-mikey

 

Here are my plots for my 2005 C6 Z51 M6 using my Gtech Pro Competition:




Note that for anyone who believes that Torque is not directly related to your Acceleration look at the TQ curve that I overlayed on the G curve.

-mikey

 

I don't know how the G-tech works but I thought it
sensed acceleration through some sort of
accelerometer and used that result along with time to
derive torque. It also looks like it is directly
calculating HP from the derived tq and measured rpm.

I do think something is wrong with your results
though. Apart from the obvious thing that the
numbers look too low, they also look strange for some
other reasons. The torque appears to increase by
almost 20% from 1st to 3rd gear! Even if that's just
error, that's a fairly large error bar.

If the device was deriving tq observed at the rear
wheel from acceleration and dividing that value by the
gearing in the rear end and the transmission to arrive
at the values plotted, one would expect to see the
torque reduced at high speeds. Because drag
increases exponentially with speed, you'd expect the
vehicle to accelerate more slowly at high speeds, due
to the extra drag. This would result in torque observed
at the rear wheel appearing reduced, if the way you
measured that value was through acceleration. That is
why I expected 3rd gear runs to show reduced power
compared to second gear runs. But in your data, the
3rd gear run produces the highest numbers!

One thing which would produce numbers which are too
low is an estimated car weight which is too light (but
that wouldn't explain the increasing power in higher gears).
What did you use for your final weight?

Pat

 

3426 pounds based on my weight some junk and the gas amount. I agree that amount could adjust for some error. Also the lack of pitch could cause some error in 3.0 firmware.

But I suspect it's more weight if the numbers are low. I'd like to see some other gtech #'s before I suspect the numbers are low. I don't doubt losses in the 24% area for rolling resistance. Drags in there, drivetrain loss in known at about 15% that puts the #'s close maybe a tidge low.

I think the early gears may be low from lack of a ram air effect helping the engine get air and also possible TQ management in the first gear?

Also consider that the spice at the front of each shift in TQ is from flywheel dump. You should drop that first peek until the clutch is fully out. In the 1-4 gear run your only seeing the gears shifted from the last gear at redline not the entire engine curve for every gear.

If I did every gear from low RPM till redline assuming I had the room to do it and not get into trouble then things might look a bit differant. Might have to try that.

If you look at the 1-4 gear run you'll see acceleration does indeed drop each gear, partially because of the new mechanical leverage of the differant gear but also from increased drag and rolling resistance.

It's not uncommon for higher gears to produce better numbers, but it is a bit odd when using an accelerometer dyno. I'll wait till I see other results and their curves before I think somethings odd.
If anyone in the LA area is interested in testing their Manual C6 with a Z51 I'd be interested in making graphs for them to compair too.

Or if someone with a Gtech would send me some results I'd love to graph them for you to compare.

I'll have to post on the Gtech forum and see what they say.

-mikey

 

Gtech Corvette Forum

My numbers match a stock Z06 at 3rd gear at 280HP.
I feel the numbers are on target.
Also found that G is measured for HP and torque is only thing RPM is needed for. You can get RPM off your speed assuming no wheelspin if you want to calc torque after the run if you can't get RPM to work. If your G is reading solid and smooth then your HP numbers will be good. Look at HP vs Time.

-mikey

 

Thanks. Obviously I was out to lunch on how the G-tech
obtained hp numbers. And the absolute correctness of the
G-tech power numbers is far less important than the
repeatability of the results. If the results are consistent,
then you've got a great tool for benchmarking mods.

Pat

 

In 1st gear there is a lot more of the rotational inertia of the engine that consumes HP (and TQ) since the engine, and driveline has to be accelerated along with the weight of the car itself. Therefore, the measured HP numbers in 1st gear will allways be less than the measured numbers in higher gears (when aerodynamic drag is not present -- like on a rolling dyno.

In the real world, there is minimal aero drag in 1st gear, minor drag in second, and then aero drag increases quadradically with speed (in TQ cubic in HP). So you should be seeing quite visible losses in 3rd and 4th on real roads.

What G-Tech measures is the rear wheel TQ applied to accelerate the car, what it does not measure is rear wheel TQ used to push the car through the air, nor the engine TQ that never shows up at the rear wheels due to rotational inertia.

No, aero drag increases with the square of speed when you are measuring TQ and with the cube of speed when measuring the effect on HP. While squares and cubes 'have' exponents (2 and 3 respectfully) it takes an infinite series of terms with constant exponents to model an exponential of a single term!

Be sure to add the weights of the gasoline and driver (and passsenger) to get the correct weight inorder to have correctly measured power.

 

[QUOTE=MitchAlsup]No, aero drag increases with the square of speed when you are measuring TQ and with the cube of speed when measuring the effect on HP. While squares and cubes 'have' exponents (2 and 3 respectfully) it takes an infinite series of terms with constant exponents to model an exponential of a single term!

[QUOTE]

Yes, I understand exponentials (and aero drag). I was
speaking casually and was imprecise in what I wrote.
In future I will seek to avoid being this imprecise in
my postings.

Pat

 

If you mean "it does not measure" you mean it doesn't seperately measure I'd agree. It also doesn't seperately measure rolling resistance.

But it does measure everything in the that all of those variables are their when you make your acceleration run. They are all adding into the results. The gtech is a decent real-world all variables included this is what is getting to the road measure. If you input consistant accurate numbers the results are quite accurate.

They are pretty darned consistant.

I don't really intend to mod. I just like to know how it's doing once in a while to make sure it's in the ball park.

-mikey

 

Hi Mikey,

Well, unfortunately the 3.0 firmware was no good for me. It behaves a bit better but RPMs still cut out under acceleration. I just got a hold of the 4.1 beta, and will give that a shot. Based on your experience, I'll not be keeping high hopes.

In my simulation, rolling resistance losses appear go
from 0HP at 0MPH to 50HP at around 186MPH.
I'll see if I can export the power loss graph from
this simulation it is pretty interesting.

As far as more power from air flow at speed,
I don't believe there would be any noticable effect
with the stock intake. It gets air from the engine
compartment, away from airflow at the nose.


Hmm.. I didn't realize HP vs time was calculated independently
of RPM. I will fire up the PASS software and look again.

Cheers,
Rahul

 

In fact export your runs to excel and calculate the torque based on speed which is also using just acceleration.
If you know your gear and you know speed then you know RPM and can get torque from that. Not counting any wheelspin.
-mikey

 

I spoke this sentance imprecisely. let me try again.

G-Tech measures the acceleration of a car of a known (given) weight.

The engine, through the transmission and rear end create a foce at the rear wheel (contact patch). This force at the rear tires must be considered as two components, the first component is overcomes aero drag and this consumed force simply keeps the car at a constant speed against the aerodynamic drag and is not used to accelerate the car; the second force is what remains after aero drag force is subtracted, and this force is available to accelerate the vehicle. Since G-Tech measures the acceleration it 'sees' only the force used to accelerate the vehicle and is 'blind' to the force used to hold the car at speed.

Rolling wheel dynos do not have aero drag as part of their operation and measure TQ and HP more directly.

 

Not trying to be **** here but...
Add in a component for rolling resistance and I'll agree with everything your saying. And nicely put too.
Aerodynamic drag is not the only thing the available HP must overcome.
-mikey