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Paul_B

What grade gas are you running?

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HubbardRA
87 octane is good up to about 8.5 to 1 compression on a non-computer controlled engine. 89 is good up to about 9.5 to 1, and 93 is good up to about 10.5 to 1. Most of these older flathead engines only had about 6 to 1 compression, so 87 is fine in them.

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ridgerunner
I get a 55 gallon drum of 110 octane (Torco)for my 67 Ford in the spring. The Big-Ten don't use that much fuel anyway so I just pull fuel out of that. Don't make it run any better so ya might say its a waste of money but it keeps me from draging a can to the gas station. And its leaded so it should help the valve and seat out some anyway being it was made 1965 before hardend seats. And nuthing like the sweet smell of burning 110 while doing the lawn.:D -Paul

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dratkinson
Am I missing something? The blue aviation gas is 100 octane (low lead) and I remember hearing stories about guys that would try to run it in their offroad bikes and melt a hole through the top of the piston. I was told this is because the AvGas burns hotter. So why does the 110 octane not burn a hole in the 67 Ford and the lawn tractor motors? Just curious... /r David in Denver

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nrallen
Must be something about aviation fuel? In the late 60's my brother put it in a real nice chevell,and only made it a few miles before it blew up. I have always used premium fuel in my 1978 16hp briggs. It just seems to run better.

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TimJr
The higher octane fuel does not burn "hotter" by itself, generally speaking. The octane rating is a rating system that determines a fuels ability to resist burning, more specifically against igniting without a spark (pre-ignition or detonation). As stated above, the higher the compression, the more of a need for octane. Spark timing, cylinder head design, camshaft and heat can all affect an engines need for more octane. Using high octane in a stock engine that was meant to run on 87 octane should not! "run better" or "make more power". Now, that does assume the engine is in the correct tune, the chosen gas is clean and fresh, and in running condition as it was meant to be. If cam, compression, timing, or maybe even severe carbon build up are added, that can change the engines needs. As I understand, AvGas is good gas, high in octane, but is designed for aircraft and their much different running conditions than a car or lawn tractor. If you read a Briggs owners manual, it says that the older engines will run safely on 77 octane, but my guess is that is for foreign countries where fuel can be questionable. Too much octane could actually cause an engine to start hard and leave more deposits because the spark and compression may not be effectively igniting the fuel. That isn't probably going to happen unless run for long periods of time like that. I have a hot rodded 7016 for tractor pulling, and like Ridgerunner says, there's nuthin' like mowing the lawn smelling that sweet 110 burning - I just use up that last bit of 110 when the pulling season is done. Tim

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HubbardRA
The higher the compression is on an engine, the faster the fuel will burn. The higher the octane number of a fuel, the slower it burns, but it actually has more energy in it. As the compression ratio is increased the lower octane fuels will burn even faster, and produce pre-ignition, or audible detonation, both are what is commonly called pinging. The higher octane fuels are used to slow down the burn rate in the higher compression engines to eliminate pinging and produce more power. High octane fuels used in low compression engines will make the engine feel like it is running much smoother, but will actually produce less power. When I was tractor pulling with motorcycle engines, I tried several fuels to find which produced the most power. I started with 110 avgas, then 100, then 93, then 89, then 87. My engine had 9.25:1 compression. I ended up running 89 octane. Couldn't see any difference between 87 and 89, but I had actually gained power as with each time I lowered the octane till I got to 89. I have also had this discussion with many of the modified, outlaw, and pro-stock friends and they all came to the same conclusions.

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ridgerunner
I almost felt like I was being called out. Theres two reasons I get the 110. First with the 67, it runs a 1970 boss 351 Cleveland with 12-1/2 compression, quenched chambers, triple valve springs and 2.25" valves. Its brutal, but any thing under 110 pre-ignites and runs hot quick. The second is I run a 77 Ford at local mud bogg competitions with a 200 horse shot of nitrous. It runs mild compression (9-1/2) runs fine on 92 but with the extra squeez from the bottle I need to turn back the timing 12 deg. and need the higher 110 octane. At the end of summer I allways end up with extra fuel anyways. So running it in the tractor, weedeater, chainsaw or the push mower don't hurt em, I've been doing it for years. Realy you would burn holes in pistons from running to low of octane not the other way around. I've herd the same stories with the aviation fuel, a friend of a friend of a friend told me that............. Took them as wives tails.

