Ask Away! with Jeff Smith: Explaining Violent Pop Sounds in Turbo Drag Cars

I was at the local drag races last weekend and we watched an LS-powered Nova with twin turbochargers run. It was very quick. My friends and I began talking about the car and somebody asked why the engine seemed to backfire badly on the starting line.

It sounded like the engine was going to blow up right on the line.

But when the green light came on, the car took off like a rocket. We’ve seen this before with some cars but not with a turbo car. Not all turbo cars seem to do this. Can you tell me what’s happening here? — J.F.

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Jeff Smith: As you mentioned, this has become more prevalent on the starting line with turbocharged cars now. The reason for the exhaust backfiring is essentially the same reason why the normally aspirated engines perform a similar dance.

Turbochargers take time to spin or spool up to their normal speed to create boost. These turbo shaft speeds are incredibly fast — anywhere from 60,000 to 250,000 rpm and higher. Newer turbos with ball bearings and the latest technology in impeller wheels spool up much quicker than older models but all designs require some time to build boost.

A common procedure is to stage the car, bring it up against the converter, and then engage an electronic boost controller.

This is digitally managed by the EFI system and closes the waste gate to create boost. When the proper boost is achieved the gate begins to open to manage the boost.

Boost is created by spinning the exhaust turbine blades with a combination of exhaust temperature and exhaust gas pressure. With larger turbos, this requires more energy than the engine can supply at the rpm limited at the starting line.

Often, a second control device is necessary to generate more heat and pressure. This is accomplished by adding fuel, retarding the timing, and sometimes electronically interrupting the spark to certain cylinders.

These last efforts of adding fuel and dropping spark will place unburned fuel in the exhaust which will ignite in the pipes causing the violent popping sounds. It does sound like the engine is about to come apart, but if managed properly these are now just part of the starting line ritual for turbocharged drag race engines.