rwhp vs hp?
Pro
Joined: Jun 2007
Posts: 631
Likes: 3
From: Joppa AL
rwhp is rear wheel horsepower or power measured at the rear wheels. You drive the car onto a dyno and it measures power at the rear wheels.
Plain hp as you see in new car ads is power measured at the flywheel. It's what the eng is really producing.
Since eng removal is kind of a pain we just drive it onto the dyno and measure rwhp.
Most say there's about a 10 to 15% loss through the drive train so rwhp is always less than flywheel hp.
Plain hp as you see in new car ads is power measured at the flywheel. It's what the eng is really producing.
Since eng removal is kind of a pain we just drive it onto the dyno and measure rwhp.
Most say there's about a 10 to 15% loss through the drive train so rwhp is always less than flywheel hp.
4th Gear
Joined: Jan 2008
Posts: 4
Likes: 0
HP is often referred to as fly wheel horsepower which is the figure most cars are rated from the factory it is the amount of max horsepower the engine outputs. RWHP is used to represent how much of that fly wheel horsepower the car is actually putting to the rear tires, rear wheel horsepower will always be lower then fly wheel horsepower because some of the fly wheel horsepower is loss due to the drivetrain. Consider this illustration two 400 hp ls2 vettes one a manual the other automatic, the manual might put down 345 rwhp while the automatic might put down 337 rhwp, because automatics usually have more drive train loss.
Melting Slicks
Joined: Sep 2003
Posts: 2,458
Likes: 105
From: Northville Michigan
So, if you get a RWHP reading from your C5 being tested on a chassis dyno, take that RWHP number and divide it by .847 to approximate the crankshaft horsepower:
400 RWHP: 400 / .847 = 472 crankshaft horsepower.
Drifting
Joined: Dec 2005
Posts: 1,720
Likes: 3
From: East Hampton CT
I believe the common % is 15% for a C5 manual tranny (due to IRS losses too) and 20% for an A4.
My 412rwhp MN6 has approx 485 hp at the flywheel.
When talking with random people off the street, I never quote rwhp because they don't understand the difference. I had one co-worker respond that his Infinity G35 put out as much as my C5 did when I had it dynoed and had 327rwhp (with LT headers and tune).....he didn't understand that he probably would only put down 250-260hp to the wheels.
My 412rwhp MN6 has approx 485 hp at the flywheel.
When talking with random people off the street, I never quote rwhp because they don't understand the difference. I had one co-worker respond that his Infinity G35 put out as much as my C5 did when I had it dynoed and had 327rwhp (with LT headers and tune).....he didn't understand that he probably would only put down 250-260hp to the wheels.
Melting Slicks
Joined: Jul 2000
Posts: 2,721
Likes: 115
From: Phoenix AZ
I just don't understand why there is a static % drivetrain loss. Doesn't any given drivetrain require x amount of horsepower to turn it? If a 350hp C5 needs 15% of its hp (52.5 hp) why would a 700 hp C5 need 105 hp to turn the drivetrain?
Safety Car
Joined: Aug 2000
Posts: 4,933
Likes: 30
From: San Rafael CA
Don't forget that these numbers are "rule of thumb" ...... each individual car has its own actual rate of drivetrain loss, and that loss will vary depending on drivetrain speed (slightly) ...... in fact there are those who argue that drivetrain losses today for a manual Corvette are closer to the 12 percent area than 15 percent area because of CNC (computer controlled) machining of the gears .... much more accurately cut than in the days of manually controlled machining.
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Pro
Joined: Oct 2005
Posts: 738
Likes: 2
From: Richmond VA
Yes, the numbers they quoted were without the air pump, water pump, ac compressor, etc. Gross HP. Today we use Net HP. The last article I read claimed that tests showed that a current RWHP rating divided by .77 to .83 would generate the gross HP ratings shown in the past. eg: LS1 345 hp x .88 = 303 RWHP (12% loss). 303 RWHP / .77 = 393 crank hp or 303 RWHP / .83 = 365 crank hp.
