my 2800 cm^3 airbox (6 long x 6 inch diameter) on the 2005 Focus ST with the 45 cm^2 inlet (3 inch diameter) the formula becomes
Resonant RPM = 19,100 *square root [(116.5 x (45.6 cm^2)/(2800 cm^3)*(121cm)]
Resonant RPM = 2400 RPM
Now I could actually hear when this system resonated by accelerating in too high a gear. Resonance was more like at 3,000 RPM. The resonance was loud. The difference between my observed value and the calculated value is probably due to the considerable volume in the intake manifold which is not included in the calcs and the fact that the intake pipe is split in two parts. I added the total intake pipe length. Acceleration felt stronger than stock and stronger than a Focus Sport Intake with this airbox at the 3000 RPM. I actually got some slight loss of traction in the middle of first gear on occasion with stock tires. This was an improvement. There was some loss of power at higher RPM which might be corrected when I change to a different filter. Might be a good setup for autocross.
Resonant RPM with 4 inch intake.
I then put on an airbox with the same internal volume but a different snorkel. 4 inch diameter in this case. Resonance was not as strong now and occurred at higher RPM by ear. 3500 RPM this time. The engine did not feel as powerful at 3000 RPM. It was more powerful at 3500 by feel and at 5000-6000 RPM on the stopwatch. The 4 inch intake yielded a faster time of 3.08 seconds for the 5000-6000 RPM range in third gear than the 3 inch intake. Still a bit slower than with a Focus Sport Cold Air Intake.
The calculated Resonant RPM was 3200 RPM with the larger 4 inch intake opening.
So here are some general rules that I arrive at by studying the formula and from the experiments that I did.
Increase the inlet size the resonant RPM goes up.
You want the Resonant RPM at a weak point in the power curve.
Increase inlet length and you decrease the resonant RPM
Increase the airbox volume and you decrease resonant RPM
It is helpful to have the resonant RPM up in the play zone RPM rather than in the highway cruise RPM. Above 3,200 RPM on the Focus ST is good. This will make highway speed changes in high gear more pleasant as far a intake sound goes.
Also See
Focus Sport Cold Air Intake Install in this Journal
Momo Pedal Install 2005 Ford Focus ST in this Journal
Focus Sport Cold Air Intake Thread on Focaljet.com
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