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Cone Filter in the Engine Compartment - A Discussion - Sort Of

New 02/17/2005, UD 02/25/2005, 03/04/2005

I was reading my February issue of Grassroots Motorsports and they had a tuning article for a BMW M5. As part of the PCM and MAF upgrade, they put in a conical air filter in the engine compartments. It dropped the horsepower. They show a graph where the HP and torque curves go quite wacky because of all the warm air. For the short term, they put a pizza box in to force the filter to pull the air from outside the engine compartment so they could finish their dyno tuning. They said they have since formed a new air box out of aluminum.

Once again, anyone who just puts a cone filter under the hood and “feels it’s faster” needs to get their butt dyno recalibrated.
Paul L Fisher

Any takers on the new position Paul created - Butt Dyno Recalibration Tech???


Hey Don, did I dyno test the cone vs. clean paper filter on your Dyno?
As I recall they were the same curve (the K&N indicated like 1-2 hp
more but that is not a conclusive proof of gain).

For drag racing I agree with not running a filter open in the engine
compartment, but it just does not have a significant impact on real
world driving in a v-8 SHO open or closed, because the air is within a
degree or two of the outside air temp 99%+ of the time.

All you need to duplicate my findings are a digital thermometer and a
remote sensor.
A few notes on under hood temps.
With an under the hood cone on sustained winter high speed cross country
drives under hood temps are not an issue. Under all other conditions intake
air may be much higher than atmospheric.

Worst case, in traffic, in August, in Texas, under hood temps may exceed

I had the under hood auto tap for a long while before it was auctioned away
at a convention. I could monitor intake air temps. Even in mid winter the
air will warm up 10F with a cone-in-fender system.

I remember 180F intake air in traffic on mild 80F days. A 100F
between ambient and intake temps with a V8SHO at legal speeds and moderate
load is is par for under the hood cone. A OE setup may have less of a temp
diff but who cares since no air can get past the mouse trap?

I think Paul Nimz, lacking a laptop ran autotap with a full size PC in the
pass seat. Ask Paul. Before anyone jumps in with a bunch of opinions,
instrument a car and drive with it a month or two.

Radio shack sells inexpensive temp probes. Get a few. Measure for a while.
A V8SHO puts out a lot of heat and in particular bakes the intake pipes.


This is just what I did. The only time I got temp spikes was sitting
at multiple lights after a long high speed run. 99% of the time the
air was the outside air temp for under hood or stock intake.

In the heat soak condition, the stock CAI worked better though it
would heat up just like under hood intake eventually.

My temps were not at the MAF which heats up, but on the surface of the
air filter itself.
Ask UL about high under hood temps...

That was low man!!:}

Guys, it just does not happen. Remember we are talking about
under hood temps at the filter, with the engine sucking HUNDREDS of
cubic feet of air per minute through. The temp in that air stream is
BARELY above ambient.

I'm not lying. I MEASURED the temps in an actual install right at the
air filter. If you disagree, do it with data. I took the data points
and it is just a fact. Don't look at data of temps under hood WITHOUT
an under hood intake - that is a meaningless exercise.


I can only report the observations I personally have see. I will say
that there is a tremendous difference between what the air is at the
OEM air filter /IAT sensor that what is actually going into the
intake. I have moved my sensor to a position about 2" in front of the
TB. I also put a digital probe at this location too.

The problem with reporting numbers is there is not good way to
document the ambient and under hood temps compared to my recorded IAT
temps. I have 100s of miles of logged data. When the ambient temp is
under 30F or so and I am running on the highway then the IAT will see
almost the same as ambient, but it will always be at least a couple of
degrees higher. I use the EATC outside temp display for my ambient

At less that highway speeds the temp is most defendant on throttle
position. The more air flowing the cooler the air. But the actual
time the throttles is open more than 50% is usually not long enough to
cool the heated intake tract on a heat soaked motor in the summer with
under hood temps over 130F.

When I went to the convention in MD in 2002 I had a sensor connected
to the surface of the intake at the top and middle of the surge tank.
The temps were in the 90Fs and I was driving at highway speeds in the
mountains. The surge tank surface temp would run around 140-150F with
the EGR active. With the EGR deactivated the temp would drop to
~10-15F above ambient.

Here is a pic of the RTDs I had connected to the car when going to MD.
I had 4 digital temp meters in the car at the time.


For a while I've had mine set up with the SHO Shop intake system with heat
shield, as well as the intake pipe from the "Porterized" setup which I did
originally. (The intake pipe actually touches the cone filter so it is
sucking air through it pretty good, especially as the heat shield seals
against the hood. I have never done anything scientific like measure temps
with a probe, but I know that this has to help, and I really don't think I
am ramming too much hot air down my intake. These cars definitely like cold
air though.

W. Gordon Finley
I had one of those SHO Shop cold air thingys. That is where I got my
air filter and MAF adapter from. I did see that the temps when
measured at the filter were a bit lower than the underhood temps.

In the end I insulated the outside of the panel, isolated the front
from the battery area and put an insulated lid on it. This gave me
almost as good as temp readings as I get now with the filter in the


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