How To Measure Your Car's Horsepower At Home (And Why It's Useful)

• Cars

A car's horsepower is one of the most fundamental measurements that automakers and enthusiasts alike rely upon. Of course, the most important reason for us enthusiasts is, put simply, is this car faster than a different car? Fundamentally, horsepower measures how fast the wheels can turn, while torque measures how easily the car turns them — both are needed to bring a car up to speed comfortably. However, that number you see in brochures actually isn't fixed. It decreases over time, as parasitic wear from other parts and internal components sap an engine's performance.

As someone who's made classic cars her passion, it's become something of a given that all of the cars I've owned have experienced this depreciation to varying extents. It's actually not difficult to measure it, either — all it takes is a few runs up to speed (done within legal limits, naturally), a camera on a timer, and a little bit of math, and you can fairly accurately measure your horsepower without the use of a dyno. Will it be as precise as a dyno? Probably not. But it can let you see if your car is experiencing such a performance decrease. This is important for several reasons. Let's say I figure out that my car has a 10% power loss from the factory. I go in and replace the filters, spark plugs, and so on, then run the test again. If it's still 10% down, I know the parasitic drag is likely somewhere deeper inside the engine, or tied to accessories. Basically, it helps me narrow down the root cause of issues as I fix up a car.

What you'll need and how to do it

Because I'm terrible at math, my principal tool is a calculator. First, look up your car's specific SAE net horsepower (not torque, there is a difference) online. It must be SAE net because, unlike gross horsepower, SAE net is measured from the rear wheels in driving configuration with all accessories attached. So, unless your engine is on a test stand, this is the car's default measurement. Next, you'll have to set up a camera to watch your car's RPM. You run the car in a fixed gear from low RPM, then do a series of pulls using only that one gear, up to the car's redline. Take multiple passes and compile an average. This demonstrates the length of time required for the car to accelerate to a certain engine RPM under load.

A couple of caveats here — always stick with level surfaces, since it'll be slower uphill and faster downhill, giving bad readings. Moreover, this can be done entirely within the speed limit, from a long first or second gear. In order to actually get the data, you must measure the tire's height, best done by lifting the tire off the ground and measuring its circumference. Then figure out the gear ratios, usually published online or in your owner's manual. Next, get the car's weight. Lastly, you'll need ambient temperature and pressure. You can plug in the numbers yourself — spreadsheets exist online to make your life easier. Or you can use an app that automates the process, like this one.

What the results will tell you

Great, the hard part's over. Now, what do you do with the number now that you have it? This falls into mechanical expertise, and analyzing peak horsepower. When you look at a power curve, it should look relatively smooth, with torque peaking first, then horsepower. This tells you where your engine is producing maximum power; if it's outside of factory specifications by a lot (peak horsepower at 4,000 RPM versus 6,000 RPM, for example), then there's likely something going on. Moreover, it'll tell you how much horsepower you lost since new.

As any car ages, it'll naturally experience general wear and tear. No system is perfect; rubber wears out, metal-on-metal contact may occur without proper lubrication, there may be leaks, spark plugs and combustion chambers catch carbon fouling, and so on. All of these factors negatively impact horsepower to varying degrees. This provides a helpful and relatively consistent tool for measuring exactly how much of a difference replacing these parts can make. It'll also tell you how much horsepower you gain from adding aftermarket parts. Obviously, none of this is a replacement for doing regular maintenance. Rather, it's more useful as a tool to see exactly what's affecting your car's performance, and to measure what certain parts may add or detract. If, for instance, you put on a new exhaust system and want to see if it's better on power than your old one, this is a fair way of testing that.