raj

So I took the rear wheel dyno charts that are floating around the forum and did some data crunching. We can talk about peak torque and peak horsepower till the cows come home but the real answer lies in the AREA UNDER THE TORQUE CURVE. Since torque (unlike HP) is a fundamental quantity, integtrating the torque vs. rpm curve and obtaining the scanned area essentially provides a true metric of the car's acceleration capabilty. Since gearing is a key factor and controls torque multiplication, the gear ratios and redline can be used to determine the limits for integration thus providing a complete and undiluted real-world basis for comparing different powertrains.

To cut a long story short, I plotted the empirical torque curves of both cars based on the data gleaned from the rear wheel dynos and made curve fits to trace the torque trends. Both the raw graphs and the corresponding curve fits are shown below:



Note: The Z06's curve required a 3rd order polynomial to get a reasonably good fit compared to the Viper's smoother torque trend that only required a quadratic fit.

At first sight, the torque curve of the 8.3 liter V-10 appears to dwarf the 7.0 liter V8's. Note however, that the Z06's extra 1000 rpm on the top end is a deceptive weapon. The areas under the curve-fitted torque curves can now be computed simply by integrating the curves between the post-shift rpm in each gear and the corresponding redlines. The gear ratios are identical on both powertrains but the LS7's higer redline allows the Z06 to have a wider spread to sample the torque curve which is the crucial deciding factor in this comparison! No doubt the Viper's torque curve is impressive, but look at the results of the analysis below and the advantage clearly belongs to the Z06. The area under the torque curve that is sampled in each gear is calculated between the shift point and redline. For 1st gear, I used 2000 and 1500 rpm as the launch/hook-up point for the Z06 and Viper respectively. Even conceding the low end torque advantage to the Viper and allowing it a scan that starts at a lower rpm in 1st gear cannot take the Z06's edge away. Such is the advantage of high rpm torque and gearing! The LS7's ability to zing to 7000 rpm is clearly the most powerful weapon in it's arsenal and the key to its ability to outperforman an engine with an 80 ft.lbs advantage in rear wheel torque!



Now, throw in the lower weight of the Z06 and there are no prizes for guessing which way the margin will swing!

roSSco

My head hurts, but I like the outcome

Foosh

My head hurts too, but I'm impressed. Do you teach at the USAFA?

ghoffman

Very nice work! How about a further integration that includes aero drag?

raj

Thanks!

Drag calculations are certainly important in any acceleration comparison involving high speeds. While the Cd of the Z06 is lower (0.34 vs. 0.40), the only frontal area estimate I have for the SRT-10 is from C&D and is a somewhat hard to believe 18.0 sq. ft. compared to the Z06's 21.5. If these frontal area estimates are indeed accurate, the overall drag area is roughly comparable for the two cars(with the Viper holding a slight edge), so the effect of drag may just be a wash. If the 18.0 sq. ft. estimate for the snake is inaccurate and it is actually closer to the Z06's, then the Vette should have a decisive aero advantage due to its lower Cd.

ghoffman

I don't know what the Viper Cd(A) product is, but the total Cd(A) for the ZO6 I read somewhere is 7.2, so that confirms your Vette estimate but are you sure the Viper is only 18 Sq.Ft?

StickShiftCorvette

I can easily believe your analysis. However, on the steet VERY fer people are willing to rev their engines to the red line, as I routinely do.

The other question on the table is how long will a Z06 live if you run it to 7000 RPM 2 to 12 times a day. My luck with modern GM products suggest that you will not see a long life with that duty cycle.

That said, while it holds together, the Z06 will be a thrilling ride. Too bad there are no convertibles (with steel frames and 136 pounds of extra weigh!).

Bigben

Nice work . How about we just go with the stop watch next time

vettemaster1

i prefer timing lights!!

gonzalezfj

To each his own, but I have a 98 Coupe with over 60,000 miles and an 02 Z06 with over 7,000 miles of purely autocross/track day use. Both see runs to the rev limiter (not just the redline) regularly with no problems at all. No oil consumption. Dyno horsepower above average for the Z06.

