In the car hobby, technology consistently moves forward in terms of horsepower and performance capabilities, especially on street-legal vehicles. We live in a world where stock vehicles go 9s and 10s, and if you hit up a drag-and-drive event, you’ll see four-figure output and machines that can lay down five-second passes in the quarter mile—all while maintaining real street capabilities. Sizing a turbo system correctly with the right parts is one of the ways racers can achieve this level of performance.

It’s amazing that the term “big power” in a street/strip machine used to mean a stroked big-block with nitrous, probably on race gas, making 800-900 horsepower and running in the eight-second zone. And let’s face it, this came with a trade-off of poor fuel economy, so you were limited to local drives.

Thankfully, the affordability of boost and tunable EFI changed that forever. Sure, there’s still an art to engine building and creating a setup that delivers loads of power and maximum smiles for miles, but big power today is as easy as adding a power adder to your existing platform.

The Turbo Size Game

Today, the “art” part of the equation lies in selecting a power adder that’s sized for your application and budget. Of course, you’ll need a proper driveline, suspension, and an adequate cooling system for street applications, but that’s a story for another day.

When Project Swedish Meatball needed some boost, we turned to the team at Summit Racing to see what they had to offer. The team at Summit listened to our goals, asked what parts we were running, and then made a recommendation based on that information.

“Summit Racing Performance turbochargers have the latest in compressor aero technology with quick-spooling, yet free-flowing turbines,” said Mike Schmidt, Private Brands New Product Supervisor at Summit Racing Equipment. “Starting with 66mm inducer sizes and working up to 79mm, the SUM-2600 Series turbos are reliable and a great value. The SUM-2700 Series are true race turbos starting at 76mm and go up from there. T4, T6, and V-band turbine housings are available, too. These are specially designed, balanced, and precision assembled in North America for class-leading racers. They feature 2618 billet forged compressor wheels that offer superior flow and dependability at high boost.”

Now, you need to understand that the size of the turbo isn’t always going to dictate how it performs with a combination. You need to pick the right turbo for your application.

When selecting a turbocharger, size selection is critical to overall performance. The “bigger is better” mindset often leads to problems. An oversized turbo may produce impressive top-end horsepower but usually introduces lag, compromising throttle response and acceleration. Conversely, a turbo that’s too small limits maximum power potential.

The ideal turbocharger balances responsiveness with peak output and should be matched to the specific racing style or application. Noelle stresses that success in motorsports comes from a complete, well-engineered setup—not just the pursuit of the largest horsepower numbers. For best results, racers are advised to consult both their engine builder and a technical representative before making a final turbocharger choice.

Part of dialing in the best parts for YOUR combination will lie in defining how the vehicle will be used. There’s a distinction between cars that are predominately street driven, versus a full-on drag strip car. The difference is that a streetcar, no matter the max power level, will tend to use a broader RPM range. The driver may want to go from low-RPM cruise to max power in an instant, or they may want tip-in power for passing or a quick blast from 30-80 mph. In contrast, most drag cars will only be at idle or wide-open throttle, save for the burnout.

“We’ve got a selection of different-sized turbos to meet the wide range of applications. Some are more efficient than others in certain areas. This way, we can meet the demands of the customers based on price point and performance. Some turbos feature journal bearing like the economy VS, and then there’s the ball-bearing versions,” Schmidt says.

Wastegates and Blow-Off Valves

A perfectly operating turbo kit system will require a wastegate and blow-off valve. The wastegate is mounted on the exhaust side (or sides) and has the ability to bleed down exhaust pressure, thus limiting flow to the hot side of the turbo to control boost. The blow-off valve (BOV) is found on the inlet, or “cold” side, and bleeds down inlet pressure when the throttle is shut. In some cases, the turbo can still produce boost when you lift off the throttle. The blow-off valve prevents damage to the turbo from the buildup or backflow of compressed air in the inlet tract.

We asked Schmidt, what do you need to consider when selecting wastegates and BOVs? Does how you’re going to use the car matter? Is a street setup different than one for all-out drag racing?

“There are ways to properly size a wastegate for a turbo kit. The key is to match the wastegate’s size to the turbo’s size, your type of driving (street vs. race), and the desired boost levels you are looking to achieve,” explained Schmidt. “A smaller wastegate can lead to ‘over boosting’ because it cannot adequately control exhaust flow and pressure, while a large, oversized wastegate, will result in sluggish boost gains.

“I think a 44 will handle most anything up to 600, 700, or 1,000 horsepower. Two 44s will handle over 1,000. We did our 1,200 horsepower LS motor with a pair of 44s, and the wastegates were fine. At some point, you reach a point where the wastegate doesn’t flow enough. When it’s wide open, it’s still making too much boost, and you’ll need larger units to control boost. But that happens mainly on more of your race applications than your street stuff.”

