A friend of mine from work and I are considering going in together on a gtech and I was wondering if anyone's ever done a gtech reading (1/4 mile, 0-60, etc) at the same time they were doing a timed run at a track? Are the gtechs that trusted? Even knowing that an accelorometer is the best way to measure acceleration outside of a timed track, it still seems to me that there might be a bit of a "fudge factor" built into the gtech computations.

Thanks!!!

 

It has more to do with the installation of the GTech than anything else.

I have run mine against the lights at Atlanta Dragway on several occasions. Worst error to date was 0.15 with about an average error in the 0.05 - 0.075. But this in ET only. Trap speed is always high, mine is about 4 - 5 MPH typically.

I know they state that the difference is because the track averages the last 60 to establish ET and the GTech does it at the very end. But there is no way I gained 2 - 3 MPH in the last 60 feet.

 

i asked this question recently and was told that although the g tech could not be the same as a timed track, the times were fairly consistent. with the g tech, the time does not start until you move. i have had some fun with it. if you are going to mod your car, whether for speed or handling, it will give you a good "before and after" sharing is cool too as you do not "need "it all the time like a radar detector

 

Cool. Thanks for the replies! I'm not going to do any performance mods (save the K&N drop-in filter) because then I couldn't be honestly racing in G Stock at an autocross. When you hit the SP and P (Street Prepared and Prepared) classes, you find some WELL honed machines. I'd never be able to touch some of those without a LOT of money and time.

I'm just looking to use it to see the difference tires, etc can make - and I heard (pure conjecture)you can use it to record ET and average speed over a course - could come in handy during practices!

 

Next time I go to the track, I'm going to take my GTech with me and when I line up for the pre-stage I'll start the G-tech too. We'll see how close I get to the 1/4 time.

 

I wonder about that. I would think you would be busy driving as opposed to looking at the speedo.


Why is my G-TECH/Pro consistently 2-3 mph above the track?


G-TECH/Pro is a very accurate machine, and the trap speed result that you get from the G-TECH/Pro is actually more accurate than the racetrack. Reason is that the racetrack averages your speed over a 60 feet stretch between two beams and G-TECH/Pro measures your speed at the exact 1/4 mile point.


Hopefully, I don't have to pull out Maple and do some acceleration simulations to back up GTECH's claims...

 

Eric, I've posted my before and after CAI installs and have got flamed that GTECH is so inaccurate. I have explained that where I live plays a part but they don't want to admit that my car is that fast with only CAI!
Well my CAI is better than,yes thats right better than AEM! I also want to thank you Eric for posting the dyno the other day for header and CAI!

 

Yes,and I do plan on running at the local Samoa track for official 1/4m times soon so will post the times when I do. Cheers!

 

Actually I have tried to develop my skills to be somewhat multi-tasking, jk...

Come on Eric, you can calculate better than that.

To accelerate from 134 feet/sec to 139 feet/sec in under a half a second at that speed is significant.

Do the math, and I'll do the numbers in the morning and we can come up with somewhat definitive answer. I could be wrong....

 

Please do, so there can be a conclusion to these ideas.

And in reply to your previous post, I hope it wasn't for me as I never flamed you. I did disagree though.

But relating to the post made recently about your intake making more power than others, I would never dispute a reasonable claim. But 200% - 300% more power is ridiculous.

Unassociated GTech times are meaningless unless quantified against another established value. You mentioned my "unscientific" method when in fact it was one of the most precise testing reasonably available.

 

Well, I just posted the data straight from GTECH site -- they make the product. The difference is going to be different for different acceleration (with some caveats).


So, I did some rought calcs, and depending on the actual G acceleration you are running, the unit is not that far off. I did find a link to a European drag strip spec (sorry, couldn't find the US version), but the UEM spec calls for an accuracy of +/= 1 KPH (there is almost a .66 mile / hour right there).

Here are the calcs. I did not want to back calc the initial velocity, as I didn't want to start a math Jihad holy war, if you don't like the amount the GTECH says they are off -- go to the site -- they posted the data...

100 MPH is equivalent to 147 fps (feet per second) – I’m going to just use a round figure to see if I’m in the ballpark.

Velocity * Time = Distance hence, Time = Distance / Velocity

I would like the time first (over the 60 feet).


60 f / 147 fps = .408 s ( I am going to keep the time fixed)

(If you notice, lowering the speed (through the trap) increases the time which increases the delta V.

a * t = delta V

Assume only .1 G acceleration:

3.2 fps^2 * .408s = 1.3 fps difference

.9 MPH difference between start and finish...


Now assume .2 G acceleration

6.4 fps^2 * .408s = 2.6 fps difference

1.8 MPH difference between start and finish...

Now go too high with .3 G acceleration

9.6 fps^2 * .408s = 3.9 fps

2.7 MPH difference between start and finish...

The average Velocity would be (Vstart + Vend) / 2.

(Remember the car with headers is on cam and making good hp/torque/acceleration. I get above .5 g in my car in 1st gear)

BTW -- here is some data churned with your 134 fps used as average speed...

Now, I’ll use your 134 fps.

