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Mustang Dyno - Dyno Tuning

Know What's Up Before You Go

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Carb And Air/Fuel Ratio
We were fortunate that Greg hadn't milled the air horn off the carb, because the reference numbers are printed there. Using the numbers, we checked the Holley catalog to find the baseline jets and discharge-nozzle sizes. Dual-metering-block Holley carbs are sent from the factory with "square" jets, meaning the front are smaller, proportional to the power valve, compared to the rear jets. On this example, the front jets were the stock 68s and the rears were 81s. The power valve is worth 13 jet sizes when it opens below 6.5 inches of engine vacuum, making both the front and the rear flow the same at WOT. Tom will occasionally find jets that are unequal and crazy oversized discharge nozzles, so he will set them back to stock before going any further.

On the baseline, the air/fuel ratio was about 10.1:1 and it even dipped into the 9.8:1s for a few thousand rpm. Too rich. Tom kept the jets square by dropped them two sizes front and rear.

Let's face it, with 4.56:1 gears and a 3,400 stall, this Mustang is more strip than street, so the timing curve can come in hard and fast. There is not going to be any detonation down low, because there is less of a load below the stall speed so our timing curve can have all the timing in by 3,500. In the MSD distributor, we used the largest-diameter bushing (black in the MSD kit) to limit the total centrifugal advance to 18 degrees and used the lightest springs in the kit (two silver). The rule of thumb we were using is this: You'll want 34-38 degrees of total timing without the vacuum advance. With a higher stall speed, more cam, aluminum heads, and a lighter car, you can dial in the timing at a quicker rate than a car with the opposite characteristics and still not detonate. Bigger cams will allow more timing because they tend to bleed off cylinder pressure and need more initial to smooth out the idle. We ended up with 20 degrees of initial for a total of 38 to make best power.

After making the timing and air/fuel adjustments, Tom ran it up to 6,600 and recorded a best of 337 hp at 6,400 and 319 lb-ft at 4,800. The air/fuel ratio was at a healthier 11.5:1 average, and we had already found that a leaner jet didn't make a difference. Tom took the engine up a little higher in the rpm range to see if there was any more power there, but it became evident that the engine had quit making power at around 6,400 rpm.

Greg admitted to shifting the C6 at "about 6,800" at the track. Even if the transmission shifted instantly, that's about 400 rpm to high. If you've ever felt an engine lay down, you know that shifting right at peak rpm will often improve the results. This piece of information alone made the trip worthwhile.

Ready For The Track
Aside from correcting glaring problems like WOT, air cleaners, and slipping transmissions and such, the primary goal of the chassis dyno is to get the engine making as much power as possible while finding where the peaks and corresponding shift points should be. Greg only had 90 percent of his throttle, was overshifting the engine by about 400-500 rpm, and was a full point rich in the air/fuel ratio. Since his primary goal was about 0.30 at the track, it's easy to see how these problems could compound to cost him at least that much. We shall see. What's important is to have these things out of the way before you tackle the issues between the car and the track that also conspire to slow you down. But that is another tale.

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