The SDI engine is generally utilised in applications where reliability and fuel economy are of primary concern. These engines lack any type of forced induction, hence the use of 'suction' in the title, and as such, their power output is lower when compared with a similar displacement turbocharged engine. For example, the 2.0 SDI engine fitted to the Volkswagen Golf Mk5 has a peak power output of 55 kilowatts (75 PS; 74 bhp); whereas the same engine in Turbocharged Direct Injection (TDI) form is rated at 103 kilowatts (140 PS; 138 bhp) or 125 kilowatts (170 PS; 168 bhp), depending on specification. However, in the SDI engine, performance at very low engine speeds and when moving off from stationary is often preferable to forced induction engines due to the lack of turbo lag.
Why is the performance of SDI engines better at low engine speeds? I mean, they're the same engines with the same combustion volumes with the only difference that there is a turbo fitted but does that really restrict air flow?
The SDi might be a bit better at low revs due to the higher compression ratio of the engine but not enough to make any real difference in a race situation. Over 2000rpm the turbo is better all day and clearly these cars were revving over 2000 rpm.
If you add turbocharger on SDI then is like a4 I think better then A4 because SDI have very good condition piston and pistons rings because SDI is the some engine like A4 just without turbocharger