Burning Brakes
Joined: Feb 2004
Posts: 1,231
Likes: 0
From: edmonton ab
When you see cars with a 10rwhp change due to a swap from a light wheel tire combo to a heavy one, this is pretty significant when vehicles lose only 50hp from output shaft to the ground. 10hp for a steel to CF driveshaft, and 5hp+ for a steel to aluminum flywheel swap, depending on the type of car.
Gear tooth size is constant. Fluid viscosity doesn't measurably change with every 50hp increase. Dropping the rear end ratio will hurt RWHP, I guess because of the tooth contact relationship, but not as much as a light set of wheels will help it.
Comparing same day engine to chassis dynos moving the same drivetrains with different hp would give an accurate measure but I have never seen this done.
The best arguement for weight being the major component is how much hp is measurably changed when the drivetrain weight is altered, and this is just by reduction, not by removal of the weight altogether.
Burning Brakes
Joined: Sep 2007
Posts: 945
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From: Tigertown Ohio
Melting Slicks
Joined: Dec 2007
Posts: 3,371
Likes: 5
From: Mission Viejo, CA
St. Jude Donor '10, '15-'16
Here's a question: Since our cars have a rear-mounted transmission, is the driveshaft spinning at engine rpm, or is there any gearing involved pre-transmission? It seems that you would have more losses with our setup than having the driveshaft after the transmission. A post-tranny driveshaft would be spinning slower.
(pardon my ignorance)
(pardon my ignorance)
Race Director
Joined: Apr 2007
Posts: 11,150
Likes: 890
From: South Western Ontario
Here's a question: Since our cars have a rear-mounted transmission, is the driveshaft spinning at engine rpm, or is there any gearing involved pre-transmission? It seems that you would have more losses with our setup than having the driveshaft after the transmission. A post-tranny driveshaft would be spinning slower.
(pardon my ignorance)
(pardon my ignorance)
Peter
Burning Brakes
Joined: Sep 2001
Posts: 893
Likes: 38
From: ATX
Couple comments:
15% is an estimate.
The reason why it's given as a percentage as opposed to a fixed number: yes, the gear faces, fluid viscosities, etc. stay the same, however the force on the gear faces, bearings, etc. is somewhat linear in the power you will make, and therefore drivetrain friction will also be.
Again, the number given is an estimate, not a precise model.
The reason for heavier wheel/tire combos causing parasitic loss is both because of their increased friction, and because they have a larger moment of inertia. This is the same reason a lighter flywheel may be advantageous. The moment of inertia effects will be larger under higher angular accelerations and less pronounced at lower accelerations. i.e. the flywheel will 'create' significant additional RWHP in first gear WOT but almost none in fifth gear WOT. Same goes for the wheels, except they have a larger friction component which remains regardless of acceleration.
The last sentence is key. It has a very, very small moment of inertia, so very little power is wasted accelerating it.
15% is an estimate.
The reason why it's given as a percentage as opposed to a fixed number: yes, the gear faces, fluid viscosities, etc. stay the same, however the force on the gear faces, bearings, etc. is somewhat linear in the power you will make, and therefore drivetrain friction will also be.
Again, the number given is an estimate, not a precise model.
The reason for heavier wheel/tire combos causing parasitic loss is both because of their increased friction, and because they have a larger moment of inertia. This is the same reason a lighter flywheel may be advantageous. The moment of inertia effects will be larger under higher angular accelerations and less pronounced at lower accelerations. i.e. the flywheel will 'create' significant additional RWHP in first gear WOT but almost none in fifth gear WOT. Same goes for the wheels, except they have a larger friction component which remains regardless of acceleration.
It should theoretically. You don't have to accelerate the driveshaft and the couplers or U-joints) as quickly when it's behind the transmission. But, the shaft is aluminum and the weight is all very close to the centre of the rotating axis so it's not a lot to accelerate it.
Peter
Peter
Le Mans Master
Joined: Mar 2002
Posts: 5,955
Likes: 161
From: PNW
Couple comments:
15% is an estimate.
