Brake HP, Net HP what's the truth?

Unit 5302 · 02-22-2001, 08:28 PM

For a while now I've been thinking about the differences between brake hp and net hp.

From what I understand, net hp is the engine's rating with all the accessories hooked up, running like it would on the street.

I've also gathered that brake hp is the engine's rating when only the necessary accessories like the charging system and waterpump/fan are connected. So it'll be less accurate than net.

How much will net hp differ from brake? I've heard anything from a few ponies to 30%.

I've decided a commonly quoted 30% is unreasonable. That's like saying the 87 5.0HO actually has 322bhp, and a modded 5.0 with 300rwhp is making 500bhp naturally aspirated. How many people really believe it cost 100hp to run the accessories? You'll only pick up a couple tenths by bypassing them, which would indicate maybe 20-30hp. The air pump uses about 5, the power steering pump maybe 10, and the A/C pump maybe 5 when not in use. That makes 20hp difference vs brake hp. Not 100hp.

I've begun to become annoyed when people re-rate the older hi-po motors in net hp by taking 30% off their power ratings. Say a 427 dual quad, rated at 425bhp would now have 298hp, and a sick 250rwhp. Now we all know the old motors were conservative, but come on. Even if we say 500bhp, that's still 350 net. Knowing the 427 is pretty wigged out from the factory, could a 300hp or 350hp car move a 5500lb '63 convert to a low 13 stock? (my dad ran it, I've seen the timeslips) I say, no way! Plus it would seem to me, since the 427 had no A/C pump, or air pump, it would not suffer as much as a modern engine in rating.

Anybody want to chime in?

bigwhitecobra · 02-22-2001, 10:05 PM

Ok, I'm going to try to follow you here. From what you've heard, a 500hp motor would be making 300 at the wheels. That just don't sound right. I have always heard that the difference between brake and net hp ratings were relatively non-existant. But then again, that is coming from a few "shade tree" mechanics and guys at the track. No reliability there. I'll have to ask around on that one. But let me know if you get the answer before me.

bigwhitecobra · 02-22-2001, 10:25 PM

Ok, talked with the father in law, and he tells me that there is no difference between brake and net hp ratings. Well, none other than brake hp refers to the use of an engine brake, where as the net is just every accessory hooked up. I think you may have meant gross hp.

Unit 5302 · 02-22-2001, 11:24 PM

Whoops, yeah gross hehe.

smooth · 02-22-2001, 11:40 PM

brake hp is the measurement on an engine dyno using a load at a given rpm..

Skyman · 02-23-2001, 04:47 AM

All this crap is so hard to figure out, and people always love to quote horsepower numbers. I basically don't give a crap how much horsepower a car has anymore. Let me ride in it and see how fast it is.

Skyler

------------------
-1989 Saleen Mustang #406-
TFS Heads, E-303 Cam, Edelbrock Intake, and a whole lot more.
13.2@106mph
-Shooting for 12's and a 351 slowly in the works!

Stock-94 Integra 15.48@91mph

02-23-2001, 06:28 AM

All the power in the world is useless if you can't get it all transfered to the pavement

------------------
Paxton Blown 87GT

Jaydee · 02-23-2001, 11:17 AM

Best as I can tell, net hp these days is the actual output at the flywheel of the motor, carrying its normal as installed production accessories. Everything I have ever read on rwhp shows that the net hp is reduce by an average of 20% to 30%, depending on the type of transmission used and the overall efficiency of the balance of the components oclutch, rear end etc.) f the drive train.

Unit 5302 · 02-23-2001, 01:23 PM

The numbers that seem pretty accurate are
17% loss T-5
19% loss C-4
22% loss AOD

Those I'm comfortable with.

It's this gross vs net BS that drives me nuts. Well that motor was 425 gross, so actually about 300 net, and with drivetrain loss 250rwhp.

Rev · 02-23-2001, 05:57 PM

From what I can gather, gross was used 1970 and before. Net has been quoted since 1970. When gross was used, manufacturers wanted to inflate HP numbers as much as possible for advertising purposes. Gross numbers were without any accessories at all. Only fuel, oil, and water pumps and no fans or anything else. Headers were dyno headers and no mufflers. The headers didn't even need to fit in the car. These engines also were tuned especially for the dyno test by someone like Smokey Yunick or some other factory racing guru. These engines were well broken in with fairly loose tolerances, thus minimizing friction. The idea was to maximize the numbers. Factory engineers referred to these numbers as the "A" curve.