Wastegate Sizing Tips:

• Turbocharger Size: Larger turbos commonly require larger wastegates, especially at lower boost levels, to effectively manage exhaust gas flow. Smaller turbos functioning at higher boost levels may be better suited to smaller wastegates.
• Desired Boost Levels: If your goal is lower boost, a larger wastegate is often preferred. This allows for a wider tuning window and more efficient venting of exhaust gas. For higher boost requirements, a smaller wastegate is typically more efficient because it keeps more pressure in the exhaust system to spin (spool) the turbo effectively.
• Wastegate Size: Wastegates are sized by the internal valve size, with 44mm, 50mm and 60mm being common examples. A 44mm wastegate, for instance, has a valve that measures 44mm.
• Applications: A 44mm wastegate with a softer spring is often recommended for street cars, as it can efficiently vent the larger volume of exhaust gas needed for lower boost. A 44mm wastegate with a stiffer spring might be more efficient for race cars, as it keeps more exhaust pressure in the exhaust system to spin the turbo. In summary, you really can’t have a blow-off valve that is too large if it fits in the desired location. As it relates to wastegates, proper sizing is far more critical.

Blow-Off Valves

“There’s no strict or ‘proper’ sizing method for a BOV. However, there are a few factors to consider when choosing a valve for your turbo application. Commonly, the key in selecting diameter (usually measured in mm) is to stay within the range of 38-50mm for most applications. Simply put, there is no such thing as a BOV that is too big. Factors like boost level, as it relates to airflow and available space in the engine bay, is what influences the choice.

Blow-Off Valve Sizing Tips:

• Boost Level: Higher boost levels translate into higher airflow when the throttle body shuts and the BOV opens. The purpose of the BOV is to eliminate compressor surge on deceleration.
• Horsepower: Said differently, higher horsepower targets usually require larger blow-off valves to accommodate the increased airflow on deceleration.
• Engine Bay Space: Larger valves historically require more space for installation. The SUM-260070 50mm BOV is compact, yet it is capable of eliminating compressor surge on 1,000hp engines on deceleration.
• Applications: For drag racing or high-horsepower street cars, a larger valve like the 50mm is preferred. In short, the ideal BOV valve size depends on the specific needs of your turbo application. A valve that’s too small may not adequately vent the excess pressure, while a valve that is too large might create installation issues.

Most street enthusiasts don’t want to overspend, and they’re not likely to buy the parts two or three times like a drag racer will continue to refine a combination.

“And mind you, this is the basic information for getting started. Every combination can be refined, and typically the all-out drag racing stuff is custom fabricated, and the parts are chosen based on very specific parameters that have been previously defined,” Schmidt explains.

Kevin Schweizer’s Godzilla-powered Mercury Zephyr wagon is a regular on the drag and drive circuit. It packs the punch of twin 72mm turbos and is seven-second capable in the quarter mile. This shows you how well a turbo system will work when all of the parts are properly sized for the application.

Tying It All Together

To further assist in your quest for boost, we asked Schmidt how a beginner can avoid overspending, failures, or falling short on performance goals. You can easily get lost in the weeds when looking for information about how to put a combination together. Having too big of expectations without the right parts can lead to problems and disappointment.

“It’s just like cams or carburetors, where everybody’s like, oh well, that turbo will make 900 horsepower, but then it’s a dog down low, it takes forever to spool, and then the car doesn’t build any boost until it gets up into. It’s like when you over-cam or over-carburate something, it falls on its face down low, then eventually it picks up and goes,” Schmidt states.

A smaller turbo that’s more efficient is the best way to avoid these types of problems. It will provide better performance in the lower RPM range, make more low to mid-range torque, and spool better. All of this will assist with getting your vehicle moving quicker.

Another thing to keep in mind is what the engine you’re going to build can handle. If you’re going to try and make big boosted horsepower with a turbo, make sure that engine is ready for it.

“So, know your RPM range and what the engine can really handle. You know, the hot setup is a junkyard 5.3L LS where they take the thing and pop the pistons out and open up the rings and ring gaps, stuff the pistons back in, and throw a good set of head gaskets on it and then throw on a 78- or 80-millimeter turbo. It will make 800 or 900 horsepower for a while, and then they drive over the crankshaft because the bottom blows apart. And then they’ll do it again. I kind of feel like if you’re going to build a turbo motor, you should put in forged pistons, good crank and rods, and ARP fasteners, if nothing else,” Schmidt explains.

A turbo system needs to work together just like any other part of a vehicle. When you select the correct parts that are properly sized for your application, it becomes much easier to achieve your horsepower and performance goals.