SBC_and_a_stick

I think I can answer that for you.

The TVS 1740 builds boost linearly from 2500rpm to about 4000rpm, and then increases boost about one PSI fairly gradually on this platform. Unlike the TVS2300 on the ZR1 this is a far better boost curve. It peaks at high rpm instead of mid rpm and it produces more boost around 6000rpm than the ZR1 blower. In effect, the supercharger should produce a steeper, more naturally aspired like power curve than the old ZR1 blower. Furthermore, we know it's more efficient by 10%. An additional benefit for those who tune their cars is that you can maintain lower boost at low rpm so there is less chance of detonation on extreme tunes. Rev high only on high octane and you are good. Ok, so it's not the blower.

If it's not the blower than it has to be the head. Two things going on, both of which impede the ability of the valves. The introduction of direct injection meant that the fuel inectors needed to be directly coupled for injection. The optimum location required a lower diameter intake valve than that in the LS9. There is your first factor. The introduction of AFM required AFM capable lifters. These lifters don't like a lot of lift. GM engineers specified lift at the max AFM lifters can tolerate, which unfortunately is less than that of the LS9. There is your second factor.

The single largest factor torque drops at high rpm has to be AFM, indirectly by restricting maximum lift on the exhaust valve. I don't think the intake valve limitations matter as much given there is boost.

GrantG

Those guys are Formula One commentators, so for that conversation you can change "never being below 4,000 rpm" to "never being below 6,000 rpm" because F1 cars idle at 6k and will stall at anything below that (at least that was the case before this year when they went turbo - not sure of the exact idle speed now, but sure it's at least 4k rpm).

RedLS6

The AFM explanation makes sense. Looks like the peak output could easily have been 35-50hp higher if not for the drop.

sunsalem

Last year's 2.4L NA V8s revved to 18k (don't know their idle speed).
Went to Austin last year for the race and the sound was glorious.

This year's 1.6L Turbos rev to 11K, with much higher torque than the 2.4L NAs.
They can rev higher, but because the cars are restricted to 100kgs of fuel per race they don't.
Hopefully next year they will loosen the fuel usage rule and the engine revs will climb back up where they should be.

SBC_and_a_stick

Another piece of evidence is the SRT Hellcat comparison. Same basic achitecture and displacement. The supercharger does generate more boost but there is a lot more HP than torque on the Hellcat, ie. more top end torque. Why? No AFM or DI. The result is bigger intake and exhaust valves and more lift.

I would have preferred a non DI non AFM LT4 engine. Compared to the actual LT4 produced: lower weight, similar torque, lower noise, cheaper, a lot more power.

Ching Ho

Looks like Autoblog & Motortrend may have borrowed their idea from our thread on Corvette Forum. Funny how so many posters discounted torque to weight as a legitimate metric until the major car media started using it.

https://www.corvetteforum.com/forums...-in-class.html

stevebz06

Not being an expert in DynoJet dynos, I'm going to go out on a limb and say that I think you have it backwards. Horsepower was just a way for people to connect to new contraptions like the automobile.

One horsepower is defined as 550 foot-pounds per second. Foot-pounds? Kind of sounds familiar, doesn't it? Like the way we measure torque?

If you go to the farm and grab a couple of horses to see how they rate against this standard, some will probably do it easily, maybe some will have trouble or can't do it at all. It's an arbitrary number made up by some human engineers so that ordinary people transitioning from real horses can relate.

I know that a lot of turbines are rated by horsepower. I used to fly helicopters and they were all rated by horsepower, and turboprop airplanes, as well. None of them made 100,000 hp, though! I was talking about jets that fly from thrust instead of going through a gearbox and driving a propeller, although most, if not all, commercial jets these days produce most of their thrust from a geared fan. If you use the correct formula for calculating horsepower that it takes to get a Boeing 777 to fly at 540 knots, I'm pretty sure it would come out well above 100,000 horsepower because of the speed component.

I'll do some research.

SBC_and_a_stick

So facts are decided by a popular vote nowadays?

If that were true, and scientific reason did not exist, an LT4 would be nothing more than a bottle opener.

glass slipper

We've had these discussions before (not just you and I) and it always bothered me when people put up the F=mA formula (a first order equation as you stated) when talking about translational acceleration but I never put a lot of thought into it and even agreed with the peak acceleration happening at peak torque crowd. I mean who can argue with Newton. I made a few posts several years ago relating HP and kinetic energy to acceleration and I became more curious. KE=½mV², a second order equation. Acceleration is a second order equation, you take the 2nd derivative of the equation to find acceleration. As you know, HP is also defined as 550 pounds lifted one foot in one second or 550 ft-lb/sec with the pound being weight, not mass. When you convert to mass, the unit of HP becomes lb-ft/sec². To bring everything together: KE, HP, and friction/drag losses are all 2nd order equations. F=mA works because it's for that falling apple where gravity has units of ft/sec², a 2nd order equation. But we don't have a vertical acceleration situation, this is translational acceleration (accelerating in a straight line).

