Tom Wilson
November 1, 2009
Photos By: Edelbrock Corp.

On The Dyno
We did watch the XT engine, heads, cam, intake, and throttle body during prototype runs on one of Edelbrock's three in-house engine dynos. Initially there was the usual monkeying around with ignition timing and XT electronic mapping as the XT intake was tuned for maximum power.

The baseline the XT was running against was the single-plane Pro Flow II. The Pro Flow II had put up nice numbers with 486 hp and 432 lb-ft of torque maximums, while the XT just matched the horsepower at 485 hp at the 6,500 rpm peak, along with better torque at 439 lb ft at 4,700 rpm.

Frankly, no one was overly excited by the XT's power output compared to the Pro Flow II, so McCarthy took matters into the grinding booth. There he introduced the XT's fuel-injector bosses to the die grinder, hand-whittling them down to about half their generous as-cast size. This didn't surprise us as peering into the runners showed the seemingly overly large bosses were at the runner's base, right where the air is passing into the cylinder head's ports.

Hand-detailing the injector bosses was the right move, the power moving up to exactly 500 hp and the torque peaking at 445 lb-ft. That is a healthy 347 stroker in a sporty, rev-happy package. Looking at the bottom of the dyno sheets, we find the XT engine opens at 2,500 rpm with 335 lb-ft of torque, so it isn't hopelessly soggy down low like your dad's carbureted tunnel rams. The torque curve does make a notably steep jump between 4,000 and 4,500 rpm, however. Torque swells from 381 to 440 lb-ft in that range, with power ramping from 290 to 377 in the same span.

Edelbrock's XT testing reported in this article was done at a cooling water temperature of 172 degrees using a standard correction factor, plus a step (not a sweep) test and the air/fuel ratio was fully leaned at 12.7 to 13.0:1 at the power peak. These parameters will give a slightly higher, say 5 to 7 hp, reading than a sweep test, SAE correction factor, and slightly richer mixture.

Also interesting, Edelbrock uses a long tube to organize the airflow into the throttle body. They call it Old Faithful, as it seems to help power and repeatability a little. It did give a place to mount an air meter on an EFI engine, so we can accurately report air consumption was 655 cfm at 500 hp.

Part of the dyno work done during our visit was crate-engine calibration by Troy Hooker. He was operating the dyno engine with the XT Plus electronics with laptop interface, a system he really likes. He characterized the system as offering plenty of adjustability without drive-you-nuts detail, and pointed out that the system's ability to read on the fly and flash download to the XT box is a speedy help. He also gave us a quick XT Plus tour, demonstrating the tiltable maps and table-viewing options for fuel and spark, plus an encyclopedic data-logging capability.

It clearly offers comprehensive tuning and a variety of viewing options to make sense of what can be an arcane adjusting process.

RPMPowerTorque
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3,000215376
3,500250376
4,000290381
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5,000423445
5,500459438
6,000489428
6,500500404

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