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Discussion Starter #1
This is just a general question about horsepower.

I was hoping someone would give me a refresher course on the various ratings. My father had a ’67 Corvette (327/300 bhp). I remember him explaining to me that the ‘old’ hp rating system was different than the one currently used. He said the old figure was measured at the crank, and without any limiting agents (driveline, exhaust, etc). That is why his 300 bhp Vette wouldn’t run with an ’88 Mustang GT (225 bhp). He spoke of some ‘general conversion’ to compare these figures…but I forget it, now.

So…I was always under the impression that the horsepower ratings that are currently published for new cars were the most accurate. However, after spending some time in this forum, it seems that those ratings are not actually ‘rear wheel’ horsepower. From what I gather, there is approximately a 20% loss of power (through the driveline) when the figure is measured at the wheels.

And to complicate things even more, you’ve got the DIN rating that has been used in Europe (and Japan, I believe) for some time. That’s at least four different ways to measure horsepower.

How do they all relate? Would somebody kindly educate this newbie?
 

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I believe the old system was crank HP without accessory loading.
The new system includes accessories.
 

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Discussion Starter #4
After postion my question, I did a little research. I found a few more details.

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SAE Gross:
Horsepower is measured at the crank with a brake dynometer (bhp), without any accessories (exhaust system, air cleaner, air-conditioning compressor, and power-steering pump). This method was used in the United States until 1972.

SAE Net:
Horsepower is measured at the crank with a brake dynometer (bhp). The engine is set up just like it will be installed in the car. This method has been used since 1972 in the United States, and is what most manufactures use in their published HP figures. This value can range from 20-40 percent less than the same engine rated by SAE Gross.

DIN:
Horsepower is measured at the crank with all of its accessories. It is set up like it will be installed in the car, and is always a ‘NET’ figure. The SAE Net horsepower is roughly the same as DIN, with some minor uncertainties and a small 1.0139 scaling factor. The 1.0139 factor comes from the conversion of the 'standard' horsepower (745.7W) used in SAE to the metric horsepower (735.5W) used in DIN. Additionally, the two standards differ on the temperature/pressure conditions and exactly which accessories must be on the engine at the time of rating. Typically the DIN figures are slightly higher, but not by much. DIN ratings are common in Europe and Japan. DIN ratings have been used in other countries prior to the adoption of SAE Net.

RWHP:
Horsepower is measured at the driving wheels, with the engine installed in the car (along with all of its accessories). This value can be approximately 15-20 percent less than the same engine rated by SAE Net, due to the loss through the driveline.
 

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Thanks for the clarification. I new some of it but not all of it.

- Charley
 

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Damillster said:
This (SAE Net) value can range from 20-40 percent less than the same engine rated by SAE Gross.
Wow -- thats alot of lost HP due to all the things being driven off the engine. I thought it would be more like in the 10~20% range.

Good info ... thanks for posting it :D.
 
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