Hi guys,
Can someone explain to me how it seems whenever a redline rises, the max torque figures decline.

LS2: 400/400
LS7: 505/475 with a 500 rpm higher redline

Then the Honda S2000, 8000 redline, 240 hp/hundred something ft/lb torque.

Is there a relationship between DOHC type engines and torque? Seems the new BMW V-10 is low on the torque, and all Porsche engines have significantly lower torque ratings than HP ratings.

Seems the higher tech the engine, the lower the torque is.

Does a higher redline make up for a lack of torque?

Just trying to understand. Thanks.

 

Here are a few things that may help or get you on your way.


"Torque is the only thing that a driver feels, and horsepower is just sort ... 300 foot pounds of torque will accelerate you just as hard at 2000 rpm as it 4000 rpmsin the same gear, yet, per the formula, the horsepower would be *double* at 4000 rpm. Therefore, horsepower isn't particularly meaningful from a driver's perspective, and the two numbers only get friendly at 5252 rpm, where horsepower and torque always come out the same."

http://vettenet.org/torquehp.html


Torque = [( HP * 5252 )/ RMP ]


at 5252 RMPs HP = TQ

So the higher the RMPs the lower the torque.

 

TRR

 

<drool>

 

Torque[rpm] = Displacement * Breathing[rpm] * [compression**(gamma-1)]

Notice that the 6 litre LS2 at 400 lb-ft would be 400*7/6 = 466 lb-ft. So the GM guys found another 10 lb-ft beyond the L2 through either better breathing or a touch of compression. So, essentially the TQ numbers are "right what you should expect" with a 6 litre engine being punched out to 7 litres!

Another name for breathing is Volumetric Efficiency. and Gamma is 1.39 the ratio between heat of adiabatic heating and isothermal heating.

HP, on the other hand is simply: HP[rpm] = TQ[rpm]*rpm/5252

Many times when a more agressive cam is installed in an engine, the TQ at high RPMs increases (causing an increas in high end HP) while the TQ at low RPMs decreases (slightly). This is a function of the time of intake valve closing. With a LS2 cam the intake closes around 30 degrees ABDC*, a more agressive cam might not close the valve until 40 degrees ABDC (or more). During these 10 degrees, the piston have moved upwards in its compressing stroke, and at low RPMs has pushed some of the mixture back up into the intake runners. This shows up as a loss in low end TQ. At higher RPMs the momentum of the inflowing air continues to fill the cylinders even while the piston is comming up, resulting in more TQ (at these higher RPMs).

So cams for high RPM operation loose low end TQ, in efffect, moving the powerband up the RPM range.
[*] can someone supply the actual valve closing timing WRT BDC.

 

Great info!

 

Others have added some good info. I'll add some generic
rules of thumb:

1. To a first approximation, on normally aspirated engines,
torque is correlated with engine displacement. Therefore,
larger engines tend to have higher torque outputs.

2. HP depends on the rpm range in which an engine
develops its torque. All things being equal, it's better to
develop torque at a higher rpm than a lower rpm because
then you can take advantage of steeper gearing to give
higher torque at the rear wheels. Torque at high rpm
results in high hp figures. For example, diesels make huge
amounts of torque but at very low rpm so typically produce
low HP values.

3. Larger engines, unless you go to 10 and 12 cylinder
models and DOHC solutions (and even more exotic in the
case of F1 or motorcycles), have greater reciprocating
mass per piston/cylinder and therefore need more exotic
metals and technologies to achieve high rpm. So if you
want a cheaper, more reliable hi-perf engine, you are
better off lowering max rpm and increasing displacement.

4. Some argue that there are benefits to high hp engines
of smaller displacement (a la BMW M5) because the torque
the rest of the drivetrain needs to tolerate is much reduced,
allowing smaller, lighter components.

Again, rules of thumb, not absolute physical laws.

Pat

 

Torque is a twisting force. Nothing needs to be moving for their to be a measurement of torque. There can be a measurement of torque at zero rpm's. Think of it this way. You can tighten a bolt to 100 ft lbs of torque. Then come back and apply another 100 ft lbs on the torque wrench to the same bolt - already tightened. That bolt will still be seeing 100 ft lbs, even though it will not turn any more.

Horsepower is a factor of torque, but it's measurement involves something moving. In the case of an engine, the movement is the crankshaft rotating. The more times it rotates in a given period of time - the farther it is moving. Therefore, spinning at a faster rpm, the meaurement of horsepower increases dispraportionately to the measure of torque because the crankshaft is moving at a greater rate of speed per time (revolutions per minute, in this case)

Now, someone said that toque is what moves a car. Technically, that is not correct for the reason I stated above. I agree that torque is the important measure of the driving force, but because there can be torque at zero rpm, there must be horsepower to actually move a car.

 

Nice job!

Rich

 

MitchAlsup has it right with regard to torque production, and jschindler makes an excellent point that torque by itself is meaningless with respect to acclerating a vehicle. That's because acceleration is a time function, and torque is independent of time, but horsepower is torque per unit time (with a constant thown in to make the units come out right), and that does relate properly to acceleration.

This latter is why a small high revving engine of a given hp can produce accelerations similar to a larger displacement lower revving engine of the same hp, provided gearing is properly selected in each case. In other words, we can make rear wheel torque anything we'd like with gearing, provided the engine has enough horsepower. This ties in with catpat8000's rules of thumb because horsepower matters.

This is a somewhat simplified view, of course, since real cars with real transmissions have a finite span of gear ratios, and pausing to change gears costs time. So, given two engines with the same peak hp, the engine with the broader torque curve will be faster, because it has to change gears fewer times. Larger displacement engines typically will have broader torque curves, which is why in the real world a larger displacement engine will usually beat a smaller displacement engine with its much peakier torque curve.

 

Apart from the launch and a small amount of time in
1st gear at lower rpm, all that matters in 1/4 mile run
is the torque curve over the rpm range from where
you land after a shift to redline. In the C6, this is
approx 4500rpm - 6500rpm. This isn't a very broad
curve, as internal combustion engines go.

Also, I respectfully disagree that large displacement
engines have broader torque curves than smaller
displacement engines. They simply make more torque.
I don't see any reason why the curve shape itself is
a function of displacement.

My old M3 developed maximum torque at 4800rpm
and max hp at 8000 rpm. That's a pretty broad curve
and it was a 3.2 litre inline 6.

With the modern combo of variable valve timing and
high rpm engines, there is no reason for an engine of
any displacement to be peaky.

Pat

 

Take the S2000 vs the 350Z

Very differant means but the ends are close.

There are advantages to the smaller displacement method.
You can build a cheaper smaller more agile car. The S2000 handles better than the 350z. A good amount of the money put into the s2000 is that wonderful engine. I came to my C6 from an S2000 and it's a great car. That said modding an S2000 would be expensive compaired to modding a larger displacement engine, you have few choices and all the good ones involve forced induction. That's getting pretty serious and kind of scary on an engine that's already so tightly wound. But many have built 350HP s2000's with superchargers, and they seem pretty reliable.

-mikey