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Originally Posted by HotRoddin
...that measuring HP is incredibly crude, compared with a million other things that we routinely measure now days ... how could roller speed or signal noise be a concern when measuring something that only has to be measured within a couple HP... barometric pressure, temp. humidity etc can be very easily measured with radio shack stuff now days, and if you can measure it, you can easily compensate for it.
As for the calculations, you could set that up in a spread sheet, that will, with one keystroke, solve all the calculations in a couple hundred thousands of a second.
If you mean the motor or vehicle measured is varying from one machine to another, i can kind of understand that, but if you're saying the machines themselves vary that much from one to the other then i'm still as baffled as before
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You've answered your own questions...in a way. You're right, the individual measurements aren't difficult, however there's a difference between a direct measurement and the calculation (inference) of a different quantity from those measurements. The speed at which you can make those measurements (as you imply with the spreadsheet analogy) has an effect on the outcome, but probably not nearly as much as the tolerances, variabilities and such as the sum of the whole on all the measurements. If you've ever studied Probability and Statistics, you learned that the total variance is not necessarily linear with the sum of the individual variances, but rather each individual variance compounds the overall variance exponentially.....say to the nth power where you have n variables. Let's take for example the speed sensor on the roller from which acceleration is inferred. The speed sensor is an inductive device that picks up on 60-teeth machined into the roller support shaft and works just like your Hall effect sensor in your EFI distributor. Now as you roll into the throttle on a power curve, the pulse signal is not constant, but is getting shorter between each of those 60 pulses for each turn of the drum. Acceleration of the drum is calculated in part as the delta in pulse width. Obviously, the more pulses measured over a period of time, the finer the resolution of the delta and hence the more accurate estimation of acceleration, correct? Oh, but lets not forget that the inductive pick-up has an error factor of its own, lets say .1% of full range, so there's an error in the signal before we've even sent it to the computer. Lets also remember that the 'roll' into the throttle isn't a perfectly smooth event either so we're going to complicate the matter by a non-linear acceleration of the wheels, which by the way, may be slipping on the drum to boot. Now then, the 50-feet of pick-up cable between the sensor and the computer also has a resistance to it that hopefully we've taken into account, and calibrated into the signal value as it arrives at the computer. Let's hope also that Ohio Edison is supplying us with a reasonably steady 230VAC electrical supply (which we all know isn't true) and that the line noise isn't affecting our signal too much, maybe only another .1% or .2%. Now we place our trust in the computer's I/O card (made in Taiwan) that hopefully doesn't introduce too much more variability into the signal. OK, so now the speed signal has made its way to the CPU to be acted upon by the computer. The calcuation is not an exact one. What? Not exact? Engineering calcuations are rarely exact, but rather mathematical approximations of the physical events. So, if the engineers are approximateing the acceleration as a fourth order polynomial (instead of the theoretical exact derivative over an infinitesimally small increment of time), then there's at least five coefficients that are 'approximate' in the equation. WOW! All this just for the acceleration component and I've over-simplified tremendously. So anyways Rod you can see where I'm going with this (at least I thought I did myself when I started this diatribe). Oh yeah, I completely forgot about the temperature compensation! Scratch another couple of percent off the accuracy!
In your response, you're allowing the motor to vary with the test, so why not the dyno. We've built billions upon billions more motors than we have dynos, should we not expect the motor to be more repeatable and consistent than the dyno?
Sure we MEASURE light and radio signals from distant galaxies, but do you think that these measurements aren't without error? And we even spent billions putting together Hubble to try and take such measurements. Truth is that all those pretty pictures coming off Hubble have the color artifically applied based on the light spectrum being measured (most people don't know this happens). We work with simple AC & DC electricity everyday, but take two different meters attached to the same voltage source, display them to more than one significant value and see what you get. Hell, see if you can measure the same voltage with the same meter for any more than a tenth of a second.
Again people, don't expect exact repeatability or likeness from these machines and you won't be disappointed. Take the data that you're given and try to make reasonable decisions based on that data. The dyno is just a tool, that's it, a tool. Its not the end all to tuning problems and its not the only assessor of your car's performance or potential. Use it and use it wisely for what it is, don't expect more than it can give you and you won't be disappointed in the end. If you like one brand more so than the other, then use that brand, but use the same one each and every time. YOU have at least controlled one variable by doing this (so long as your dyno shop doesn't sell that unit and buy another). This 'he said, she said' can go on and on forever but no one will ever be declared the victor. Find a shop that you trust and use them, use them often and if you don't have a shop yet let me recommend CRT Performance in Norwalk Ohio (shameless plug).
Getting back to the original poster's question. I'd have to ask the owner a few more questions to try and figure out where the power went. However, right off the bat I could surmise that the six-speed transmission may suck up a few extra ponies and that depending on what gear he dyno'd in the final drive ratio may have had some effect. I'd also have to ask if he put the 4-degrees back into the cam timing did he see any change? I personally wouldn't consider 4-degrees a minor change and would say that this potentially robbed him of the horsepower that he's missing.
BTW: I dyno'd my bone stock 2001 GT the other day and it made 241.7 HP and 298.3 ft-lbs of TQ at the wheels. Not too shabby for an engine that was rated at 260hp from the factory.