BHP vs. WHP | HP to the Wheel vs. Flywheel
It’s very common to see confusion around the topic of brake horsepower (BHP) and wheel horsepower (WHP). What’s the difference? First let’s go over the definitions…
BHP (Brake Horsepower):
The power output of an engine measured at the engine’s flywheel. (A flywheel is a disc with teeth on it. It’s connected to the rear of the engine and its purpose is for the smooth transfer of power to the transmission from the engine).
Car manufacturers, engine manufacturers, boat manufacturers, and all other equipment sold that has an engine advertises their power rating using BHP. This is the truest and most accurate representation of the engine’s performance since it’s done in a controlled environment (a dyno room) by itself, without the influence of a vehicle.
WHP (Wheel Horsepower):
Unlike BHP, WHP is the power output measured at the wheels of the vehicle and not the flywheel. This is the engine’s performance influenced by the vehicle and environment.
The engine’s power is usually reduced by 25%-45% depending on many different variables. We must factor in the vehicle’s weight, 4WD vs. RWD, gear ratio, alignment, wheel and tire size, altitude, and more.
Alongside those factors, the numbers are also influenced differently depending on which gear it was in, what type of dyno, and who was operating the dyno. With all these variables it becomes very difficult to measure the accuracy of the engine’s actual performance.
This is why car manufacturers advertise a car using only BHP since WHP is a wild variable.
Corvette VS 4WD Truck Example
It’s fairly easy to see that a bigger, heavier vehicle with big tires and 4WD would achieve less performance (WHP) than a lighter vehicle such as a sports car.
If I put a 300 HP engine in a Corvette and the same 300 HP engine in an 4WD full-size truck and race them up a hill who would win? The easiest difference here is the weight – a Corvette is roughly 3,000 lbs and a full-size truck is roughly 5,000 lbs.
Even though the BHP is the same at 300 HP, the truck wouldn’t stand a chance to the Corvette in a race with the same engine power under the hood.
The Corvette may have about a 28% loss of HP to the wheel which would be 216 WHP and the truck would have about a 44% loss which would be 168 WHP.
How Environment Plays a Role
Altitude does effect power with both WHP and BHP. The reality is – less oxygen = less power. Jiffy Lube has a great article on their website that explains this perfectly. Below is a quote that sums-up the altitude:
“Reduced oxygen in high altitudes can lead to inefficient, sluggish engine performance because there is less air to feed the internal combustion engine. Generally speaking, an engine loses 3% of it’s rated horsepower for every 1,000 feet of altitude gained. This means you could lose as much as 20% of your vehicle’s horsepower if you drive it from sea-level to Big Bear Lake (with its 6,750 elevation).”
Now when they say 3% loss per 1,000 feet this would be ADDITIONAL to the WHP loss. With that being said, if we take the example above the Corvette and Truck are losing 3% more than they’re already losing due to their build.
So what have we learned here today? If Joe has a 5,000 lb, 4WD truck (not including tools, the driver, and any other added weight), lives in Reno, NV with 4,500 FT elevation… unfortunately he will roughly be looking at about a 45% loss to the wheels. If he upgraded his old 200 HP engine to a new 300 HP engine, that’s a 100 BHP increase, right? Correct. BUT his WHP would only change from 110 to 165. It would only be a 55 HP increase to the wheel due to the 45% loss from vehicle and altitude.
The engine industry isn’t an easy one. There’s so much to be learned and there are hidden curve balls everywhere. That’s why here at West Coast Engines, we try to provide as much information as possible while you’re looking to buy an engine. This is a high price tag purchase and it’s vital to know what you’re getting into and to do it right.
Even if you’re not in the market, we just want people to be informed and to learn more about the engine world. It’s our passion and we think you like it too!
Interested in learning more? Feel free to leave us a comment below and ask us anything that might be on your mind. We love answering questions from you guys and creating new material to inform our readers. If there’s anything brewing in your head or a myth you’d like us to clear up, let us know and we’ll get right to it!