The amazing disappearing-horsepower lawnmower engine. The Briggs and Stratton Company is honest in saying that its engines wont provide the horsepower that is advertised. From the site, The gas engine will not develop the rated gross horsepower when used in a given piece of power equipment. The website then has a long list of reasons why horsepower will be less. Included are air filters, cooling fan, and muffler. What is surprising is how much less power may be delivered to the user than is advertised by the company. Consider the most powerful Intek Pro OHV engine with the 6.75HP rating, sometimes found on ordinary walk-behind lawn mowers. The advertised power is rated at 3600 operating RPM. But residential lawn mowers are more often regulated at 3100-3200 RPM to keep blade tip speed well within EPA safety, and possibly to limit engine and blade noise. At 3200 RPM the 6.75 rated HP engine is rated at about 7.7 foot pounds of torque. The Briggs and Stratton horsepower and torque chart lists only torque, omitting horsepower ratings. However, torque, engine RPM and horsepower have a mathematical relationship that allows us to calculate horsepower if torque and RPM are known. The formula is: HP = (Torque x RPM ) / 5252. The 6.75 HP engine is rated at 7.7 foot pounds of torque at 3200 RPM, as taken from the torque chart for the largest Intek Pro engine. Using the formula, (7.7 X 3200) divided by 5252 equates to 4.6915 HP at 3200 RPM. Engine horsepower, per Briggs and Stratton, is further reduced if temperature rises above 77degrees, by one percent for each added ten degrees Fahrenheit. If mowing is done at 80 degrees, not uncommon during the summer, then power will decline by .003. Net power will then be 1-.003, or .997 percent of the power output at 77 degrees. Multiplying our 4.6915 HP by .997 produces a new temperature-adjusted rating of 4.67426 HP. Power is also rated at 328 foot altitude. Per Briggs and Stratton, Engine power will decrease 3.5% for each 1000 ft. above sea level. If the mower is operated at 2328 foot altitude, about the average altitude of Kansas, then power decreases by another 7 % (2000 ft. difference times 3.5% power loss per 1000 feet). The 93 percent retained power at 2328 ft. times our previous rating of 4.67526 HP results in 4.35 HP. As engine users, we are concerned with what power an average engine will produce. But Briggs and Stratton has a different view. Quote: The Maximum BHP curve represents maximum performance output of optimum laboratory test engines of a particular model. A statistical mean of production engines, with 95% confidence, will develop no less than 85% of the Maximum BHP when tested after run-in to reduce friction and after clean-out of combustion chamber, with valves, carburetor and ignition systems adjusted to laboratory standards. That statement tells us that we can be highly confident (95%) that average production engines will develop at least 85% of the rated Maximum BHP, using factory assumptions of temperature and altitude. How much more than the 85% power output is likely to be achieved is unanswered, but the high level of confidence (95%) suggests that most engines off the production line will put out substantially more than 85% of rated power, again defining "rated power" as engine output that has been measured under engine test conditions which omit mufflers and air filters. A reasonable guess is that a production engine will produce halfway between 85% and 100% of factory-rated Maximum Power. Halfway brings us to the 92.5% level. The previously derived power of 4.35 multiplied by .925 gives 4.02375 HP. Remember, this power level is calculated from a series of adjustments to claimed power from a test engine that featured 1) a cleaned out combustion chamber, quite unlikely to be done by most homeowners, and 2) an engine with valves, carburetor and ignition systems all set to laboratory standards. Laboratory standards are unlikely be consistently met in the hurly-burly of a high speed production process designed to minimize production costs. Where have all these numbers gotten us? From a 6.75 advertised HP engine, we have lost nearly 2.75 HP. If reasonably accurate, our powerful engine is now only about 60 percent as powerful. Put differently, a true 6.75HP engine is about two thirds more powerful than a true 4.02 HP engine. A reasonable question is this: Do these numbers make any sense? Is our estimate of a true 4.02 HP engine reasonable? I think that the 4HP number is reasonable but possibly a bit low. The Briggs is a 11 cubic inch engine running at perhaps 3200 RPM in a home-user style lawnmower where blade speed is limited through the governor setting. A one half-horsepower per cubic inch output is possible for power-optimized small utility engines operating at 3600 RPM. At 3200 RPM, with slightly higher volumetric efficiency than at 3600 RPM, the same engine might put out roughly 90 percent of the power attainable at 3600 RPM; maybe something in the 4.9HP range. 3200/3600 =.88889 Lawn mower engines are not optimized for power, and instead are compromised to meet a multitude of goals including long life, easy starting, lugging power, reasonable fuel economy, etc. My present guesssubject to better informationis that the Briggs and Stratton 6.75 advertised horsepower lawnmower engine might put out about 4 ½ horsepower when mowing on a hot day at a couple thousand feet or so of altitude. Background: From the Briggs and Stratton website section titled Small engine horsepower ratings & engine performance data: The small engine horsepower ratings shown with the individual engine series on the preceding pages are established in accordance with SAE(Society of Automotive Engineers) code J1940 (Small Engine Power & Torque Rating Procedure) (Proposed Revision 2001-09). Power curve and engine performance data are obtained in accordance with SAE J1349 (Net) or J1995 (Gross) Engine Power Test Codes. In accordance with SAE J1349 (Net) or J1995 (Gross) Engine Power Test Codes, power curves are developed from laboratory test engines and are corrected to standard conditions which are: Altitude: 100 meters (328 ft) Ambient Temperature: 77 F (25 C) If the engine will be operated under ambient conditions different from above standard conditions, the following factors should be accounted for in estimating "on site" power output: Engine power will decrease 3.5% for each 1000 ft (300 m) above sea level and 1% for each 10 F (5.6 C) above standard temperature of 60 F (15.6 C). I caution that the difference between advertised and real-life engine power of the Briggs and Stratton engine is likely to be typical for other engine firms that make residential lawnmower engines. And, you may be lucky and get the rare exception: an engine that actually produces its advertised power.