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dratkinson
Not being called out here, Paul...just trying to beat back my ignorance. I too remember when gas was sold at a higher octane than what is available at service stations today. And I remember when gas use to smell better...even when just filling the tank. The story about the AvGas and wrecked motorcycles came to me from an instructor pilot and I had no reason to doubt him. So I just have small pieces of information from many different times, sources, and was trying to make them fit into one logical whole. Not there yet, so I guess I'll just keep using the regular unleaded in Baby Alice that I also use in the car. ...whatever comes out of the gas pump. /r David in Denver

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HubbardRA
Ridgerunner, I was not trying to call you out. If you have 12.5:1 compression, then you should be using at least 110 octane. I was just trying to explain that octane needs vary with compression. A friend of mine has a pulling tractor with 13:1 compression. He runs either avgas or CamII. He has to run at least 110 octane. You know what you are doing with those machines. I also have done tractor pulling, drag racing, and circle track racing. I was just trying to clear up some observed misconceptions (not by you). I just wanted to make sure that those who don't understand octane numbers understand that high octanes were not needed in these old flatheads with only around 6:1 compression. It will run fine if you just happen to have some lying around.

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dlcentral
I dont really know if those rumors are true but I do know that if you spill some blue avgas on your hand, it evaporates like right now! and leaves a white residue on the skin.My friend who runs grounds maintenance dept. at the local airport, put some in a Toro Groundmaster,because he had no reg.gas at the time and cooked the eng pretty quickly,within 20 mins or so, dunno,seems like it has no upper cyl. lube capabilties in it like reg gas does?,or maybe exhaust temps are much higher with it?,or prob both are true but,it's wierd stuff for sure,,

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TimJr
My favorite fuel story is when a friend told me about a time when he and his buddies once had a "pretty hot" Camaro. They were apparantly street racing on an old country road. They had been drinking (real smart anyway) and figured that the moonshine, which they said was almost pure alcohol, would be great to run in the car. Race cars run alcohol, so that should be just as good. He said it ran like crazy, they beat everyone there. I just smiled and nodded my head. Guess he didn't know that race cars use methanol, which I don't think he was drinking, and that alcohol needs a mixture of about 6 to 1 air to fuel, which is twice as rich as typical gas engines which is around 14 to 1. So, seeing how he omitted the jet changes they needed to do, I didn't believe him. Fuels are a real science at times, and I don't claim to be the smartest, but some things are just too outrageous. Tim

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MikeES
Rod, I found the same thing with our pulling tractors. They pull best with 92 octane. I do not know what our compression ratios are. But they did not have as much power on Turbo Blue high octane gas. To Tim's point. That is why on like on Super Stock pulling tractors they burn Alcohol-with the 6:1 ratio you do not need as much air which is the main restrictor to engine hp (you can get plenty of fuel into the engine but you can't get enough air).

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TimJr
Yeah, and then you get to step all the way up to nitromethane, which burns at about 2:1. It's expensive, but cool. The alky Supers have come a long way in the last 15-20 years. A couple of my old favorites were when Doug Roberts had The Barnyard Beast JD, or Terry Blackbourne with the Slowride when it was Magnum bodied. Can't wait 'till Bowling Green! Tim

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TimJr
Yeah, and then you get to step all the way up to nitromethane, which burns at about 2:1. It's expensive, but cool. Plus, alcohol has a high "octane" rating. It burns "cool" compared to gas because of the sheere amount of fuel load required to get any power. The fuel can actually act as a heat sink to help carry heat out of the engine. Hard to explain. I can't remember the BTU's for gas, alcohol and nitro. I would like to see Paul's vehicles - they sound fun! Long live '77 Ford trucks! The alky Supers have come a long way in the last 15-20 years. A couple of my old favorites were when Doug Roberts had The Barnyard Beast JD, or Terry Blackbourne with the Slowride when it was Magnum bodied. Can't wait 'till Bowling Green! Tim

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