bernrex

Most use 15% loss in RWHP calculations. So .. a 405 HP Z has 345 RWHP. So it takes say 60HP to overcome friction, etc.

Now, as we add HP to the new Z's 505 level ... 15% becomes 76HP.

Is 15% a constant ... or do we only require say 60 HP to drive the C6's gearing, tires, etc.

Perhaps ... as HP builds past some inflection point ... we gain some FREE HP benefits. Then again .... ? Who knows.

Racer X

It is not simple. On the same car at the same weight with the same tires, brakes, and wheels and the same driveline (therefore same rotational inertia) at the same RPM then the driveline loss should be the same HP on an inertial dyno. However, the new Z06 doesn't have the same tires nad wheels and driveline (the trans has different gears and it has been beefed up somehow), and the peak hp is at a different RPM, so the loss at peak hp would be different.

Since I don't think frictional and inertial drag are strictly linear, then I don't I don't see how the loss can be a single percent at all RPMs and inertial weights. We know this is not true because people have shown losses or gains jsu by changing wheels.

I know this has been discussed here many times, so a search may be in order.

Greg

EuG

Loss is not a percentage, but rather a constant HP number…

This is why new Z06 will dyno ~450rwHP and although most will scream that the car is severely underrated (applying the 12-15% loss rule) it will be more or less on target (save for ~5HP)...

Scissors

Those who's actually removed the engine from C5 Corvettes and put them on an engine dyno after doing a chassis dyno have found an approximate 12.5% loss for the manual-transmissioned cars.

And drivetrain loss is neither a percent nor a fixed number. It's a combination of both when comparing different cars and also varies based on RPM.

itchynackers

I agree.

EuG

Obviously it’s different for different drivelines (cars)
This is part of the reason Vipers have less loss through their driveline (they have 3.07 gears = more rwHP vs 3.42)

Yes, it varies based on RPM, but since quoting HP number always implies peak HP and assuming the peak RPM range is similar, we can ignore the rpm part in these discussions.

As far as it being 12.5%, well I guess we should wait ‘till actual dynos to start showing up and we’ll see who was right

Scissors

The shorter the gearing, the greater the loss. Which you and I both know is the reason for the 1:1 4th gear and the reason for dynoing in 4th.

As I said, for the C5. But the fact that the average C6 w/ M6 is dynoing 350 RWHP shows that it, too, has a 12.5% loss.

I have no clue what the Z06 will be. My guess is that it'll be higher (numerically) due to beefed up drive train components.

bernrex

Sounds intelligent and well researched. But, there probably is the usual bell shaped curve representing percent loss.

For example: STS (Squires Turbo Systems) has developed a new rear mounted TT system for all C5 cars. They have published the Dyno chart for a stock Z car. Results were HP 345.05 @6000 and TQ 341.13 @ 4500. This is exactly 15% loss from certified Fly HP.

Now I wouldn't be surprized if some lose only 12% ... but, others might lose 17%.

I'd bet that GM dynos cars they send to the Rags ... and give them the top 1%. But, for R&T ... they probably give them the muttdogs ... since they would somehow screw up a top Ringer anyways. Leave on the parking brake or leave the gate @ 750 RPM.

Would be interested to see some Z owners stock dyno #'s .. to see if my theory is valid.

Scissors

C5 Z06 HP isn't certified. The LS7 is the first SAE Certified engine.

One would have to assume that that particular engine was producing the advertised 405 HP. And we all know what happens when you assume.

The only way to be sure would be to remove the engine, dyno it, then put it back in the car and dyno again. And also to make sure that the dynos are calibrated.

Doubtful. The HP loss will be greater because of the C6 Z06's heavier wheels, rotors, and tires and because of beefier drivetrain components, but it is extremely unlikely that it'll have losses of 15% or higher.

No. The LS7 is SAE Certified, which means that the production cars (which are what the magazines tested) must be within 2% of the advertised number. Even if they did take the very best one, that's still a maximum of a mere 10 HP above advertised--not enough to make a noticable difference in the times achieved.

You have to be careful with those for a few reasons:

-Some people lie
-Some dynos are not properly calibrated
-Certain types of dynos give different results than other types
-Sometimes the numbers are not SAE corrected to standard conditions (77 degrees, 0% humidity, 0 feet above sea level, and 29.235 inches of mercury.)

bernrex

Phenomenal answer Dr. Scissors. You answered all questions and defused all doubts.

I'll never question your mastery of Corvette/Automative Facts again

glass slipper

For the differential ring and pinion, the shorter (higher numerical) gearing does give greater friction losses. Higher numerical hypoid gear ratios incorporate a change in pitch to maintain ring gear diameter. This change in pitch results in more sliding action (thus more sliding friction losses) for the higher numerical ratios. For the helical gears in the transmission, this is not true as pitch (and thus sliding friction) is held constant for the different ratios (except first gear in some transmissions). Transmissions vary ratios by changing the relative diameters (and number of teeth) of the gears.
...There are many reasons for dynoing in 4th. One is to make the HP required to overcome the inertia of the cars own rotating mass very small in comparison to the roller on the chassis dyno. It's simply a time factor, it takes more time to go from 3k RPM to redline in 4th than 1st, besides, the Z06 wouldn't hook up on the dyno in 1st. (Thus the bigger wheels, tires, etc of the Z06 will not have a measurable effect on HP on the dyno. In 1st gear on the dragstrip in comparison to the weight of the car, it might be measurable assuming traction can be found.) 4th gear is the most efficient gear (least HP loss) in the transmission because the 1:1 ratio provides zero relative velocity between the input shaft and output shaft meaning the bearing between the two shafts has zero velocity. This means 4th is the only gear where there is zero HP loss associated with that bearing. If it was related to "shorter" gearing in the transmission, we'd dyno in 6th.
...My guess is the Z06 HP percentage loss will be the same since the beefed up drivetrain components will have no effect on friction losses in a dyno test done in 4th gear. 1st gear is hardened, but 4th is the same.

