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  #11  
Old 07-06-2011, 04:40 PM
Kyle Boatright Kyle Boatright is offline
 
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Location: Atlanta, GA
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Quote:
Originally Posted by rocketbob View Post
Where are you guys getting the data from that shows what compression ratio works with different octane levels?

I ran my -6 for over 1000 hours with 87 pump gas on 8.5:1 with absolutely no problem.

As I recall in CF Taylor's book there is no clear relationship between compression ratio and octane. I know plenty of motorcycles that have a 12:1 compression ratio that run fine on 87.
Those motorcycles are running at 10k rpm with small cylinders. That's a very different proposition than our engines. Relatively speaking, we have slow turning engines and large cylinder volumes, which are both contributing factors for ping/detonation/preignition - whichever verson you worry about.

That's why cars manufactured back in the pre- automated engine management system days would ping at high loads and low RPM (like our airplane engines), but wouldn't ping at higher RPM and high loads...
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  #12  
Old 07-06-2011, 07:32 PM
Mark7986 Mark7986 is offline
 
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Talked to ly con a week ago and they said you could use 91 oct with 10:1 compression
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  #13  
Old 07-06-2011, 07:43 PM
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rocketbob rocketbob is offline
 
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Lots of conjecture here about knock detection, EI, fuel injection, etc.

My point is that nobody's ever done any real testing of different octane levels on Lycs that I've ever seen published.

87 octane worked fine in my engine, but according to the conventional wisdom aka "old wives tales" should have blown up a long time ago.
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N9187P PA-24-260B Comanche, flying
N678X F1 Rocket, under const.
N244BJ RV-6 "victim of SNF tornado" 1200+ hrs, rebuilding
N8155F C150 flying
N7925P PA-24-250 Comanche, restoring
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  #14  
Old 07-06-2011, 07:44 PM
Peterk Peterk is offline
 
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Default Ethanol

How do these 0320's (experimentals of course) do with the ethanol that the certified STC's don't allow?
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  #15  
Old 07-06-2011, 08:12 PM
Kyle Boatright Kyle Boatright is offline
 
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Location: Atlanta, GA
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Quote:
Originally Posted by rocketbob View Post
Lots of conjecture here about knock detection, EI, fuel injection, etc.

My point is that nobody's ever done any real testing of different octane levels on Lycs that I've ever seen published.

87 octane worked fine in my engine, but according to the conventional wisdom aka "old wives tales" should have blown up a long time ago.
I, too, know people with 8.5:1 engines who have happily run 87 octane. I just wonder what happens when you hit the "worst case" conditions. Hot, dry day, high atmospheric pressure, low altitude, gas that is slightly "off" for some reason, and an aggressive fixed pitch prop.

And I bet someone has done the testing. We just haven't seen it. ;-)
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  #16  
Old 07-06-2011, 08:31 PM
foka4 foka4 is offline
 
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Location: Ankeny, IA
Posts: 210
Default Low on compression and high on attention to detail

The low compression E2D is one of the most durable and dependable engines available for GA aircraft. How about building it to its original low compression ratio and focusing instead on efficient propeller, cooling, induction, and exhaust?

That's the route I took in my -4 and I'm delighted with the performance. The ability to run cheap gas worry-free just adds to the RV grin. To each his/her own, though. I'm sure I'd be just as happy with more power.

M
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  #17  
Old 07-06-2011, 08:59 PM
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Av8torTom Av8torTom is offline
 
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Default Back in the day...

airports used to have 80 octane avgas. Our O-320, 150 HP preferred that to 100LL - too much lead.

I've also not seen any published data on the effects of EtOH in airplane engines. If someone could point me to those data I'd appreciate it. Always been a mystery to me why automobile engines seem to run fine on it.
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Last edited by Av8torTom : 07-06-2011 at 09:02 PM.
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  #18  
Old 07-07-2011, 08:17 AM
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rocketbob rocketbob is offline
 
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Last night I thumbed thru my CF Taylor vol 2 book. In chapter 2 he has a chart that shows octane requirements on the Y axis, with bore diameter on the X axis. With a 5" bore (Lycomings are 5.125") the octane requirement is about 70, and it will go up or down from there based on spark plug arrangement, valve design, inlet temperature, etc.
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Bob Japundza CFI A&PIA
N9187P PA-24-260B Comanche, flying
N678X F1 Rocket, under const.
N244BJ RV-6 "victim of SNF tornado" 1200+ hrs, rebuilding
N8155F C150 flying
N7925P PA-24-250 Comanche, restoring
Not a thing I own is stock.
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  #19  
Old 07-07-2011, 03:05 PM
Sig600 Sig600 is offline
 
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Quote:
Originally Posted by rocketbob View Post
Lots of conjecture here about knock detection, EI, fuel injection, etc.

My point is that nobody's ever done any real testing of different octane levels on Lycs that I've ever seen published.

87 octane worked fine in my engine, but according to the conventional wisdom aka "old wives tales" should have blown up a long time ago.

I don't think it's that it should have blown up, or that it doesn't work (obviously it does), but that you're more succeptable to detonation.
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  #20  
Old 07-07-2011, 06:07 PM
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rocketbob rocketbob is offline
 
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Quote:
Originally Posted by Sig600 View Post
you're more succeptable to detonation.
And what data did you base this statement on?
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Bob Japundza CFI A&PIA
N9187P PA-24-260B Comanche, flying
N678X F1 Rocket, under const.
N244BJ RV-6 "victim of SNF tornado" 1200+ hrs, rebuilding
N8155F C150 flying
N7925P PA-24-250 Comanche, restoring
Not a thing I own is stock.
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