Hi Ross thanks for you reply, it seems we are communicating better which always good for all "We agree that the BSFC for 4 stroke diesels is better than most SI engines- no argument. Here pump gas is around .83c/ L and diesel is 1.05. This means the diesel is not going to save any money over a comparable SI powered car." Lol...Luky you! here in Portugal and in most other European countries its the other way round, diesel is cheaper than gasoline! here is a link that should make you guys across the pond realise how good you have it https://www.energy.eu/fuelprices/ (these are reference prices, they can be a little cheaper or expensive) Here you only have unleaded 95 @ 1,48 euro/L and 98 @ about 10 cents more! Diesel costs 1,25euro/L Just for the record my home made Biodiesel has a final production cost between 30 and 40 cents/ L ! so you can understand why I bother to make it and dream of a flying machine powered by it ! Depending on where you are in Europe diesel powers between 50 and 65 % of all traffic on the road even in countries where diesel is more expensive! Just stating a fact that explains why the best diesel engines are developed here! "At the low specific output levels you are talking here, SI engines easily last a very long time also. Our shop turbo 240SX has 330,000 km on it now after 17 years of being turbocharged, beat on at the racetrack etc. Compression is still excellent, oil consumption 0.5L in 10,000 km. I just change oil in it and drive it. I think you will find that engine life in the aero application (continuous high power) is reduced considerably compared to automotive, especially when the hp is increased over stock levels. My point was simply that diesels can't match the same power to weight ratios as SI engines and weight is important in aircraft. Any turbo engine can have better power to weight ratios simply by increasing manifold pressure." Well Ross there are good and bad engines out there, both SI and diesel , but the point is the engine examples I mention have decent power to weight ratios that are stock and can maintain high power output all day without undue stress! So you can increase power levels safely between 20 to 25% and some makers actually offer that as an option! But you dont need to , because the power is there already ! While you are right that Si engines can be boosted to produce more power then diesels that comes at greater cost in terms of fuel cosumption and engine life ! and lets not forget that torque plays a part as well ! (no wish to go into the torque vs hp debate) but in aircraft applications it seems to be a bonus in certain conditions . So correct me if I´m wrong take off on 80 to 100% power for 2 minutes, then back of to 75 to 80 % on climb , and cruise on 60 to 65 % power ! In a current diesel engine that translates into 3600 to 4000 rpm then back of to 3000 to 3200 rpm and cruise at 2400 to 2600 rpm ! For a diesel engine that is easy, in fact they thrive on stable power settings , when I drive across Europe my diesel loves it , I set cruise control for 2400 rpm (good for 120kph). In Germany I push it a bit 2800 rpm and keep there for hours! no problem and this stability also delivers great fuel economy! My point is a diesel is built precisely for long periods of high work loads sustained by their natural low end torque that is constant from as low as 1800rpm , and max out at around 2800 rpm, which is apparently the sweet spot for best prop efficiency.