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Old 06-04-2002, 03:53 PM   #1
R. Bruno
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Default Net Horsepower vs. Gross Horsepower

A question for the engineers in the house. I have seen both mentioned. I assume the difference between net and gross has something to do with what actually gets to the wheels (?) :confused: So when the horsepower is reported on our cars and modern cars for that matter what are they using, gross or net?
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Old 06-04-2002, 04:00 PM   #2
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Default Re: Net Horsepower vs. Gross Horsepower (R. Bruno)

Gross - used exclusively before 1971 (then used in conjunction with Net for one year before disappearing) is the measure of HP with the engine on a dyno with NO accessories attached.

Net - used exclusively after 1971 (used in conjunction with Gross for one year before being the only measure in 1972+) is the measure of Hp with the engine on a dyno with ALL accessories attached as to be delivered by the factory.

Rear Wheel - NEVER used by manufacturers it is the measure of HP given with CAR on chassis dyno and measured through drivetrain at the drive wheels.
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Old 06-04-2002, 04:01 PM   #3
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Default Re: Net Horsepower vs. Gross Horsepower (R. Bruno)

Nothing to do with the wheels. Gross hp is the total potental hp. It is an engine on an engine dyno at the flywheel without an accessories attached, has open air intake, headers and open exhaust, no alternator, no emission equipment.

Net hp is what the engine is capable of in a car. It is also taken on an engine dyno at the flywheel, but with the engine equiped as it would be in the car. Stock air filter assembly, stock exhaust mainfolds running through stock mufflers, alternator, A.I.R. pump and all emission controls.

Rear Wheel HP is a speed shop thing. Factory never used that rating.

tom...
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Old 06-04-2002, 05:33 PM   #4
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Default Re: Net Horsepower vs. Gross Horsepower (Tom73)

Wow- great answers! Thanks. Now I can amaze my friends this weekend over :cheers: !!
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Old 06-04-2002, 05:54 PM   #5
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Default Re: Net Horsepower vs. Gross Horsepower (R. Bruno)

Generally speaking, rear wheel HP less drive train losses of about 20% will yield Net HP.
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Old 06-04-2002, 06:28 PM   #6
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Default Re: Net Horsepower vs. Gross Horsepower (flynhi)

Quote:
Generally speaking, rear wheel HP less drive train losses of about 20% will yield Net HP.
And net to gross divide by .78. I recall a '71 350 (base) rated at 270hp gross, was also rated at 210 net, hence 210/270= 78%.

Put another way, if a car has 230 RWHP that would equal approx. 287 net hp (assuming the 20% drivetrain loss above for auto trans, little less for stick) and about 369 gross. This is an approximation anyway. Hope this helps!!
PS. Now it's easy to see why a new Z06 with 405 net hp is such a screamer.....in gross hp would be well over 500!!
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Old 06-04-2002, 07:54 PM   #7
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Default Re: Net Horsepower vs. Gross Horsepower (68shark)

The formulas are all well and good but engines from different era's had different draws on them. For a 1960 engine there would not be any emission equipment to factor in, for a 1978 engine there would be a huge draw for emission that would have to be factored in, and then for a 1999 engine there would be emission equipment but their draw would be much less so their factor would be less. In other words, one formula will not fit all years or all applications.

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Old 06-04-2002, 09:30 PM   #8
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Default Re: Net Horsepower vs. Gross Horsepower (Tom73)

cool. thanks. i learned something new :cool:
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Old 06-04-2002, 10:38 PM   #9
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Default Re: Net Horsepower vs. Gross Horsepower (Tom73)

How about a ZZ4? They claim 355 hp. Is that gross hp? I don't think so. Maybe they made an average with the most used accessories.

Just a tought

Stephan
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Old 06-04-2002, 10:55 PM   #10
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Default Re: Net Horsepower vs. Gross Horsepower (American Boy)

Stephan, that's gross. Keep in mind that, except for a little bigger cam, no emissions devices, and a carbed intake instead of the TPI, the ZZ4 is basically an L98 engine that was rated at 245 hp net. Those differences would definitely provide a power boost but not 110 net hp worth.
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Old 06-05-2002, 10:06 AM   #11
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Default Re: Net Horsepower vs. Gross Horsepower (American Boy)

Quote:
How about a ZZ4? They claim 355 hp. Is that gross hp? I don't think so. Maybe they made an average with the most used accessories.
Stephan,
I had wondered the same thing so I sent an e-mail to GM Performance Parts and asked them. The response from GMPP was that the Performance Parts Crate Engines are rated at GROSS hp as they do not know what the engines will be installed in and what the final configuration will be.

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Old 06-05-2002, 10:33 PM   #12
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Default Re: Net Horsepower vs. Gross Horsepower (Tom73)

Thanks for the response. :)

My stock L48 at 186 rwhp ain't that bad after all. It would be about 220 hp at the flywheel.