Velocity * Time = Distance so, Time. = Distance / Velocity

I would like the time first…


60 f / 134 fps = .447 s ( I am going to keep the time fixed)

a * t = delta V

Assume only .1 G acceleration
3.2 fps^2 * .447s = 1.43 fps difference

0.95 MPH difference

Now assume .2 G acceleration

6.4 fps^2 * .447s = 2.86 fps difference

1.95 MPH difference over 60 feet

Now go a bit too high with .3 G acceleration

9.6 fps^2 * .447s = 4.3 fps

2.9 MPH difference over 60 feet


FINALLY....

(Just to backup the accel figures as best I can -- I dug up my old MT issue with the 2001 CLS vs the 328ci. Bummer -- they only had the times to 80 mph for the stock CLS)


a * t = delta v, so a = delta v/t and (v2 – v1) / t = accel

I have the data from the July 200 Motor trend magazine.

Here is the times:
<pre>
<font size=+1>

speed time delta t time
------------------------------------------
0..30 mph 2.5 sec so, 0 - 30 takes 2.5s
0..40 mph 3.6 sec so, 30 - 40 takes 1.1s
0..50 mph 4.8 sec so, 40 - 50 takes 1.2s
0..60 mph 6.4 sec so 50 - 60 takes 1.6s
0..70 mph 8.1 sec so, 60 – 70 takes 1.7s
0..80 mph 10.5 sec so 70 - 80 takes 2.4s
</font>
</pre>

I take the 70-80 MPH time and change the 10 MPH difference to feet per second and divide by the time from 70 to 80 MPH to get the accel:

14.7 fps / 2.4 s = 6.125 fps^2

Now I get the "G-force":

a “G” is 32 feet per sec^2, so 6.125 / 32 = .19G

If you notice the acceleration is good from 70-80 (off the VTEC), and from 60-70 (is on the VTEC) and this is a stock car. If you plug in those goodies you have in your car, you will see that I get pretty close to the 2-3 MPH off...

So toss in a possible error of 1 MPH of the track (remember -- they are measuring ETs in thousands of a second. How accurate is the time reading requirement for US tracks? As I said, the Euro requirement was +/- 1 KMH.

Now toss in a slight accuracy error -- the figures I came up with are not THAT FAR OFF -- you need to do some calcs to see, so it is not a "2-second in my head problem." Finally, as the speed goes down, so the delta time over the 60 feet goes up! Can you say iterative process? Finally -- you do have a modded car WITH headers and as such the torque curve is extended right where it needs to be at the end of the track. I even got the figures of a STOCK Cl-S to back up my data...

(And I know about the average speed --> (V1 + V2)/2t too...

[ 07-18-2001: Message edited by: EricL ]

 

Eric:

What a way to silence the crowd. I think myself along with many others are in complete 'awe' right now as to the time you spent on proving this

Thanks for the understanding....

 

Eric,
I love the Physics and Math. Sins said it best you just silenced everyone. What was your major. Anyhow I did see one error that effect nothing but a comment you made. 60-70MPH takes 1.7 secs not 3.3 So therefore it isn't faster than 70-80. Who cares though great way to work out problems. Work one out for light rims next, convince me to get 16 pounds rims. Numbers don't lie. Thanks

Oh I see you looked at 50-70 it wasn't a math error at all just misread. I think we can forgive you hahah just prove me light rims work cause I'm hearing from most they make no difference and I don't want to beleive that.

[ 07-18-2001: Message edited by: pgatour1 ]

 

This is a link I did for the wheel issue -- it uses rather "simple" logic to compare rotational weight to static weight, but the "values" should be conservative...
http://www.acura-cl.com/cgi-bin/ulti...c&f=1&t=008076

 

Very nice math, I should send you something for all of that work...

I do feel you are being overly optimistic about the Gs being pulled. I do see 0.4 - 0.5 in 1st gear, heck I could see 1.2Gs in my Typhoon at launch but that dropped to about 0.1 Gs at 90 MPH.

Drag does increase significantly as speeds get higher. My bet, and it will be tested, is that 90 - 100 will take about 3 seconds. If that is the case then 3 MPH is around a second, not 0.4 something.

Secondly, of all vehicles I have run at the strip while using the GTech I don't see 2 - 3 MPH error, as mentioned I see 4 - 5. Now this was typically in a vehicle pulling over 100 MPH trap speeds. So I would assume that the Tesla's statement about the 2 - 3 MPH might be incorrect as it should be percentage of reading and not an absolute. Almost all error values in measurements are either % of reading or % of FS (Full Scale).

I'm not knocking the GTech, I've owned one for about 5 years and have used it hundreds of times. I also know the value of an accelerometer as I use them nearly every day in test systems. I just felt that this error value for trap speed might be more significant. I'm comfortable with its ET capability.

But getting back to the CL-S. Even on the higher cam profile accelerating in the 90 MPH range as opposed to 70's will have an exponentially greater amount of drag. Even if it were the same as the 70 - 80 MPH rate as mentioned in your reply, 2 MPH would be the maximum delta. But this delta would increase as trap speeds increases. If it took 3 seconds to accelerate from 90 - 100 (our trap peed range) we would only gain 1.4 MPH over the last 60 feet assuming an average velocity of 95 MPH and constant acceleration.

