That “Adds 25 HP” Claim May Not Mean What You Think: How Performance Parts Are Really Tested
Performance parts are entering their busiest launch season of the year. The 2026 SEMA Show New Products Showcase is accepting entries ahead of the November show, while the PRI Featured Product Showcase is built around helping racing buyers and media discover new products and technology.
That means enthusiasts will soon see a fresh wave of headlines such as “adds 25 horsepower,” “supports 1,000 horsepower,” “flows 30 percent more,” or “drops two tenths.” Some of those claims are backed by excellent engineering and controlled testing. Others may be technically true but presented without enough context to tell you whether the part will produce the same result on your vehicle.
The goal is not to distrust every horsepower claim. It is to understand what was measured, how it was measured, and whether the test conditions resemble your combination.
First Question: Crank Horsepower or Wheel Horsepower?
A manufacturer may report power at the crankshaft, at the tires on a chassis dyno, or at the hubs on a hub dyno. Those are not interchangeable numbers.
Crank horsepower is measured before drivetrain losses. Wheel or hub horsepower is measured after the transmission, differential, axles and other rotating components have consumed some of the engine's output. A claim of 25 crank horsepower does not automatically equal 25 horsepower at the wheels.
The percentage of drivetrain loss also should not be treated as a universal conversion formula. Transmission type, fluid temperature, tire size, tire pressure, gear selection, differential, wheel mass and test method can all affect what reaches the measuring device.
For a deeper explanation of test standards, correction factors and dyno types, read our guide to why dyno numbers change between SAE, STD, hub and roller testing.
Peak Gain Is Not the Same as a Better Power Curve
“Adds 25 horsepower” often refers to the single largest difference between two curves. That difference might occur only near the rev limiter, or it might represent a broad improvement across thousands of rpm. Those are very different outcomes.
For a street car, truck or road-course vehicle, average torque and horsepower through the actual operating range can matter more than the highest point on the graph. A part that adds 12 horsepower almost everywhere may feel stronger and accelerate better than a part that adds 25 horsepower for a narrow 200-rpm window.
When possible, look for the full before-and-after graph rather than one peak number. Check where the curves separate, whether torque is lost at lower rpm, and whether the engine reaches the same final rpm in both tests.
A Good Before-and-After Test Controls the Variables
The strongest evidence is a controlled A/B test on the same vehicle, using the same dyno, fuel, gear, correction method and operating temperatures. The baseline and modified pulls should be close enough together that weather, adaptation and vehicle condition are not doing more work than the part.
Useful test documentation should answer these questions:
- Was the same vehicle tested before and after the change?
- Were both tests performed on the same dyno?
- Were intake-air, coolant, oil and transmission temperatures comparable?
- Was the same fuel used, including the same ethanol content?
- Was the same gear and correction standard used?
- Was the tune unchanged, or was tuning part of the advertised gain?
- Were required supporting parts installed for both tests?
- Were the results repeatable across multiple pulls?
A single hero pull can be interesting, but repeatability is what makes a result useful.
Heat Can Create or Hide a Horsepower Gain
Heat management can dramatically alter the result, especially on supercharged and turbocharged vehicles. A heat-soaked baseline followed by a cooled-down modified pull can make an upgrade look stronger than it really is. The opposite can also happen when the modified setup is tested before the intercooler system has recovered.
Charge temperature can affect ignition timing, throttle control, boost, knock response and torque management. That is why our recent article on diagnosing IAT2 recovery and supercharger cooling-system limits focuses on repeated pulls and cooldown behavior rather than one number.
For a fair comparison, the vehicle should begin each test in a similar thermal state. The datalog should also be reviewed for timing, knock, throttle, boost, lambda and fuel pressure, not just the final horsepower figure.
Fuel and Calibration May Be Part of the Gain
An intake, exhaust, pulley, camshaft or forced-induction system may require calibration changes to operate correctly. In that case, the advertised result may represent the part plus tuning, not the hardware alone.
That is not misleading when it is clearly disclosed. Some combinations are designed to work with calibration changes, and the tune is part of extracting the safe performance available from the hardware. The problem is comparing a tuned modified pull to an untuned baseline without explaining the difference.
