I have seen your threads out there, you are brilliant but haven't seen anything recently. I do hope you're still following, your advice on headwork is fascinating.I think you should bear in mind that using twin-turbos on an inline 4-cylinder is more 'gimicky' than practical. It will look very cool, no doubt, but in reality a properly sized single turbo will have better response and make more power. Running two small turbos seems a better choice, but when you take into consideration the fact that you now have nearly twice the friction to deal with (though less inertia per turbo), and only half the engine heat/impulse power to drive each turbo, I think you can see it is not all it's cracked up to be.
A properly size single turbo with a ball-bearing center section can make more than enough power, and packaging won't be a nightmare. You can also reduce the length of the underhood plumbing this way, again reducing potential lag. BTW, this imformation comes direct to me from numerous people with far more knowledge about turbos than myself. I have two aquaintances who have vehemently stated this fact. One is a Garrett turbo engineer, the other is a Nissan engineer with much hands-on experience (Mike Kojima, a writer for Sport Compact Car).
As far as the Opel's ability to accept boost, it makes sense that if a project of this magnitude is going to be attempted, the engine will be overhauled anyway. The stock compression ratio of the '9:1' engine is in truth closer to 8.4:1. This is certainly in the realm for a turbo system, assuming engine management is adequate. The real issue at hand is that the pistons are cast aluminum. They should either be thermally coated, and boost kept moderate (8-10 psi) with an intercooler utilized and possibly water injection (such as Aquamist), or the pistons could be swapped for custom forged pistons with lower compression, and a slightly lower ring stack to aid in ring cooling. Venolia has made many turbo pistons for me for Opel engines over the years, the crown is thicker to aid in dealing with heat.
Head gaskets are no real issue in Europe. I even have some copper head gaskets I had made for these engines. I see that zuxx is from Sweden, and in Sweden and neighboring Finland, Opel Tuners are more prevalent than Honda tuners here in the States. In fact there is one Opel Tuner in Finland that can lay claim to the world's fastest CIH-powered Opels. They have an Ascona B with a turbocharged 4-cylinder engine running 7's in the 1/4 mile. And a naturally aspirated version running 11's.
If the engine management is good, and the engine is not revved too hard, the stock Opel internals are tough. A good set of rod bolts and polished/shotpeened rods, plus forged pistons will make you a bulletproof shortblock. Strong enough for 350+ hp with a turbo, or 8500+ rpms without. I have run stock rods to 9400 rpms regularly in N/A racing engines. The real issue with the Opel engine is in the valvetrain. But with proper roller rockers, a stud girdle, and good springs/retainers, Opels have handled a few missed shifts where the rpm's exceeded 10,000.
Yes, Bob is still around and still Opel'ing and still posting and sharing his huge wealth of knowledge with those who ask nicely.I have seen your threads out there, you are brilliant but haven't seen anything recently. I do hope you're still following, your advice on headwork is fascinating.
I don't know if you realized it or not, but this thread is 15 years old. Just figured I would mention it.If you really wanted to get crazy, I think a single turbo, centrifugal supercharged CIH with EFI would be out there. As Bob said, a twin turbo 4 banger doesn't make a lot of sense. You simply don't have enough exhaust heat to spool two turbos properly. But, there is a lot of merit in a supercharged-turbocharged engine, especially if the supercharger has an electronic clutch. Volvo is doing this with one of their engines, the clutch disengages at higher RPM's.
Realistically, economically, best bang for your buck option is to just stick with a single turbo. Keeping things simple means less crap to worry about. A single turbo will provide all the boost you need. The really crafty task would be to create an intake manifold with an integrated water to air intercooler. Most turbocharged vehicles don't go this route because it's harder to do. Done correctly though, it's more efficient.