Turbo technology moves at a quick pace and one doesn’t have to look any further than the heads-up drag racing ranks for proof. In the early days a Turbonetics T-100 turbocharger was the baddest of bad, helping Pro 5.0 cars go 7.60s in the late 1990s. Just 20 years later and not only do QA1 True Street cars crush those performances; they do so with turbochargers that are far smaller in size.

It would be foolish to think that turbo technology is solely responsible for better performances. You can also thank exceptional strides in areas like chassis design, suspension components, engine efficiency, torque converters and other driveline improvements, and of course, engine and power management systems for those lower elapsed times. However, we think you get the point that when talking turbochargers the improved aerodynamics of compressor impellers, cover designs, and turbine sides — along with better understanding of turbochargers in street/strip applications — helped produce better products than yesteryear.

As the bigger is better crowd focuses on the compressor side for gains in power, the turbine side of the turbocharger offers significant horsepower increases, as well. Excessive backpressure in the turbine housing will choke an engine, reducing its horsepower-production capabilities and possibly leading to catastrophic engine failure in extreme cases. A turbocharger uses expanding exhaust gases to spin the turbine wheel, which in turn rotates the compressor. As the turbine spins faster with increased gas pressure from more efficient engines, the more pressure builds up in the housing and ultimately not allowing the engine to breathe. After all, what goes in must come out.

The DiSomma Racing Engines 348ci utilizes a Dart Iron Eagle engine block, a steel crank, steel rods, custom Diamond pistons (with 10:1 compression), a custom camshaft, and Total Engine Airflow-ported Trick Flow High-Port heads (250cc version). The engine has been revised over the last several years to be far more efficient and currently produces approximately 1,500 horsepower. B&G Customs built the turbo kit with 1.75-inch primary header tubes and a 3-inch crossover. The cold side uses 3.5-inch pipes that are connected using Race Parts Solutions dual-seal connectors. A giant Garrett air-to-air intercooler sits behind the bumper.

The optimum backpressure-to-boost ratio is 1:1; meaning for every 1 psi of backpressure in the turbine there is 1 psi of engine manifold pressure. The number you don’t want to see on a back-pressure datalog is 2:1 (2 psi of backpressure for every 1 psi of boost). The real-world reading for most drag cars falls between those two numbers, preferably on the lower side. The higher side is usually seen on vehicles in certain heads-up classes with turbocharger restrictions. Though not optimal, they merely live with the higher ratio since the category doesn’t allow them to correct it due to performance advantages over other combinations. There are some tricks that help, like using multiple wastegates to keep the backpressure reasonable and system design, but those are stories for another day.

Turbocharger manufacturers balance the backpressure through turbine-wheel size, wheel design, and housing dimensions. That’s why it is critical to work with companies with extensive experience in matching the right turbocharger to an engine combination. If the compressor/turbine relationship isn’t correct, the result will be poor performance due to slow spool times or a choked up exhaust side that restricts the capabilities of the compressor. In a world of bigger is better, going too large on either side of the turbo is a common problem, so work with the professionals on purchasing the correct turbo for your application.

We tested a Garrett GTX5533R Gen II turbocharger with a 94mm compressor and it worked flawlessly compared to the turbo that was mismatched for the engine combination. It made roughly the same power with less boost, less backpressure, and worse atmospheric conditions than the previous 94mm turbo. The lower manifold pressure has also helped reduce head gasket maintenance.

We took a step inside the garage of Mike Jovanis, eight-time Overall Winner in NMRA QA1 True Street to get a real-world look into the progress of compressor and turbine evolution in the turbo industry. The car has seen quite an evolution over the last 20 years, as the goal is to run quicker and more reliably. Each passing year Jovanis will upgrade one major system of car, bringing it from a 14-second ride to its current 7-second/180 mph status.

In the last five years he concentrated on working with Matt Wirt RaceFab Engineering on making the chassis safer and better with a SFI 25.5 upgrade, TRZ Motorsports 9-inch housing, coil-over shocks, and mini-tubs to fit a set of Mickey Thompson ET Street Pro 315/60-15 drag radials.

Another area that received a lot of attention was power management through the Haltech Elite 2500T ECU and working with Hughes Performance for the right torque converter that balances street driving and on-track performance. Jovanis’ 1989 Mustang LX instantly became more consistent on track as the 3,320-pound street car pushed into 1.20-second short times, regardless of track conditions.

Until this point Jovanis has run a variety of turbochargers on his Mustang, ranging from 76mm to 94mm, and he’s had great success. However, backpressure in the system increased each year due to more boost and an improved DiSomma Racing Engines 348ci powerplant. He sourced the issue back to the original turbo system, which utilized a PT88-based turbo with a T4 exhaust housing. Retaining that setup made swapping turbos easier, but the long-term downside with greater engine efficiency, power, and boost was high backpressure.

