EFILive horsepower calculator
EFILive horsepower calculator How accurate is this measurement?
How is it calculated? Does it know what gear I'm in for an M6 tranny?
I thought it is a neat gizzmo but just wanted some more info on it.
How is it calculated? Does it know what gear I'm in for an M6 tranny?
I thought it is a neat gizzmo but just wanted some more info on it.
It's based on speed change over time (during acceleration) and it depends on the vehicle weight. You should look at this feature as an estimator tool, not a lab accuracy measurement instrument. There are lots of similar comparisons. For instance, some of the G force acceleration devices that can be installed in a vehicle have similar accuracy issues (consider wheel spin for both EFILive and the G Meter, both have issues here). Also consider the differences between eddy current and inertial dynamometers.
Steve
Steve
TECH Senior Member
Joined: Apr 2002
Posts: 6,080
Likes: 17
From: So.Cal.
It's a physics calculation for the power used to change the vehicle's velocity:
P = m * v * dv/dt
As you can see, anything outside of m and v is ignored.
Coming from the OBD-II port, v may be delayed, and has a sampling rate of 10 samples/sec (or whatever rate the scan tool pulls it at), so the accuracy of v and it's gradient suffers.
Also m is hard to determine since the vehicle is carrying varying fluid levels, passenger(s), and various other junk in the trunk.
P = m * v * dv/dt
As you can see, anything outside of m and v is ignored.
Coming from the OBD-II port, v may be delayed, and has a sampling rate of 10 samples/sec (or whatever rate the scan tool pulls it at), so the accuracy of v and it's gradient suffers.
Also m is hard to determine since the vehicle is carrying varying fluid levels, passenger(s), and various other junk in the trunk.
TECH Enthusiast
Joined: Sep 2004
Posts: 564
Likes: 0
From: Omaha, NE
HP calculators are very crude at best. The ones I've seen never indicate if it's RWHP or FWHP, and Dynojet or other if it is RWHP. Some calculators try to compare to "gross" FWHP that would normally come direct from an engine dyno instead of "net" FWHP that the manufacurers use. Also, these calculators can only compute average HP throughout various RPM sweeps per gear that can cover as much as 2000 RPM for autos. Thus an engine with a very low but fat power curve could show much more HP then a higher HP engine with a peaky HP curve.
Since this is looking at your dv/dt this is trying to calculate the rwhp. I am unsure of the diff between front wheel and rear wheel hp. Does it matter if it is FWD or RWD??
It maybe inaccurate but it is a good glimpse of useful power... if you used this averaged number to compare two similar cars I am sure it would always gradify the faster car.
With an auto I can see it being inaccurate as you are multiplying torque off the line. If you consider how much they can mutiply even at 1000 RPM you could rationalise that 800 hp figure better.
Starting from a roll might help you out on that one. MY G force meter always threw out reasonable numbers but then again it had dedicated hardware to perform this exact calculation and our cars lack those solid state g meters.
It maybe inaccurate but it is a good glimpse of useful power... if you used this averaged number to compare two similar cars I am sure it would always gradify the faster car.
With an auto I can see it being inaccurate as you are multiplying torque off the line. If you consider how much they can mutiply even at 1000 RPM you could rationalise that 800 hp figure better.
Starting from a roll might help you out on that one. MY G force meter always threw out reasonable numbers but then again it had dedicated hardware to perform this exact calculation and our cars lack those solid state g meters.
Originally Posted by 2002_Z28_Six_Speed
Since this is looking at your dv/dt this is trying to calculate the rwhp. I am unsure of the diff between front wheel and rear wheel hp. Does it matter if it is FWD or RWD??
It maybe inaccurate but it is a good glimpse of useful power... if you used this averaged number to compare two similar cars I am sure it would always gradify the faster car.
With an auto I can see it being inaccurate as you are multiplying torque off the line. If you consider how much they can mutiply even at 1000 RPM you could rationalise that 800 hp figure better.
Starting from a roll might help you out on that one. MY G force meter always threw out reasonable numbers but then again it had dedicated hardware to perform this exact calculation and our cars lack those solid state g meters.
