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mfennell
mfennell Reader
11/5/19 4:10 p.m.
Dave M said:

In reply to Snrub :

I mean, that Carrera S is, what, $120k as tested? And faster than a GT-R as well.

$143!  Sheesh.

Knurled.
Knurled. MegaDork
11/5/19 4:41 p.m.
Keith Tanner said:

I have to leave to go to SEMA, but here's something to look at. Same car, same gear, back to back runs. The difference? We changed the sweep time. The difference you see is the difference in inertial losses. That's why you see different dyno numbers in different gears, because your inertial losses change.

https://flyinmiata.com/tech/dyno_runs/NC_sweep_times.pdf

I added 10% more torque to a Pontiac on an engine dyno by sweeping it down instead of up! Went from 500ft-lb to 550.

 

Really, the only accurate measurement is steady-state.  Or just seeing what it will do in a car from a standing start, because we care about engine output for the purpose of accelerating a car.  If it makes 10% lower numbers but is 5% more effective at doing the job of accelerating the car, then it is "better" no matter what the dyno sheet says.

Dave M
Dave M HalfDork
11/5/19 6:08 p.m.
mfennell said:
Dave M said:

In reply to Snrub :

I mean, that Carrera S is, what, $120k as tested? And faster than a GT-R as well.

$143!  Sheesh.

At that price not even dentists can afford it! Only orthodontists.

Adrian_Thompson
Adrian_Thompson MegaDork
11/12/19 2:40 p.m.

Interesting observation around here.  I think in the last few weeks I've seen more C8's on the road than I have C5's.  I've been seeing approx one C8 per day and few if any C5's.  I also saw more C8's in the week prior to going to SEMA than I saw at SEMA where I only recall seeing three.

Keith Tanner
Keith Tanner MegaDork
11/12/19 2:48 p.m.
Knurled. said:
Keith Tanner said:

I have to leave to go to SEMA, but here's something to look at. Same car, same gear, back to back runs. The difference? We changed the sweep time. The difference you see is the difference in inertial losses. That's why you see different dyno numbers in different gears, because your inertial losses change.

https://flyinmiata.com/tech/dyno_runs/NC_sweep_times.pdf

I added 10% more torque to a Pontiac on an engine dyno by sweeping it down instead of up! Went from 500ft-lb to 550.

 

Really, the only accurate measurement is steady-state.  Or just seeing what it will do in a car from a standing start, because we care about engine output for the purpose of accelerating a car.  If it makes 10% lower numbers but is 5% more effective at doing the job of accelerating the car, then it is "better" no matter what the dyno sheet says.

That's one way to determine driveline loss, a coast-down. I think that's how the Brits do it, they like to calculate crank HP. The problem with steady state is that it's not a realistic test of the engine. A 650 hp engine at WOT will not find itself at a fixed RPM, and you have to load the bejeebus out of it to get there which has all sorts of knock and heat management implications.

Standing start has waaaaayy too many variables to be useful for anything but dick measuring or actual racing. You can even take the exact same datalog and give it to different magazines, and they'll calculate different results. It's not useful for development.

There was a baby blue C8 at SEMA on the GM stand. I would like to request that nobody ever EVER order the baby blue leather interior. It looked atrocious.

kazoospec
kazoospec UltraDork
11/12/19 6:04 p.m.

Today we learned (perhaps surprising no one), the C8 is unlikely to be a good winterbeater.  

Allegedly, this was a GM test mule.

Keith Tanner
Keith Tanner MegaDork
11/12/19 6:28 p.m.

So, Motor Trend has come clean. The dyno runs were jacked up in two ways.

First, the dyno was set to "all wheel drive" mode, which made the dyno assume a much higher inertial load. I'm not as familiar with Mustang dynos as some other models, but I expect this basically doubles the inertial load for calculations. It's like trying to calculate horsepower using a 0-60 run and accidentally doubling the mass of the car.

Secondly, it has some sort of "road load" compensation which is intended to simulate the aero drag in the testing range. Haven't come across this myself, but it again messes with the inertial load assumptions. I don't know this particular parameter, but apparently it was off by 20%. 

They're also pulling a 15% drivetrain loss number out of - well, let's say thin air to be charitable - so that adds a random fudge factor to the numbers. We've seen that modern drivetrains are far more efficient than older ones because this is a way to get MPG, so using a blanket 15% is a recipe for inaccuracy.