For 2019, Jovanis met with Tim Coltey of Garrett Motion and discussed the options for a change in the turbo department to reduce backpressure and gain horsepower. Those goals and the solution fit perfectly in proving our point that reducing backpressure along with an improved impeller design will yield positive gains. Coltey recommended a GTX5533R turbocharger, which is offered with a 94mm compressor — the same size as the turbo that was coming off Jovanis’ Mustang.

The change over required reconfiguration of the hot side to fit the larger exhaust housing mount. NMRA Open Comp racer Stan Bachonski handled the fabrication work, which included modifying the crossover and a new turbo mount to secure the larger unit. The passenger-side frame rail required a little massaging to clear the Garrett turbo. Bachonski also built a new dump pipe coming off the V-band housing and connected it to the four-inch exhaust under the car.

The Garrett GTX turbo replaced one that had a T4 exhaust, requiring a crossover modification for the V-band clamp, a new turbo mount, and a new dump pipe to connect to the exhaust.

A trip to a chassis dyno was made in order for Brian Friedentag to develop a baseline tune-up with the Haltech Elite 2500T. Then Jovanis ventured to All-Out Automotive and the night of tuning and bad stories concluded impressive output. While the first combination wasn’t on the chassis dyno, the data from the Haltech showed reduced backpressure. The real test would be on track during the Nitto Tire NMRA Spring Break Shootout as no other major changes were performed over the winter.

It would take one lap down the famed Bradenton Motorsports Park quarter-mile to reveal the first of many observations. The Mustang stood on the bumper, a rare occasion for the finely tuned machine. Jovanis short-shifted and made quick work on the pedal to bring the nose down. A quick study of the Haltech data log showed the turbocharged boost followed the boost curve that Friedentag built into the boost controller.

The Garrett GTX turbo was making boost a lot quicker than the previous 94mm unit. The launch boost was identical, something that both turbos had no problem achieving. Once Jovanis released ‘brake, the old turbo struggled to keep up with the boost controller’s commands. The Garrett GTX had no problem doing so, giving the driver a view of the sky.

Friedentag worked on the boost curve and timing controls to keep the front-end down and the car running quickly. By the end of the weekend, Jovanis managed a best time of 7.66 at 180 mph with the new turbocharger, which is slightly off the car’s previous best of 7.61. Adding to the impressiveness was the car’s personal best was accomplished in exceptional Fall air at Atco Raceway while the Garrett turbo testing was done in heat of Florida.

All-Out Automotive was kind enough to let us occupy its chassis dyno on a cold winter night. We didn’t do any back-to-back testing, but the chassis dyno served as a way for Jovanis and tuner Brian Friedentag to create a baseline tune-up before embarking on a trip to the Spring Break Shootout.

The Garrett GTX effectiveness is blatantly obvious in the data — the new turbo performed those times with just 26.5 psi of boost whereas the previous turbo was run at 33.5 psi. The backpressure was reduced by 20 psi, going from 55 psi with the old turbocharger and was just 35 psi with the Garrett GTX unit. An additional benefit has been head gasket longevity since there is nearly 8 psi less manifold pressure, solving a problem that has plagued Jovanis’s 348ci engine for the last year when he turns up the power to run quicker.

The improved impeller design and properly sized turbine side of the Garrett GTX5533R-94 helped the DiSomma Racing Engines 348ci small-block Ford make more power with less boost and reduced backpressure. It proves that turbochargers can be mismatched to an engine and turbo technology continues to march forward.

Here is a comparison of the turbochargers on the drag strip, the top is the data log from the Spring Break Shootout and the bottom is a test session at Atco Raceway at the end of the 2018 racing season. The Atco test session netted Jovanis a personal best of 7.61 while the best run from Spring Break Shootout was 7.68 at 180 mph. The boost pressure with the Garrett GTX turbo is 26.5 psi and the old turbo required 33 psi to run similar times. Moving to the backpressure, the Garrett produced only 35 psi while the former unit saw 55 psi with its smaller turbine setup.
Friedentag works with a Haltech Elite 2500T engine management system for air/fuel, timing, control dozens of inputs/outputs, boost control, torque management (i.e. traction control), and data acquisition.
VP Racing Fuels C16 was used for all dyno and track testing
The Garrett GTX turbo makes boost quickly on the DRE 348 as evident by the off-the-trailer pass in testing at Bradenton Motorsports Parks. As the previous turbo struggled to accelerate the impeller, the Garrett unit matched the commanded boost numbers of the controller, sending Jovanis into a wheelstand.

DiSomma Racing Engines


Garrett Advancing Motion




Mike Galimi
Mike Galimi
Mike Galimi is the Director of Content & Marketing at ProMedia Publishing and Events with nearly 20 years of experience in motorsport writing and photography.