The early Ghibli IIs (like mine) came without blow off valves (see earlier post) and as a result, there's a good chance that the turbos on these cars will fail sooner than those fitted on the later cars, due to the constant spooling and stalling that results from not having blow off valves. However, my Ghibli had only done low mileage (around 55,000 km at that time) so the turbos were still in good shape. So it was really a bit premature to change them out... but the opportunity arose.
The turbos on the 3200 GT are technically more advanced than the ones fitted on the Ghiblis, since they incorporate a roller bearing package over the plain bearing package on the Ghibli turbos. The turbos for both cars were manufactured by IHI of Japan. The power side of the turbo on the Ghibli and the 3200 GT is the same except for the bearing package, denoted by the suffix letter in the product description, RHB for the Ghibli and RHF for the 3200 GT. The compressor side is larger on the 3200 GT turbos, which is not suprsing given that it's a 3.2l engine versus the Ghibli's 2.0l. However, I'm eventually planning on further increasing the maximum RPM, polishing up the heads and fitting a free-er flowing exhaust, so the additional capacity that the 3200 GT turbos cater for will eventually be utilised in my Ghibli.
The cost of the increased capacity of the 3200 GT turbos is increased lag time. However, this is offset by the bearing package on these turbos versus the Ghibli turbos; the lower friction co-efficient of the 3200 GT turbos means less resistance to rotation, which means that like for like they should speed up quicker than the Ghibli units. So, factoring in the fact that the compressor is larger on the 3200 GT turbos and therefore doing more work, the net effect is that the 3200 GT turbos spool up at about the same rate as the Ghibli turbos.
I was fortunate to acquire some nearly new 3200 GT turbos from David Askew Maserati Parts Service, with only 14,000 miles on the clock. I had them shipped directly to the Maserati specialist Auto Forza, where Jorrit promptly fitted the units for me, taking about 2 days since there was a bit more messing around than he had anticipated. The exhaust manifold on the 3200 GT and the Ghibli is obviously different, so the old manifold from the Ghibli had to be retained and re-used. This meant that they had to be carefully removed from the engine block and the old turbos, which is easier said than done given that the bolts seize up pretty tight here given the heat they're subjected to.
The new turbos were a very snug fit and the engine had to be lifted slightly at the front of the car to accommodate them. This was done by inserting some washers underneath the engine supports between the engine and body, lifting it about about 10 mm. Some material also had to be shaved off one of the new turbo housings (the right one I think) to prevent it from touching the body, particularly when turning.
Once fitted, it was a bit tricky to determine what effect the new turbos had had on performance. When I initially took the car for a dyno, it was giving around 310 hp but only producing 0.7 bar boost pressure. No one could figure out why the boost pressure was low, although 0.7 bar just happened to be the fail safe pressure for the OEM boost controller. There was no engine warning light and the chip tuners confirmed that the fuel map was being followed. We could only think that the car was running in safe mode, but no one, including Jorrit, could figure out why? It didn't seem to be the solenoid, particularly since this had been recently changed out after a similar previous incident when the car was found to be running again at 0.7 bar due to the solenoid not working.
Since we couldn't find a mechanical problem and the chip tuners at Rica Engineering swore that it wasn't a problem with their software, I decided to purchase a new boost controller, an e-Boost 2 from Turbosmart in Australia. I fitted the boost controller and completely removed the OEM solenoid (seems there's no feedback loop from the solenoid to the engine ECU since the car runs fine with the solenoid not connected). I took the car back to Rica and set up the e-Boost on the dyno. However, despite the car now producing about 1.2 bar of inlet pressure, the power output was down at about 270 hp!
It took a lot of thinking and running of a few tests (including a cyclinder leak test, which showed that the engine was in pretty good condition, with only 5% leak from most of the cyclinders, but oddly with 20% leak from the cylinder on the rear right). I was convinced that it was the exhaust that was the problem, so asked Jorrit to try swapping out the mid section with a known good unit (which he fortunately had in his workshop). He took the car for a drive and instantly noticed a difference! So it was definitely exhaust related.
When putting back the old exhaust, he happened to notice that one of the pins on the lambda sensor was bent and not connecting properly. He straightened the pin and connected it back. When driving the car with old exhaust re-installed, it was driving much better than previously, although I suspect that not as good as with the replacement exhaust. So the problem appeared to be with the lambda sensor, which means that the engine had probably been running in open loop mode, unable to correctly measure and interpret the exhaust gasses. This means that the car had probably been running rich, which would explain the drop in power. It also means that the catalysts have become junked, since running a rich mixture through a catalyst will end up seizing it up. I find it a bit odd though that the engine warning light did not come on when this was occurring? So I'm tempted to install a data acquisition system so that I can monitor the mixture myself and not rely on the ECU's electronics doing the right thing.
This would explain the results from the next visit to the dyno at Rica. Although the results were better, now putting out 328 hp, it was not as good as I had anticipated. I'd figured that the car should deliver about 360 hp given that I had been getting 327 hp before the turbo upgrade (since the car had been chip tuned previously and was now red-lining at 7200 rpm). I also set up the e-Boost again while on the dyno and noticed that the turbos were by no means hitting their limits; I was not able to get the sensitivity of the controller to overshoot the dialed in maximum boost pressure of just over 1.2 bar. This means that there's probably insufficient exhaust gas flowing through the turbos (i.e. that the turbos are too large for the current application).
I find this a little bit hard to swallow. Given the evidence, I'm convinced that there's a restriction across the catalysts that's robbing some power from the engine. Also, the restriction may cause sufficient pressure drop in the exhaust to restrict the gas flow from the engine, which would result in the turbos not spooling up so quickly.
Of course, there's only one way to tell and that's to try fitting a new exhaust and take the car on the dyno... I'm planning on doing this later in the year...