At some point in the late '60's, muscle cars were criticzed by insurance companies and by the Feds. In '68, Ford started advertizing the "B curve". That was just an off the line engine with factory exhaust system, but no accessories. This was to appease insurance companies and the Feds. This is how the 428 CJ was rated.

After about 1970, manufacturers used net HP or the "C" curve. That is just net HP that we use today. It's just as the engine comes in a new car with all accessories except PS and AC. Factory exhaust and no special tune. Also a tight new engine with no break in.

Basically, Manufacturers tried to make the HP numbers be what suited them at the time. It created a lot of confusion. The above mentioned numbers are flywheel dyno numbers (hence "brake"). Rear wheel numbers are about what Unit said they are percentage-wise.

I know this is long, but thought it might help a little.

Rev

------------------
'66 Coupe, 306, 300 HP, C-4, 13.97 e.t., 100.3 mph
1/4 mi.

QuantumMotorsports · 02-23-2001, 06:12 PM

Check out www.howstuffworks.com and look for the section on horsepower. It tells the relationship between Horsepower, torque and RPM. It makes it pretty easy to understand. I'm not sure if it will answer your question, but it does have a lot of info.

------------------
Michael Black
Quantum Motorsports
Norman, Oklahoma

1988 Merc Cougar 5.0 HO, P&Ped heads, 2.25" custom mandrel bent dual pipes, T5 five speed tranny
15.43 @ 91.08mph (not shabby for a 3600 pound car)
60' 2.453 Street tires suck!!!

02-22-2001, 08:28 PM	#1
Unit 5302 Registered Member Join Date: May 1999 Posts: 5,246	Brake HP, Net HP what's the truth? For a while now I've been thinking about the differences between brake hp and net hp. From what I understand, net hp is the engine's rating with all the accessories hooked up, running like it would on the street. I've also gathered that brake hp is the engine's rating when only the necessary accessories like the charging system and waterpump/fan are connected. So it'll be less accurate than net. How much will net hp differ from brake? I've heard anything from a few ponies to 30%. I've decided a commonly quoted 30% is unreasonable. That's like saying the 87 5.0HO actually has 322bhp, and a modded 5.0 with 300rwhp is making 500bhp naturally aspirated. How many people really believe it cost 100hp to run the accessories? You'll only pick up a couple tenths by bypassing them, which would indicate maybe 20-30hp. The air pump uses about 5, the power steering pump maybe 10, and the A/C pump maybe 5 when not in use. That makes 20hp difference vs brake hp. Not 100hp. I've begun to become annoyed when people re-rate the older hi-po motors in net hp by taking 30% off their power ratings. Say a 427 dual quad, rated at 425bhp would now have 298hp, and a sick 250rwhp. Now we all know the old motors were conservative, but come on. Even if we say 500bhp, that's still 350 net. Knowing the 427 is pretty wigged out from the factory, could a 300hp or 350hp car move a 5500lb '63 convert to a low 13 stock? (my dad ran it, I've seen the timeslips) I say, no way! Plus it would seem to me, since the 427 had no A/C pump, or air pump, it would not suffer as much as a modern engine in rating. Anybody want to chime in?

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02-22-2001, 10:05 PM	#2
bigwhitecobra Huh? Whatcha said? Join Date: Oct 2000 Location: Fayetteville, NC Posts: 1,073	Ok, I'm going to try to follow you here. From what you've heard, a 500hp motor would be making 300 at the wheels. That just don't sound right. I have always heard that the difference between brake and net hp ratings were relatively non-existant. But then again, that is coming from a few "shade tree" mechanics and guys at the track. No reliability there. I'll have to ask around on that one. But let me know if you get the answer before me.

02-22-2001, 10:25 PM	#3
bigwhitecobra Huh? Whatcha said? Join Date: Oct 2000 Location: Fayetteville, NC Posts: 1,073	Ok, talked with the father in law, and he tells me that there is no difference between brake and net hp ratings. Well, none other than brake hp refers to the use of an engine brake, where as the net is just every accessory hooked up. I think you may have meant gross hp.

