Engine bay mounted filter vs Fender mounted filter
I posted this on another forum, but I'll post it here too.
What’s the best cold air intake (CAI) kit for my Mustang? How many times have I seen a thread with that question? The CAI has to be in the top 10 most debated topics in the Mustang world. Some argue that these CAI kits aren’t worth the money. Some argue that it’s one of the first mods a Mustang should get. Personally, I didn’t care if it was worth the money when I decided to get one. I wasn’t really concerned at the horsepower per dollar ratio of the CAI. I like how the CAI looks and it was within my budget and it was a really easy mod to do. That meant a lot for a desk jock like me.
When I decided to get a CAI, naturally the next question was which CAI to get. I read thread after thread on different message boards comparing the different cold air intake (CAI) kits available for the Mustang. I realized there were so many choices but I narrowed it down to two general types. There are ones with the filter in the engine bay protected by a heat shield, and ones with the filter in the fender. The arguments around the filter location are all about the air temperature. The engine bay mounted filters are said to suck hot air, while the fender mounted filters suck cooler air. The popular engine bay mounted kits are the Steeda and
K&N kits. MAC and Densecharger are popular fender mounted kits.
I’ve seen this air temperature argument so many times and thought to myself, this was something I could easily validate. I went to the hardware store and looked for indoor/outdoor thermometers. I found this digital thermometer with an outdoor sensor on a 10’ wire for $10. Perfect! I bought two so I could take temperature readings in the engine bay and in the fender at the same time. I hurried home and opened up the hood. I attached one sensor along the front brace, just behind the passenger side headlight. The sensor was about two inches in front of the K&N filter in the engine bay. I attached the second sensor to some cable inside the fender. It was about 4 inches into the cavity for the air silencer. I ran both wires into the car and attached the two thermometers to an old CD. I rechecked all my connections and I was ready to start my data collection. The geek inside me was happy and I’m sure I got a few weird looks from the neighbors. Time to drive around.
[These message forums don't make it easy to post data in a tabular format so I'll just summarize the data.]
The data tells me that once I am moving above 15 mph, the temperature difference averages around 3.5 degrees. With the a/c running, the engine bay was around 4-5 degrees warmer than the fender area. At speeds less than 15 mph, the difference goes above 10 degrees. The engine bay is definitely hotter but only by a few degrees when the car is moving.
Does the air temperature matter? I can’t answer that. I’ve seen some claims on the Internet about losing 1 hp per 10 degrees increase in air temperature. If we assume that is true, then having the filter in the fender gives you ¼ to ½ hp over the guy with the filter in the engine bay.
Do I miss that fraction of a horse because I got the K&N FIPK? No. If we assume the 1 hp per 10 degrees is accurate, then my car will produce 2-3 hp more in the early morning than my drive home at 5:30 pm only because of the weather. The ¼ to ½ hp is negligible.
Does the extra half hp result in better mileage or performance? In a laboratory or on paper, Yes. But in the real world, a slight incline in the road, or a light wind, or having a passenger, or a little traffic will have so much more impact on mileage and performance that the ¼ to ½ hp is again neglible.
My recommendation is to choose a CAI that has the larger filter for more surface area, and one that has the shortest path to the throttle body.