Having said all of the above, max acceleration does not occur at peak torque because torque is just a force which is linear or 1st order, you have no way of resolving all of the 2nd order influences unless you include the time factor. Before you get ahead of me here, I'm not saying peak acceleration occurs at peak HP. Since KE is a 2nd order equation, the amount of KE added to the car to accelerate from 30- 40 MPH is ¼ of the amount of KE added to the car to accelerate from 60-70 MPH even though both are 10 MPH increments. It takes the same amount of HP to add a certain amount of KE (let's say 1000 units) to the car no matter what speed you're going, therefore the higher your speed, the slower you accelerate with the same HP. I haven't run any numbers but it's obvious peak acceleration is past peak torque but before peak HP if the HP curve is flattening out as it reaches peak HP. In the case of the LT4/LS9 where the HP curve climbs at a steep rate up to peak HP, peak acceleration will occur closer to peak HP.

The main reason I investigated this is because I have a DOHC engine in my C4 ZR-1 that has an almost flat HP curve from ~5600-7000 RPM, it only varies about 20 HP through the whole range. On the other hand, torque peaks at ~4800 RPM and continues to drop out to 7000 RPM where it's ~65 LB-FT less than peak torque yet I don't feel acceleration dropping off. The first time I raced the car, I hit the 7100 RPM rev limiter hard in 1st gear because it just didn't experience any decrease in acceleration that I came to expect with a pushrod engine. I was used to holding a gear until I felt acceleration drop off and then shift...it just doesn't happen with this DOHC engine.

Road racers use a program where they load the HP curve and the speeds on track into it and the program tells them the optimum gears and shift points on track to use. Notice it doesn't use the torque curve, torque is a 1st order force that is really meaningless in the overall scheme of things. One day I'll sit down and run the numbers to see where peak acceleration occurs.

glass slipper

Except that it isn't a legitimate metric, it's absolutely meaningless. I saw your "calculations" on post 42 and it was based on statistics, not physics. Statistics is a way to make numbers say what you want them to say.

nitrojunky

yep. horsepower is going to determine acceleration at a given speed. lotsa torque misinfo ramping up again with newer low RPM engines coming out again. the only reason peak torque numbers come up is because a high peak torque number often means a fat power curve and because people can't wrap their head around basic physics.

for those that want to argue,

defn of kinetic energy: T = 1/2*m*v^2
defn of power: P = dE/dt (where E is energy; can also use W for work. same thing. J)

so some quick calculus to illustrate my point:

dE/dt = dT/dt = d(1/2*m*v^2) = 1/2*m*2*v*dv/dt = m*v*dv/dt = m*v*a, where a is acceleration.

therefore

P = m*v*a -> (P/m)/v = a. at a given speed, the power to weight ratio determines acceleration. for spherical corvettes in an intertial frame, in a vacuum, and with frictionless tires of course.

nitrojunky

his R^2s were very poor (weak, weak correlation) and driven by three outlier points, two of which were corvette and one viper.

glass slipper

You need to do a lot of research...I don't have anything backwards. I was in charge of the internal combustion engine lab my senior year at Georgia Tech and it had every kind of dyno you could imagine, I think I know how different dynos work.

A DynoJet inertia type chassis dyno measures HP by measuring the change in RPM of the roller drum to determine the Kinetic Energy added to the drum of a known mass/moment of inertia and then dividing that amount by the time it took to increase the KE to determine HP. From that, torque is calculated...there are no strain gages to measure torque.

Yes, HP=550 ft-lb/sec...as in 550 pounds lifted one foot in one second. The "ft-lb" term is not torque, it's a unit of work...HP is the rate work is done therefore the units are ft-lb/sec where the "lb" term is a pound mass. Torque is lb-ft which is a unit force where the "lb" term is a pound force. They are two different units of measure. Torque is a cross product of two vectors resulting in a vector with magnitude and direction while energy is a dot product of two vectors resulting in a scalar that has magnitude but no direction. The unit of HP was defined before cars came along.

