When Aaron Otstott began researching a new car, he wanted to know something many buyers don’t care about: How easily could he make it faster?
“I bought it from the beginning knowing that I would be doing modifications,” he said.
He settled on a 220-horsepower Volkswagen GTI, a vehicle beloved for its practicality and strong performance. And then he set to work joining the proud fraternity of enthusiasts who possess the kind of hubris that can make automotive engineers like Richard Parry-Jones shake their heads.
“I have no doubt in my mind they cannot do it better,” said Mr. Parry-Jones, who from 1998 until his retirement at the end of 2007 was in charge of Ford’s worldwide vehicle development, including models from Lincoln, Jaguar, Volvo, Land Rover and Aston Martin.
An owner might make a vehicle go around corners faster than when it came from the factory, he said, but it will come with a cost in comfort or even safety.
“They can never achieve the finely balanced trade-off we have achieved,” Mr. Parry-Jones said.
Even so, the desire to make modifications demonstrates an owner’s passion, he said. And that’s not a bad thing — as long as the owners know what they’re doing.
“If you are a total ignoramus and you are just not really doing it carefully and getting advice and doing research and so on, you shouldn’t touch anything but appearance items,” Mr. Parry-Jones said. “However, many enthusiasts are not stupid at all. They are quite knowledgeable.”
Mr. Otstott, for sure, knew what he wanted. He bought the GTI knowing he’d be trying to wring every ounce of performance out of every penny he spent.
“I had done the research and knew that for less than $1,000 I could get it above 300 horsepower,” he said.
Indeed, after buying the car in March 2016, he purchased an electronic upgrade that he estimated had raised the GTI’s output to 300 horsepower. He has made other changes, both cosmetic and mechanical. Next up: altering the suspension to improve handling.
Mr. Otstott, 44, a video game programmer and father of two from Austin, Tex., is far from alone in thinking he can improve on vehicles that automotive engineers spent years testing and creating.
Car, truck and sport-utility owners spent $41 billion in 2016 on automotive accessories, ranging from stereos and wheels to engine and suspension modifications. That was up from about $37 billion in 2014, according to a report last year by the Specialty Equipment Market Association, the trade association for aftermarket equipment makers.
About $400 million in 2016 went for “engine control and computer products,” a category that includes the device purchased by Mr. Otstott, which tricks an engine into producing more power. Some $600 million was spent on superchargers or turbochargers. And about $2 billion was spent on suspension changes — alterations that range from making a vehicle lean less when cornering to increasing the ground clearance of a pickup.
Drastic changes, however, do raise the possibility of making a vehicle less safe, said David McLellan, who was the Corvette chief engineer from 1975 to 1992.
“You see this guy with his pickup truck another foot in the air,” he said. “His rollover stability is greatly diminished. Is he smart? Is he better than the original equipment engineers? I would have to say absolutely not.”
When owners begin making changes, the results can go either way, said Thomas Gillespie, author of the textbook “Fundamentals of Vehicle Dynamics” and a co-founder of Mechanical Simulation, a Michigan company that develops software to simulate vehicle performance.
“You can pick parts that would make the vehicle behave better in the more extreme handling situations,” he said. “On the other hand, you could also screw it up.”
It’s also possible to run afoul of federal safety regulations.
Owners are permitted to modify their vehicles, a spokesman for the National Highway Traffic Safety Administration said in an email. But aftermarket equipment makers may not do anything to “make inoperative” federally required safety equipment.
For example, starting with the 2012 model year, Federal Motor Vehicle Safety Standard No. 126 requires vehicles to have electronic stability control. Such systems are designed to detect and end a slide, reducing the chance a vehicle will “trip” and roll over — a particularly dangerous form of crash.
All members of the Specialty Equipment Market Association are encouraged to make sure suspension changes resulting from their products are not so drastic that they can overcome the electronic stability control, said Christopher Kersting, the president of the association.
Some of that testing is done by Link Engineering of Plymouth, Mich. “We’ve tested products on well over 100 S.E.M.A. members over the last two or three years,” said Terry Ledwidge, Link’s director of business development.
Testing costs about $15,000, and Link has found that the electronic stability control systems on most vehicles are flexible enough that they work despite suspension changes, Mr. Ledwidge said.
“There is a lot of robustness built into the system on the production vehicle,” he said.
But one of the changes that can cause problems involves significantly increasing the ride height of a vehicle, such as a pickup, because it raises the center of gravity — a change that can impair the vehicle’s handling.
“If you get too outrageous, you are going to run into problems there,” Mr. Ledwidge said, although he added that test failures were rare.
Mr. Kersting said that his association did not require its members to have such tests, but that they were aware that they had to be careful because there was “a stout product liability tradition in this country.”
“They don’t want to be sued out of existence,” he added.
There are no federal safety regulations regarding increasing an engine’s power, although doing so is almost certain to void the automaker’s powertrain warranty. (Some enthusiasts theorize that if they have a major mechanical failure they can quietly remove the guilty gizmos and perhaps avoid being caught.)
Increasing an engine’s horsepower is much harder than in the old days, when engines had carburetors instead of sophisticated computerized systems, said Jason Siegel, an assistant research scientist at the University of Michigan.
Getting more power now involves modifying the software, which is “neither easy nor inexpensive, and maybe not even possible without some inside/leaked information,” he wrote in an email.
All challenges aside, Mr. Kersting expects continued growth in the aftermarket industry.
“The reason the market grows the way it does is that the carmakers have a mass-production model and it does not leave a whole lot of room for people who want to improve or personalize or upgrade their cars and trucks,” he said.
Social media — such as websites or Facebook pages for owners of specific models — is also playing an increasing role, Mr. Kersting said.
“Now people who have similar interests are able to find one another and share expertise and the passion they have for whatever segment of automotive lifestyle or hobby they enjoy,” he said. “It has been very good for the market.”
Once upon a time, owners making changes irked Erich Heuschele, the manager of Fiat-Chrysler’s SRT Vehicle Dynamics, which is responsible for the automaker’s high-performance models, like the Dodge Challenger SRT Demon. But he has learned not to get emotionally attached to the vehicles he designs.
“Cars go to a diverse audience and diverse customers, and people don’t always agree with the balance or compromises, and you can’t get upset with that,” he said. “I don’t get miffed. I kind of get a chuckle.”
