What Octane? Reply #60 – December 31, 2011, 01:20:45 PM That's no big deal. You just had a brain fart. Everyone has those.Imagine trying to get a plane to perform well at low altitudes AND high altitudes. P51 Mustangs (the later models) had 2-speed superchargers (the pilots hated the superchargers, because they were junk) on their engines to compensate for the thinner air at higher altitudes. That thin air can really change how an engine performs. Quote Selected
What Octane? Reply #61 – December 31, 2011, 04:44:33 PM Same reason why avaiation fuel is higher octane, and most airplane engines had a timing advanced control in them. Have not read much past worl war 2 planes, but they ran those as lean as possible with high timing to help with mileage and power. I'll have to ask my dad what he knows about aviation fuels, but I can't imagine he would say much different. Quote Selected
What Octane? Reply #62 – December 31, 2011, 05:32:21 PM Quote from: 88 Blackbird 5.0;376585That's no big deal. You just had a brain fart. Everyone has those.Imagine trying to get a plane to perform well at low altitudes AND high altitudes. P51 Mustangs (the later models) had 2-speed superchargers (the pilots hated the superchargers, because they were junk) on their engines to compensate for the thinner air at higher altitudes. That thin air can really change how an engine performs.The Rolls Royce (Packard) Merlin with the supercharger was faaaar superior to the Alison V16 it replaced in the P-51A. Quote Selected
What Octane? Reply #64 – December 31, 2011, 07:57:27 PM The Merlin engine was far superior in power, but the 2-speed superchargers that were on the Merlins were pr0ne to sticking. That left the pilots who flew Mustangs in the war (like my Dad) in some touchy situations at times. Imagine being in a dogfight and the supercharger on your plane's engine is stuck in the wrong gear. You need 1st gear and the supercharger is stuck in 2nd. Not good. I remember asking Dad what he and the other pilots that flew Mustangs thought about the Merlin engines when they first came out. The first thing he said about them, was that they were junk. When they worked correctly, they were a dream though. I think dad said they finally resolved that problem though. I wish I could have gotten him to tell me more stories about the war though. He sure seemed to hate talking about it. Quote Selected
What Octane? Reply #65 – December 31, 2011, 08:00:18 PM Yes it was poorly worded, but I knew what he was trying to say. I suppose a different kind of cam is why Chrysler was able to have so many of the newer cars without EGR. I think this would be a better way of controlling emissions because it does not leave a point of failure. Had no idea cam selection was so critical when it comes to compression ratio. I would love to learn more about this. Quote Selected
What Octane? Reply #66 – December 31, 2011, 08:04:33 PM Tom's just being pedantic. I did word that poorly, but knew you'd understand what I was saying was that there is less air being compressed in your engine's cylinders at high altitudes than at lower ones. Quote Selected
What Octane? Reply #67 – December 31, 2011, 08:13:23 PM The compression ratio does not change. But the cylinder pressures do. Quote Selected
What Octane? Reply #68 – December 31, 2011, 08:26:06 PM That's right. That's what I should have said. :) Quote Selected
What Octane? Reply #70 – January 01, 2012, 01:17:32 AM with our older motors and to answer the original post, I tend to stay clear of higher octanes as of the past couple of years.I tend to stick to the middle grade but now I am seeing EPA fuels (test bed areas like Indiana) where there are like 4 or 5 fuel selections and even one selection that had a different nozzle on the end but it was not diesel. It was some sort of Eco fuel or something... so much to choose from all i know is that the the higher the octane, the odds are against my motor being capable of putting up with it for long periods of time.answer, im using 87 and 89. Quote Selected
What Octane? Reply #71 – January 01, 2012, 01:29:43 AM When you know the actual cylinder compression, and the compression ratio, you know almost exactly what you can and cannot run. Even though the cam does help with cranking pressures, it is not the sole determining factor. You might have a big cam with 8.5 to one compression, or lots and lots of overlap, which will run completely differently then another engine, all else being equal, but with 11 to 1 compression ratio. Then there is the style of piston, heads, quench area ect. There really is no other way to know exactly what octane you can or can't run, until it is running. But guessing off of you compression ratio and actual cylinder pressures, you will be right 99% of the time. Quote Selected
What Octane? Reply #74 – January 01, 2012, 03:47:03 PM Tom, the thing to keep in mind is most of us are running well under 400 HP. Most of us would be happy with 350 HP. For these numbers, we don't have to spend the kind of money that the engines you are building cost. Most of us have no access to a dyno! This is why we share what works for us and what doesn't. I don't know about anyone else, but I don't want to visit Dallas every time I build an engine. Quote Selected
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