Text and Photography by Chirag
Asaravala
The moment of truth has arrived. We have initial dyno
results on our blown and stroked 1967 Mustang. Before
we delve into the numbers here
is a recap of the events which brought us to this
point. Earlier this year we picked up a used Vortech S-Trim
5.0L Mustang supercharger kit. We spent a couple of months,
working at a snails pace, trying to figure out how to
adapt the kit to our carbed 331 motor
and '67 Mustang engine compartment (see Part
I: Mock Up.) Once all the mounting issues were sorted
out we bolted the blower on for good and plumbed all the
ducting. Part II cumulated in the firing of the motor
with blower attached. Days after you read and heard the
symphony of blower whine and lumpy idle (see Part
II: The Pressure Mounts) we took the '67 out for its'
maiden voyage under boost. As expected we were faced with
detonation. We knew this would be an issue to contend
with due to the 10.5:1 compression ratio of the 331. We
tackled these issues, as you'll see later in this article,
but before that we took the blower belt off and drove
the '67 up to the dyno for some naturally aspirated pulls.
Naturally-Aspirated
Dyno Results
The 331 stroker engine which was built nearly two years
ago (see Build
a 302 Stroker) has been a stout performer in naturally
aspirated form. The '67 Mustang has gone as fast as 11.6
@ 119 mph with only track side tuning. Needless
to say we were curious to learn what sort of power it
was laying down. We drove the car up to our local Dynojet
facility, Advanced Dyno in Suisun, CA, and arranged for
a three pull baseline session. We made an initial pull
with the engine in the same tune it was when it ran its'
best ET. It put down 356 horsepower at 6100 rpm, but the
graph showed some important shortcomings (see blue curves
below.) The air-fuel ratio (AF) was way rich at wide open
throttle. On a naturally-aspirated engine we like to see
AF between 12.5:1 and 12.75:1. On our initial pull we
were at 10.0:1 from 5000 rpm onward. That's not only detrimental
to fuel economy but to power as well.
Dyno Results
(RWHP): Naturally Aspirated, 331cid
| |
Peak Horsepower |
Peak Torque
(lb.ft) |
Initial |
356.9 @ 6100rpm |
343.4 @ 4700rpm |
Best |
372.2 @ 6100rpm |
346.2 @ 4900rpm |
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On the subsequent two runs we started to jet the
750 Holley downward. We had started with 74's in the primary
block and 79's in the secondary. For run two we brought
the secondaries down to 75. The engine actually responded
with a loss in horsepower, from 356 down to 350 at peak.
Air-fuel ratio came up slightly, but still terribly rich
at 11:1. This is an appropriate place to make an important
point about dyno tuning, especially when dealing with
air-fuel ratios and jetting. Carbureted engines will often
times appear to respond unfavorably to a jet change. We
were running rich so one would think a drop of four jet
sizes would shown a power gain. In fact we lost power.
This could easily have been interpreted as a sign that
we were headed in the wrong direction. Fortunately experienced
dyno operators H.H and Mike at Advanced Dyno have seen
this all too often and know that the basic principles
of engine tuning do not change from motor to motor. You
have to press on. The AF ratio was still rich and engines
simply do not make their best power with that much fuel.
We took another 4 jet sizes out of both the primary
and secondary sides. With the carb now jetted at 70/70
we made our last pull. Low and behold the engine responded
as expected and power went up across the rpm range, with
peak now at 372 horsepower (orange curves in the above
chart.) Air-fuel ratio could still be improved upon still
hovering in the low 12's.
While we were thrilled to find new power in our naturally-aspirated
motor we knew our ultimate goal was to get this motor
dialed in for the blower. We were also detecting signs
of valve float, evident by the sharp and choppy power
drop right after peak horsepower. While not visible in
our graph above the original power curve falls hard after
6100 rpm. We think this cam should be pulling a few hundred
more rpms. We suspect the valve springs require replacing.
Something we'll address in the near future.
For now we're packing the car up and taking it back to
the shop. We'll bolt up the blower ducting, connect the
belt, and prepare for some boosted dyno runs. (Continue)
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(Dyno Results - Supercharged) |
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Supercharged 331 stroker packed
in the '67 Mustang bay. It took work and creativity
but the power gains were worth it. |
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Behind
the Boost Gauge
To monitor boost we selected the Autometer
Pro-Comp 2 5/8" boost-vacuum gauge (PN: 4477).
We also stepped up to matching mechanical oil
pressure and water temp gauges.
The boost gauge
uses a MAP (manifold absolute pressure) sensor
to convert vacuum/boost pressure into electronic
signal. It features peak recall and a max-boost
warning LED. The sensor
requires finding a 12V key-on source that does
not cut out upon cranking.
We located the MAP sensor
under the dash and behind the firewall in our
'67 Mustang. The vacuum tube connects to manifold
vacuum (we used the full-time vacuum port on our
Holley carb.) |
Source:
Autometer
413 West Elm Street
Sycamore, IL 60178
Tel. (815) 899-0800
www.autometer.com
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