The 550 ft-lb/sec was a measured number from watching the average work a horse could do, there was nothing arbitrary about it. However, the two terms in torque, foot and pound-force, are very arbitrary. The foot is based on the length of an average man's foot which of course varies. Over the years, depending on the location and time, the length of a foot has varied arbitrarily. The foot was defined in the US in 1893 and then internationally in 1959. It appears the foot is not only arbitrary but a moving target. The pound was initially as 7000 grains of wheat or barley and has experienced the same arbitrary changes into modern times as the foot.

nitrojunky

unfortunately, it's a different foot*pounds. for hp, it's pounds pushed through a given number of feet, a unit of work. for a torque unit, it's also foot*lbs, but it comes from a cross product and does not mean pounds pushed through a length. also, it is a vector quantity, not a scalar.

to get work (energy), torque also needs to be multiplied (actually, dotted) by an angle, given in radians (dimensionless, which doesn't add another unit so it stays ft*lbs) to convert it to a unit of work (a scalar) as opposed to a unit of torque (a vector). also, you missed the important point: per second. energy (work) per second is power.

nope. the concept of energy and power is very fundamental in physics, came waaaay before the car, and has an incredible number of well understood uses.

ChevyChad

So I’m guessing you didn’t have an Eddy current dyno there in your lab did you? Because they DO measure torque and then calculate HP as a result. And NO, even a dyno jet does NOT measure HP directly; it measures acceleration of the drum and acceleration of the vehicle and relates the 2 to calculate HP. But, as I’m sure you already know, the units of acceleration are distance/ time^2. So again, it uses a calculation to figure out HP. And it’s very convenient in the imperial system that 1 pound of force equals 1 pound of mass (on earth at least). Bottom line is, no matter how you slice it, horsepower is a CALCULATED value. There is absolutely zero way to measure it directly. We CAN measure force, mass, distance, and time and then calculate a slew of things from there.

nitrojunky

torque is also a calculated value. one measures the length and direction of the vector pointing from the axis of rotation to the point at which force is to be applied, then one measures the magnitude and direction of the force applied, then one calculates torque by crossing the vectors in the proper order producing a handed and perpendicular vector. so torque is also a 'calculated' measurement

bottom line: at a given speed and all other factors being the same and allowing for proper gear selection, the car with more (horse)power will be able to produce a larger acceleration. period.

speed is a function of energy and power is how fast you can change the energy level. it's very basic physics, and torque at the flywheel is nowhere to be found.

glass slipper

Actually, I did run an Eddy current dyno at Georgia Tech but you have failed to make a point. Read my post again, I was talking specifically about a DynoJet inertia type chassis dyno which does measure HP. He stated HP is calculated from torque which is measured first before HP can be calculated...all I had to do was show one instance where that isn't true and I did exactly that, try to keep up.

You continue by splitting hairs about measuring HP...by your reasoning, torque isn't measured either. The strain gage measures the strain in the shaft and correlates it to torque based on the diameter, material, and temperature of the shaft. Or, after the lever arm is measured and the force applied is measured, torque is calculated. Splitting hairs works both ways but what he was really saying was torque has to be measured first before you could calculate HP and he was wrong, your weak attempt to defend his position makes you just as wrong. The bottom line is you can't directly measure torque transmitted though a spinning shaft, you can directly measure the work done by a spinning shaft and divide it by the time it took to measure HP or HP=dW/dT. By measuring the rate at which the roller drum is accelerated, there is a direct correlation to HP so the output is just given in HP, not α. Therefore, we just measured HP with torque calculated...again, this is on a DynoJet inertia type chassis dyno.

FYI, one pound of force/weight does not have the same units of one pound of mass.

raj

In your example, the KE increase in going from 30-40 should be a bit over one-half (~0.54) the KE increase in going from 60-70, not 1/4 - what am I missing here?

We could keep things closer to a first order by limiting ourselves to first gear acceleration runs (both drag loss and frictional loss should be negligible). I would imagine in this scenario, that peak acceleration would be close to the torque peak and, if anything, it should occur slightly lower than the torque peak due to increased drivetrain loss at higher rpms. If you do run some simulations, I'd be very curious to see how this plays out.

nitrojunky

peak acceleration in a given gear will occur at the torque peak. but select the gear ratio at that peak torque speed such that you're instead at your peak power RPM, and your acceleration will be greater. if you understand basic calculus, see my simple motivation a couple posts up. it is provable, it is not an opinion.

ChevyChad

I was not trying to defend anyone. I was simply stating that you can not measure HP directly. It is a calculation. That was my point. Guess you missed that.

And an Eddy current dyno does indeed calculate HP based on its derrived torque measurement. The conversation was not limited to dynojets until you said it was just now.

And yes I know one pound of force does not have the same units as one pound of mass. However, one pound of mass creates one pound of force on earth.