Miata Mailing List: August 1993, Message #159
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   From: Mike Rother 
Subject: TEST OF INTAKE TEMPERATURE
   Date: Fri, 20 Aug 1993 15:54:18 -0400



Following are results of a test of Miata intake-air temperatures.

PURPOSE:

Determine temperature of intake air at various underhood locations.

TEST CONDITIONS:

- August 20, 1993, Ann Arbor, Michigan, 1-2:00 pm
- Sunny with some clouds.  Ambient air temperature = 90 deg F.
Moderate humidity.
- Same stretch of road driven in same direction for each test run.
Some shady spots along the route.
- Four (4) speed & gear combinations were measured.  The same
combination and sequence was used for each test run.

CAR:

- 1991 Miata.  Base model.  Red.  10,000 miles on odometer.  No options.
- Timing set at 14 deg BTDC.  No other modifications or changes from
stock.
- 89 octane Amoco Silver gasoline.
- Heater and ventilation fan off.  Top down.

TEST EQUIPMENT:

- Omega 871 Digital Thermometer with remote probe.
- Real-time temperature readout at dashboard.
- Airflow effect on measured temperature was calculated out of results.
- Passing shade and other factors caused some fluctuation in
measurements during the tests.  Temperature accuracy is estimated to
be +/- 3 deg F.

RESULTS:  (Pretty Interesting)
---------------------------------------------------------------

PROBE LOCATION		---TEMPERATURES in DEGREES FARENHEIT---

AT DRIVING SPEEDS & RPMs	AT IDLE
(see "test conditions" above)

1) At Stock Intake
Snorkel Opening		126-131 		156

2) At Airbox Opening
With Snorkel Removed		121-124			145

3) 2" Left of Driver-Side
Radiator-Fan Shroud		100-102			129

4) 5" in Front of Airbox.
Up Toward Hood.		114-117			149

---------------------------------------------------------------

COMMENTS, OBSERVATIONS, & OPINIONS:

- If you remove the stock intake snorkel, the intake air is about 5
degrees cooler - even though the intake is then right next to the
exhaust manifold.  Air here may be drawn partly from under the car,
which would be cool but dusty.

- The stock intake is hot, but apparently not as hot as some say.  Miata
Magazine measured air-filter surface temperature at 160 degrees, while
I measured the flowing air to be approximately 130 degrees.

- Intake air gets considerably hotter during idle.  This may heat up
the air filter and associated tubing, which could heat the air passing
through.  Thus the intake air your engine receives may be hotter then
my measurements.

- There is a one (1) horsepower gain for every 11 degree reduction in
intake air temperature.  The Jackson Cold Air Induction Kit claims a 10
hp increase.  According to my measurements, on a 90 degree day, their
kit could only reduce intake air temperature by a maximum of 40
degrees.  That would account for about 4 of the 10 horsepower.  The
remaining improvement must come from Jackson's shorter & less
restricted intake tract, and perhaps from a forced-air effect at
speed.

- Based on this test, here's a recipe for what may be the least
expensive Miata performance improvement.  Advance timing to 14 deg BTDC
for a 2 hp gain.  Switch to a K&N air filter for another 2 hp.  Install
an intake tube off the front of the airbox - collecting air from just
to the left of the fan shroud.  Intake temperature will drop by about
25 degrees on a 90 degree day, giving another 2 hp gain.  The result is
a 6+ hp gain for under $60.