Vette960

Yes, it is that SIMPLE. Whatever the determined drivetrain loss in a specific vehicle, and assuming no change or alterations to said
drivetrain, the energy (HP) lost to the drivetrain is a CONSTANT. The
only variable factor in the computation is lubrication temps ,and it is
insignificant. Our C6s expend ~ 60 HP of the rated 400HP to get us down the road at full tilt. To calculate FWHP using a % x RWHP only INFLATES the FWHP number as we MOD the motor.This is strictly a feel good exercise.In any case, it's the RWHP numbers that count in the real world. If you feel better calculating with a % - it's your privilege.
Dyno (chassis type) operators/tuners/entrepreneurs have no problem
with that methodology - why should we?

Scissors

I meant shorter relative to 1:1.

Racer X

It has been shown many times on chassis dynos that going to larger diameter wheels, with more weight or the weight further from the center will decrease whell horsepower. the same is true of larger heavier brake disks. It is sometimes a small impact, but it is an impact.

If the only difference in the transmission is hardening first gear, then it may have no impact. However, as i recall the ring gear is larger and heavier, and that may have an impact. If that is positive or negative, I do not know as it would also affect the gear angles changing friction losses, as you described.

Driveline losses for the same driveline is constant at the same RPM. The driveline speed of the C6 Z06 is higher at peak horsepower, especially the the heavier portions of the driveline (wheels and tires). So if you are measuring on a inertial dyno, it will impact measured HP.

The same car with lighter tires and wheels will accelerrate faster, so the effective HP (at the ground) must have increased. Rotational mass does matter.

Greg

MitchAlsup

Minor quibble: and question::
To me the 2% number with a 505 certification number means that the duds come in at 505 - 2% = 494.9 and the beasts come in at 505 + 2% = 515.1 a span of 20.2 HP.

Or does the SAE spec use 2% as -1% to +1% leading to a range of 505 - 1% = 500.05 to 505 + 1% = 510.05 or a span of 10.1

Scissors

For SAE Certification of the Net HP of the tested engine(s) must be within +/- 1% of the advertised number. For actual engines in production the engine dyno result must be within +/- 2% of the advertised number.

So the maximum difference between the weakest factory engine and the strongest is 20.2 HP.

The point in the SAE Certification is to stop the corporations from underrating or overrating their engines significantly. GM was underrating slightly, so that's why the Z06 gained 5 HP. Toyota was overrating by quite a bit, which is why their 3.0 liter V6 dropped by 20 HP.

glass slipper

HP loss to the drivetrain is a function of percentage (with the same drivetrain) at a specific drivetrain RPM. An engine making 400 FWHP at 6000 RPM and 15% loss to the drivetrain will make 340 RWHP for a 60 HP loss to the drivetrain. Mod the engine to 500 FWHP at 6000 RPM and it'll make 425 RWHP for a 75 HP loss to the drivetrain. Chassis dyno operators/tuners DO use the constant % methodology...go talk to a few of them. HP loss through a gearset is always expressed as a percentage in engineering books...good spur/helical gearsets are usually 95-98% efficient.
...What you are proposing is a car that won't budge until the output from the engine exceeds 60 HP...not a plausible situation considering it only takes about 15 HP for a car to maintain 60 MPH.

Scissors

I'm going to go out on a limb and re-predict 442 RWHP for the average 2006 C6 Z06 on properly-calibrated dynos.

bernrex

Rumor from GM reps in March was that all new cars would get dynoed. Seems the new SAE Certification will ensure this occurs.

Hope new owners will get a copy of their engines dyno. May have to ask to get it ... or perhaps it will be in the glove compartment.

I'm betting it won't be provided w/o some arm twisting .... or personal connections with Dave.

MitchAlsup

Consider two shafts supported by bearings and a single multi-toothed gear on each shaft.

Let us assume that the gears are horizontal to each other in plan view.

Now apply a TQ to the input shaft, and consume that TQ multiplied by the gear ratio on the output shaft.

The input shaft will develop high forces pushing the shaft upwards, while the output shaft will develop high forces pusing that shaft downward. The forces the bearings much support are proportional to the TQ being applied. And thereby, since the friction in the bearings remains constant under load, the heat produced is proportional to the TQ load times the rotation rate (which by the way is in units of power).

I submit that there is an additional component of power consumption of an unloaded shaft rotating in its bearings. I believe that for any high power applications, this rotational power consumption is VERY SMALL compared to the power consumption of the loaded part of the gearset.

Therefore, to a first order, the loss in a transmission (or differential) is proportional to the power (or TQ) going through it, and not a constant (as proposed by Vette960).