Go figure...

phantasms

i must hit 6800 rpms on the street 100 times aday in my daily driver Z06. zero issues.

trumper Z06

Excellent info....

This has gotta be an ABSOLUTE nightmare for the Dodge Boys driving their snakes !!!

All you see posted over on their boards is...

look at the V-10's higher TORQUE !!!

SWCDuke

Torque is no more "fundamental" than power, and it is the area under the power (energy per unit time) curve using the rev range dictated by gear ratio spacing that represents the total mechanical energy input that is converted to kinetic energy.

Take a F1 engine for example. Torque is in the same ballpark as any other hi-performance 3-liter engine - a little more than a street engine because a F1 engine has a very high CR and low internal friction and inlet and exhaust tuning to create VEs of 100-120+ percent over a fairly narrow range, but torque is basically a function of displacement and CR. Since the gear spacing keeps them in the range of 15000-18000 revs, integrating the torque curve over this range won't yield anywhere near the number that you get integrating the LS7 torque curve from 4500-7000, but the F1 engine makes an average of at least 800 HP from 16000-18000 and would surely accelerate a Z06 faster than the LS7 and OE gearbox.

Torque bandwidth is more important than peak torque as this tells you the rev range where the engine is strong and responsive. The Viper and Ford GT engines are virtually flat, though the dyno pulls were started too high so we can't see the low end, but they likely have better bandwidth than the LS7. The eighty or ninety percent bandwidth is what counts, and LS7 looks like the eighty percent bandwidth is about 2000-7000, which is very good, but there is a hole in the 2500 to 4000 range. The eighty percent torque bandwidth of a F1 engines probably starts at about 12000.

Here's another way to look at it. What accelerates a car is thrust force at the drive wheels, and power is proportional to drive thrust times speed, which is a key relationship. If power averages 450 through the gears, then drive thust decreases as the vehicle accelerates and acceleration is zero when drive thust equals total drag - otherwise known as top speed.

To achieve 450 HP you can have an engine that produces about 200 lb-ft in the range around 12000, or 400 lb-ft in the range around 6000. If gear spacing was the same for both, the high revving engine rev range dictated by gear spacing would be double that of the low revving engine, which is why very high revving engines - like F1 and high-performance motorcycle engines - have much closer gear spacing than lower revving automotive engines.

Duke

Tom Steele

??? Why not? Come on over - bring a BRICK. I'll show you how the rev limiter works in the driveway if you want. Until the neighbors complain.

I do a lot of backroad driving where I spend most of my time in third gear, and most of it between 5,500-6,500 rpms. We are talking 30-45 minutes straight.

Probably now, more than ever before, the manufacturers have the ability to engineer the engines so that they can control 99% (until you make an improper downshift) of the engine's usage. Anything it CAN do, it can do for a long time. They warranty them for 3 years/36,000 miles built into the cost, and you can buy extended warranties beyond that. Trust me, they are building them to survive in the environment they are putting them into.

I believe you've had bad luck. The engines are the most bulletproof part of the car these days. Your seats, gas tank sending guages, column locks and batteries may fail, but engine failure is rare. The transmissions probably fail more often than the engines.

See the above paragraph. GM has a great deal of ability to control the environment of the engine, and it will last just fine within that environment.

WHY would they build an engine that wouldn't last at 7,000 rpm and allow you to rev it to 7,000 rpm when they can CONTROL that aspect of the engine's duty cycle? They believe that the engine will survive indefinitely within the parameters they have given the PCM.

What a load of crap. "While it holds together..."

SPIN SPIN SPIN. In other words, "For the first hundred thousand miles or two, the Z06 will be a thrilling ride."

Come on...

bernrex

I don't think you need to worry about a Stock engine holding up to frequent redline trips. Now, modified is where the risk escalates.

PS. If you're not hanging out at redline ... your vette experience is boring.

Tom Steele

Duke,

Good post. The tried-and-true waterwheel page has always been good for me and may be helpful for others as well...

http://vettenet.org/torquehp.html

"OK. If torque is so all-fired important, why do we care about horsepower?

