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Originally Posted by exdos
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Originally Posted by elwesso
Here's what I don't understand. I don't understand how that device can simply use speed and RPM to determine torque. As we all know, torque = force x distance (lb-ft or N-m). HP is simply torque at a given RPM.
Using one of these scanner tools, you can measure all of those things except a force. The only way this scan tool can calculate that is by essentially back-solving using the A/F ratio, airflow, and air intake temps. My guess is that scan tool isn't setup specificially for any given engine, so assumptions have to be made on volumetric efficiency, thermal efficicency and mechanical efficiency. The one thing that this CANNOT account for is friction inside the engine, which can be significant.
That said, I don't really think dyno numbers really mean a whole lot, as a number itself. They are only useful to compare setups. In other words, you do a baseline run of a car in it's stock form, then install your modifications, then re-run the car ideally on the same dyno at similar ambient conditions. I don't care if the dyno says you make 30RWHP or 3000 RWHP, if you show a 30HP gain, it's still 30HP regardless. That is where I think dyno's are most effective in showing the CHANGE.
Remember you can see EASILY see a 10HP swing just by changes in ambient temperature.. It's two fold because by decreasing the ambient temperature you increase the density of air, which in turn makes the engine injest more air, and by increasing the temperature DIFFERENTIAL of a heat engine, you increase the overall power output and to some extent the thermal efficiency of the cycle.
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In order to use the DashDyno in "dyno" mode you have to set up a "profile" for each specific car in which it's necessary to programme the following figures into the profile:
vehicle weight, Cd, frontal area, gear ratio (i.e. gearbox ratio x diff ratio), tire diameter, ambient temperature, ambient atmospheric pressure and geographic elevation above sea level.
Since the actual method of calculation is not published I'm guessing that the method used by the DashDyno (and other dataloggers) uses engine rpm and calculates distance based on the tire diameter as a unit of length multiplied by engine rpm x gear ratio. Since Force = Mass x Acceleration, this can be computed from the data logged and used in conjunction with the constants in the vehicle profile so that the torque equation can be solved. The environmental figures are used as corrections to the data so that the results are "standardised' so that all results are comparable.
Like you say, I agree that dyno figures don't actually mean a lot. What matters to me is that the DashDyno produces repeatable and reliable figures for any specific vehicle so that I can compare the differences that might occur when modding of the intake and exhaust system. A gain or a loss is a gain or a loss on any scale, and if the DashDyno shows a significant gain, then I believe it is a genuine gain.
In any event, I don't particularly rely on the dyno features of the DashDyno, I prefer to use the actual raw data logged from recording various specific PIDs, which also show significant beneficial changes from successful modding, such as airflow through the MAF and the difference between IATs and ambient temperature. |
A lot of variables there that can contribute to error or can be "fudged" to prove your point.
Just sayin
Especially with Cd, weight, and instant tire diameter...those values WILL be off.