hashman626

If one car is said to have 300 hp and another 300 rwp, is one better than the other or is it two terms for virtually the same thing?

RotaryEvolution

iTrader: (30)

HP=horsepower is usually a term that is power measured at the output shaft of the engine.

RWHP= rear wheel horsepower, this is the true figure of power that the car puts down to the road surface after all the power hogs such as transmission and differential take their share.

RWHP is a more common term these days because it gives a true figure of a cars power versus the manufacturer's claims that a car puts out 350HP, when in fact it is more like ~290 horsepower to the ground. you can see why the manufacturers pick the bigger figure because it sounds more impressive. induvidual sports car builders and owners use RWHP figures because it is a true figure that applies to all cars. different cars have different loss amounts through their drivetrain. all wheel drive vehicles for example the EVO and WRX lose more horsepower through their drivetrain than we do because they have 2 differentials, a transfer case and a transmission to power the wheels whereas we just have the transmission and 1 differential.

a 7 generally eats up about 15% of its power from the engine as a loss through the drivetrain, all wheel drive vehicles generally lose no less than 20% which is pretty substanital and why they use the term AWHP(all wheel horsepower) to keep themselves in a different category.

automatic transmissions usually eat close to the 20% mark as well due to their heavy rotating masses and fluid transfer, they also can be significantly heavier than a manual transmission to weigh the car down and make it naturally slower by weight drag.

jjcobm

iTrader: (18)

HP by itself without any prefixes is horse power at the crankshaft of an engine. WHP is wheel horse power which is horse power measured at the wheels of the car.

There is power losses in the drivetrain and all the components that deliver the power from the engine to the wheels. So a car with 300HP is not really delivering all those 300HP to the ground. A car with 300WHP is getting 300HP to the ground.

hashman626

interesting, thanks

RotaryEvolution

iTrader: (30)

also note that most honda owners calculate horsepower on paper by adding up their mods list that they purchased from ebay. in some communities this is considered "legit".

Jdrift

^ Burn

hashman626

HAHAHAHA, that's actually what I was going to do for an unofficial count, or ball park figure because my car is a '91 coupe, 160 hp, corksport headers about 13hp, high flow cat i found produces about 9, but i wont run one, race pipe instead, so 10hp, then the catback and intake, ill just say 10 more, haha, that's why i call it unofficial but still need those things first, at the moment, 160hp unofficially on a rebuilt with like 30k

SirCygnus

iTrader: (1)

not 160. try 120 at the wheels, and with all that stuff, you end up with 140 to 150.

remember, even with a fresh rebuild, its rare for a stock na under any circumstances will hit 160. even with I/H/E, you might hit 160.

what you should focus on after that is the engine management. get the stock fuel setup.

doing so will net far greater HP than I/H/E alone. hell, even slapping on a stand alone will net more hp compared to an I/H/E combo.

Evil Aviator

Just to clarify, RWHP is more accurate for evaluating cars, while FWHP is more accurate for evaluating engines.

Also to clarify the anti-corporate cynicism in the above quote, I have never seen misleading advertising using the FWHP vs. RWHP issue, but I imagine it happened once or twice. In most cases the car manufacturers mislead the consumers or insurance companies with gross hp vs. net hp, or they rate the engine at an rpm that produces less than optimal power.

"Better" isn't as simple as that. Because it varies with engine rpm, horsepower is better represented by a curve rather than a single peak value. The shape of the curve will affect the car's performance. Some modifications will have a more positive effect in the lower rpms, while some will have a more positive effect in the higher rpms, even if the peak hp remains the same. Some modifications will help power in a certain rpm range while hurting it in others. For example, porting the engine will raise high-rpm power but reduce low-rpm power.

Then there is the issue of torque vs. horsepower. Torque helps a car accelerate. For example, the Mazda 787 race car's engine was fitted with a variable intake system in order to optimize torque for the twisty track at Le Mans in 1991. It worked, and the car won even though the fancy torque-enhancing intake reduced the engine's peak output by 30hp. The intake was not intended for the longer tracks in the remainder of the racing circuit in which the extra 30hp was more valuable than the torque.

arghx

iTrader: (3)

You can't rig a manufacturer horsepower test these days. They have to follow strict guidelines from SAE J1349. Then they have to be certified by an independent witness using a certification form similar to SAE J2732. I have both of these guideline sets if anyone is interested. Now I suppose the manufacturer can still be creative with its advertising, but the tests themselves don't lie. They can't. The tests are too controlled. You can't change inlet air conditions, you can't manipulate the number of accessories on the engine, you can't use some weird fuel blend, you can't run the engine really hot or really cool, you can't play with the boost or knock control.