Because (to quote a friend), "It is better to make torque at high rpm than at low rpm, because you can take advantage of *gearing*.

For an extreme example of this, I'll leave carland for a moment, and describe a waterwheel I got to watch awhile ago. This was a pretty massive wheel (built a couple of hundred years ago), rotating lazily on a shaft which was connected to the works inside a flour mill. Working some things out from what the people in the mill said, I was able to determine that the wheel typically generated about 2600(!) foot pounds of torque. I had clocked its speed, and determined that it was rotating at about 12 rpm. If we hooked that wheel to, say, the drivewheels of a car, that car would go from zero to twelve rpm in a flash, and the waterwheel would hardly notice :-).

On the other hand, twelve rpm of the drivewheels is around one mph for the average car, and, in order to go faster, we'd need to gear it up. To get to 60 mph would require gearing the wheel up enough so that it would be effectively making a little over 43 foot pounds of torque at the output, which is not only a relatively small amount, it's less than what the average car would need in order to actually get to 60. Applying the conversion formula gives us the facts on this. Twelve times twenty six hundred, over five thousand two hundred fifty two gives us:

6 HP.

Oops. Now we see the rest of the story. While it's clearly true that the water wheel can exert a *bunch* of force, its *power* (ability to do work over time) is severely limited."

Enjoy grasshoppers! (P.S. Go read the whole thing, valuable information in there!)

JasonR1974

Let me know when you are ready to put your car up for sale. Would love to buy it

MitchAlsup

I think you are confusing some data here.

It is my contention that running the LS7 engine up to 7000 RPMs with the oil in the 200dF range is "not very stressful", while running the LS7 engine to only 6500 RPMs and the oil at 300dF is "very stressful". And a whole spectrum exists in between.

With the oil at 10 cSt of viscocity there is plenty of bearing surface area to deal with the forces of reciprocation at 7000 RPMs, whereas with the oil at 2.4 cSt of viscocity there is not enough surface area to deal with the reciprocation forces (at a lower RPM level).

gonzalezfj

If this is of concern to you, just put 20W-50 Mobil 1 in the engine and go. You may lose a couple of horsepower to increased engine friction due to the heavier oil, but you would not have to worry about a spun bearing or other engine damage due to insufficient lubrication.

By the way, GM has set the high-temp oil alarm at 320 degrees in the C5 Z06 (I don't know what it is in the C6 Z06). Why do you suppose the set it at this value? Hint: Mobil 1 is good to over 400 F so at 320 there is plenty of headroom for safety.

jchazr

I think you are both correct. Torque, when viewed with respect to RPMs and gearing, is all that matters. Engine Torque, by itself, is only half the picture.

Even though F1 engines are only creating maybe 200 ft-lbs of torque (at the flywheel) at high RPMs (18000rpms), you are not taking into account the gearing multiplier, which will increase the actual torque produced at the wheels (which is what counts).

For instance, 1st gear in a C6 Z51 has a final gear ratio of about 10. That means when at the peak engine torque of 350 ft*lbs @4200rpms, you are going about 33mph. The torque you are actually applying to the rear axle, due to the gearing, is T=350ft*lbs x 10 = 3500 ft lbs.

Now, let's look at what an F1 car puts down. 1st gear final drive ratio is going to be in the neighborhood of 25, probably more though. That means at 18000rpm, when he's going 55mph, and his engine is *only* producing a measily 200 ft*lbs, he's actually putting T=200 * 25 = 5000ft-lbs to his rear axle. Let's not forget that 18000rpm is probably not his peak engine torque either, while for the C6 i DID pick the peak engine torque.

So yes, even though the LS7 engine is going to produce a lot more TQ than an F1 engine, keep in mind the only thing that matters is TQ at the axle (not the engine). Since the F1 engine has ridiculous RPMs, it can also have a ridiculous gear ratio to multiply its little torque, into massive torque at the axle.

Sure, you can derive this all from power, since power is a function of torque and rpm. But all you'll be doing is going right back to torque if you want to calculate acceleration.