Playing with a G-Tech Pro

[JeffM]

I bought a G-Tech off ebay new for $37. Some guy bought like 500 in bulk..

Anyways...

I was playing with it on the way to work this morning. Got a 5.76 second 0-60 time. Didn't have time to try anymore, but I'm sure I could of done better.

Then I went to lunch with a friend, and did two back to back ¼ mile runs. Pulled over on the side of Rt25 till traffic cleared, then pulled out. Level both ways, the passenger is about 165lb or so.

First run with 2.5krpm or so launch, and a good 2-3 seconds of wheel spin: 14.14 @ 105.4

Second run with a lower launch, and just a squeal from the tires: 14.29 @ 105.4

That's with one passenger. That was the first time I really ripped on the car too.

That seems pretty accurate, though I have no way to prove it since the car hasn't been to a track. Also, the high MPH numbers is because the G-tech takes the MPH at the instant you cross the 1/4m mark, whereas at the track it's the average over the last 60 feet.

I'll have to do more experimenting this weekend by myself. This car is so easy to shift really fast. I love it.

BTW - the car is a stock six-speed 96 Z28.

Just figured I'd share my results.

[Jeff_in_Atl]

Would you still have the link to this guy selling the G-Tech?

[Injuneer]

Just to clarify your misconception, the g-tech is NOT off because of the fact that it gives you the actual MPH at 1,320-ft, and the track averages it over the last 60-feet. If you do the math for a 100 MPH car pulling 0.20 G's (
a generous assumption) at the end of the track, you can prove that the difference between these two numbers is LESS THAN 1 MPH.

The problem appears to be in the way the g-tech (as do all similar devices) calculates speed and distance. It can only measure two things... G's and time. It calculates speed by integrating G's over time, then it calculates distance by integrating speed over time. If the sampling rate is low enough, these iterative calculations become very inaccurate.

There was an interesting thread on another board recently that was able to correlate the MPH discrepancies in part to the "rollout" distance that appears to be set as standard in the g-tech. Newer, more expensive models let you adjust the "rollout" distance and that seems to allow a better "calibration" of MPH.

Typically, the g-tech show MPH 4 to 8 on the high side.

[JeffM]

Jeff 95 Z28 - go here: http://cgi.ebay.com/ebaymotors/ws/eB...tem=2442376200

He's got like 60 left or so, but they're $39.

Fred, thanks for the insight! It seems pretty complicated. I do want to bring it to the track and see how it fares against the track clock.

[NJSpeed]

how accurate is the horsepower reading on the G tech?

[89GTA]

What's the general concensus on this device? For $39 I can't see really how you could lose. Worst being, hey you just lost a weeks worth of gas money heh. I've never been to the track but I'm curious as to where it would mark me with the quarter mile times. Is that part pretty accurate?

[JeffM]

From the research I've done online - meaning real-world research, not advertisements, I've seen that it is usually about one to two tenths off in the 1/4. The mileage like fred said is usually 3-5mph off, so, jsut do the math. The 0-60 is supposed to be very accurate too. I haven't done the HP yet because you need to input vehicle weight, and I don't want to do that until I weigh the car.

I've seen six counts of people running it while at the drag strip, and the mileage is off by 2-5mph (high), and the time is off by three tenths at the most. I'd say it's pretty dam accurate!

[89GTA]

Yea no kidding, that's not bad at all. Cool deal for those of us that can't get to the track that much. (or me at all yet!)

[Joe K. 99 Zeee!!]

Originally posted by 89GTA
Yea no kidding, that's not bad at all. Cool deal for those of us that can't get to the track that much. (or me at all yet!)

The only problem is finding a place one can safely do a 1/4 mile run. My unit is shelved right now. North Jersey is a bit too crowded to do much safely.

[89GTA]

Good point. However, PA has some nice open roads. Sort of lucky there. Now finding ones without a bunch of potholes is a diff story, but they're out there.

[JeffM]

I have several highways/main roads that get pretty vacant during the non-working hours..

Just pull over with your hazards on, make sure the road is clear, then pull out, stage, and go! hehe

Seriously though, I made sure NO ONE was coming for at least a half mile in either direction.

[89GTA]

man, those things are going pretty fast. There was about 68 20mins ago, now 40-something.

[JeffM]

He had 250 the day I got mine, 4 hours later, 0!!

He's making bank I'd guess.

Get 'em while they're hot!

[Injuneer]

The g-tech is very useful if you just use it to measure the effects of mod's to your car. It may not exactly duplicate the results at the track, but it is a way to measure your performance before or after a mod. There is a similar device made by Vericom, and I used one almost from the day I bought my car new in 1994. At the time, the Vericom was very "advanced", including a "full tree" countdown, reaction time, adjustable rollout dimension, and time, speed and G's at 60-ft, 330-ft, 660-ft (1/8-mile), 1000-ft and 1/4-mile. I think maybe the new g-tech PRO has some of these features. The Vericom had a very high sampling rate, and tended to be more accurate than the g-tech, but it also cost 3 times as much.

With the g-tech....

-insure it is accurately aligned on all three axes
-insure it is firmly attached
-try and verify that the road is level
-run the course in both directions if possible and compare the results to verify "level"
-always use the same stretch of road.
-correct the g-tech results for temperature, humidity and barometric pressure using one of the online calculators. Always use the same calculator... they are not all the same.

0-60 time will probably be very accurate, as will ET. In fact, for the ET/MPH that our cars are running, I can show you that you can estimate 1/4-mile ET very accurately from 0-60 time. Just add 8.5 seconds to 0-60 time to get 1/4-mile ET. Just check the data that JeffM posted as an example of this.

[Joe 1320]

I ran my g-tech at the track right alongside the track timers. In each case, the ET was within .1 of the timers, but the g-tech read 3-4 mph higher than the dragstrip timers. I also found that the quicker your car is, the greater the mph error. For example, my T/A logged a 13.15 according to the g-tech and the strip timer registered 13.09. The MPH on the g-tech said 108.9 but the strip timer was 105.1 Best 0-60 on the street was 5.05 before swapping to 3.73s.

Now my GN registered: (g-tech) 11.44, (strip): 11.38, still pretty accurate. The mph: (g-tech) 130, (strip) 121- way off. Perhaps the lack of traction had thing messed up. I was able on another run to get a 0-60 of 3.23 with only mild wheelspin, but aborted the run due to blowing off the intercooler hose.