The reason why it's given as a percentage as opposed to a fixed number: yes, the gear faces, fluid viscosities, etc. stay the same, however the force on the gear faces, bearings, etc. is somewhat linear in the power you will make, and therefore drivetrain friction will also be.
15% is an estimate.
The reason why it's given as a percentage as opposed to a fixed number: yes, the gear faces, fluid viscosities, etc. stay the same, however the force on the gear faces, bearings, etc. is somewhat linear in the power you will make, and therefore drivetrain friction will also be.
If you put a car on a load dyno and found the RWHP at a constant load at WOT, then the HP lost from the mass acceleration factor would disappear, and you would theoretically see slightly more RWHP if only the drive train frictional losses were in effect. If you are not accelerating mass, you are not losing HP.
Last edited by ZeeOSix; Jun 5, 2008 at 02:42 AM.
Burning Brakes
Joined: Sep 2001
Posts: 893
Likes: 38
From: ATX
If you put a car on a load dyno and found the RWHP at a constant load at WOT, then the HP lost from the mass acceleration factor would disappear, and you would theoretically see slightly more RWHP if only the drive train frictional losses were in effect. If you are not accelerating mass, you are not losing HP.
Team Owner
Joined: Sep 2003
Posts: 43,084
Likes: 33
From: Raleigh / Rolesville NC
dynos are for tuning purposes only. and there are several differnt types of dynos, Dynatech and Mustang are two that come to mind. Dynos read differently on different days. There is a SAE ( Sociaty of Automotive Engineers ) correction factors that should take into account temperature, humidity, and several other things
HP numbers are for bench / bar / forum racing and selling cars. ie does really not mean that much.
HP numbers are for bench / bar / forum racing and selling cars. ie does really not mean that much.
Drifting
Joined: Sep 2007
Posts: 1,576
Likes: 11
From: Negley Ohio
Ques....An 01 coupe 6-speed is 350 hp stock.So what would it be at the rear wheels stock.Where are these people in the past coming up with there numbers...I mean,they will tell you,,,,,,got vararam,lg pro longtubes,corsa indy,royle purple,new plugs and wires,got it tuned and putting out 380 rwhp.Is that possiable.I mean to the rear wheel.
Drifting
Joined: Sep 2007
Posts: 1,576
Likes: 11
From: Negley Ohio
And another.Whats my vette going to read,,,give or take.I have new plugs and wires american racing longtube H.corsa indys, breathless performance vortex air bridge,and ram air box.Ported throttle body.I did all that work and have not had it tuned yet.Im going to have it tuned at carlisle this year.Its an 01 coupe.What kind of hp am i looking at after dynoed and tuned.Give or take......
Team Owner
Joined: Sep 2003
Posts: 43,084
Likes: 33
From: Raleigh / Rolesville NC
Ques....An 01 coupe 6-speed is 350 hp stock.So what would it be at the rear wheels stock.Where are these people in the past coming up with there numbers...I mean,they will tell you,,,,,,got vararam,lg pro longtubes,corsa indy,royle purple,new plugs and wires,got it tuned and putting out 380 rwhp.Is that possiable.I mean to the rear wheel.
a 2002 or newer LS6 motor on 100 octane race gas, stock parts though from Intake though to the cats, then open exhaust could be tuned to 380 rwhp.
Not an a 2001 or older engine.
Bolt ONs help some what. any great single CAI over stock makes a difference.
LTs yes makes a different
x-pipe with free flowing open exhaust - YES
Mufflers or cat back - nope those are just for sound
plugs and wires - nope the AC Delco are still just about the best plugs and wires. Most racers still use these
any special fluids - nope
Changing to a dry sump oiling system - YES but very expansive.
the TUNE is the biggest source of more HP and TQ
unfortunityly some shops will make a dyno reading almost anything they want to get higher numbers and happy customers.
So take dyno numbers with a grain of salt. Dont believe what you read in advertising. "THIS PART will ADD 50+ HP to your Corvette"