H.P. vs Torque

[rjhat]

So, can someone explain waht the differences are between horsepower and torque? I have been around cars for as long as I can remember, including working at a mechanic shop for 2 years. But the real answer to this question is elusive. I have heard so many different answers that its getting hard to remember the question! Can anyone clear this up once and for all?

[Ranger]

This is probably not the "official" answer but HP is an engine output rating and the rated HP is only realized at peak RPM. Torque is what turns the rear wheels and is what gets the car moving (probably more important than HP). As Guru say's "people buy HP and drive torque". I like that one and have adopted it.

[Willie Hank]

try this link out:

Ford Muscle

[rjhat]

Good Description ob the ford site! thanks!

[JasonA]

Perfect explanation...helps a lot!

[Bruce Nunnally]

Another factor in hp discussions is hp at the wheels, as measured on a chassis dyno. This includes drive-train losses, rolling losses, frictional losses, because it is actually measured by putting the drive wheels on rollers and determining how much power the car puts to the road. Dyno average experience is that wheel hp is often 15% less than rated hp at the crank for manual transmission cars, and 20% less than rated hp at the crankshaft for automatic transmission cars.

Confusingly, based on their on-the-road performance, the rated 300 hp Northstar engine/4T80e transmission often get low marks on a chassis dyno, showing 206-216 hp on the ones I have seen. Example:

With the average automatic losses, one might expect the Northstar 300 hp drivetrain to put around 240 hp to the wheels (80% of 300).

I recognize there are some engine to engine power variances based on normal production tolerances. And I recognize that the 4T80e is a 'beefed up' version of previous front-wheel drive transmissions, in order to tolerate the torque; this perhaps makes it less efficient in transferring power. I also have experienced variation in the amount of torque allowed through by the 4T80e torque management system in back to back on the road runs -- variances in 0-60 times are often from 40-60 mph and not from 0-30 mph. On the other hand, Front-wheel drive is theoretically marginally more efficient than rear wheel drive in this respect, due to fewer power transfers.

What are the predominant causes of the difference between expected and actual chassis dyno results for the Northstar?

[justgreat]

this question goes in line with my earlier post regarding the power/torque curves for the 3.6. manufacturers make a big deal out of the "ultimate" numbers for hp and toque: 315 hp at 6500 rpm!!!...so what...it's much more important and useful to know WHEN in the power band the hp or torque is made...ideally, you want to see 90 per cent of the total output being produced between 2000 and 5500. especially, the low end of the rpm range: this is where we do most of our driving. jackg

[tjtjwdad]

To: bbobynski

A damned good explanation. I like your simple analogy!

For anyone else:

Does a FWD or RWD have more HP/TQ loss? It seems to me back in the 1973 the Tornado was rated 25 HP better than the Olds 98 with the same engine. Of course FWD systems are much different today.

Jim White

[jschunke]

horsepower = torque x speed.

[JasonA]

Does a FWD or RWD have more HP/TQ loss?  It seems to me back in the 1973 the Tornado was rated 25 HP better than the Olds 98 with the same engine.  Of course FWD systems are much different today.

In theory, the FWD drivetrain is more efficient because there are fewer frictional losses. There's no propeller shaft to turn, no heavy solid axle to turn (in the case of live axle RWD cars). I think when they rate engines, they do so at the crank, before any drivetrain losses.

In 1973, it was actually the reverse -- the Toronado was rated at 250 net HP with the 455 and the 455 installed in the Custom Cruiser and 98 series cars was rated at 275 net HP. The reason was the intake manifold and exhaust system in the Toronado. The engine was mounted longitudinally, and higher up to make room for the front drive shafts, and the intake manifold had to physically be much shorter than that on other engines (to be able to mount the 4MC carburetor and air cleaner and still have clearance under the hood). It was much more restrictive than the "normal" intakes on other 455s. But those rated HP figures, again, are at the crank and don't take into account the drivetrain losses. If the Toronado's drivetrain was 10% more efficient than that of the 98, actual wheel horsepower might have been very similar.

[tjtjwdad]

I was beleiving the HP ratings for the 1973's was 250 for the Toronado and 225 for the others. Didn't know the Toronado had a different intake but that would play a role with the HP/TQ rating of any engine.

Also I thought:

In 1973 the HP/TQ ratings were at the rear wheels.

1970 was the last years for just the engine.

1971 the readings were from the engine with accessories attached.

1972 and to sometime in the 80's/90's the reading was at the rear wheels.

Not sure where the HP/TQ is now but I think it is BHP ... what ever that is.

Jim White

[Bruce Nunnally]

Before 1972 hp was rated as SAE Gross hp. From 1972 on hp was rated as SAE net hp. The two are not directly comparable, because they are two different tests under different configurations and different environmental conditions. In general, SAE gross is without accessories or exhaust, and SAE net is closer to how the engine performs in its road configuration with all accessories and exhaust. Because the tests are under different conditions, there is not a direct mathematical relationship between the two. However, the 1972 Cadillac 472 cubic inch engine was rated at 220 hp SAE net, and 345 hp SAE gross for example. In this case, the net hp was 64% of the gross hp rating.

Both SAE Net and SAE gross were still hp at the crank, or at the flywheel, measured on an engine dyno, not at the wheels as measured on a chassis dyno.

The chassis dyno measures hp at the wheels, with losses through the transmission and drivetrain and rolling losses added in. Wheel hp is less than Net hp as rated by the manufacturer.

[JasonA]

I was beleiving the HP ratings for the 1973's was 250 for the Toronado and 225 for the others.  Didn't know the Toronado had a different intake but that would play a role with the HP/TQ rating of any engine.

Yes -- the first thing you do when you pickup an Olds 455 (if you're an Olds fanatic) is check the intake. The Toronado intake, compared to a "regular" 455 intake, looks like a squashed pumpkin. The carburetor pad is sunken a good bit vs. on the regular intake, which necessitated smaller (or "choked") intake runners. No good for ultimate performance. Rumor has it, this was the reason for the net horsepower rating that was lower than in the other cars.

To bring my message back into relevancy on this board, I'm thankful for the advances made in powertrain design, so these systems are much more compact and less dependent on the design of the vehicle. Having a transverse DOHC V-8 engine making 300 hp in a car as "small" as the Seville would simply be unheard of just a few decades ago!

[JasonA]

manufacturers make a big deal out of the "ultimate" numbers for hp and toque: 315 hp at 6500 rpm!!!...so what...it's much more important and useful to know WHEN in the power band the hp or torque is made...ideally, you want to see 90 per cent of the total output being produced between 2000 and 5500. especially, the low end of the rpm range: this is where we do most of our driving.

Excellent point. I wish the auto magazines would publish the power curves for the vehicles they feature as well as the ultimate peak numbers. The peak power figures are heralded as the mark of a good engine or a bad engine, but it can't be that simple. To use the Northstar as an example (because that's really the only DOHC V-8 I have a number for)...the LD8 version produces 240-250 lb*ft of torque at just 1000 rpm, according to GM Powertrain. That's about 80% of peak (300 lb*ft) -- right off idle! 90% of it is available from about 1800-5500 rpm. Are other manufacturer's DOHC V-8s this usable in everyday traffic?

Of course, there are so many other factors too, like final drive ratio (axle ratio), tire size, transmission ratios, etc. I guess the real measure is the smile-to-mile ratio. It's pretty damned high in a Cadillac!

[tjtjwdad]

jadcock Posted on Feb 8 2004, 07:59 AM

I guess the real measure is the smile-to-mile ratio. It's pretty damned high in a Cadillac! 

Ya' darn right it is, and roger that WRT published torque curves!

Well said!

Jim White