Spent the last 4 days at Sebring Road races as a spectator. I missed last year so this observation may be dated. Seemed like a lot but not all of the turbo cars, under braking had a rather different sound from years past. I'd describe it as a fast staccato, and not downshifting related, almost like cross between a rev limiter and a SemiJake exhaust brake. No way were engines at high revs. My suspicion it was some trick to keep turbos spooled for corner exit, However if those were power pulses, it would seem in a 12hr endurance races, that bwould eventually reek havoc on the powertrain, as I doubt cars were coasting in neutral under braking. There was the occasion a back fire under full throttye during corner exit, that is a completely different sound then I am mentioning.

So what was going on?

I am assuming this is an efi related question, as everything is high dollar BMW, Porsche, Aston Martin, Caddy, etc factory supported, without a carb on the track.

I haven't ever seen one of those tunes so I'm not sure what they are doing but EFI systems do allow you to play with the fuel during decel. There is a menu in the Holley system for Decel Fuel Cutoff but I've never messed with it. I would imagine that you can do all sorts of things such as cut fuel off completely or continue to add small amounts. I'm sure those guys play with all the knobs to keep the cars up front.

Anti-lag is common on turbocharged/EFI cars in many forms of racing. It's a combination of wastegate control, retarded timing and very late injection that will keep the turbo spooled up. You're essentially moving combustion to the exhaust manifold and turbine housing. Without fueling, if the turbos have variable geometry you can accomplish almost the same thing, but I'm not sure VGT's have appeared on gasoline powered race cars yet (almost all diesels now are VGT).

Durability isn't a problem but I wouldn't expect the IMSA guys to be using it as aggressively as something like a WRC rally car. IMSA is much more dependent on fuel strategy as a whole.

Thinking your all 3 touching on the potential answer. Jake Brake with variable turbo and wastegate and timing and decel enleanment strategies. I could see a jake type system to help braking. Variable turbos and wastegates that help keep it spooled up. The different sound may be from all 3 types of strategies. I could see the decel fueling actually being on a richer side. So not to lose your port wall fuel film. When they get back into throttle because a higher boost. The occasional backfire was probably a slight lean condition. At that instant of rpm kpa timing and boost. Because of lack off fuel film from off throttle. Like Andy would love to see some of the technology and strategies they use for those cars

You are correct. Keeping the turbo spooled on decel is accomplished by using the low map signal/ tps/ timing to keep fuel burning in the exhaust. Some go as far as to allow air into the header in an electronic controlled manner to get it hotter.