To be frank, I was a little disappointed with the results. The good news is that, as I suspected, the mid-range torque has improved significantly since the last time I was at the dyno, with power and torque shifting down the rev range and maximum torque now accessible at 4100rpm (where before it was at 5000rpm). This performance correlates directly with the set up of the boost controller, with the boost limit of 1.20 bar achieved at 4100rpm and held steady to the rev limit at 7200rpm.
But what happens after this is a little odd and I'm finding it difficult to determine what is causing it. From 4000rpm onwards, the rate of increase in engine power drops significantly and the torque curve drops away. The result is that the maximum engine power is 30hp less than it was previously, peaking at just under 300hp, where in the past I've had it running at just under 330hp.
I asked Roel at Rica to plot the air/fuel ratio, since I know from the last time I visited them that the mixture was running a bit rich. Indeed, at high revs the ratio was running below 0.8, so after a bit of convincing, I got him to lean it out a little. As you can see from the chart, the ratio is now maintained above 0.8 throughout the high rev range, but unfortunately this made no significant difference to the curves. As a side note, Roel seems reluctant to run the mixture any leaner than this, preferring to err on the side of caution and keep the mixture slightly rich so as not to over stress the engine and to help cooling a little at high revs.
I discussed the curves with Roel and it was his opinion that either the exhaust or the inlet temperature are constraining performance. I've given this some thought overnight and I'm not convinced.
On Enrico's pages there's a chart of a stock Ghibli torque and power curve (the torque is in different units, but just multiply by 10 to get approximate Nm values) which I've attached above. On these curves, there's no sudden change in gradient that I'm seeing on my curves. There seems to be a bit more inertia with my turbos spooling up too... mine have a bit more lag, but once they start spooling up they do so much quicker than the stock ones appear to do.
So, things are looking ok up to 4100rpm. At this stage, the boost controller opens up the wastegates and starts bypassing the compressors and suddenly there's a drop in the rate of power increase and a drop in torque. On the stock curves, the torque stays flat until about 5000rpm and on the Ghibli Cup, this is extended further to 6000rpm with more efficient bearings, as illustrated below.
I'd expect my results to be more akin to the Ghibli Cup ones, since my turbos have roller bearings and are "geared" for greater flowrates and should therefore be more efficient with higher revs. I also have a sports mufflers fitted which should flow better with high revs over the stock car.
For reference, here's an old dyno chart.
You can see that the car used to spool up much slower, which has been rectified by installing a free flowing inlet system and optimising the boost controller settings. However, the car used to produce considerably more power at the top end?
I don't think the inlet temperature can be the cause for the change since nothing has changed that could have increased the inlet temperature unless the efficieny of the intercoolers has changed since last time, which is unlikely. There may be a problem with the exhaust since I know one of the cats is not working and may be causing a restriction? But then I'd expect this to effect performance through the entire rev range, not just from 4000rpm and above?
I was thinking that it was maybe the timing that is out, but this hasn't been changed since the last dyno run. So I'm confused now to explain the difference. Any ideas?
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