They attempted to reclaim some legitimacy by doing an on-road dyno simuluation in 2nd and 3rd, and ended up with 395 at the wheels. That's in the realm of plausibility.

TL:DR - dynos are a tool that take some knowledge to operate properly. This wasn't completely MT's fault, the dyno shop should have caught it the moment the car put down unrealistic numbers. But you get more clicks when you publish outliers.

Snrub
Snrub HalfDork
11/13/19 7:16 a.m.

Interesting.

Keith - I disagree with you on one point; IMO it is Motor Trend's fault. I don't expect them to be as knowledgeable as say yourself, but the results clearly made no sense. They are presenting themselves as having a certain amount of expertise. When the results seemed too good to be true, it would have behooved them to take the car to a second dyno. Or don't publish the results.

z31maniac
z31maniac MegaDork
11/13/19 7:21 a.m.
Keith Tanner said:

TL:DR - dynos are a tool that take some knowledge to operate properly. This wasn't MT's fault, the dyno shop should have caught it the moment the car put down unrealistic numbers. But you get more clicks when you publish outliers.

That's what sucks about journalism in general today. Any car guy knew that dyno number was COMPLETELY wrong, but they knowingly published something incorrect for clicks. 

#gladididntbecomeajournalist

Dave M
Dave M HalfDork
11/13/19 9:03 a.m.

In reply to z31maniac :

I mean, they're car journalists not investigative reporters. Half of us think they're just the PR outlet for OEMs (e.g. Gulia "car of the year"). I don't expect anything except snappy writing.

Keith Tanner
Keith Tanner MegaDork
11/13/19 10:45 a.m.

In reality, they're just guys up against a deadline. I agree that MT shouldn't have published, I left that part out. The dyno shop should not have provided the data to MT, MT should not have published and this whole distraction wouldn't have happened. But MT only had the car for a few days and the automotive audience was roaring for C8 content, so they went for it without double-checking.

GRM kinda did the same thing with the 2019 Miata when they put it on the dyno and discovered it had gear-related rev limiters. It was easy enough to get the whole story (and Miata specialists knew this was the case on the earlier ND) but in the rush to be first to print there wasn't enough investigation done and the conversation at the launch of the car veered off into the weeds. It's easy to do.

I do have to give MT some credit for not only explaining what happened without publicly throwing the dyno shop under the bus, but also attempting to develop some meaningful data using other tools they had available.

z31maniac
z31maniac MegaDork
11/13/19 11:25 a.m.
Dave M said:

In reply to z31maniac :

I mean, they're car journalists not investigative reporters. Half of us think they're just the PR outlet for OEMs (e.g. Gulia "car of the year"). I don't expect anything except snappy writing.

I get your point, but it was pretty obvious it was incorrect information. 

But then I don't read MT outside of the occasional online article, so I have no idea about the quality of their writers...........but if you're writing about cars, it seems it would be helpful to understand basic things about cars. 

Keith Tanner
Keith Tanner MegaDork
11/13/19 11:28 a.m.

I once took a turbo ND to one of the major magazines so they could check it out and we could say hi. They had their online editor take it for a drive. It became pretty clear that he'd never actually driven a stick before. He published a flattering little listicle based on the car and I'm thankful for that, but I was cringing the whole time as we drove through LA.

STM317
STM317 UltraDork
11/13/19 11:36 a.m.

In reply to z31maniac :

They understood it enough to suspect/accuse GM of delivering a ringer, and didn't really back down from that in their explanation of why their previous test was wrong.

Step 1: Publish obviously incorrect, sensationalist article that generates tons of clicks/discussion

Step 2: After initial fervor dies down, publish explanation article that gets similar attention but only includes non-scientific runs and a teaser for "the next time we get a C8 we'll do it again and get to the bottom of this..."

Step 3: After some time passes, and the attention fades, get another C8 and test it again for the "Did Chevy give us a ringer?!..." story and still  more clicks.

Whether it was intentional or not, they're now going to monetize the situation and milk this thing for all they can.

Keith Tanner
Keith Tanner MegaDork
11/13/19 8:17 p.m.

Could be worse, they could be publishing "the 50 wildest Supras we saw at SEMA!!"

