Dan87951

Well I just purchased a G-tech Pro performance meter went out and tested it in a semi snowy condition (couldn't wait). Well I first started out by testing HP. I put in a vehicle wait of 3420lbs did the step by step instruction and flowered the car up to about 100mph and let off. It registered 295HP at the rear wheels, and this was with a really bad start with constant spinning (hey it was snowing). How accurate are these things? Wish I could have done more but the weather was just to slippery.

Thanks

Tabor

It is pretty accurate, except that when you launch it guesses how much squat your car has. This number is essential in its calcuations and can not be changed. Also, if you want an accurate HP number, you need to know how much your car weighs with you in it.

Ganja Porsche

The more slippery it is outside the higher the HP reading is going to be. You need to do it in DRY conditions. If you do your weight correctly the G-Tech is a very accurate instrument. You need to look in your yellow pages for some construction places or gravel places. Then call and ask you could use their scale for free. Then you can get an exact weight, then go to a freshly black topped street and get note worthy readings... =). 295 sounds wrong.

Dave

They are suprisingly accurate...unless it's snowing . As mentioned above, accurate weight is important, your number sounds a little high. With your list of mods, I assume that 295 RWHP is posible, but considering that any wheelspin or other wasted HP is not measured I'm thinking that there might be toO much snow on the ground.

Dan87951

Isn’t the dry wait of a 944 Turbo around 3250lbs? I figured with me in it 3420lbs was a pretty high estimate (I weigh 141lbs). How would be the more slippery the condition the more HP the meter will read? Doesn't this meter use some kind of g-force calculation to get all of its numbers? So if I was sitting there spinning it would be like me in a slow car not going anywhere therefore poorer times. Please enlighten me Ganja Porsche to your theory. I was also running around 17psi-18psi of boost as well.
Thanks

Ganja Porsche

I didn't know you had all those modifications.

Well, I WEIGHED my car and added my exact weight into the calculations.
Then went to try it out. IT gave me 147-155 for my RWHP, and I have a n/a. It was slippery.
Next day, same exact place, dry. 130-134RWHP. The correct #'s. I think that it being wet makes it break the tires free to easily, and G-Tech assumes the conditions are ideal. And if you could break your tires free that easily on dry ground you'd have a MEGA car ya know what I'm saying? It thinks you're ripping it like that on dry ground... I konw this instrument doesn't 'think' but that's just my uneducated guess on the subject. =). Peace.

Dave

While I can't explain Ganja's numbers, (OK, a phat spliff would explain it )the GTech doesn't know if there's smoke comming from the rear wheelwells, it just knows your velocity isn't changing very fast. Therefore I consider your theory valid.

Danno

Well, the Gtech measures ONLY acceleration in a single direction (forwards & backwards). It then integrates with respect to time to get speed. Then this is integrated again to get distance.

As for misalignments from the initial squat, it's canceled over a longer time period as the car accelerates and the tail-end of the car RISES (if it can sense squat, it can sense rise as well). Actually on the very first gear-change, the deceleration that pushes your head forward (look at your passenger out of the corner of your eye) effectively cancels out the initial squat. This explains how it can give the SAME speed readings regardless if your car squats like a Cadillac with worn shocks or a super-stiff racecar. After all, you never ever enter a squat amount (inches or degress) anywhere in setting up that G-tech thingie.

And the HP rating it uses comes from F=MA. It measures the acceleration (given a known mass, which is the total mass for that run car+driver+fuel). This gives it the force pushing the car at any given moment in time (thrust at rear wheels as generated by final torque through gearing delivered through moment-arm of 1/2 tire diameter). So... integrating this thrust force with respect to time, gives power.

Regardless of the precise mechanisms behind the scenes, what matters it how the darn thing works. And I've found that it works very well. The 1/4-mile times are exactly right on with dragstrip timeslips. The 1/4-mile speeds are 5-10mph higher than the timeslips because the G-tech computes instantaneous MPH at the end of the run, instead of the average over the last 100ft like the dragstrip timing lights.

Also the HP rating is at the CRANK. Ganja's numbers only confirm the 147hp-crank rating that Porsche claims. All the 147hp 944NA cars I've ever seen have registered 120-125hp at the wheels. If you read the manual, it states that it uses correction factors to account for drivetrain loss and aerodynamic drag as well.

But the HP numbers depend a lot on your launch and tire grip. If you spin your tires, you'll get a lower HP reading because the G-tech will sense lower acceleration. That's why they tell you to consistently run it up to the same redline and to go through several gears. This makes the initial conditions (squat and tire-spin) affect only a small percentage of time during the entire run.

Dan87951

I went out today and ran it again. I pluged in a weight of 3490lbs this time and did HP, 1/4 miles, and 0-60. The highest HP I could get was 304HP, 1/4 mile time was 13.23 @ 114mph, and a 5.01 0-60 time. I think my clutch is really starting to go, towards then end it wouldn't even grab 1st anymore. The clutch has been worn all summer I think I just used the last of it today.