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Old 06-06-2002, 12:52 AM   #13
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Default Re: Net Horsepower vs. Gross Horsepower (American Boy)

OK.... I am familiar with the gross vs net ratings, and have been pretty comfortable with the definitions.... but I just thought of something:

:lightbulb:

Our corvette engines come with only one net hp rating. But the definition of net hp supposedly has all the accessories hooked up.

So, shouldnt a 72 base engine coupe with A/C and power steering have a lower net hp rating than a 72 base engine coupe without A/C and manual steering? (Presumed answer: NET hp rating must be done with the base standard equipment?) MJ
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Old 06-06-2002, 01:44 AM   #14
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Default Re: Net Horsepower vs. Gross Horsepower (MNJack)

MJ, your presumption is correct. They also published curb weight figures without options but they did also publish added weight for individual options. While there is a power loss for engine driven accessories, that loss is pretty small if those accessories aren't being used (though installed). There's usually enough "slop" in their ratings to allow for the difference between an engine running one more or one less pully. Ever wonder how factory ratings tend to end with a 5 or a 0? It's because they round the actual figures to whatever looks best in print. Typically, the variance in hp from one engine to another can easilly be as great or greater than the difference that an option might make.
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Old 06-06-2002, 12:06 PM   #15
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Default Re: Net Horsepower vs. Gross Horsepower (Vetterodder)

Also, GM has tended to rate their engines somewhat low for quite some time. Case in point, the late model LT1 in the f-body's (which I worked on extensively) were rated at 285 hp in '96 but really had between 295 and 300 hp at the flywheel.

Then, of course, there are "political" ratings... The '96 LT4 is rated at 330 hp, but dynoes higher than the '97 LS1 which was rated at 350 hp. GM couldn't let the 96 LT4 be rated higher than their new "uber engine", even though the Lt4 was stronger.
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Old 06-06-2002, 01:26 PM   #16
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Default Re: Net Horsepower vs. Gross Horsepower (American Boy)

Quote:
Thanks for the response. :)

My stock L48 at 186 rwhp ain't that bad after all. It would be about 220 hp at the flywheel.
Stephan, just curious about your specs? What was the factory rating for your year? Stick/Auto? True duals with/without cat? Your rwhp sounds very good, as just from memory, some year L48's were rated net (flywheel) around 165 which would be in the 130's at the tires?
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Old 06-06-2002, 01:34 PM   #17
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Default Re: Net Horsepower vs. Gross Horsepower (zwede)

Quote:
Then, of course, there are "political" ratings... The '96 LT4 is rated at 330 hp, but dynoes higher than the '97 LS1 which was rated at 350 hp. GM couldn't let the 96 LT4 be rated higher than their new "uber engine", even though the Lt4 was stronger.
I've been told that the ZZ4 is the "current" version of the '96 Grand Sport LT4. Does anyone know if that's correct? I like the sound of it as a proposition, but I don't know what the underlying facts are.
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Old 06-06-2002, 02:01 PM   #18
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Default Re: Net Horsepower vs. Gross Horsepower (mg62)

Quote:
I've been told that the ZZ4 is the "current" version of the '96 Grand Sport LT4. Does anyone know if that's correct? I like the sound of it as a proposition, but I don't know what the underlying facts are.
The ZZ4 is a Gen 1 engine, the classic chevy small block. The LT4 is a Gen 2 engine. (only major component that is interchangeable between Gen 1 and Gen 2 is the crankshaft.)

I have heard the ZZ4 described as a hot rodded L98 with a carb.

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Old 06-06-2002, 09:33 PM   #19
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Default Re: Net Horsepower vs. Gross Horsepower (American Boy)

Quote:
Thanks for the response. :)

My stock L48 at 186 rwhp ain't that bad after all. It would be about 220 hp at the flywheel.
Was the above figure from a dyno pull? Best I could get from mine, when stock, using a G Tech was 152 RWHP.
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Old 06-06-2002, 09:40 PM   #20
Noel Carboni
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Default Re: Net Horsepower vs. Gross Horsepower (68shark)

Quote:
And net to gross divide by .78. I recall a '71 350 (base) rated at 270hp gross, was also rated at 210 net, hence 210/270= 78%.
Probably not particularly accurate. Accessories don't suck more power from a bigger engine. Perhaps it would be more realistic to add 60 hp than to do multiplication. From a common-sense point of view, even picking one figure is probably bogus. Accessory packages differ.

Oh, and the number I've always used for drivetrain losses was 10% for manual tranny, 15% for automatic. The different measurements make some sense... An automatic turns more power into heat (thus the cooling requirement). But then, does it have a lock-up torque converter? Maybe we need three figures... No, four... No.... AAAAAAHHHHHHH!!!!!! :crazy:

-Noel


[Modified by Noel Carboni, 12:43 AM 6/7/2002]
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Old 06-06-2002, 09:40 PM
 
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