I hope to eventually have lots of performance numbers and experience with my carbureted Biturbo. For now, I'll sketch some ideas I've heard about, and a few of my own. I don't guarantee that any of these ideas will work!
If you want to read about the limited
number of upgrades that I've done with
my car, you can read about them
here.
The air pump can be disconnected for a few more horsepower. It will also decrease the underhood temperatures since the injected air is used in the exhaust to consume unburned hydrocarbons.
Adding ceramic coating to the exhaust headers will trap more heat inside the exhaust manifold, which helps build power in the turbos and reduce underhood temperatures as well. Don't put header wrap on the exhaust manifolds because it will cause the headers to crack.
I heard about a place called Paeco Industries from the Alfa Romeo mailing list. They claim they can build a set of custom pistons and cams for virtually any car, including Maseratis. Another place that sells custom camshafts is Elgin Cams; their catalog actually lists the Biturbo.
One easy horsepower gain on the 425 is to disconnect the coolant lines running under the intake for a cooler intake charge. This may cause drivability problems in cool weather, and might also cause ice to form on the throttle plate, causing it to stick open.
I've heard some people have removed the single air cleaner housing and installed dual air cleaners, one for each turbo. These are placed either closer to the turbo inlets or in front of the engine so that they can pick up colder air. It seems the latter idea would be the better solution, as the former solution causes the intake air to be drawn from the hot engine compartment.
I've seen a few ideas for turning up the boost; the most popular and simplest involves bleeding off pressure from the wastegate circuit. The wastegates appear to be adjustable; could you turn up the boost that way?
Another idea is to swap in larger turbos. The Biturbo turbochargers are pretty small, which gives short turbo lag but it also gives low efficiency at high boost levels, because small turbos heat the intake charge more than large ones. The downside of larger turbos is more turbo lag.
There are a few restrictor pieces in the turbo plumbing; they act as vacuum delay circuits. Perhaps their sizes (inside diameters) could be optimized for performance, assuming they're not already.
I think most, if not all of the performance ideas seen in other turbocharged engines should be adaptable to the Biturbo (carbed or FI, respectively). These include, for example, blow-off valves, wastegate bleed valves, and boost controllers.
I've heard of one owner who did a "big valve" conversion on his Biturbo. I'd guess that he used the heads from a 2.8L engine, which have two intake valves of the same size, instead of being different sizes like on the early cars. I think Maserati originally used different sized intake valves to improve the amount of "swirl" inside the combustion chamber, which helps combustion efficiency (e.g. emissions), but reduces volumetric efficiency. When fuel injection arrived, the improved emissions may have made it no longer necessary to have different sized valves.
One other possibility is to use the 4 valve per cylinder heads from some of the non-US cars. The FI engine from the Ghibli Cup car produced 330hp from only 2.0L! Imagine what would be possible from a 2.5 or 2.8L engine. I understand the heads will bolt right on and the head gasket is the same. Whether they will work at all is another question. It might be necessary to swap pistons and/or cams. There's certainly a lot of work involved. But it would make a pretty interesting project!
One obvious idea is to swap out the 2.5L engine for a 2.8L with the fuel injection. You'd go from ~185hp to 225hp in stock form (US-spec). Or maybe buy a 4/valve engine over in Europe and install that.
It looks like the automatic can be swapped out with a manual transmission. Or you can try modifying the torque convertor for higher performance. A lightened flywheel for the manual-equipped car allows the engine to rev up quicker.
Some ideas for loosing weight include: switching from a full-size spare to fix-a-flat sealant, changing the seats to lighter racing seats (the power seats are pretty heavy in the 425), removing the back seats (back seat room in the coupe is somewhat tight anyways), and switching the catalytic converter to a straight pipe (it's pretty heavy). You can do all these things and still have a comfortable and usable street car.
The first thing I've done with my '86 425 was to replace the stock muffler (which had a hole in it) with a new Stebro performance muffler. Total cost was about US$350 including express UPS shipping. It is claimed to be a stainless steel free-flow design. It looks very nice from the outside, the construction is welded with fairly thick metal (you could stand on it), and it has dual chrome tips. Inside the muffler, there are no baffle plates or anything like that, only some fiberglass-looking material held in by a mesh that covers the inside of the muffler.
Since I detest going to mechanics for anything, I attempted to remove the old muffler myself. That turned out to be a mistake, because in my exuberance to remove the old muffler, I put a small hole in the exhaust pipe. I struggled with removing the muffler using various techniques that turned out to be woefully inadequate (chisel/hammer, impact chisel, heat). I finally gave up and went to a local muffler shop to get the thing installed. They installed the muffler in less than a day, and I was charged $70 for the install, and over half of that was to repair the damage I did in trying to get the muffler off (big grin). The installation apparently went pretty well, but they did have to bend some pipes to get the muffler on, and I was charged for that, along with a new hanger. Pictures of the finished installation will come shortly.
Once I had the car back from the mechanic, I started it up and immediately noticed that the engine note had a little bit more of a "rumble" to it during idle. However, on the road I haven't noticed much of a difference in sound. There is a bit more edge to the exhaust note, but it is really hard to tell from stock while cruising on the highway, or even with my foot firmly planted on the gas pedal. I will eventually have recordings of the exhaust note on these pages.
I have not noticed a significant increase in performance; the car might be a little faster, but there's no way for me to really tell. I bought a G-Tech/Pro meter after the muffler install, and will use that to measure future performance gains.
I was a bit disappointed that the exhaust note wasn't louder; perhaps the catalytic convertors are dampening the sound more so than the muffler. I bought another performance muffler, this one from MIE, and plan to install it eventually when I get the chance. It's a twin glass-pack muffler designed for the Biturbo, its construction is also very nice, and it has dual chrome tips like the Stebro.