RWHP v. HP

In your example, HP is the measured power of the engine when its not in the car. RWHP, is measured horsepower at the rear wheels, that is, the leftover power after it has transferred through the drivetrain.

ditto

One is big and the other is small

To elaborate, there is a drag added by each of the miscellaneous drivetrain parts - for manual transmissions, that is approximately 15-18% drag.

Let's say you go and get dyno'd and you get 275 RWHP. 275 / .85 =323 HP at the crank.

Ford rate the new motor at 300bhp SAE Net

SAE = Society of Automotive Engineering

They set standards for how to test the engine, it will be tested at particular temp and humidity levels (or if not correction calculations added to the numbers produced).

Net = this means that it is done on a chassis dyno, so the engine is not actually in the car. But it does have to have all of the ancilary devices fitted, such as water pum, air con, alternator, air filter, full exhaust system, etc.

This figure is often called flywheel or crank bhp. In years past (pre 1973) American manufacures used to measure SAE Gross numbers, these are the same as above but the engine did not have any ancilary devices which could sap power. This is howcome some of the muscle cars of old have 'massive' bhp numbers. However if they where rated today they would produce much lower figures. A Mopar 440 rated at 370BHP SAE Gross probably produces about 290bhp SAE Net. Different cars and ancilaries will affect the amount lost.

Other countries suck as Britain and the rest of Europe generally do not quote SAE figures, either BHP or PS is listed. PS is German for horsepower. This is a Metric and Imperial rating, however they are very very similar. An MG ZR160 produces 160PS or 158BHP (the actual power is the same it just depend on the rating system, just like Centrigrade and Farrenheight for temps)

British BHP ratings will tend to be almost identical to American SAE Net numbers.

Along with the different ratings there are also different methods of producing the numbers. HP stands for horsepower which refers to how fast a horse can lift a particular weight. BHP stands for brake horse power, this uses a braking device to measure the force, ie torque of a motor. Then using a calculation the bhp numbers can be produced

**Confuesed yet?**

Countries such as Australia use yet another rating system kw (kilowatt) this in reality is probably the most accurate measure, but it again depends on how the numbers are produced.

RWHP - this doesn't really exist as it could be claimed as RWBHP if it was measured using a brake device, yet there are also SAE ratinging for it as well.

These are measured on a rolling road as a rule by the wheels rotaing drums in the floor, these devices will then usually measure the rotaional force (torque) and calculate the bhp from these numbers. There are many types of rolling road dyno, in America the Dyno jet and the Mustang dyno seem to be the most common. The Dyno jet just uses a weighted drum which measures the amount of force required to turn it, these dyno's tend to produce higher numbers, a Mustang dyno uses varying loads so is much better for tuning the engine over the entire rev range (eliminating flat spots etc.). But because the load varies the numbers produced will appear lower.

For example if a car makes 220rwhp on a Mustang dyno it may make 230+rwhp on a dyno jet, this does not mean it produced more power on one dyno than the other, just that they are measured and rated differently. In the same manor that 1US dollar has a different value to 1 Australian dollar. They are both dollars but they are not the same REAL value.

Rolling roads only really have one use - and that is tuning, they let you see how the engine is performing. The curves on the graphs are much more important than the peak numbers produced. As they are to often inaccurate, temp, humidity will affect them, even the type and pressure of your tyres will have an affect on the peak numbers produced (however they will not affect the profile of the curves on the graph so the engine will still be performing the same), if everybody added correction factors to their dyno runs then you could compare them, but very few people do. So if you car dyno's is 20rwhp less than someone elses they may infact be producing the same power as the results are flawed. If you and a mate take your cars at the same time to the same dyno and have them run then these figures could be compared to an extent although they can still be out.

The only true way to ascertain true bragging rights it to remove your engine and have benchmarked on a chassis dyno conforming to the same standards as the manufacure.

Chassis dyno's produce higher numbers than rolling roads, because the drivetrain (clutch assy, gearbox, prop % drive shafts, wheels, brakes, bearings, etc.) all produce friction and need power to rotate them, thus giving the lower figures at the wheels.

