David - If I understand you, the car will give different dyno figures on different circumferences (?)
If that's so, then gearing must also affect the readings?
Yes, and so does the tyre pressues as they can change the tyre diameter.
Will 3rd read higher? than 4th?
Yes, but the readings between different gears are not that hugely different (actually I'm not sure why they are different?). We're talking about 10kW difference in the reading, while in truth the power to the ground between the 3rd and 5th gear is significant ... like 60% difference if I'm not mistaken.
If so, how's different cars/models get compensated for in a dyno shootout/comparison?
I'm no expert on dynos, let alone the 'shootout mode', but I'll say that the readings between 2 different dynos are not really comparable even if both are in the 'shootout mode'. They might be in the same ball-park, but considering that every kW counts, 10 - 20kW variation does not make for a good comparision. Best thing between twodynos is to find a 'benchmark' figure that will tellyou the variation between the dynos. ie. if dyno A reads a stock '04 WRX at 115kW, and dyno B reads it at 140kW, then we know that there 25kW difference between the dynos, and you 'should' be able to use that to compare car dynoed on one dyno to a car dynoed on the other dyno. Though, I'd take all this as approximations, and that's all.
Here is what I pressume about dynos:
- dyno will read the exact speed that the car will reachin any gear ... and that is no matter what the car is. Afterall, it measures the tyres spin the dyno rollers.
- It needs to know your gearing ... or at least the final drive if you pressume that every car has a gear 1:1 ratio. I pressume that this is something that is downloded in the software and they have this info for every car. Same goes for the cut-out.
- From gearing and speed it should be able to figure out your tyre diamter.
- I don't see why the dyno would not have archives of all the gear-ratios and if that was the case, then it would not make any (significant) difference to what gear you're running in as it would be able to work it out and hence compenstate the figures for that.
- Though, dynoing in a 'taller' gear will produce 'smoother' and more accurate reading just due to the smoother power delivery. Lower gears are more likely to produce 'slip' on the dyno.
- dynos read the ambient temperature as well as humidity and compenstate the figures accordingly.
Last of all, and most importantly, as far as I know, dynos do not measure acual power, but lots other things from which it then calculates a figure that 'looks like' power. This calculation is based on the reading and all the figures that are stored (or punched in) the dyno. Not only that, but then it will scale this number by an arbitary constant just to print a value that looks kind of in the ballpark.
Therefore the figure that you see onthe dyno is not really related to the power of the engine ie. you read 160kW, and hence pressume that there are 40kW of drive-train losses. That pressumpion is not correct. Also the figures are scaled by a bunch of constants that vary from dyno to dyno (the shootout mode tries to standardize these, but I don't know how strong this standard is from dyno to dyno).
So the best thing you get out of a dyno is the shape of your power curve. And if you go back after changing something, then the dyno should tell you what difference it made. Also, if you bring bunch same make cars there, then it should show you the differences between them. Last of all, if you bring bunch of different cars toone dyno, and that dyno has all the gear and tyre specs for each car in it's software, then you should be able to compare the figures between the cars.
ps. I would not take the above as gosspell, as I'm making a lot of pressumptions ... I'd love someone know knows about this kindof stuff to chime in.