Why does my Mustang put down less HP than Ford advertises?


From time to time, someone will ask us “Why does my car put out 290 horsepower on the dyno if it was rated at 300 horsepower stock?”  Dyno results can be confusing for anyone who hasn’t read up on the topic.  How can the same car put down different numbers just because the dyno manufacturer isn’t the same?  These answers are simple much easier to understand if you know how a dyno calculates or measures horsepower and torque.


Ford30V6TestCell.jpgHow Manufacturers Measure HP & TQ
So if Ford says my car makes 300 horsepower, how come the dyno results are lower? It’s actually really simple. Vehicle manufacturers will use a base horsepower (bhp) figure, also known as flywheel horsepower, in advertisements. These are the horsepower and torque numbers before the power leaves the engine and travels through the transmission and the rest of the drivetrain. As this energy is transferred, it will decrease as much as 10-20%, or even more depending on the vehicle. A typical Mustang will lose anywhere between 12-20% of the base horsepower number when tested on a chassis dynamometer. So while the car is advertised at 300bhp from the factory, it may put down as little as 265rwhp on a chassis dynamometer.

How Chassis Dynos Measure HP & TQ
So what is a chassis dynamometer and how does it measure horsepower and torque? Most chassis dynamometers (known as “dynos”) are large single or dual rollers that can weigh as much as 2500lbs. These rollers can either be free-rolling (inertia-type dyno) or equipped with a large drum brake (known as an eddy-current brake) to simulate load and put resistance against the vehicle.

amcarshow_dyno.jpg Eddy current dynos use a load sensor in order to measure the amount of resistance being applied with the large magnetic brake. Load force, along with vehicle weight and aerodynamics factored in will allow the dyno computer to properly measure torque. It will then take a derivative of that number in order to calculate an approximate horsepower figure. Inertia dynos know the weight of the drum, and measure the acceleration of the drum as well as the rate of acceleration of the vehicle in RPMs. It arrives at the measured horsepower number and then calculates an approximate torque figure from there.

As always, the real testing is done on the street and at the track!  Since there are so many different dyno makes with different ways to calculate horsepower and torque, these numbers can only be taken for what their worth.  A dyno can be a very important tool in properly calibrating your Mustang to ensure that every aspect of the tune has been optimized.  Just like anything else, your dyno results will largely rely on proper mechanical operation, temperatures, altitude, etc.

Tell us your thoughts, or even post up a link in the comments below to your dyno results and show us what your Mustang put down!

Written by Andrew

"You're telling me it's got four wheels, two seats and goes faster than the speed limit? Good, I'm driving."


  1. thank you very musch AM for posting this i was very confused when they said my 03GT automatic with boltons ( CAI 75mm TB+ Plenum) was only making 240hp


  2. the 70 mm combo works good with the 5 speed, I gave my dad my 75 mm parts and he has automatic along with the bbk intake. It really woke his car up. Even with 3000 grand of parts into my car and the 87 octane tune, I still couldn’t beat him on the drag strip, I could creep up on him but not before the finish line.

    So it pushed me to put in the 93 octane tune, Look out pop I’m coming to get ya.

    If your running stock or 87 octane tune, your burning money.


      1. about 1200 dollars in suspension, the rest in HP, i can beat him in top end , but that automatic is better in acceleration. when he gets his automatic tuned, I am going to have to get gears to beat him.

        curious to what the HP differences are on a stock automatic and stock 5 speed. On the PI mustangs

        now i put in 93 octane, we have raced since then, laugh it up. what are you running.


      2. No, no, you misread that.

        You said 3000 grand. Which is my interpretation of 3000 thousand. 3000 grand=3 million. I.E. “You spent 3 million and you still can’t beat him on the strip”

        Better now?


      1. both 02 GT’s but one 5 speed, and one Automatic. The automatic’s come with bigger injectors, It seems to give him an advantage.


    1. I had my 04 GT (with lots of bolt-ons:  legal Magnaflow hi-flow cats + Magnapack, BBK 75mm t-body, plenum, gears, CAI, NGK iridium plugs, BBK pullies, etc…
      I travel a lot and in Oklahoma you can ONLY get 91 octane gas (same as California), but in OK you can get NO ETHANOL gasoline!  I am very glad I went with a 91 octane SCT tune.


  3. I love when “mustangs and fast fords” put the articles in where guys will dyno after each part they put on.

    There’s nothing like seeing a claim of 11 HP on an electric water pump, and ends up only putting an extra 2 HP.


    1. I have a 99 v6 with bbk cai, und Pullies, throttle body, pypes off road full exhaust kit, bbk hedders, and a sct x3 and it put down 192 rwhp and 218 rwtq


      1. Also has aluminum driveshaft I think that helps alot with rwhp… The stock one was out of balance so that was why I replaced it in the first place… and it loves to eat hondas haha


  4. Very nice write-up AM! so many people get confused with factory rating vs dyno… even supposed car experts (i.e. Top Gear with the GT500 for example :p) there are only a few cars where the factory rating is what ya get… but far and large the majority will always be bhp.

    2002 GT – 302whp/320wtq
    Mustang dyno

    Diablosport chip
    85mm maf housing
    75mm intake tube
    75mm tb & plenum
    24 lb/hr injectors
    OR x pipe and magnapack catback
    3:73 gears on 5 speed tranny


  5. i have a 2000 gt automatic with C&L cai & intake plenum, bbk 70mm tb, slp cat-back, o/r x-pipe, and a diablosport tuner and i had it dyno tuned when i put 4.10 gears in. the numbers were 225whp & 262tq which seemed low, the shop guy said it was a mustang dyno which reads lower. anybody have an idea what my car would put down on a “regular” dyno ? thanks


    1. yeah like i said i went to this speed shop that had a regular dyno and my 03 GT auto with similar mods put down 240 hp /260tq, im not tuned though so u probly makin a good bit more than me


      1. okay thanks man, i was a little disheartened to say the least at the dyno chart lol. however i thought it may be off because ive ran a few 05-09gts that come with 300hp and before gears i lost by a half car at 110mph and after the gears ive pulled from the dig by a few car lengths. thanks again though. you getting a diablosport or sct tune ?


      2. well i think i might go with the SCT tune because what ive heard is that the diablosport is good but you have to adjust the parameters yourself and i dont know very much about that, i think the SCT is preloaded already from the Bama tuners, what i really want more than anything is better shift points


      3. yeah that is true i had mine done at this shop locally when i got gears the guy adjusted all the shift points. im clueless on that myself. i think you’ll definately be happy with the sct. let me know how much it helps.


      1. were you talking to me ? lol. i have stock heads and manifold if you were. i was lookin at getting a local shop to go stage 3 port & polish and pairing that with a typhoon intake manifold. it was a cheaper setup still aimed to produce 60 plus horsepower


  6. This is so true… I have a V6 2007. My base line was so low, they had to stop the test! I was running TOO LEAN, from factory! After I had a retuned and dyno, I’m pushing 270rwhp! Of course, aftermarket; exhaust, intake, etc. I wonder what it will be like with a pro charger with intercooler. Give me about a year and half and everyone will know;)


  7. I had my 04 GT (with lots of bolt-ons) “dynoed” and the result showd a gain of 35+ HP nd more than 50ftlb of torque.  The paper said only 255.9hp + 299.2 ftlb of torque.  It smokes just about anything else short of top-line Corvettes and Vipers (or Italian supercar thingies) and it is totally streetable. 
    I do not know what type of dyno it was…but WOW am I happy with the results.


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