Yeah, I think that I pretty much understood all of that

haha, nope

Wow 300bhp/ton, what an incredibly well worded explanation. Perhaps then you could answer a question I have along the same lines. When a car is on a dyno, you state the HP is determined by measuring the Torque. What I don't understand is, if that is the case, how can torque curves be different than HP curves and how can the relation change from vehicle to vehicle? I.E. how can one engine have more torque than horsepower while another has more HP than torque? If one is derived from the other, well I just don't understand how that's possible.

Thanks for any help you have time to give to those of us that may be slightly less informed

Rwhp

I have been reading all the threads about how much RWHP is gained by adding intake and tune. Suppose I gained 25RWHP, does that mean that I actually gained about 30HP at the front end (assuming 18% loss) or am I missing something?

Aipaloovik said:

Wow 300bhp/ton, what an incredibly well worded explanation. Perhaps then you could answer a question I have along the same lines. When a car is on a dyno, you state the HP is determined by measuring the Torque. What I don't understand is, if that is the case, how can torque curves be different than HP curves and how can the relation change from vehicle to vehicle? I.E. how can one engine have more torque than horsepower while another has more HP than torque? If one is derived from the other, well I just don't understand how that's possible.

Thanks for any help you have time to give to those of us that may be slightly less informed

Click to expand...

Remember, you asked ...

Max engine HP or torque refers to a "peak" number at a certain engine speed. It all depends on the torque curve of the engine. Torque is not linear across the RPM range of the engine. It generally peaks at a low RPM and falls off as engine speed increases. Since HP is a function of torque and engine speed, how quickly the torque falls off will determine if the HP peak is above or below the torque peak. If the torque falls off relatively quickly as RPMs increase, then it's not as likely that HP will peak above torque. If, on the other hand, the engine can continue producing torque higher into the RPM band, keeping the torque curve more flat, then there's a greater chance the HP curve will peak above torque. As a rough calculation...

HP = (Torque x RPM) / 5252

300bhp/ton & cheezsnake

You guys are good, are you both teaching in a school?

Very good explanation, couldn't say it better... !

Outstanding explanations - THANKS!! Care to tell us about Newton Metres of torque - those figures mean a lot but mean nothing to me since I don't have even the foggiest idea what they mean.

Cheers,

cheezsnake said:

Remember, you asked ...

Max engine HP or torque refers to a "peak" number at a certain engine speed. It all depends on the torque curve of the engine. Torque is not linear across the RPM range of the engine. It generally peaks at a low RPM and falls off as engine speed increases. Since HP is a function of torque and engine speed, how quickly the torque falls off will determine if the HP peak is above or below the torque peak. If the torque falls off relatively quickly as RPMs increase, then it's not as likely that HP will peak above torque. If, on the other hand, the engine can continue producing torque higher into the RPM band, keeping the torque curve more flat, then there's a greater chance the HP curve will peak above torque. As a rough calculation...

HP = (Torque x RPM) / 5252

Click to expand...

If your wondering where 5252 came from that is the magical # that all cars have where the hp and torque curves cross, just incase anyone was wondering

cheezsnake said:

Remember, you asked ...

Click to expand...

Yes, indeed I did. And you answered very thoroughly. Thank you

I think I'm starting to get my head around the concept now.

Turbo 05 said:

If your wondering where 5252 came from that is the magical # that all cars have where the hp and torque curves cross, just incase anyone was wondering

Click to expand...

its acutally 5250 to clearify and its the place where tq an hp cross on a vehicle with an "in tune" intake and exhaust

kystang1889 said:

on a vehicle with an "in tune" intake and exhaust

Click to expand...

Its true on any vehicle. It just a constant in the equation. If they dont cross at 5250, then usually people didnt notice the scales for hp and tq were different on the chart.

Very close 300bhp, but Ford tests its engine output on an Engine dyno not a chasis dyno, a chassis dyno tests output while in the vehicle.

007 said:

Its true on any vehicle. It just a constant in the equation. If they dont cross at 5250, then usually people didnt notice the scales for hp and tq were different on the chart.

Click to expand...

Yes, it's all math and so is true with any engine. I stand corrected on the 5250 ... shows what I get for trying to recall from memory.

The only thing i have to disagree with on here is the difference between a dynojet dyno and a mustang dyno. From what ive seen if something puts down 230 on a mustang dyno, its usually in the 265 range on a dynojet dyno, its a pretty good difference.