I've recently been emailing with a scandinavian Maserati enthusiast who got me thinking about the inlet side of the Ghibli (again).
Having seen my dyno charts and then looking at Modena Performance's torque and power figures for their tunes Ghiblis, it appears that my Ghibli is being restricted on air flow which is capping the performance of the engine. My attentions have been focused on the exhaust side of the engine and problems with the catalyst, but Jarle seems convinced that the problem lies on the inlet side.
So I've been doing a bit of research today, looking at air filters and boxes and stumbled across a new product from BMC that looks interesting. I've always found the air boxes amusingly small on the Ghibli, but this kind of makes sense when you consider there's two of them. I've also always found the fact that there's now decent route for air to flow from the front of the car into the intake system a bit strange on such a high powered car. But to be honest, there's not really an awful lot of space to stick anything exotic in. What's also a bit odd is the dimension of the inlet piping to the turbo compressor. Although it starts off at a pretty healthy looking diamter as it leaves the air box, it appears to rapidly constrict down to a dimater that looks to be in the region of 50 mm? Have a look at the picture below and you'll see what I mean.
From talking with Jarle, he seems pretty adamant that once you start increasing the power output on the Ghibli, the inlet side of the engine (filter, piping and inlet manifold) quickly constrain performance. He might well be on to something... I'll have to do some calculations to determine the pressure drop across the inlet piping from the air box to the compressor, but if you just look at the increase in area going from 50 mm to 70mm diamer, it almost doubles! If you also factor in the increased efficiency of say, a cone type filter versus the OEM filter, it's certainly not going to harm performance increasing the capacity of the intake system.
If you could also get some dynamic charging effect by directing cold air from the front of the car into the intake system, you could improve performance in further, but I've not been able to figure out an easy way of doing this on the Ghibli (although removing the plastic housing around the headlights like this guy has below would certainly help (see page 79 on Enrico's site for more details)).
I've always been put off cone filters by the fact that they're prone to suck up hot air from the engine bay when driving at low speeds (which in todays world is most of the time!) which effect both performance and engine longevity. I've therefore been looking for a replacement air box, to shield the air flow from heat as much as possible. I've thought about an itg Maxogen airbox, which in principle is an ideal replacement for the OEM airbox given the orientation of flow in and out of the unit. However, it's a bit too big for the Ghibli and given that you can't easily run a hose from the front of the airbox to the front of the car, it's function is a bit wasted, making the value proposition for this unit (remember you need two of them at about €250 each!) not very positive.
I've also been very tempted by the BMC Carbon Dynamic Airbox. However, as for the itg unit, the value proposition for this unit is not great for the Ghibli by virtue of the fact that it's dificult to route the flexible ducting from the front of the airbox to the front of the car.
Then I stumbled on the new BMC Dynamic Intake Airsystem, for which I found a nice picture on the Dutch BMC distributors website pictured on the right. This is supposed to be the little brother of the BMC CDA above, made from cheaper materials and hence selling at a cheaper price. But given that it can be run withough the inlet ducting, it could just be perfect for the Ghibli, particularly if the plastic housing around the headlights is removed as in the picture above.
So, time to do some sums. Using Bernoulli's equation (see eFunda for some background), good old Microsoft Excel and a bit of logical thinking, I come up with the following:
My Ghibli has a 2 liter engine and has a rev limit of 7200 rpm. I'll assume the volumetric efficiency of the engine is 90%. The engine inlet pressure is about 1.2 bar and I'll assume that the inlet temperature is about 70 deg C. Air only flows into the engine half the time per revolution of the crankshaft, so to determine the flowrate of 1.2 bar air into the engine, multiply everything together and divide it by two to get 6.48 m3/min or 0.108 m3/sec. This is air at 1.2 bar and 70 deg C, but I'm interested in air flowing through the inlet ducting upstream of the compressor. I'll assume that the air here is at atmospheric pressure and temperature, so 1 atm and 20 deg C. Since PVT = constant, the volume of air on the inlet side of the compressor is about 0.936 m3/sec assuming 90% volumetric efficiency. Since the Ghibli has two inlet conduits, this volume must be halved to give 0.468 m3/sec.
I'm assuming that the inlet piping from the airbox to the compressor inlet on the Ghibli is 5 cm diameter, but I still need to measure and confirm this. If this is the case, then the velocity of the air flowing through the piping is 238 m/sec. Considering that the speed of sound is about 340 m/sec (at which speed all sorts of funny effects start to occur), this seems to be an unneccessarily high velocity for the inlet system? The air flow is undoubtedly turbulent at this rate (this is verified by calculating the Reynold's number). You can get all the air properties also from eFunda.
Next thing is to determine the pressure drop across the inlet piping. To do this requires a bit of Bernoulli. Assuming a friction factor of about 0.02, that the compressor inlet is 30 cm below the airbox and that the length of inlet piping is 50 cm, I calculate that there's about a 0.07 bar pressure drop across the inlet ducting. Dosen't sound like much, but it means that the turbo is having to do work just to suck air into the compressor. If I now calculate the pressure drop for a 7 cm diameter intake pipe, the pressure drop is reduced to about 0.01 bar.
So what does this mean? Well, looks like I'm loosing about 6% flowrate of air due to the choking effect of my current intake piping over increasing the diameter to 7 cm. Since power is directly proportional to the volume of air, this means I could increase the power output of the Ghibli by 6% just by increasing the diameter of the ducting. So that's a 20 hp increase over the current 328 hp, taking it up to just shy of 350 hp.
Looks like I'll be ordering some new stuff pretty soon!