Fuel quality matters too. A test performed on high-octane race fuel or a known ethanol blend should not be treated as a prediction for a vehicle using ordinary pump gasoline. Our lambda, E85 and wideband guide explains why ethanol content and fuel scaling need to be measured rather than assumed.
For owners who plan to log and tune their own supported vehicle, SMG carries the HP Tuners MPVI4 and AEM Wideband Tuning Bundle, along with application-specific bundles for the 2010-2015 Camaro and 2016-2024 Camaro.
Required Supporting Parts Must Be Included in the Story
A part may be capable of supporting a certain horsepower level without adding that horsepower by itself. A fuel pump that supports 900 horsepower, for example, does not create 900 horsepower. It supplies the fuel capacity needed for an engine combination capable of producing that output.
The same distinction applies to injectors, intercoolers, throttle bodies, exhaust systems, clutch assemblies, transmissions, driveshafts and cooling upgrades. “Supports” describes capacity. “Adds” describes a measured change. Those words should not be treated as synonyms.
Our guide explaining why a 1,000cc injector is not always a 1,000cc injector is a good example. Flow rating, fuel pressure, ethanol content, injector data and fitment all determine whether the advertised capacity is useful on the actual vehicle.
Horsepower Claims Cannot Simply Be Stacked
If an intake advertises 15 horsepower, headers advertise 25 and an exhaust advertises 12, the combined result is not automatically 52 horsepower.
Each component may have been tested on a different vehicle with a different baseline and combination. Multiple parts can also address the same restriction. Once the first upgrade removes most of that restriction, the next part may produce a smaller incremental gain than it did on a stock vehicle.
There can also be interaction effects. An intake that produces a small gain on a stock engine may become much more valuable with a larger throttle body, camshaft, ported cylinder heads or forced induction. Conversely, a large exhaust may reduce low-speed response on a combination that does not need the additional flow.
This is why parts should be selected as a system rather than as a pile of independent advertised numbers.
Not Every Valuable Upgrade Should Be Judged by Peak Horsepower
Some of the best upgrades may not add measurable peak power at all.
- Wheels and tires: Improve traction, braking, steering response and the ability to use available power. Browse SMG's wheel and tire catalogs.
- Suspension: Controls weight transfer, alignment and tire contact. See our suspension collection and the guide explaining why lower is not always faster.
- Brakes: Improve repeatable stopping performance, pedal control and heat capacity. Shop the SMG brake catalog.
- Cooling: Helps preserve power and reliability on repeated pulls, laps or towing loads.
- Drivetrain: A clutch, converter, differential or axle may make the vehicle quicker and more dependable without changing engine output.
A quality product can also provide better fitment, serviceability, durability, sound, reduced weight or improved response. Peak horsepower is only one measurement of value.
How to Evaluate a New Performance Product
Before buying a newly released component, use this checklist:
- Identify exactly what the claimed number represents.
- Find the complete parts list used in the test.
- Confirm the fuel, tune and vehicle configuration.
- Look for a full curve and repeatable results.
- Check whether the product requires additional hardware or calibration.
- Compare the test vehicle to your own engine, transmission and existing modifications.
- Decide whether your goal is peak power, average power, response, traction, cooling, reliability or a balanced combination.
For example, the JLT cold-air intake for the 2018-2021 Mustang GT is clearly identified as requiring a tune. That disclosure matters because it tells the buyer that installation and calibration should be planned together.
Build the Combination, Then Prove It
SMG Speed Shop carries the parts needed to build a complete combination, including exhaust systems, superchargers and forced induction, fuel-system components, tuning hardware, wheels, tires, suspension and brakes.
For customers who want the complete process handled correctly, North Texas High Performance can install the combination, verify mechanical and fuel-system health, calibrate supported vehicles, review datalogs and test the result on the Mainline ProHub dyno. A controlled baseline and repeatable after-test are more useful than guessing from a collection of catalog claims.
The right question is not simply, “How much horsepower does this part add?” It is, “What did this part add on this combination, under these conditions, and will it help the vehicle do what I actually want?”
Browse SMG Speed Shop for your next upgrade, or contact the SMG and NTHP teams to plan a matched package instead of chasing disconnected peak numbers.