At least this has some mildly interesting technical content wink

GCrites80s
GCrites80s Reader
11/13/19 8:51 p.m.
Knurled. said:
Keith Tanner said:

I have to leave to go to SEMA, but here's something to look at. Same car, same gear, back to back runs. The difference? We changed the sweep time. The difference you see is the difference in inertial losses. That's why you see different dyno numbers in different gears, because your inertial losses change.

https://flyinmiata.com/tech/dyno_runs/NC_sweep_times.pdf

I added 10% more torque to a Pontiac on an engine dyno by sweeping it down instead of up! Went from 500ft-lb to 550.

 

Really, the only accurate measurement is steady-state.  Or just seeing what it will do in a car from a standing start, because we care about engine output for the purpose of accelerating a car.  If it makes 10% lower numbers but is 5% more effective at doing the job of accelerating the car, then it is "better" no matter what the dyno sheet says.

This is why dirt bike dyno numbers haven't been a good comparison to how a bike actually feels on a track. When traction, weight loading/rider position and the fact that the wheel is off the ground so often become severe limiting factors a dyno queen can become covered in pig's blood. That's why dirt bikers say they don't race dynos.

Roenar
Roenar New Reader
11/28/19 1:43 a.m.

I was thinking of buying this car but its way out of my budget. 

Appleseed
Appleseed MegaDork
11/28/19 11:03 a.m.
Keith Tanner said:

Could be worse, they could be publishing "the 50 wildest Supras we saw at SEMA!!"

At least this has some mildly interesting technical content wink

There was a Supra st SEMA?

Keith Tanner
Keith Tanner MegaDork
11/28/19 3:14 p.m.

Yeah, it was hidden away in the back corner. Most people didn’t see it. 

Snrub
Snrub HalfDork
12/20/19 2:14 p.m.

MT did a comparison at VIR (3.3 mile layout) to the GT500. C8 (high camber setup -3 front/-2.5 rear) 2:00.96 vs. GT500 1:59.68 vs GT500 carbon track package 1:56.30. First two were on Michelin PS4S tires, last one on Cup2 tires. Randy Pobst driving.

Thoughts?

Here's some of mine:

While these results aren't directly comparable to C&D's lightning lap (4.1 mile layout), there's some correlation (3.3 mile layout probably more favorable to power). The GT350R did a 2:51.8 vs. C7 Gran Sport 2:47.1 (picked those due to similar tire class) vs. C7 Z06 2:44.6. The C7 Z06 gains 3 seconds with an addition 190hp vs. the C7 Grand Sport. Presumably C7 Grand Sport handles better, etc. which accounts for a 4.5 second gap over the GT350.

Taken together, if we assume the GT500 chassis performs a bit better than the GT350, and it has 225hp more, we might predict a C7 Grand Sport (with tires in same class) would be close to equal to the GT500. It's obviously difficult to say, but I suspect the lower level C8 doesn't gain much lap time vs. the lower level C7s. This kind of aligns with the C&D C7 vs. C8 comparison around smaller Gratten.

T.J.
T.J. MegaDork
12/20/19 3:17 p.m.

Factory-corvette-c8-r-confirmed-for-cota/

I'm glad they will continue racing in GTE class. I still have not seen a C-8 in real life yet.

z31maniac
z31maniac MegaDork
12/20/19 3:58 p.m.
T.J. said:

Factory-corvette-c8-r-confirmed-for-cota/

I'm glad they will continue racing in GTE class. I still have not seen a C-8 in real life yet.

Probably because the only cars on the road have been prototype/media cars? Production on the customer cars doesn't start until February IIRC.

T.J.
T.J. MegaDork
12/20/19 4:08 p.m.

In reply to z31maniac :

That makes sense. The post above about seeing more C8 cars on the road than C5 cars and the post of the C8 off the road in the snow is why I posted that. We don't all live in MI and get to see things like that before the rest of the country.

classicJackets
classicJackets Dork
12/20/19 4:19 p.m.
T.J. said:

In reply to z31maniac :

That makes sense. The post above about seeing more C8 cars on the road than C5 cars and the post of the C8 off the road in the snow is why I posted that. We don't all live in MI and get to see things like that before the rest of the country.

Seeing them on the road is a treat. I've been fortunate enough to have seen nearly all the colors, too - still waiting for an "Adrenaline Yellow."

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