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Dyno Time
With the heads installed on our 88k mile '93 Cobra we were ready to rock n' roll. We headed up to Advanced Dyno Technologies in Suisun, Ca, our local chassis dyno operator. Shop owner 'H.H.' strapped the car down and installed the wide-band air-fuel sensor which would allow us to do some tuning. As the engine cooled from the drive up, we chatted a bit about what we expected the car to put down. I told him, cautiously, that if I made 270 peak horsepower and over 300 ft-lbs. I'd be content.

Figure 1. Dyno Results 1993 5.0L Cobra engine, AFR 165cc heads, ported Cobra lower intake. Red line is baseline pull, blue line is final pull.


Pull 1
The first pull was made in "as is" condition to obtain a baseline. The only difference between how the car would be run on the street and these dyno pulls was that we used an ice bag to allow the intake to cool rapidly between runs. The initial pull was with 42lbs. fuel pressure and timing at 13 degrees initial advance.

The results immediately settled my nerves - 293 horsepower and 328 ft-lbs. of torque. Perhaps even more exciting was the air-fuel (A/F) ratio at a

Adjusting fuel pressure was a big factor in getting over 300 HP.

fat 11.5:1. It was clear that the motor still had considerable power to give once the A/F was leaned out.

Pull 2
Using the adjustable fuel-pressure regulator we backed off pressure by 2 to 40lbs. After allowing the engine to cool for 15 minutes we made another pull. (Each pull is being made in 4th gear 1:1 gearing ratio from approximately 2500rpm to 6000rpm.)

As anticipated the car picked up power across the entire curve due to the slightly leaner A/F ratio. In fact the 1lb drop in fuel pressure barely registered on the chart, showing A/F still in the mid 11:1 range.

Pull 3
For the finale in our three-pull session we continued to work on getting the air fuel curve closer to the stoichiometric ideal of 14.7 parts air to 1 part fuel. While in chemistry this is the perfect burn, in a real engine it is a little to lean and can result in detonation if factors such as colder air or lack of fuel were to occur. Most naturally aspirated gasoline engines run best at around an A/F of 13.1.

We pulled out another 1lb of fuel pressure for the last run. We would like to have pulled out slightly more, however we were concerned by the dangerously lean A/F ratio of over 18:1 (the large peak in the A/F curve) before 3400 rpm. This actually was an indicator that the vehicles O2 sensors needed replacement. The O2 sensors are only used by the engines computer during part throttle, at wide-open throttle the engine is strictly using the mass air meter and look up tables within the computer memory. So while the bad O2 sensors were not hurting our dyno numbers, they were causing detonation during the transition from part to wide-open throttle. Any detonation is something to be worried about, so we played it safe with just a 1lb drop in fuel pressure for the final run.

This time the chart responded with a A/F ratio slightly over 12:1. Still a little rich, but power improved remarkably, to a best of 303 horsepower and a stunning 341 ft-lbs of torque!

Conclusion
There was a time when making 1 horsepower to 1 cubic inch was a big deal - and that was at the flywheel! Today achieving those numbers at the rear wheels is a testament to the advances in engineering and technology of the aftermarket. What is even more impressive is that this stout little 5.0L motor is in a fully-comfortable, no hassle to drive, daily commuter Mustang. The idle and driveablity are like-stock, and the power band is all in a very usable, stoplight to stoplight rpm range.Look for an upcoming article on track results from our new project car.

 
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Dyno Pull #3 (Best) Details
1993 Cobra + AFR Heads
Ambient Temperature: 83.4 F
Barometric Pressure: 29.91 Hg
Correction Factor: 0.99 SAE

RPM HP Torq. A/F
2800 149.5 280.5 18.0
3000 169.3 296.5 18.0
3200 188.9 310.0 18.0
3400 206.9 319.6 13.6
3600 226.9 331.1 12.1
3800 245.2 338.9 11.9
4000 259.0 340.1 11.7
4200 272.9 341.3 11.7
4400 283.3 338.2 11.9
4600 292.2 333.6 12.0
4800 297.7 325.7 12.2
5000 300.2 315.3 12.2
5200 301.8 304.9 12.1
5300 303.1 298.9 12.1
5400 301.9 293.7 12.1
 
Sources
Air Flow Research Heads
10490 Illex Avenue
Pacoima, CA 91331-3137
818-890-0616
Advanced Dyno Technologies
96 Railroad Ave.
Suisun City, CA
707-425-DYNO

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