Thanks

Ganja Porsche

All that sounds very reasonable considering your monster of a car. G/J

DanD

Dan87,
You're numbers look good. I got almost identical numbers out of my 86 with similar mods. I crept down into the 4.8 for one of the 0-60 runs, but I suspect I'll have a hard time duplicating that. 5.2 was more like it. 13.2 at 112 was my best 1/4 time on the Gtech. I never got a decent hp number, never the same twice. Maybe I'm not putting in enough weight. The FAQs say 2758 plus driver and fluids. A trip to the scales is in order.
DanD
(Sick on Christmas)
86 951 *2 K&N cone intake, HR MAF Stage I (upgraded to III ), custom intercooler hard pipes, ARC II fuel controller, ARM1 air/fuel meter, AutoThority Stage II chips, APEXi AVC-R electronic boost controller, shimmed wastegate, free flowing PowerEffect adjustable muffler. Autopower Rollbar, upgraded suspension, Koni Adjustable shocks, autocross alignment.
Dans rennlist site

Danno

Hey DanD! Merry New Year!!! I just got back from the Bay Area. Went to my cousin's wedding in Fremont, reception at the Fairmont. Hung out with my brother in Berkeley for a couple of days. Then Christmas with the fam in San Jose. Stayed a couple more days with my other brother in Cupertino. Then last night with a high-school friend in Saratoga.. Ahhh, never enough time to visit everyone. Hmmm, I think '87 951 is about 3150-3200lbs depending upon options. Primary weight gain over '86 cars was airbags and ABS.

Got my '86 car corner-balanced a couple of weeks ago. Total weight was 3055lbs with no driver and 1/2 tank. This should be pretty close to curb weight since I took out the back seats, but put in a rollbar. Then add my porky weight of 180lbs and I enter in 3235lbs total for the G-tech.

Last June when I dynoed 290rwhp, I was getting similar low 13-sec 1/4-mile times as you guys, with G-tech HP rating of 330-340bhp. However, after Huntley blew my headgasket (on that same dyno "tuning" session), the car never ran that fast again. Now I get low 14-sec 1/4-mile times and G-tech HP of 240bhp or so, arrrghhh....

ribs

Your weight sounds a little high, and the higher the weight of your car, the more HP the g-tech is going to display for the same change in velocity. I did a few runs with my friend's g-tech this summer, and I added up everything in my head and came up with a total weight of 3350 or so, and that was including a 170 pound passenger (my friend with the g-tech), a full tank of gas, and some crap I was moving out of my apartment that I left in the car. I got RWHP #'s of 238-250 something over several runs, and I have a lindsey boost enhancer (probably no addn'l HP), APE stage II chips, and a 3" cat back exhaust. I was able to nail off consistant 5.15ish 0-60 times, but I was getting really good traction where I was experementing with the gadget. I didn't have enough road for 1/4 mile tests that day, so....

Later in the summer I took my car up to the local drag strip. I was not getting my car to hook up off the line (the tires kept catching, the clutch would catch, and I would bog the revvs down below 2000 on almost every launch, easily costing me a few 10ths...I don't know what my problem was, but on the street I can get the car to hook up perfect almost every time...just never at the drag strip) and my best 1/4 mile time was a 14.0 flat at 102 MPH. I know from my timeslips and pathetic 60' times that I could easily run a mid 13 second 1/4 mile, but, anyways...

I had the g-tech hooked up in my car for my last run of the night the first time I took my 951 up to the strip, and was actually able to arm it and run the 1/4 mile at the drag strip with it. I believe my timeslip told me I ran a 14.4 @ 101 MPH, while the g-tech told me I ran a 14.3 @ 107 MPH. The trap speed at a drag strip is averaged over the last 60' or 100' of the dragstrip while the g-tech number is instantaneous at the end...this could count for the discrepency a little bit, but 107 MPH still seems a little high. The time was within 1/10th of a second, as per the commercial.

In low traction, you are not going to be able to accelerate as well, therefore you will not get as good of HP #'s as you will with dry traction. Also, the weight of your car is only used to calculate HP...the rest of the tests don't use weight in their calculations.

Robby

What Ribs said

I've always liked mine, and found it to be consistent, but have never actually used for it for HP readings. If you were to change your tires,you would get more traction, thereby accelarating stronger, and giving yourself a better hp reading, while not having actually gained any. In a way, this would have given you more power to the ground, but not the same w/a dyno. I BELIEVE this is how it works. Anyway, Ribs and Danno are also right about the weight issue. I've always seen curb listings of 2998 w/a test of ~3150, and have a test or two that puts it at 3150 and ~3350 (sounds too high), but, I have weighed mine (Turbo S) w/a full tank of gas, spare, etc, (no people) and came up w/a 3160 at a local truck scale. The same scale read me right at 180, which was correct when I got home (was working out more diligently at the time and carrying more muscle). So, your #'s may be able to be adjusted even lower... BTW, Ribs and Danno, those are really pretty good times. Mine has no mods, but I was running about a 6.0 and a 14.6 @ 101.1. I only did two runs at the time, and I didn't get a perfect launch by any stretch, also granny shifted a little bit- haven't gotten around to doing it again, I'm kind of leary of it right now- too many little problems adding up...

Danno

Yeah, running 1/4-miles and going to the dragstrip can be fun. One time, I actually had to shift up to 4th gear! This kind of driving can be a little tough on the clutch. My Cup clutch kit that was installed in the car only lasted 50k-miles with that kind of abuse. Put in a Centerforce Dual-Friction kit and it's held up great. No slipping or fried-clutch smells. Only half worn at 70k miles, so this is a good sign.