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Originally Posted by mclaren1885
I have heard that dyno's from different manufacturers with the same numbers set for different parameters put out different figures. Some higher, some lower. This in itself can turn around quite a few tables. Now which one would you believe if you were to know the real BHP of your car? |
True. But I would use the dyno more as a tool for comparison rather than as an absolute indicator.
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Originally Posted by mclaren1885 Also, doesn't one have to feed in the values for barometric readings, total weight, axle weight, vehicle's rotating mass, gear ratios, co-efficient of drag, vehicle frontal area, tire size, dyno roll mass etc? Now what if a common man who has no clue what CD his car has, or what his gear ratios do? |
You dont need to do any of these things. Look at Dynojet Research's page to understand what their data acq takes care of:
Automotive Dynamometers - Innovations & Performance Products from Dynojet.com
Although, I've spoken to a couple people who use inertia dynos on a regular basis, and they seem to think the variation comes more from the operator himself. In other words, how and when you roll on the throttle during the run or "pull" as they call it MAY change the reading, but
only by a couple of tenths of a HP. You decide if thats significant.
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Originally Posted by mclaren1885 Also in which gears do you peak out to check the final output? Does it depend on a particular dyno? Or are there any standard rules? |
That depends on the rotating mass of the drum. If its a small drum, obviously there wont be enough oomph in it to load up a high-horsepower engine in the lower gears. For example, a 600 hp Nissan Skyline cannot be loaded in the 1st or 2nd gear enough to produce an accurate run. So you do the "pull" in sixth. But for a stock Zen with 50 hp, the run might take forever to do in fifth. So you do it in third or second just to get a quicker reading.
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Originally Posted by mclaren1885 Also aren't inertia type dyno's less accurate compared to load type dyno's? If so, how will this translate into measuring HP say for a common man like me? |
No. Inertia dynos are accurate in practice because you dont have to calibrate them every other run like you need to with some brake dynos (calibration errors). For measuring HP, doing back-to-back runs with even small changes like switching from 20W40 to 0W20 oil
CAN be measured.
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Originally Posted by mclaren1885 Also, cant dyno's be manipulated? Easiest ways are:
1. Entering the wrong set of values for air temperature & density.
2. Depending on which gear you take the run. Is it possible to do a dyno run just in 2nd gear? Suppose we do a run in 3rd gear wont the figures change? Also is there any standard procedure to this?
3. Getting the engines upto optimum temperatures for comparative runs.
And so on. I am sure you will be aware of all this. |
1. not if you have a data acq that has sensors for these. See link above.
2. Figures wont change in second or third because you are sensing RPM as well. In other words, drum acceleration is normalized against rpm change per second.
3. Well no one who does a pull on a cold engine should be allowed NEAR a dyno.
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Originally Posted by mclaren1885 I did infact read that KS had mentioned it to be a Inertia Dyno only after I had posted. Agreed ID doesn't need much of a calibration except for setting of the air gap between the Hall Effect sensor and the trigger wheel. |
I wouldnt know why dynojet would want that adjustment from the user end (KS is dynojet right?). Also, I doubt if you can find a chassis dyno that's not an inertia type. The risks in holding a car or motorcycle at high speed for long times are too many.
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Originally Posted by mclaren1885 But my question is since there are too many IF's & BUT's how does one go about it? Suppose KS' dyno shows my car putting out 130bhp @ crank and tomorrow another tuner gets another dyno that shows 110bhp @ crank whom am I to believe? All parameters being the same. |
This is a common problem and the only workaround is go to the tuner you trust the most. And do back-to-back runs with and without the changes you made that mandated the dyno run in the first place. Then the cat will be out of the bag.
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Originally Posted by mclaren1885 How do I go about if I were to get a definitive indication of my cars HP without having to think twice. |
Why would you wanna do that? The dyno is only a tool to understand what changes have affected the engine and by how much. Not to tell you your car has x amount of power and thats it.
Its a step in the right direction if Karan publishes baseline readings for popular stock cars. Then the confusion will clear itself.