If you've been around cars for a while you've probably heard that an engine
is like an air pump. The more air in the motor the more fuel you can burn the more power you'll make.
A commonly over looked essential part of this equation is air density or pounds of air. Air density will
change from several factors including, air temperature, air pressure / elevation, and humidity. You will not
be able to change the air pressure or humidity in the air but you can change the air temperature entering
your motor. I'm not saying you'll be able to change the ambient air temperature. What I'm saying is the air
your engine breathes is all too often HOT air. I cannot count how many times I go to car shows and see
beautiful cars with thousands of dollars into the motor and it has an open air element under the hood.
I have to admit I was one of the guilty and ran the open air element on top of my carburetor robbing power.
How much power are you losing? From my testing I found with an open air filter under a sealed hood, no
scoop I averaged approximately 140°F with a high of 158°F when the ambient air temperature was 80°F.
That's 60°F above ambient air temperature. Now let's see how that difference in air temperature effects
the air density. The Air Flow calculator on the site has an input
for air temperature. This calculator is nice because it shows the lbs of air based on air temperature.
Using my intake air temperature measured numbers, 355 ci, 6000 rpm, 90% volumetric efficiency resulted in:
554.69 cfm and 40.9 pounds of air. Now only changing the air temp to 140 you get 554.69 cfm and 36.81 pounds of air.
NOTE: The cfm did not change but the pounds of air did. The increase in air temperature from 80°F to
140°F resulted in a loss of 4.09 pounds of air. That might not sound like a lot but a general rule of thumb is
each pound of air equals approx 10 horsepower, therefore I was giving up almost 41 HP.
How do I fix the problem?
I ordered a Harwood fiberglass hood from Jegs and made the
cowl functional. A note about the Harwood hood it is a very nice
hood, it's more expensive but it is much nicer than the other cheaper
Unlimited hoods. If your thinking about cheaping out and going
for the Unlimited hood I recommend bite the bullet and cough up
the extra dough for the Harwood it is a quality product. It uses
the stock hinges and latch mechanism. The cowl is approximately
3" in height. I made the cowl functional by getting a
sheet of aluminum approx. 26" X 30", enough to seal
the opening in the hood. I cut a piece out of the aluminum,
approx. 12" circle, and bolted the base of the K&N air
filter to the aluminum and foamed the outside of the aluminum so
when the hood closes it seals around the air cleaner and keeps
out the hot engine bay air. The foam I used was a quality
packing foam. My only concern is if the foam is flame retardant,
I hope so! The result is the air temperature is pretty close to
ambient air temperature.
Now it's time for some 0-60 runs. I ran a
5.1 second 0-60 time with a bog off idle and the intake air
temperature was 70°F. I generally averaged about 0.5 seconds
faster and did not even rejet the carb. With proper jetting the
car should run even faster. With a 0.5 second improvement in
0-60 I'm hoping for about 0.7 to 0.8 improvement in the 1/4
mile. Unfortunately the track closed before I was able to fix my
An interesting note I tried getting a 0-60 time on nitrous. I RAN SLOWER than off
the bottle. I could get no traction in first or second. I basically smoked the tires
through first and most of the way through second. The time was 5.5 seconds on the Nitrous.
Slicks should cure that problem but I don't think my axles are up for slicks,
that's why I haven't put them on the car yet.
Read Part 2 and see how the car did at the track with cold air induction.