Don't be discouraged
Mikemo is spot on the X is light/nimble, 150 is heaps as that Nissan is a truck by comparison if that is where you got the E85 number let go of 200 target first time up, but I disagree about the X remaining unsuitable for a turbo. I'm agreeing with Karl, "we'z talking about da management" modern high CR engines exist because they are managed by smart ECUs that didn't exist when Legend Industries was putting puffers on 124's.
Cooling is key, no rain tray, opening up the side scoops ( adding engine bay underside scoops !! ), some underside fans on the IC, correct sized in and out pipework. Work on making incremental steps on this motor, while you are busy breaking it start hording parts, you guys have so many 1500's laying around find a decent (straight) 14 bolt block and take the lessons you learn from your first one and build a built proof puppy. You'll have a $#t grin you will find hard to wipe off your face when you turn it off and get out of it.
There are a lot of folks here that want you to succeed, (some have said so by asking you to document it because emulating what works is easier) its just that the spare 'parts bin' in the America's didn't get Fiat SOHC Turbo bits and the natural choice for such a nice chassis is NA. The previous boosting experience you have had with the SX, MR2 and importantly with the MSquirt may make you their poster child. The availability of a cheap ECU has held boosting the SOHC back as we are predominately Fiattightfisters with our limmited cash. So spending $2k on a proper ECU/compressor setup/dyno time makes us think twice.
I get the feeling you understand that once you step outside of standard equipment and hardware the stock ECU will suffer and "premapped" chips can't cope. From personal experience you do not want to lean burn the engine out - go rich once you stitch it together and work you way back to a leaner burn for greater HP. Blowing a SOHC can be done but it isn't easy as folk first imagine by just bolting a blower on, the 240SX engine bay has acres of space around a far more modern engine designed to accept more air into the cylinders and space for a big front mount IC in lots of air that can take the abuse from errors of poor thermal design, the X because of its packaging its problematic as a single inlet valve and low airflow over most of the hot bits in mid mount config' make it harder to shove air in and dispose of heat.
Individual velocity stacks are good for a broad torque curve when compared to a helmholtz plenum setup, but in a boosted application to get each intake runner to act as an IR requires a rather large plenum volume. 200 300 percent of displacement if I recall correctly. With a plenum of that size it starts to become less important where the TB is mounted in relation to the intake runner entrances but where are you going to find space for that plenum volume?
There are routes to a reliable 150bhp (91 octane) on a fresh ~1500cc motor but the heat you will generate pumping ~ 20 or +psi will see losses as you chase 200. If you don't manage them it will end in tears. The harder you try to shove air into a small hole the bigger the thermal load gets and just like you may have experienced with the MR2 there's little air flowing around in the little 'hot box' engine bay, the reverse flow head in the X is also ancient by comparison. Head over to the Punto forums and hoover up the examples of what works with the 8v Fiat turbos the MkI / GT is so similar you can just transfer the examples across. If you are going to Fab your own dump/exhaust then look for the 'Dr Evil elbow' and attend to the correct diameter and length, there is enough room between the turbine exhaust flange and the alternator for a decent radius - use all of it and shield the alt' from the latent heat.
That turbo is too big for my taste, your engine proposal is 8.5:1 (1500cc) so perhaps consider the smaller compressors.
Let me try and explain ECR ( apologies if I am telling you how to suck eggs if you already understand this ), I am going to exaggerate to illustrate what I'm trying to convey so take a pinch of salt.
Stay low as a higher static compression creates more power throughout the entire RPM band, but it'll lower your maximum allowed boost before the onset of detonation. Boost is worth way more power than compression, because boost raises your compression and your total air flow at the same time. With the down side of, when you're not on the boost, you have slightly less (or if you choose the wrong compressor a lot less) horsepower.
As I understand it (but anyone please chime in if you have better info):
Effective Compression Ratio = static compression ratio x (1 + boost/14.7)^1/2
For a car running 8.5:1 pistons and 18psi (~max on 91 octane unleaded) 8.5 x (1 + 18/14.7)^1/2 = 12.67 ECR
If you run 9.0:1 pistons and want to maintain the same 12.67 ECR (~max on 91 octane unleaded), you'll have to lower your boost to: 14.4psi
So you have to run 3.6 psi less boost to maybe pick up a tiny bit of bottom end due to the NA nature of the higher static compression. Or to take it even further for 9.5:1 CR you can only run 11.4psi. I'll point out right now that the difference between 11.4psi and 18psi is massive.
At some point say for giggles 12.5:1 you can run very little boost and be maxed out on the ECR for 91 octane unleaded. And how fast is a N/A car with 12.67:1 compression, I'm sure you can ask the Honda guys running ~15's.
Of course if you swing too far in the opposite direction things don't end well either and you turned the car into a gutless ride on lawnmower. A car with 6.0:1 isn't much good on the street, but it can run 50.8 psi boost on 91 octane unleaded if boasting about big numbers floats your boat. But the kind of turbo that could support that much boost would hardlyr spool up in the life of this little engine. You have to find what's right for you, but 8.5:1 is what most of us old turbo guys like. I know of a bloke that just built a motor for his 8 valve 4 pot SOHC Pug motor with 9.5:1 pistons and he is quite upset to find that he's getting lots of knock running just 16psi. While lately in the cold weather, I've been running 20psi (8.5:1) on 91 octane unleaded * WOT without any knock. I drove his car and the difference in bottom end grunt isn't decidedly noticeable, the kicker is his top end performance is sad compared to mine.
I don't think it is a case of the journals being too small, I have a firm belief that the the lack of thermal containment gives the oily bits a hard time when you aim at high levels of boost. Anything will collapse and die under those conditions. The Fiat Turbo blocks had oil sprays tapped into the mains supply and an oil feed to the turbo drilled into the main rising feed in the centre of the block (which wouldn't fit with a 1500 bore so don't try). The pistons are not just low compression, the ring landings are taller as well to prevent the rings getting overheated and avoid cracking of the piston crown (that's why I say just find some turbo pistons and see how far you can go). The turbo rods had two pin hole oil sprayers in the big end, so all this tells me the engineering boffins at Fiat were trying to get rid of heat and keep the reciprocating parts slippery.
The piston "ring landing" refers to the gap between the first ring recess and the crown of the piston. On NA cars this gap is quite small but on turbo cars where there can be a substantial thermal gradient between the two points it can lead to fatigue of the metal with obvious detrimental effect. To counter this the ring recesses are cut further down the piston.
The validity of the oil spray is a debated subject - it isn't unique to Fiat or even to turbo cars but many engines do perfectly well without them. The addition of these sprays is a matter of tapping into the main oil galley and simply screwing the spray heads in. The one thing I don't have for reference is where to find a spray setup with an arm long enough so they can be off to the side and allow it to be pointed in the right spot as the rods will hit the BMW ones I thought might work, something you would perhaps need to investigate if you do build one from the ground up.
The extra heat causes some of the blocks to distort (IMHO these engines have basically the same problem but less apparent), the engine still runs but you will never get the extra power you were hoping for. I'd be real interested to hear what you end up achieving. Drilling two pin holes in the rod big ends isn't rocket surgery - maybe that will be enough * < 15psi.
That basically skims over the top of some of it. I try to make sure my information is straight before posting but I am a novice so I make the regular mistake. So don't take my posts as the complete and only truth. But take this one to the bank :
With turbos pick the smaller one with the better efficiency where the engine speed is most of the time for your driving style. Bigger is not always logically better.
If you go bumping up the power on N/A then you will be pulling down anyway - boosting has bigger gain so don't chicken out now.
Were all rooting for you!!
My best
Sandy
