| Paul:
Didn't mean to upset you with some facts.
There is a difference between efficiency and Hp. At the same Hp, the two engines will result in the same TAS. The difference is that the one with the lower BSFC will do it on less fuel IF, IF and ONLY IF they are both at the same mixture--as in the BSFC(min). If the timing is the same, the 8.5:1 engine will be significantly more efficient at turning fuel into Hp than the 7.5:1CR engine. With the advancing of the timing, the difference is less dramatic due to the effects that has on the BSFC. One might appreciate the effect these changes have on TIT in these engines which make one easier to manage than the other.
If you are not interested in these issues, that is fine. Others are interested in the "whys", so I spent the time to post the answer.
Walter
OK, Paul, let's review the issue.
Compression ratio: The higher CR WILL ALWAYS result in a lower EGT/TIT if all else is held constant. Why? Boyle's Law. PV = nRT. As the volume expands, the gas temperature goes down. More expansion; more temp loss = lower EGT. Walter: There is no issue here to review, so please step away from the tutorial podium. I fat fingered my previous email and met 17.5 rather than 7.5 so I was intending to compare a rate to a rate. Your simplistic physics above is all just dandy, but all else is never held constant. The timing recommended by the engine builder for a 8.5 and 7.5 are different, the MP is different to recapture some of the detonation margin and so on. All this has little to do with my original question which was at the same rate of consumption of a precious resource, disposable income, what sort of KIAS are LIV drivers achieving. You said in a previous email that a typical cruise fuel flow LOP for the TSIO-550 is 17.5 gph so fine, what indicated airspeeds are we seeing and if anyone now cares, if there are differences what accounts for those differences. If higher CRs are more fuel efficient as has been stated by many, well I would expect to see LIVs equip with those engines cruising at 17.5gph attaining a higher indicated airspeed. I chose KIAS because I thought it would simplify data collection. Simple stuff, and since I did not spend my time building a test stand, meaningful stuff at least for me. paul |