Guest Andys@coffs Posted January 29, 2009 Share Posted January 29, 2009 I immediately went to the Jaba Chat site as I can recall a historical jaba chat newsletter that dealt specifically with the question of cruise RPM. However Jabiru has cleaned up the page and only the last 3 are there.....Perhaps someone else can answer as I cant exactly recall the reasoning. The j230 POH identifies cruise as 75% = 2800rpm. The 3300 installation manula identifies - Recommended Cruise: 2750 RPM – 3100 RPM. - Max continuous 3300RPM There isnt a similar manual for the 5100, unfortunately, however Im guessing given that the 2200 and 3300 are the same and a 5100 is just a 3300 plus 2 pots more that again it will likely have the same limitations. So given that there is 30hp more, turning at the same speed then the prop diameter must increase or the pitch increase. Iif diameter I pressume climb = better, if pitch speed = better (or worse if close to or beyond VNE). With regard to that nugget about a V8 getting better efficiency than a 4, its often talked about but if I constrain it as follows:- 1) Both vehicles are operating within their expected operating range 2) Both vehicles are of similar age and technology 3) The weight and aerodynamic efficiency of both vehicles is similar Then would we really see the V8 returning better fuel efficiency than the 4? Most modern 4's are around 6-9 lts per 100kms, does a V8 really get those returns? I only ask, not disparagingly because I truely dont know. I would have thought the V8 would be around the 10lts/100km, however in that comparison in general the weight of the car is significantly higher than the similar weight of the 4.... Andy Link to comment Share on other sites More sharing options...
Tomo Posted January 29, 2009 Share Posted January 29, 2009 Now thats an interesting comment that in the context of a 5100 needs more debate. On my 6cyl jab, Im told to cruise with RPM at 2850. to cruise with much lower RPM which is underutilising the available power causes damage to the engine Im told through Jabiru and will result in a shorter operating life. Yes that's true... If you have a big tractor, say 300hp, doing the job that a 100hp can do. yes it will probably be a lot more economical, but it wouldn't be doing any work. ie there's not much load so its practically 'just running'. and what that does is glaze up the piston bores, and therefore wears the engine out a lot faster... Same with idling your car, you shouldn't idle your car for to long, particularly if it's a diesel... So the morel to the story is: Engines like working, not just running:big_grin: Link to comment Share on other sites More sharing options...
Guest Flyer40 Posted January 29, 2009 Share Posted January 29, 2009 Of course that was a broad generalisation, but from my own experience, I had a V8 that averaged 10.5 overall but would turn in under 8 on the highway where it was at 1800 rpm and never needed to change gears when a hill was encountered. At the same time I had a 1.8l Corolla work car that spent a lot of its time at red-line rpm trying to keep up with freeway traffic and used a lot more than 8. That was almost ten years ago and car engine technology has advanced where aircraft engine technology has not, so the analogy wouldn't work with newer cars. Just to show how incomparable this can get, I now have a 4 cylinder car that uses more fuel than my older 6 cylinder, but it makes 90Kw more than the 6 cylinder. Link to comment Share on other sites More sharing options...
Guest pelorus32 Posted January 29, 2009 Share Posted January 29, 2009 Just to show how incomparable this can get, I now have a 4 cylinder car that uses more fuel than my older 6 cylinder, but it makes 90Kw more than the 6 cylinder. And even more bizarre I have a 4 cylinder that doesn't even make 90kw, let alone 90kw more...and it's still a great car to drive - go diesel!! (75kw) Regards Mike Link to comment Share on other sites More sharing options...
gofastclint Posted January 29, 2009 Author Share Posted January 29, 2009 It doesn't take much load on an engine to prevent glazing of the bores. Some generators have a built in feature that will only just barely load the motor while no electricity is being drawn to prevent this. general flying would be enough as the prop always loads the engine unless in a dive. Link to comment Share on other sites More sharing options...
Guest brentc Posted January 29, 2009 Share Posted January 29, 2009 And even more bizarre I have a 4 cylinder that doesn't even make 90kw, let alone 90kw more...and it's still a great car to drive - go diesel!! (75kw)Regards Mike I had picked you as the diesel type Mike. Fossil fuels - using the past to power the future. Link to comment Share on other sites More sharing options...
Tomo Posted January 29, 2009 Share Posted January 29, 2009 I had picked you as the diesel type Mike.Fossil fuels - using the past to power the future. Haha... While where on the subject... our 4 cyl pushes out 120kw using 8.7 liters per hundred klm... and this is the hard bit to believe...'it moves nine people in the process;) It's amazing what you can do when you blow into an engine.... Link to comment Share on other sites More sharing options...
Guest pelorus32 Posted January 29, 2009 Share Posted January 29, 2009 I had picked you as the diesel type Mike.Fossil fuels - using the past to power the future. Just moving from one turbo - the Rex - to another - the Polo diesel. Pass a distillate soaked rag in its vicinity and it'll run for a week.!! Regards Mike Link to comment Share on other sites More sharing options...
Guest Andys@coffs Posted January 29, 2009 Share Posted January 29, 2009 And even more bizarre I have a 4 cylinder that doesn't even make 90kw, let alone 90kw more...and it's still a great car to drive - go diesel!! (75kw)Regards Mike Yep I to did something similar, out with the old commodore V6 and in with a ford focus 2Lt turbo diesel 100kw but 300Nm of torque. 5.6lt / 100kms, which means that the range is just in excess of 1000kms per tank.... And then the price of petrol and diesel came tumbling down which killed any perceived $ advantage.... Logic (maybe shear optimism!!) tells me thats only a temporary thing.... Andy Link to comment Share on other sites More sharing options...
Yenn Posted January 29, 2009 Share Posted January 29, 2009 It has been taught in the flying syllabus that full throttle operation is more economical than part throttle. That is the reason for controllable props, at cruise you pull the revs back and keep the throttle open, just like running a car in a very high gear. Link to comment Share on other sites More sharing options...
Guest brentc Posted January 29, 2009 Share Posted January 29, 2009 I get 6.5 lp/100 in the car at 205kw. Turbo 2.5 petrol. Makes the Jab look thirsty. I'm not sold that the 8 cylinder will use less fuel than the 6 at the same speed. My old boss used that argument on his V8 SS without success versus my 2.5 4 cylinder - he never won. Link to comment Share on other sites More sharing options...
gofastclint Posted January 29, 2009 Author Share Posted January 29, 2009 Subaru has a 2.0 turbo diesel for the Euro market, its the only production boxer diesel car engine. Can't wait till this engines start showing up at the wreckers. It would sit nice in an RV-8. Link to comment Share on other sites More sharing options...
Mick Posted January 29, 2009 Share Posted January 29, 2009 I'm already dreaming of a MoTeC Duel EMS with EFI and a redundant fuel delivery system. GFClint, Check this out......... Sport Aircraft for the Sport Pilot Looking at the list of area rep's there is one in Aus. Cheers Link to comment Share on other sites More sharing options...
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