Engines, horsepower and torque...please help clear this up!

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Hi everyone,

I was just wondering: Why do some cars with the same horsepower/torque ratings, curb weight and transmission perform differently? The reason why I ask this question is because it just does not make sense!

For example- I Drove a 2003 Chrysler Intrepid. It had the 2.7 liter DOHC V-6 which is rated at 200 HP and 190 lb-ft of torque. It's acceleration and passing power was adequate, but nothing special... I then drove a 2004 Pontiac Grand Prix. Although this car had a bigger V-6 at 3.8 litres (pushrod), the horsepower was also 200, and the torque was only 40 lb-ft more than the Intrepid. It seemed a little underpowered for me, well with a bigger engine and all. The Grand Prix was a fast car! I was surprised at the acceleration from a stand still, and it really performed nicely on the express way. The dash to 100 km/h took maybe 7.5 seconds. It felt a lot faster than the Intrepid...

Now why is this so? How come the Grand Prix put the Intrepid to shame? Both cars have practically the same curb weight. Both have a 4-speed automatic transmission, and both had 16" wheels and about 20,000 kms on the odometer. The Intrepid, although with a smaller engine (2.7 VS 3.8) had practically the same power output. Should it not perform just as good??? Does the size of the engine really make that much of a difference even if the horsepower/torque are almost the same???
Hopefully someone can explain this...many thanks

Cheers,

posted by  donvito2005

Forget about engine torque ...leave that to the internet mechanics. Look at the spread/gradient of power and how effectively it's transferred to the ground through gearing and transmisson.

Remember that power is power no matter what guise it takes. A small engine producing 100kW @ 6500 rpm has the same instantaneous power as a big V8 producing 100kW@ 3200 rpm.

A longer stroked engine will tend to be laggy in comparison to a shorter stroked one. So if your bigger engine displacement is via stroke it may feel less responsive, but feel gruntier.

And you are implicitly correct, there are lots of other variables that contribute to a good handling car.

posted by  Wally

Although other factors will affect how a vehicle will acelerate (effective gear ratio is the biggest), the Grand Prix likely felt more powerful because it has over 20% more torque than the intrepid.
Torque is very important and the torque rating (and the rpm where it reaches peak torque) both contribute greatly to the "seat of the pants" feel of a cars acceleration potential.
A big block V-8 producing 400hp and 450ft lbs of torque will feel way more powerful off the line compared to a similar vehicle with a 400hp small block with 350 ft lbs of torque. Remember: horsepower is simple a function of torque x rpm divided by a constant (5252, if I remember correctly).
Another example would be my Cummins diesel pick-up. It's rated at 325 hp and 600 ft lbs of torque. Torque peak comes at around 2200rpm, but the torque curve is virtually flat from 1400 rpm to 2700 rpm. The Hemi in the same truck is rated at 340hp at a higher rpm and much less torque. The Cummin's increased torque means that it has WAY more power at a far lower rpm, and although the hemi has more hp, the Cummins can easily outrun it when loaded up to maximum GVWR, where the torque can be put to work.

Dave

posted by  DPelletier

Torque does not produce power, it defies all the laws of physics. You are confusing a crude method of establishing power by measuring (yes measuring) torque. Power is liberated in an engine by burning fuel, not by some magical motive force called torque that's sucked out of the air. It's very basic high school stuff.

The fact the equation for torque and power is a simple algebraic one should tell you that you can't have large low speed torque without a corresponding high power rating. And your truck analogy tells you the answer:- a truck has lots of gears and takes a lot of time to get up to speed.

When was the last time you saw linear motion of a piston with gobs of pressure behind it rated in torque units = never because its not annular. And one more time for the record Torque (NM) = Power (watts)/ (rps x 2Pi) :- note the Pi value. Torque is a product of power applied to an annular application.

posted by  Wally

Wally...you're getting way too complicated for some of the lesser minds here. They don't understand about BMEP or BSFC and how they're related and effect each other......an engine with loads of low end torque can take the proverbial nose-dive on high-end horsepower...when you take you're vehicle to get dyno'ed...the torque produced to the wheels is what is measured..it isn't an imaginary number...torque is a torsional force(twisting force) which can be measured...which is what dynamometers measure power in, then use the previously stated formula to calculate hp(hp is just a number calculated from the torque curve, nothing more, nothing less). In the case of the 2.7 Vs. the 3.8, well, the 3.8 has a broader torque curve.

Why do you think flywheels hp/torque numbers are higher than what is measured to the wheels? Because the torsional force of the engine(known as torque) has to transfer through gear clusters(as well as a torque convertor and drive the pump in an auto trans) through the drive shaft and u-joints as well as change directions at the differential(rwd example). And remember, in an auto trans, the convertor is basically two fans working against each other, pushing transmission fluid.

Too many people just look at peak torque and hp numbers....when in reality it's the entire rev range that's important...if you have a car that peaks 220 lbs-ft of torque at say, 3200rpm, but produces over 200lbs-ft of torque from 2500-5500rpm and another car that produces a peak of 220lbs-ft at 4000rpm in which the torque quickly ramps up to that peak then dies off...well, guess what, that car will feel like a dog compared to the car with the wider power band.

posted by  Sick88Tbird

Sorry if I get to complex, I really try to keep things at a minimal level....I do I tell you, argh :mrgreen:

posted by  Wally

I am aware of all the physics involved thanks to years of university. I don't have the energy for another in-depth post on the physics involved in an internal combustion engine. Your first post implied that torque isn't an important measurement for engine performance and I disagree. While an engine with 200 hp and 200 ft lbs can perform work at the same rate once it gets to the rpm that it produces 200 hp as an engine that produces 200 hp and 400 ft lbs of torque, the torquier engine will get to its hp peak quicker due to the relationship between hp and torque.
As far as the truck taking longer to get to speed, that's a function of its GVW and gear ratios involved, not the engine.
Sick88Bird has hit the nail on the head; peak numbers don't tell the whole story, you need to look at the power curve. Due to it's larger displacement and higher torque rating, the Grand Prix got to it's 200hp faster than the Intrepid did. Ergo the resulting "seat of the pants" feeling.
Where higher rpm engines have an advantage is the ability to use lower gears and maintain the same peak velocity, thereby multiplying its available torque.

Dave

posted by  DPelletier

I stand by what I have said. I too have spent many years at uni and I'm sorry you no longer have the zeal for even shallow conversation :mrgreen:

posted by  Wally

Thank you so much for the explanations!

Cheers,

posted by  donvito2005

I stand by what I have said too.
Bottom line is the GP felt faster because it has more torque and more power at a lower RPM.

Dave

posted by  DPelletier

I think that's really all he needed to know, rather than see us blather on with our technical jargon. DPelletier, I think you've explained it perfectly.

posted by  Sick88Tbird

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