Worst Dyno Graph you've ever seen?
Discussion
My rx8 dyno was the worst Ive ever seen given the amount I`d spent on the car. Uprated Hi flow exhaust, decat, manifold, air intake with cold air ram, re-map, new coils, new spark plugs and it achieved about 260bhp. All I ended up doing was ramping up the max bhp and the peaky torque curve still remained so very little real world performance difference. Then the engine block cracked.
R26Andy said:
My rx8 dyno was the worst Ive ever seen given the amount I`d spent on the car. Uprated Hi flow exhaust, decat, manifold, air intake with cold air ram, re-map, new coils, new spark plugs and it achieved about 260bhp. All I ended up doing was ramping up the max bhp and the peaky torque curve still remained so very little real world performance difference. Then the engine block cracked.
danyeates said:
Mr Sparkle said:
Why does it have that loop effect, can someone explain how it can be going back on itself? Does this mean that the revs were dropping at those points rather than increasing or something?
It's not real! 
A graph can't go back in time!slinky said:
Can you show me the time axis on the graph please?
Oh dear... the graph is HP + RPM. What happens if the RPM drops (because of a cut out?), it goes backwards. You assume all cars on the dyno are increasing in RPM (and may lose some power), but in this case, the car was cutting out. Hence the graph.Remember that the car is "fighting against" the dyno in "speed control" mode. The dyno is programmed to control road speed in a slowly increasing ramp over the period of say 20 odd seconds.
Hence, as torque is applied by the engine, the dyno/car rotating mechanical inertia is accelerated, this is recognised by the dyno speed control loop, and it applies a counter acting torque to try to hold back the speed rise to it's targeted value.
If you get a sudden massive cut in torque (like the usual over boost limit that just turns off the sparks/injectors) the dyno, at the point of said cut, is applying a large negative torque. The engine torque cut is virtually instanteous but the dyno control system will have been calibrated to provide a smooth oscilation free ramp under more "normal" torque fluctuations. Hence, the speed control loop will be unable to remove the load as fast as the torque drops, as a result, the road speed will actually fall, and with it engine speed. The measured torque direction will also reverse, but most dyno's do not show this (as it is used to calculate the coastdown losses on a trailing throttle)
As a result, the dyno graph shows "loops" where the torque reversal and speed reduction come into effect. After a period of time, the engine controller will reinstate the sparks/fuel, resulting in the speed and torque ramping back to the cut point again. And this will repeat over and over again.
The unusal bit is that the dyno operator "kept his foot in" during that sequence of events, usually the first large cut and resulting chassis oscilation is enough to get the throttle shut rapidly......... ;-)
Hence, as torque is applied by the engine, the dyno/car rotating mechanical inertia is accelerated, this is recognised by the dyno speed control loop, and it applies a counter acting torque to try to hold back the speed rise to it's targeted value.
If you get a sudden massive cut in torque (like the usual over boost limit that just turns off the sparks/injectors) the dyno, at the point of said cut, is applying a large negative torque. The engine torque cut is virtually instanteous but the dyno control system will have been calibrated to provide a smooth oscilation free ramp under more "normal" torque fluctuations. Hence, the speed control loop will be unable to remove the load as fast as the torque drops, as a result, the road speed will actually fall, and with it engine speed. The measured torque direction will also reverse, but most dyno's do not show this (as it is used to calculate the coastdown losses on a trailing throttle)
As a result, the dyno graph shows "loops" where the torque reversal and speed reduction come into effect. After a period of time, the engine controller will reinstate the sparks/fuel, resulting in the speed and torque ramping back to the cut point again. And this will repeat over and over again.
The unusal bit is that the dyno operator "kept his foot in" during that sequence of events, usually the first large cut and resulting chassis oscilation is enough to get the throttle shut rapidly......... ;-)
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