National Aeronca Association Bulletin Board

I've read alot about WW2 and the Germans inability to get hold of 100 octane. (We apparently had it all, thanks to primarily Jimmy Doolittle who worked for Shell before the war.) I always thought that 100 octane , due to the lead, increased horse power of high comp. engines, (and made them quit knocking.) The Germans made ALOT of high test out of synthetic fuel which they made out of coal. Expensive process but they made lots of it after we bombed Polesti. When I had the Stearman in the 80's I was told by a GOOD engine man to use only Amoco hight test car gas as the 100 would mess up the low comp. 220 continental. (I think he said it was about 5 to one) On the panel of the Stearman it read: Use Octane higher than 86 only in emergency!. So what is the story? Today I use 100 as the airport needs the business and its easy. Whats the compression on a 65 cont.? Thanks JP

JC,
I think the compression of a 65 is about 6.3:1 if my memory is correct. I'm thinking on your Stearman (if the compression was actually 5:1) that excess lead like in 100 LL (blue)or 100/130 (green) octane would load up the valves, guides, etc. if not able to scavenge it due to less cylinder heat. To be honest, my 65 doesn't seem to care much what it burns, although I tend to use a lot of auto fuel so if I do get 100 LL it's probably pretty much mixed with auto fuel and I used MMO once in awhile. Again, if my memory is correct, the 65's were originally 'certified' for 73 octane which I think is the old 'white gas' we used to get years ago. I only ever saw it used on old tractors. When I was in the Air Force some of our big radials used 115/145 (purple). Unsure what the compression ratio was but most were supercharged and needed the higher octane to prevent detonation.

Mike

Back in the 70's, the FAA published a document advising pilots to NOT use 100LL in an 80 oct. engine. If 80 oct was not available, only use enough 100LL to get you to an 80 oct pump. The FAA stated, as Mike points out, that the excess lead would load up the valves, guides and the plugs. As we all know, that changed after the first oil embargo, plus the size of the valve stems and the valve stem to valve guide clearance were increased.

Like Mike said, the purpose of adding octane to gas is to eliminate "knock". If you have engine "knock", you are not developing the power the engine was designed to produce. To be correct, octane does not increase the designed power output of a non modified engine but rather allows the engine to develop the designed power output. The energy content of all gas is about the same, unless it is contaminated with something like ethanol. Octane in excess of what is necessary for an engine to produce design power is wasted. Kinda like a 40 DDD.

The lead content of 80 oct avgas is approx. 1/2 gram/gal.
The lead content of 100LL avgas is 2 grams/gal. 4 times as much
The lead content of 100 oct avgas is 3 grams/gal. 6 times as much
Don't know about 100/130 avgas but it must be a bunch.

Dennis

Thanks for info ! Spoke to old pro yesterday who said " the 65 Cont. is a cave man engine! If put together (overhauled) correctly it will run on damn near anything and likes to run at high RPM so don't think you'll hurt it if you run clean oil and change it often, 20-25 hrs.) unquote. I usually throw in a few drops of MMO every other tank or so. JC

According to the logs on my 65, it's been run for many years on 100, & for many years has had a disassembly for decarbonization about every two yrs. Since I purchased it, I run a mix of 1-gl of 100, with 4-gl of auto (non ethinol), & 100-150ml of synthetic two stroke oil. I't runs clean as a whistle & hasn't shown any need for disassembly for decarb since

On the other had my plane ran a 45" pitch prop at a max RPM of 2,000 since 1947, I have since had Levans twist it back to 42", it now runs as high as 2,300 or so, that MAY have been a contributing factor to the carbonization problem too

Whadda-ya-do when you're placarded for 100, and all that's available is 100LL?
I've never seen 100. unless it's th' 100/130, which th' placard does not specify.
And I've not seen 80 since the early 70's.