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Old 10-02-2013, 01:59 AM   #1
TheB00T8930
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Default Interesting question

I am curious as to how subaru figures the specs of their vehicles. Or any car manufacturer for that matter. Example, the 11 wrx stock is 265hp and 244 ft-lbs at sea level. But how are those numbers figured? Is is put on a dyno? If so what kind? A fairly precise one I would imagine.
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Old 10-02-2013, 03:06 AM   #2
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Quote:
Originally Posted by TheB00T8930 View Post
I am curious as to how subaru figures the specs of their vehicles. Or any car manufacturer for that matter. Example, the 11 wrx stock is 265hp and 244 ft-lbs at sea level. But how are those numbers figured? Is is put on a dyno? If so what kind? A fairly precise one I would imagine.
An engine dyno, that's all.
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Old 10-02-2013, 08:16 AM   #3
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Many car manufacturers calculate a vehicle's HP by the engine. The engine HP will be higher than the wheel HP (WHP) and it is a better figure to proclaim for marketing since it is higher. The reason why WHP is lower is because you lose power from the transmission to drive the wheels. You will lose more power from an AWD vehicle rather than a FWD or RWD since power is going to all four wheels rather than two.

Here is a very informative video about engine HP vs wheel HP:

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Old 10-02-2013, 09:26 AM   #4
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Originally Posted by TheB00T8930 View Post
I am curious as to how subaru figures the specs of their vehicles. Or any car manufacturer for that matter. Example, the 11 wrx stock is 265hp and 244 ft-lbs at sea level. But how are those numbers figured? Is is put on a dyno? If so what kind? A fairly precise one I would imagine.
Serious question??

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Old 10-02-2013, 09:44 AM   #5
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As stated, engine specs are reported from an engine dyno. An engine dyno is basically an electrical motor hooked up to the engine. (Water brake dynos can be used, but they are not as common anymore. These are basically just a water pump).

They specify sea level because temperature, pressure, and humidity all affect the power output of an engine. Dynos measure these values and apply a compensation to correct for conditions that are not at STP (Standard Temperature and Pressure).
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Old 10-02-2013, 09:52 AM   #6
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new optima turbos are dynoing 220-230 HP stock and 280-330torque stock

They rock
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Old 10-02-2013, 10:05 AM   #7
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Serious question??
The reason I ask, is how do the engine dynos manufacturers use compare to the dynos that a performance shop would use. It seems like there is such a great variation between different dynos that they would need a more precise system.

Last edited by TheB00T8930; 10-02-2013 at 10:10 AM.
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Old 10-02-2013, 10:12 AM   #8
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Quote:
Originally Posted by Kaimande View Post
Many car manufacturers calculate a vehicle's HP by the engine. The engine HP will be higher than the wheel HP (WHP) and it is a better figure to proclaim for marketing since it is higher. The reason why WHP is lower is because you lose power from the transmission to drive the wheels. You will lose more power from an AWD vehicle rather than a FWD or RWD since power is going to all four wheels rather than two.

Here is a very informative video about engine HP vs wheel HP:

Engine Horsepower vs Wheel Horsepower - Explained - YouTube
THIS^^

The final number on a dyno is a pretty useless number. The only useful number achieved by dynoing your car is when you subtract base line dyno reading from the final tuned dyno reading. The goal is not the end number rather its to see increase in performance from the begining number.

Last edited by stikilla; 10-02-2013 at 10:30 AM.
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Old 10-02-2013, 11:30 AM   #9
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Quote:
Originally Posted by TheB00T8930 View Post
The reason I ask, is how do the engine dynos manufacturers use compare to the dynos that a performance shop would use. It seems like there is such a great variation between different dynos that they would need a more precise system.
Yep, they are a lot more expensive and precise. A typical engine dyno setup at an OEM can cost a million dollars or more. A typical performance shop doesn't have that kind money to invest. Nor do they really need to. Performance shops are looking for big gains, whereas OEM's are usually trying to eek out every little bit of efficiency, so they can give HP while still having fuel economy.

Technically, the final torque output isn't as important as the pressure inside the cylinder. There is a lot of expensive equipment being used to accurately measure in cylinder pressures. From this, things like burn rates, peak pressures, effective pressures, etc. can all be calculated.
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Old 10-02-2013, 11:32 AM   #10
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Quote:
Originally Posted by stikilla View Post
THIS^^

The final number on a dyno is a pretty useless number. The only useful number achieved by dynoing your car is when you subtract base line dyno reading from the final tuned dyno reading. The goal is not the end number rather its to see increase in performance from the begining number.
I wouldn't call it useless. It depends of the dyno operator knows what they are doing with the compensations, and if the dyno equipment is properly calibrated. If this is true, then the corrected HP should be a relevant number.

Having said that, when you are looking for a HP gain, yes, the delta is more interesting than the actual number.
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Old 10-02-2013, 12:14 PM   #11
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Originally Posted by TheB00T8930 View Post
I am curious as to how subaru figures the specs of their vehicles. Or any car manufacturer for that matter. Example, the 11 wrx stock is 265hp and 244 ft-lbs at sea level. But how are those numbers figured? Is is put on a dyno? If so what kind? A fairly precise one I would imagine.
All manufacturers use engine dynos and follow the procedures in the standard defined by SAE J1349.
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Old 10-02-2013, 02:18 PM   #12
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Originally Posted by Cocoa Beach Bum View Post
All manufacturers use engine dynos and follow the procedures in the standard defined by SAE J1349.
This spec gives formulas for the contribution of many factors into the final power numbers and so if you want to increase hp it tells you where best to put your money and may also tell you why you had a good day or a bad day at the track.
Some of these factors have very little influence on the hp.
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