74-19 AW
EVALUATION OF THE STP
MODULATING AIR BLEED
December 1973
Environmental Protection Agency
Emission Control Technology Division
Test and Evaluation Branch
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Background
The Test and Evaluation Branch of the Emission Control
Technology Division is responsible for the testing of
devices designed to reduce emissions from automobile
engines. The "modulating air bleed" manufactured by
the STP Corporation is one such device. EPA was supplied
with three systems for evaluation. The vehicles used in
the evaluation included one uncontrolled car, and two
cars with a moderate degree of emission control.
Device Description
The device is essentially an air-bleed used in conjunction
with a delayed spark advance. The air bleed is a modulating
type working off a ported vacuum source. The vacuum delay
valve is installed in the vacuum line between the distributor
and carburetor.
The system contains an adjustment to regulate the amount
of air bleed at idle. Under wide open throttle conditions
there is no bleed.
For cars that do not have a ported vacuum source, a valve
is installed in the vacuum advance hose and operated through
a mechanical linkage connected to the throttle linkage. The
air bleed and vacuum delay valve receive their vacuum
through this valve.
The device was tested on three vehicles, a 1963 Chevrolet,
a 1970 Plymouth Valiant, and a 1971 Ford. The 1963 Chevrolet
was equipped with a 283 CID engine and a 3-speed manual
gearbox. The 1970 Valiant was equipped with a 225 CID engine
and a 3-speed automatic transmission. The 1971 Ford was
equipped with a 351 CID engine and a 3-speed automatic trans-
mission.
Test Program
All three cars were adjusted to manufacturer's specifications
prior to baseline testing. Two tests were run in the baseline
configuration. The devices were then installed on the cars
and two tests were run on each vehicle.
All tests were run according to the 1975 Federal Test Procedure
(Federal Register, Vol. 37, No. 221, Part II, -November 15, 1972)
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Results
The 1963 Chevrolet was run in two configurations. One con-
figuration used nominal 20-second vacuum delay valve and
the other used a nominal 60-second vacuum delay valve. With
the 20-second vacuum delay valve there was a 38% decrease in
HC, a 42% decrease in CO, a 120% increase in NOx and a 4%
increase in fuel economy over baseline testing. With the
60-second vacuum delay valve, the HC and CO decreases remained
about the same while the NOx increased only 20% and fuel
economy increased 1% over baseline.
The 1970 Plymouth Valiant showed a 23% decrease in HC, a 66%
decrease in CO, a 9% decrease in NOx and a 6% increase in fuel
economy over baseline.
The 1971 Ford showed a 10% decrease in HC, a 46% decrease in
CO, a 35% decrease in NOx and a 1% decrease in fuel economy.
Conclusions
The effectiveness of the device varies from car to car.
Significant reductions in HC and CO concentrations were
measured for all three vehicles. Nox concentrations were
reduced from two of the vehicles but large increases
occurred from the 1963 Chevrolet.
Changes in fuel economy varied from -1% to +6%. The 1970
Valiant showed a 6% increase and in one configuration the
1963 Chevrolet showed a 4% increase. The 1971 Ford was
characterized by a 1% penalty in fuel economy.
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STP - 1963 Chevrolet
Mass emissions in
grams per mile
HC
CO
NOx
Fuel
Economy
Baseline
11.54
100.61
1.36
14.5 mpg
Device Test
(20-sec. VDV)
% Change
Device Te'st
(60-sec. VDV)
% Change
7.12
-38%
7.30
-37%
58.8
-42%
58.9
-41%
2.99
+ 120%
1.63
+ 20%
15.1
+ 4%
14.7
+ 1%
mpg
mpg
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Baseline
Pevice Test
(avg. of 2)
% Change
STP - 1970 Plymouth Valiant
Mass emissions in
grams per mile
HC
1.84
1.42
-23%
CO
30.0
10.1
-66%
NOx
4.83
4.41
-9%
Fuel
Economy
18.7
19.8
+6%
mpg
mpg
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STP - 1971 Ford LTD
Mass emissions in
grains per mile
Fuel
HC_ CO NOx Economy
Baseline 2.68 14.64 4.75 12.7 mpg
Device Test
(avg. of 2) 2.40 7.85 3.11 12.6 mpg
% Change -10% -46% -35% -1%
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