You can call a whp number whatever you want to call it ("true horsepower" or whatever), but some roller/hub dyno in some converted warehouse isn't the same testing done under the proper controlled conditions with lab-grade equipment. It's a good tool to use for our purposes but they are vastly inferior to the repeatable testing used by engine manufacturers. And the so-called "correction factors" on your typical dyno are a complete joke. They pull numbers out of thin air and use SAE formulas in a way for which they were not intended.

Evil Aviator

Unless something has changed recently that I am not aware of, a manufacturer is only required to use SAE J1349 if they advertise their product to be "Certified to SAE J1349". They can also certify to SAE J1995 when they want to use gross hp rather than net hp. I don't think any particular method is required for advertising, so they can make up any method they like as long as the fine print covers them for truth in advertising lawsuits. Please correct me if I am wrong, as I have not been reading my SAE email lately, and I don't work in retail (thank God).

The calculations are pretty good for NA engines, but they tend to lose accuracy on supercharged engines. I have used similar calculations with extremely valid results for over 20 years, and the numbers are also accurate for supercharged engines if I have the manufacturer's performance charts. The main problems with ghetto dynos are the inferior equipment and operators.

rx7racerca

iTrader: (1)

Just to add a little to what both Evil Aviator and arghx were saying - current manufacturer HP levels are pretty reliable in terms of not exaggerating HP (bhp [brake hp, because it's measured with an engine brake, not a wheel dyno] aka flywheel HP).

If anything, some understate their HP numbers - BMW with the *35i engines for example - rated at 300 bhp, but when guys dyno them they routinely come in at nearly 300whp, so once you allow for the 15-20% driveline losses typical in rwd cars, indicating they're actually putting out well over 300 bhp. The Ford 4.0 Cologne V6 in the '05 -'10 Mustangs took a lot of flack for being rated at only 210 bhp, when a lot of other manufacturer's 3.5L v6's were in the mid to high 200's - but dyno'd, lots of reports again of whp numbers higher than Ford's reported bhp numbers, indicating actual power in the 240 range. In that case, it was pretty clearly a case of Ford wanting to make the leap in HP between the V6 and V8 seem more substantial, to match the price jump and performance image of the GT better.

BMW may do it so the *35i - equipped engines in cars also offered with larger V8's don't crowd in on the rated numbers of the higher trimline. Or they may just do it for insurance purposes, as was common in the late Sixties and early Seventies, when exaggerated HP numbers co-existed with under-stated numbers, depending on whether the marketing guys thought it better to play up the rated power or reduce the insurance hit buyers took, because insurers looked at the advertised HP numbers in setting rates.

In the early to mid 2000's there did seem to be a common pattern of "overly optimistic" HP numbers, from many manufacturers, including Mazda, Ford, Toyota, Honda, as some I remember specific instances of, although there are lots more. Which is why the more rigorous SAE testing process was developed. When introduced, you suddenly had a lot of cars making anywhere from just a few less HP than the unchanged previous model year, to drops of 10-15%.

arghx

iTrader: (3)

Technically that's true. But did you notice that all the manufacturer horsepower claims changed when they updated SAE J1349 a few years ago? That's why a Honda K20 in the Civic Si is now rated at 197hp versus the original 200, while a Corvette Z06 LS7 engine was rerated at 505 from the 500 at early testing.

You're right about forced induction skewing correction on gas engines. Check out the dyno sheet section ("proven power bragging") on the Subaru forums (nasioc.com). You'll see what I mean about people abusing correction factors. They'll take their car at high altitude dynos and have an absurd correction on top of that. SAE J1349 clearly states that +/- 3% is the maximum allowable correction factor, otherwise it will be considered a "non standard" test.

arghx

iTrader: (3)

J1349 (2004), straight from the horse's mouth:







as rx7racerca was saying, you can't have an "optimistic" engine dyno reading when you follow these OEM-grade procedures. But you can "juice" a dyno number from a shop easily. On dynojet, turn the smoothing off (set to 0) and set correction factory to "STD" (whatever that is really) and the WinPEP software will show the highest peak numbers. On some other dynos you can actually manually input a correction factor directly into the software. Then you can justify that correction number to the customer however you wish. "Oh this is a low reading dyno so I threw a 15% correction on there." Thus the dyno curves are still useful to see and so are before-and-after changes. But the numbers themselves become pretty arbitrary.

The most unreliable dyno numbers are corrected numbers from boosted engines at high altitude.