[Formulafr]

i use a gtech pro ss to measure my hp at the wheels, it told me 103hp (car setup was done with the real weight of the car+passengers). engine is stock 350ci (175hp) 1975 and 2.56 ratio.

today, i went to a dyno to compare. at the dyno i have 202hp for the engine and 168hp at the rear wheels.


i think the gtech works fine for the 1/4mile time and G measuring but not for hp and torque because of rear end gear ratio.


what do you think? have you compare it too?

[cosgrove]

The G Tech has to assume you are using your power efficiently. You could probably install a motor in your car with 102 hp at the wheels and install the right rear end gear and duplicate the current performance of your car.

[Jack Gifford]

Were you at the engine's peak-power speed (RPM) when taking the G-force reading? Does the G-force factor-in any amount for aero-loss (wind drag) or is the user supposed to do that manually? My guess would be that it merely does the F=MA calculation, with no consideration of aero forces. Ideally, you'd want to be in top gear to minimize drivetrain losses; but at peak-power engine speed, that would be a very high road speed, with high aerodynamic forces.

[Formulafr]

i know the gtech considere aero loss, but it's too far from the dyno.

i will make an other run with the gtech et recheck the setup for my car to see if there's any differences.



i forget to tell that i have open exhaust and an electric fan, maybe few hp gain... but engine has never been redone

[Jack Gifford]

For the G-tech to calculate power, you have to "tell" it the vehicle's overall gear ratio (engine-to-rear-axle), right? Did you try it in various transmission ratios? Doing the test in the lowest gear would minimize the aero effects, and allow you to easily get to the engine's peak-power speed.

[69Customs]

No you don't have to tell G-tech the rear gear ratio. Only weight of the vehicle. The g-tech is more realistic because it takes in wind resistance. Run you car at the drag strip. Then plug your weight and 1/4 mile times into one of those calculators on the internet to see where your horsepower is at the rear wheels. It will proabably be closer to the g-tech.
If you read the manual it tells you for Max hp to run it in lower gear (Up to second gear) to avoid wind resistance. The pro ss will actually give you a graph of HP vs torgue vs rpm plus other neat stuff. It also tells you in the manual that hp will be lower than a dyno due to actual wind resistance and not to use it on a windy day or run up hill which can lower your reading.
I run mine at the track and it is real close to actual track time for what it costs.

[Formulafr]

i haven't done the test in second gear, i read it after. next time i drive it, i will do it.

[ta man]

I have a g-tech pro even though I haven't used it in a while here are my comments on it.It is accurate compared to its own data.Compared to a dragstip the ets will be close but the G tech mphs higher.I always loaded my data to the homecomputer.I'd study the g graphs to learn what rpm to shift at.It also helped me with my 60ft times when I had marginal traction.I also advanced my cam 2 degrees and saw the horsepower peak 200 rpm higher which is varily accurate.I will be using it soon so I can see what my current combo likes as far as shifting etc.Don't get hung up on the low horsepower numbers just use them as reference and try and tune to get them higher.

[Jack Gifford]

Originally Posted by ta man

... I also advanced my cam 2 degrees and saw the horsepower peak 200 rpm higher which is varily accurate...

Since I don't know what "varily" means, I suppose you could be right. However, it sounds backwards to me- advancing the cam should have dropped the peak-power RPM a little bit.

[ta man]

Sorry I made a mistake retarded my cam two degrees.

[bobzdar]

There are a couple of things to keep in mind:
1. The gtech should be tested in 1st gear only to minimize wind resistance. A dyno test is usually done in high gear to minimize frictional losses. Thus the gtech will read lower due to higher drivetrain losses
2. Your car will most likely have a different acceleration rate in 1st gear than the dyno (ie the engine accelerates quicker) which will often cause a lower HP reading, especially in an engine with high rotating mass. Some dynos have a set acceleration rate and measure the force required to resist the engine accelerating at that set rate to calculate HP. If it's slower than your car accelerating in 1st gear, it will most likely also read a little higher as the rotating mass of the engine/drivetrain will be less of a factor.
3. Rolling resistance of the front wheels, probably marginal but people do pick up .05 to .1 in the 1/4 going to tiny front wheels, so that could be an equivalent 5-10hp difference.

The key is to not get hung up in the numbers and spend your time trying to increase the HP number that the gtech gives you as it's very consistent in it's measurement. If you increase the HP number on the gtech, you're increasing the power of your engine.

[bobzdar]

Originally Posted by ta man

I have a g-tech pro even though I haven't used it in a while here are my comments on it.It is accurate compared to its own data.Compared to a dragstip the ets will be close but the G tech mphs higher.I always loaded my data to the homecomputer.I'd study the g graphs to learn what rpm to shift at.It also helped me with my 60ft times when I had marginal traction.I also advanced my cam 2 degrees and saw the horsepower peak 200 rpm higher which is varily accurate.I will be using it soon so I can see what my current combo likes as far as shifting etc.Don't get hung up on the low horsepower numbers just use them as reference and try and tune to get them higher.

In regards to the mph being higher on the gtech - this is at least partially due to the fact that track mph is the average mph over the last 100' of the track (or something like that, can't remember the exact distance) while the gtech gives the instantaneous mph. If it's off by 2-3mph, that's most likely part of the reason.