August 2011 Chevy High Performance Q&A

Performance From Around The World

Kevin McClelland Jun 30, 2011 0 Comment(s)
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How much power do you think this combination should make? The shop, using a PowerDyne Mustang dynamometer, came up with these numbers: 381.2 hp at 5,265 rpm/109.9 mph in Fourth gear or 400 with WCF (whatever this means), and 398.6 torque at 4,746 rpm/98.8 mph and 418.2 with WCF at the rear wheels.

Based on this info, do you think it is possible to make this much horsepower? The shop owner is the nicest guy in the world and very honest and reasonable, but I find this a little hard to believe. He said these LS motors always make more power than they are rated. What gears would you recommend? I am near retirement and plan on touring with the ’55, and therefore want the best mileage I can squeeze out of her.

Love your magazine and look forward to it each month. I would greatly appreciate your thoughts.

David Schrumpf

St. Louis, MO

A: The topic of engine power derived from chassis dynos is a subject that is constantly being challenged. Our sister publication Hot Rod had a great article in their Mar. ’11 issue. They took at “Ford Product” to five different reputable chassis dyno shops around the Southern California area in one day. They tested the car on a Mustang, DynoJet, Superflow chassis dynos, and Dynapack axle-mounted brake system, and the procedures varied from shop to shop. The recorded output varied around 100 hp among the five different shops. Testing procedures, equipment used (dyno brand, cooling fans), and exhaust ventilation varies at each facility, all reasons for wildly varying power numbers from the same car on the same day. As you shouldn’t race dynos and flow benches, use them both as tuning tools unto themselves. If you stick to this you will be very happy with the outcome.

The correction factor is used by a dyno to correct the power numbers for atmospheric weather conditions. We believe the “WCF” the dyno operator is referring to is Weather Correction Factor. All dynos will give you “observed” power and torque numbers from a dyno run. The correction factor is to adjust the data to standard weather conditions. The Society of Automotive Engineers (SAE) has adopted several correction factors. The most widely used correction factor used in the performance aftermarket is the SAE Standard, which adjusts all power numbers based on these atmospheric conditions: 29.92 in-hg, 60 degrees F, and dry (zero humidity) air. The next would be what is called the SAE J1349 correction factor, which all of the OEM auto manufacturers have used since 1971. This correction factor uses 29.234 in-hg, 77 degrees F, and 0 percent humidity. Using the SAE J1349 correction factor alone will adjust the power number around 5 percent lower than if you used the SAE Standard correction factor. This is another reason not to race dynos unless you are sure how the data was corrected from shop to shop.

As for the power you saw on your dyno adventure, we’d speculate that they are slightly higher than we would expect from your listed modifications. The LS engines are very powerful and respond well to minor bolt-ons. With open headers and the components you listed, we would expect to see around 425-430 hp from your engine. If you assume that you will lose around 15 percent from parasitic and driveline losses, that’s around 365 hp at the tires. Since you saw 381 hp on your dyno test we’d be pretty happy. Again, from the listed dyno tests that Hot Rod did you could see the larger variances that you have.

The 3.50 rear gear should work very well with your T56 six-speed transmission. With the overdrive in Sixth and your tires, the cruising rpm should be around 2,200-2,300 at 75 mph. This will put your engine right in its sweet spot to knock down some decent mileage. You can expect low 20 mpg with your brick of a shoebox. Don’t worry about the power numbers, and finish your toy. CHP

Source: hotrod.com

Technical questions for Kevin McClelland can be sent to him at chevyhi@sorc.com.

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