It maybe inaccurate but it is a good glimpse of useful power... if you used this averaged number to compare two similar cars I am sure it would always gradify the faster car.
With an auto I can see it being inaccurate as you are multiplying torque off the line. If you consider how much they can mutiply even at 1000 RPM you could rationalise that 800 hp figure better.
Starting from a roll might help you out on that one. MY G force meter always threw out reasonable numbers but then again it had dedicated hardware to perform this exact calculation and our cars lack those solid state g meters.
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Going along with the other comments.
If it only considers P=m*v(dv/dt)
So based on this: the gear ratio and rearend ratio don't matter. --> First gear will always make the most torque at the wheels and the slope of 'v' during this time will show that. There has to be more considered than this. Dyno programs consider this and scale the results depending on the torque multiplication through the system.
Secondly, the drag force on the car is not considered and this is a simple calculation assuming the drag coeffienct is known or can be reasonably estimated. At 150 mph you might not be accellerating much and still be making 400+horsepower.
If it only considers P=m*v(dv/dt)
So based on this: the gear ratio and rearend ratio don't matter. --> First gear will always make the most torque at the wheels and the slope of 'v' during this time will show that. There has to be more considered than this. Dyno programs consider this and scale the results depending on the torque multiplication through the system.
Secondly, the drag force on the car is not considered and this is a simple calculation assuming the drag coeffienct is known or can be reasonably estimated. At 150 mph you might not be accellerating much and still be making 400+horsepower.
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Verdad Gallardo TECH Senior Member
Joined: Apr 2002
Posts: 6,080
Likes: 17
From: So.Cal.
Other than friction losses, power is the same across a gear ratio...
power = torque x speed
if a gearset has a ratio of 2:1, torque doubles and rotational speed halves, so power is the same.
P = m * v * dv/dt calculates the power remaining that actually accelerates the vehicle mass after all friction/drag has been overcome;
so this the equivalent of RWHP, and this is what a chassis dyno would measure, but a chassis dyno provides a controlled environment with accurate/precise measuring equipment (with a much higher sampling rate);
if you coast down, then the equation calculates the power losses due to friction/drag (a negative number since v is decreasing) and if you subtract this (a negative number) from the acceleration power you get a number that represents approximate FWHP.
Instruments such as the G-Force use the same equation, but they get their dv/dt and v (which is the integral of dv/dt) from builtin accelerometers.
power = torque x speed
if a gearset has a ratio of 2:1, torque doubles and rotational speed halves, so power is the same.
P = m * v * dv/dt calculates the power remaining that actually accelerates the vehicle mass after all friction/drag has been overcome;
so this the equivalent of RWHP, and this is what a chassis dyno would measure, but a chassis dyno provides a controlled environment with accurate/precise measuring equipment (with a much higher sampling rate);
if you coast down, then the equation calculates the power losses due to friction/drag (a negative number since v is decreasing) and if you subtract this (a negative number) from the acceleration power you get a number that represents approximate FWHP.
Instruments such as the G-Force use the same equation, but they get their dv/dt and v (which is the integral of dv/dt) from builtin accelerometers.
Last edited by joecar; Dec 6, 2006 at 07:17 PM.
I would think gears would have an effect on the HP numbers the when using a G-Force instrument.
It assumes you are standing still and only looks at the weight and acceleration to make the HP calc. Gears would give the affect of making the car look faster at the expense of top MPH.
It assumes you are standing still and only looks at the weight and acceleration to make the HP calc. Gears would give the affect of making the car look faster at the expense of top MPH.
going off dv/dt sucks with the resolution (speed) and frequency (time) we can scan. you'd need much better speed sensors, prefereably averaged out with some military spec gps, and at least 200Hz scanning frequency to get data that's remotely meaningful. if you dont know what i'm talking about, make yourself a simple dv/dt spreadsheet from your scans, and you'll see so much noise that the signal wont be even visible. i've tried it many times, not worth your time, unless you got a completely different approach to it than just dv/dt
