Rolling roads, dynos, or dynamometers are more than just a tool for tuning and ego boosting/bursting. They can also, in the right hands, be phenomenal diagnostic tools. Case in point, we had a car on the other day for tuning, previously it had ran the standard turbo and made some good figures but after upgrading the turbo the customer wanted to release the true potential of their work.
RPS13 RB25DET NEO
Once the car was on the dyno and we reached the stage of sweep testing it became apparent there was something wrong, the car was suffering sudden lean spots at the top end of the rev range and boost creep. Since the map was being monitored we knew it was requesting enough fuel so we began looking at the fuel system itself. Before coming to the shop the car had just had a new fuel pump and injectors fitted and the fuel pressure regulator was also reasonably new so that ruled out it being old components. We attached our fuel pressure sensor and began some more tests resulting in the graph below:
As you can see the manifold pressure (turquoise line) rises and the fuel pressure (yellow line) does as well, albeit at a slower rate and then it momentarily drops, raises then drops again. Not only is this not meant to happen but it is extremely dangerous to the engine, what should happen is the fuel pressure mirroring the manifold pressure maintaining a constant pressure differential of the base fuel pressure.
The lower than expected fuel pressure and also the drops in fuel pressure would explain the lean spots at the top end of the rev range, but what is the cause of the lower fuel pressure?
Knowing that the majority of the fuel system components were new the next thing that we checked was the voltage the fuel pump was receiving, the car and it’s wiring are 20+ years old after all. With the car idling the fuel pump was receiving 12 volts, we’d have expected to see round about 13.5-14 volts, depending on what the alternator was delivering. Since the pump was already losing out on possibly 2 volts from the cars power supply we began a sweep test, this time we added the voltage monitoring to the dynos data recorder to see what was really happening high in the rev range:
Manifold pressure, fuel pressure, voltage
This time there was no sudden drops in fuel pressure but we had found our answers, as the load (engine rpm and manifold pressure) increases the pump is struggling to deliver adequate fuel pressure because the voltage is dropping from 9.53 volts, which itself is too low, down to ~8.86 volts, a voltage drop of 0.67 volts. This might not seem a big drop but the initial low voltage paired together with the extra demand means that there isn’t enough power for the pump to deliver the required fuel pressure. With definite answers found we can now remedy this with a new relay controlled power supply from the battery, this was carried out and the same test performed, the results are shown below:
As you can see in this graph we have the voltage measured at the pump starting at 13.938 volts, an improvement over the previous 9.53 volts, and as the test ends we have 13.875 volts, a drop of only 0.063 volts which is totally acceptable, the air to fuel ratio was now back within acceptable tolerances as well. This allowed us to continue testing of the car where we found the next issue, boost creep!