As for the accuracy of the trap speeds at the track. The most accurate thing we can measure in this world is time. I have made systems that rely heavily on time-based measurements ranging from PLCs to PCs. We even calculate dimensional measurements that are based on the duration in front of a sensor.

1ms is easy and could be the base for the trap speed calculation. The photo-eyes do have latency but they typically are around 0.250ms so lets assume we could have a total system error of about 1 ms.

At 100 MPH that equates to about 0.25 MPH potential error when using this time base to calculate the speed required to travel the last 60 feet.

So, maybe Tesla was just generalizing for most cars that would be tested?? But anyway, great dialog, now I remember why I keep coming to these forums.

 

Steve:

One of the things that Tesla did do with the GTECH was to "generalize" as you say. I would expect that they used the 2-3 MPH as a generalization -- you can see from just the calcs that I did, that this changes very quickly.

Some comments about Gs etc -- I did do a bit of extrapolating with the data to the 1/4 mile point (using M/Ts data) and the stock car is still accelerating very well to the 1/4 mile. Our car also has a very good drag coefficient, however, I don't know the frontal area, so it would just be easier to take a modified car (headers and CAI) and measure the Gs at the end of the 1/4 mile.

One of the "problems" with the GTECH is the lack of a differential accelerometer. When the car is setup, the vertical and perpendicular pull from gravity is eliminated by the GTECH's zeroing-out process. The bummer is the "tilt" that the unit will experience under acceleration (can you say squat).

Here is the amount the unit would be off with 2 degrees of tilt (for example):

The force of gravity (1G) acts via the sine of the tilt (and increases the perceived forward Gs), and the forward acceleration force is reduces by the cosine of the tilt.

So, here is the 2-degree example:

If the car is accelerating at .5 G (this is the true forward acceleration force), if there was a tilt of 2 degrees, this force would become:

1G * SIN(2) = 1G * 0.035 = 0.035 (force of gravity gets added to the calc)

0.5G * COS(2) = 0.5G * 0.999 = 0.4995

I now add the two component vectors to get the resulting (or perceived force) of the GTECH unit 0.035G + 0.4995G = 0.5345G

Now, I will get the percent of error in the reading:

0.5345 / 0.500 = 1.069, so the GTECH is thinking that the acceleration is about 7% greater than it actually is.

If anyone ever wants to know where the GTECH gets all screwed-up, it is in the “tilt” department.

Note: The tilt would be most extreme at the start of the run and is probably less than the 2 degrees that I specified here. However, if someone can have a buddy film the car during acceleration, it would be possible to see the amount of room between the top of the front and rear tires and the wheel wells. This information along with the length of the car would provide very accurate tilt data. The other important note is that the tilt lessens as the car moves forward.

[ 07-18-2001: Message edited by: EricL ]

 

Steve part #2:

Cars have changed, and the drag is much lower on our cars (and other newer cars) than older cars.

I do have the data on a Ford Mustang Cobra Jet from the same issue of MT (it at least had data to 100 MPH):

<pre>
<font size=+1>
-----------------------------------------
0-50 MPH 3.9 sec
0-60 MPH 5.3 sec 50-60 MPH 1.4 sec
0-70 MPH 6.5 sec 60-70 MPH 1.2 sec
0-80 MPH 8.0 sec 70-80 MPH 1.5 sec
0-90 MPH 9.7 sec 80-90 MPH 1.7 sec
0-100 MPH 11.5 sec 90-100 MPH 1.8 sec
</font>
</pre>
----

I take the 50-60 MPH time and change the 10 MPH difference to feet per second and divide by the time from 50 to 60 MPH to get the acceleration:

14.7 fps / 1.4 s = 10.5 fps^2

Now I get the "G-force":
a “G” is 32 feet per sec^2, so 10.5 / 32 = .328G (This is the G-force for 60 MPH)

<--------->

I take the 90-100 MPH time and change the 10 MPH difference to feet per second and divide by the time from 90 to 100 MPH to get the accel:

14.7 fps / 1.8 s = 8.17 fps^2

Now I get the "G-force":
a “G” is 32 feet per sec^2, so 8.17 / 32 = .255G (This is the G-force for 100 MPH)

So, I got to ask this question… If this car with a power-to-weight of about 10:1 only drops .073G in acceleration between 90-100 MPH vs. 50-60 MPH, what does this say about the change in modern day cars vis-à-vis older generation cars?

So, you made a comment how one of your “older” cars would pull .5 g in first gear, but would only pull .1 g at 100 MPH.

The older cars had really bad aero characteristics. They also had bias ply tires. One of the reasons some of the “monsters” didn’t have better quarter mile times (given their amazing amount of HP and power-to-weight) was (1) traction and (2) terrible aerodynamic properties!!!

Things have radically changed due to wind-tunnel design, advanced CAD systems, and a need to keep gas mileage good at modern-day freeway speeds.

I think the CL-S does accelerate at .2G in the 90-100 MPH area! (The quarter mile times bear this out..)