The very basic specs will be as follows:
- Cast Log exhaust manifold.
- Forged low-compression pistons.
- Garrett T25-54R BB Turbo.
- Cosworth/Mercedes Intercooler.
- Dual-Piston Blow-off Valve.
- GM Throttle with Position Sensor.
- Manifold Air Pressure Sensor.
- Uprated Fuel Pump.
- Uprated FPR.
- Uprated Injectors.
- MegaSquirt 'n' Spark Management.
- Wideband O2 sensor.
I'm thinking a log manifold for a few reasons... Firstly, I can get one manufactured much cheaper than a tubular one. - Secondly, Tubular manifold gives inconsistencies in temperature due to the differing metal in the weld and pipes, which can lead to cracking. Tubular would also require supporting mounts for the turbo to prevent vibrations causing cracking. Thirdly, space is a problem in the RHD M42 engine bay and a log is more compact. Lastly, a compact 4 cylinder engine such as this will keep plenty of backpressure in a log manifold, without the need to scavenge opposing-cycle cylinders.
Pistons
Given the choice between running 7-8PSI of Boost with standard compression and the risk of hotspots/pre-det etc., or 10-14PSI with lower compression... Both would result in similar power, but the latter would give huge torque gains, and probably more stable. There is a US company which can supply forged pistons to my specifications for the same cost as a new set of standard items from BMW.
Turbocharger
A friend of mine does a lot of work with Nobles and Morgans and happens to have a brand new GT25R with a scratch on the compressor housing at a bargain basement price. - The internal wastegate is fixed at 0.5 bar, but Ԛ£70UKP should sort out a boost controller! The turbo can be seen in detail on the link below:
http://www.turbobygarrett.com/turbobyga ... 1171_3.htm
Intercooler
Plenty of other turbos on here for reference, and a couple in the UK... I'm looking at a Ford Sierra Sapphire Cosworth intercooler to fit behind the valance, but I'm sure I've read somewhere that there's a Mercedes CDi item that sits snugly between the front lights?
Blow-Off Valve
Vacuum fed from the inlet manifold, towards the entry point to reduce fluctuations from the inlet valves.
Throttle position sensor
Apparently there's a GM item commonly used by 'MegaSquirters' which I've been offered at a silly-low price... Senses position rather than the standard Idle/On/WOT switch.
MAP
Located in the inlet manifold, independantly of the blow-off valve vacuum pipe to prevent false readings.
Fuelling
I'm a bit stuck here... Do I leave the standard pump/sender in place in the tank, then use a second, in-line, fuel pump to raise the pressure at the engine? This would save having the whole system running pressurised. Would a swirl pot be necessary?
I'm not sure which injectors I can use, as my understanding is that the M42 uses a different type of injector to most other BMWs. I'm also unsure of fuelling requirements. Uprated FPR and injectors controlled by MS seems like the way though.
Timing and Tuning
Another grey area... Can I mate the crank pulley sensors to MegaSquirt'n'Spark? How does it do cylinder recognition? I'm looking at retro-fitting a knock sensor too, as per the M44, to help tune out pre-detonation.
Wideband O2 sensor makes a lot of sense, as I'm not going to have easy access to a rolling road to set the car up and I can make sure she's not about to blow up due to undefuelling!
So there... What are your thoughts? - I'm posting this around a couple of the BMW forums I frequent to get a wider response and I'll wait until I've got a solid plan in place before I start with the rebuild of my 2nd iS.
Thanks in advance for your support and comments!
Omi
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