02-22-2001, 11:24 PM	#4
Unit 5302 Registered Member Join Date: May 1999 Posts: 5,246	Whoops, yeah gross hehe.

02-22-2001, 11:40 PM	#5
smooth Registered Member Join Date: Nov 1999 Location: illinois, USA Posts: 649	brake hp is the measurement on an engine dyno using a load at a given rpm..

02-23-2001, 04:47 AM	#6
Skyman I need 110mph Trap Speed! Join Date: Feb 1999 Location: So, CA Posts: 4,315	All this crap is so hard to figure out, and people always love to quote horsepower numbers. I basically don't give a crap how much horsepower a car has anymore. Let me ride in it and see how fast it is. Skyler ------------------ -1989 Saleen Mustang #406- TFS Heads, E-303 Cam, Edelbrock Intake, and a whole lot more. 13.2@106mph -Shooting for 12's and a 351 slowly in the works! Stock-94 Integra 15.48@91mph

02-23-2001, 06:28 AM	#7
Mustanguy Guest Posts: n/a	All the power in the world is useless if you can't get it all transfered to the pavement ------------------ Paxton Blown 87GT

02-23-2001, 11:17 AM	#8
Jaydee Registered Member Join Date: Sep 2000 Location: Conn. Posts: 220	Best as I can tell, net hp these days is the actual output at the flywheel of the motor, carrying its normal as installed production accessories. Everything I have ever read on rwhp shows that the net hp is reduce by an average of 20% to 30%, depending on the type of transmission used and the overall efficiency of the balance of the components oclutch, rear end etc.) f the drive train.

02-23-2001, 01:23 PM	#9
Unit 5302 Registered Member Join Date: May 1999 Posts: 5,246	The numbers that seem pretty accurate are 17% loss T-5 19% loss C-4 22% loss AOD Those I'm comfortable with. It's this gross vs net BS that drives me nuts. Well that motor was 425 gross, so actually about 300 net, and with drivetrain loss 250rwhp.

02-23-2001, 05:57 PM	#10
Rev Registered Member Join Date: Nov 1998 Location: Houston, Tx. Posts: 3,887	From what I can gather, gross was used 1970 and before. Net has been quoted since 1970. When gross was used, manufacturers wanted to inflate HP numbers as much as possible for advertising purposes. Gross numbers were without any accessories at all. Only fuel, oil, and water pumps and no fans or anything else. Headers were dyno headers and no mufflers. The headers didn't even need to fit in the car. These engines also were tuned especially for the dyno test by someone like Smokey Yunick or some other factory racing guru. These engines were well broken in with fairly loose tolerances, thus minimizing friction. The idea was to maximize the numbers. Factory engineers referred to these numbers as the "A" curve. At some point in the late '60's, muscle cars were criticzed by insurance companies and by the Feds. In '68, Ford started advertizing the "B curve". That was just an off the line engine with factory exhaust system, but no accessories. This was to appease insurance companies and the Feds. This is how the 428 CJ was rated. After about 1970, manufacturers used net HP or the "C" curve. That is just net HP that we use today. It's just as the engine comes in a new car with all accessories except PS and AC. Factory exhaust and no special tune. Also a tight new engine with no break in. Basically, Manufacturers tried to make the HP numbers be what suited them at the time. It created a lot of confusion. The above mentioned numbers are flywheel dyno numbers (hence "brake"). Rear wheel numbers are about what Unit said they are percentage-wise. I know this is long, but thought it might help a little. Rev ------------------ '66 Coupe, 306, 300 HP, C-4, 13.97 e.t., 100.3 mph 1/4 mi.

02-23-2001, 06:12 PM	#11
QuantumMotorsports Registered Member Join Date: Jul 2000 Location: Norman, Oklahoma, USA Posts: 597	Check out www.howstuffworks.com and look for the section on horsepower. It tells the relationship between Horsepower, torque and RPM. It makes it pretty easy to understand. I'm not sure if it will answer your question, but it does have a lot of info. ------------------ Michael Black Quantum Motorsports Norman, Oklahoma 1988 Merc Cougar 5.0 HO, P&Ped heads, 2.25" custom mandrel bent dual pipes, T5 five speed tranny 15.43 @ 91.08mph (not shabby for a 3600 pound car) 60' 2.453 Street tires suck!!!