Shhh! Quieter GM H.D. pickups get 38-hp boost

James R. Healey | USA TODAY

How about 38 extra horsepower for free? That's what General Motors is getting from the gasoline V-8 engine in its new-generation, 2015 heavy-duty pickups.

GM says it has boosted the published rating of the 6-liter gasoline V-8 in its Chevrolet Silverado 3500 and GMC Sierra 3500 -- the so-called one-tons -- heavy-duty pickups to 360 horsepower. That's up from the 322 hp it announced when it unveiled the trucks in Texas Sept. 26.

The same engine in the Silverado and Sierra 2500 models -- aka three-quarter ton -- is rated 360 hp.

Researching why the two engines had different ratings in substantially similar trucks, in response to a an inquiry from USA TODAY, GM found that the trucks could pass government noise regulations while the engines were running faster, allowing it to use the higher rating for the 3500 models, according to GM powertrain spokesman Tom Read.

The heaviest-duty pickups -- the 3500 series -- fall into a category that has required them to meet the same "pass by" noise regulations that apply to even heavier-duty trucks, the cab-chassis rigs that are the basis for ambulances, tow trucks and other commercial models.

To keep the noise down, an automaker sometimes must keep the engine running slower during the pass-by testing. It only can advertise the horsepower the engine makes at that reduced speed, even though the engine might make much more power when revved faster.

The noise rules haven't applied to the 2500 series vehicles because they are in a different category. Now, Read says, the 3500s are quiet enough running under hard throttle to pass the noise restriction.

"No longer do we need to 'de-rate' the full-size pickups, as we have done for over 20 years, because new technology and design make them quiet enough at full power to pass the noise test," Read explained.

As a result, "we are revising our information for the 2015 model year" 3500-series pickups, he said.

The heavy-duty versions of GM's new-generation full-size pickups go on sale first quarter of next year as 2015 models. The standard-duty models -- designated 1500 and sometimes called half-ton models -- have been on sale since the summer as 2014 models.

The 2500 and 3500 account for about 25% of all Chevrolet Silverado and GMC Sierra sales. Within that, the 2500 models are 75% to 80% of the mix. The 3500 models are the balance.

Usually powered by diesel engines, heavy-duty pickups have a broad array of buyers. They range from those who need simple work trucks with more capability than standard-duty pickups, to people who tow horse or race-car trailers all over the U.S. They want something powerful enough to shrug off nine or 10 tons hitched behind, even on hills, and they buy leather-lined, tech-infused interiors to keep them comfortable while doing it .

Boosting ratings due to a regulations issues is rare, perhaps unique. On the other hand, automakers over the past few decades have had to cut power ratings as new rounds of testing were conducted to verify compliance with tightening government fuel-economy and emissions regulations.

And several times lately automakers have had to cut their advertised fuel-economy ratings after the government discovered they were too high.