Racelogic tests the G-Tech with interesting results

[EspenW]

Found this test on the Racelogic site:
http://www.driftbox.com/download/GTECHvPB_Test.pdf

Very interesting reading indeed. They tested the G-Tech RR vs. their own GPS based Performace Box. They used the super accurate and super expensive VBox as the reference. The Vbox is what all the major magazines and manufacturers use in their testing.

A quick summary:

1/4 mile ET (average of 5 runs)
Vbox: 17.843
Performance box: 17.78
G-Tech: 17.857

1/4 mile velocity:
V-Box: 80.618
Performance Box: 80.50
G-Tech: 80.36

In other words, the G-Tech is only off by 0.014s and is closer to the Vbox than the Performance Box is. It is only off by 0.25mph too.

Pretty impressive.
The tests from the BMW forum and the Car and Driver test shows the same:
http://www.e90post.com/forums/showthread.php?t=90939

http://www.caranddriver.com/reviews/hot_lists/high_performance/performance_files_tested_by_c_d/do_inexpensive_performance_meters_work_feature

Since it is so accurate at measuring acceleration, the horsepower measurement will be very accurate too. As long as the correct weight is entered. Horsepower is nothing but force x velocity.

I think more people would take the G-Tech more seriously if it cost 4 times as much.

[A80Avant]

Indeed,it seems that the differences are small,but i think that street factors,non linear acceleration and movement of the car when boost comes,does affect the accelometer which needs a horizontal line to do the correct measurement,which is why i chose o buy the perfomance box.GPS is much more consistent and you only have to worry about climate after that,especially if you have a designated area of testing

[EspenW]

True that. The Racelogic is immune to dips in the road that might affect the G-Tech.
The Racelogic would be perfect if they had incorporated corrections for aero drag and environmental factors. Both are easy to do, and my MR Dyno has both.

[A80Avant]

so how much does the MR dyno calculates for air drag ? assuming that it is not a fairly windy day?

[S2central.net]

I think more people would take the G-Tech more seriously if it cost 4 times as much.

Ain't that the truth brother Espen...

All credit to Racelogic for posting such a report of comparative analysis. The G-Tech RR has the right price point for people like me... Getting that last fraction of a percent accuracy is interesting and laudable, but its just not necessary except for the most exacting applications under ideal conditions where all other parameters are known.

[EspenW]

Ain't that the truth brother Espen...

All credit to Racelogic for posting such a report of comparative analysis. The G-Tech RR has the right price point for people like me... Getting that last fraction of a percent accuracy is interesting and laudable, but its just not necessary except for the most exacting applications under ideal conditions where all other parameters are known.

Absolutely. Major props to Racelogic, as it will cost them a few sales. It shows, however, that they are very serious (as if anyone had any doubt about that..).
Also note that the G-tech actually shows slightly more pessimistic times than real life when testing is done w/o rollout. This is the contrary to popular opinion that G-tech overstates performance.

[EspenW]

so how much does the MR dyno calculates for air drag ? assuming that it is not a fairly windy day?

You can do a coastdown and note the force at three different speeds. However, as that will include some unloaded drivetrain loss and rolling resistance, so I prefer to calculate and enter the drag force at 10, 30 and 60mph, so that my whp numbers are comparable to a chassis dyno. This is very easy when you know Cd and frontal area. The Mr. Dyno then creates a loss curve and will correct hp measurement, so that I can compare numbers generated in 2nd to 4th, etc. The shape of the whp curve will also be more correct, as the raw, net whp including loss will provide a curve that is too flat at the top, due to exponentially increasing aero loss at speed.

G-Tech actually has a calculator where you can find hp loss to drag at various speeds.

http://www.gtechprosupport.com/support/AeroDragCalc.php

I have entered some speeds from my loss curve, and the G-Tech numbers for hp loss at various speed are spot on (no surprise)

Correct numbers made at different temp/pressure to SAE with the dyno correction calculator:

http://www.csgnetwork.com/relhumhpcalc.html

[S2central.net]

@Espen - what numbers do you have for Cd and Fa on the RS2 then ?

I looked at this ages ago and can't dig up the data I was using for a Coupe but I think Cd was around 0.3

[A80Avant]

perfomance box also has some coast down to measure losses.i will have to look into that.they say you need a fair amount of road to do it.

[EspenW]

perfomance box also has some coast down to measure losses.i will have to look into that.they say you need a fair amount of road to do it.

That is in order to estimate a flywheel number, but will be next to worthless as mentioned many times before as there will be no load on the drivetrain. Better to get numbers that match whp generated on a RR

[EspenW]

@Espen - what numbers do you have for Cd and Fa on the RS2 then ?

I looked at this ages ago and can't dig up the data I was using for a Coupe but I think Cd was around 0.3

According to the road test in "Motor" it is 0.35.
The RS4 is listed as 0.34 and 2.04, so I use 2.04 on the RS2.

[S2central.net]

I found my old data for the Coupe... I seem to recall calculating the FA from one of those drawings with dimensions from an old sales brochure. I had 19.2sqft or ~1.8sqm which maybe smells a bit low.

I had 0.32 for Cd which I also think came from a sales brochure or mag article... Makes sense to be slightly lower than AVant and its larger (more brick like shape)...

[A80Avant]

2.04 is m2 or sf?

[A80Avant]

also doing my calculations of the AX-22 GPS meter,
i had 318,5whp @ 176Kms (4th gear)
with the calculations i add another 65,7hp
Do i then add about 20% for driveterrain loss or less than that? And is that 20% of the number i find above?

[S2central.net]

As I understand it - something like Gtech RR calculates nett power delivered at the wheels - it does this from time and acceleration measurements knowing the weight of the car. Assuming test on flat road and adjustments for pitch correction (etc etc).

Lets say the Gtech data says you had 318whp at 176kph... Then to know how much power is lost to aero drag you add that ~65hp and you have the figure delivered by the drivetrain... Then its 10-25% drivetrain losses thru tranny and rolling resistance. I've always used 25% as rule of thumb but I could be convinced that 15% is more like it on quattro.

Using 15% gets you to 452bhp at the flywheel...

Using 25% loss on 318.5whp gets you 425bhp at the flywheel...

Which of those numbers is closer to reality (or previous dyno runs) ?