Apologies for bumping a month-old thread, but this board's fairly slow anyway.
To completely avoid the UFO noise, remember these 'rules':
-If you are fitting a N/A engine to a stock turbo car, remember to REMOVE the turbo kit 'stage 0' that it comes with, and preferably set the boost to zero for tidiness.
-If you are fitting a turbo engine to a N/A car, remember to ADD a 'stage 0' turbo kit, from any stock turbo car. If you want the car to appear naturally aspirated, leave the boost values at zero to hide the gauge (note that the game will still class the car as turbo, as it has a turbo kit equipped). Otherwise, take the boost values from the car you got the engine from.
-If you want to turbo a N/A motor, make sure it can be turboed normally. For example, I recently created a turbo Camaro. The Camaro's V8 cannot be turboed, thus fitting a turbo kit will produce the dreaded whistle. Instead, I swapped it for the Vector W8's engine, which already comes with a turbo, and thus solves the sound problem. On the other hand, fitting a N/A Golf V6 to a VW Polo then turboing that works fine, as you can buy a turbo for the Golf V6.
-Similarly, unless the donor car can equip one, don't fit a race exhaust to your swap. However, an engine that only ever comes with a race exhaust, ie. an Opel Tigra Ice Racer's V6, will work without a race exhaust fitted in a swap.
If anyone wants example hybrids to test the above, I can provide part codes.
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