How do you reduce the manifold pressure an a normally aspirated engine 
The manifold has vacuum except when the throttle is wide open.
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The SNOWMAN motto don’t let facts get in the way of nonsense. 
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I made a new topic about it here. https://community.cartalk.com/t/can-lower-octane-high-altitude-mountain-grade-gasoline-be-used-with-efi-or-a-turbo/185508
My not using full throttle. In the case of throttle by wire, the computer can not open the throttle all the way and have more precise control than your foot can.
It wouldn’t be unexpected. It would limit your power to something like 85% once pre ignition was detected. That’s all the power you have in Denver Colorado, the mile high city anyway.
The engine has 11 to 1 compression ratio and I use 93 pump gas unless I am at the track. It will run ok on 87 but better on 93. It has a 93 octane “tune” in it.
Manufacturers retard spark because it works, and because it can be accomplished very, very quickly… several orders of magnitude more quickly than diddling the throttle plate. Fact is, spark can be used to modulate idle speed in a fast loop inside the throttle plate loop.
In an old car, 11:1 would have required 96 octane fuel and the drivers foot (and ears) may not be quick enough to avoid damage… Yay computers!
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not to the person doing it, but it would be to the drivers coming up from behind.
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Just for kicks and giggles just what is considered high compression in these modern times
That’s how my 50 year old carb’d Ford truck varies its idle rpm when it needs to prevent overheating during long periods of idling. Bumps the idle rpm, which increases coolant flow and airflow through the radiator. Uses a dual diaphragm distributor, one for advancing timing during accelerations, and the other retards the idle timing except when coolant gets overly hot, then a temp-actuated vacuum switch disables the vacuum to the retard diaphragm, which effectively advances the timing and idle rpm.
13 to 1… Mazda’s Skyactive system… runs on 87 octane I think.
A few experimental programs out there working on spark ignition combined with compression ignition gas engines 14 to 1 and higher.
To be fair, that’s not what he said.
Not familiar with that idea. Could you point me to a fairly simple explanation?
The knock sensor picks up vibration and sound coming from the engine block, turns it into an electronic signal and sends that signal to the engine control unit (ECU). The car’s computer then judges the information and determines whether or not ignition timing should be altered.
This came from here if you want to read the rest.
Tester
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Retarding the spark reduces power by reducing peak pressures inside the cylinder. Pressure makes power and heat so reducing it a bit reduces the heat that causes pre ignition and detonation. A little goes a long way.
Regarding the so-called mountain grade fuel octane, most of it in OK is 87, 89, with a few places carrying 91 and 93.
In Boise City, OK which is the last stop in the far western OK Panhandle before entering NM the gas pumps there carry 85 octane and they are the only ones that do in OK as far as I know.
Boise City has an elevation of over 4000 feet so they are the highest spot in OK other than Black Mesa which is a few miles northwest and is a place rather than a town at 5700 feet.
Current Skyactiv-G is running 14:1.
Their prototype compression ignition gasoline engines are running 16:1
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Ok I’ll concede he was referring to the do not use full throttle with lower than recommended octane. I still don’t believe it was in that manual. But I will maintain he is very misinformed (bordering on spouting nonsense) on almost every other aspect of this discussion. He says he is/was teaching for many years yet apparently hasn’t learned to research the facts on theories he puts out there, other than internet anecdotes, all while admitting he knows very little on the subject. After all if it’s on the internet it has to be true, right? Is that fair enough? If it’s not, tough, grow up, life isn’t fair. If some of the things said by myself seem harsh so be it. The world and life is harsh. Not everyone gets a trophy just for showing up.
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I guess it’s about as fair as this place gets. But the thing is, I don’t want this guy to be able to poke holes in your argument. Otherwise he’ll throw the error back in your face and claim it invalidates any point you were trying to make. That’s how debates–and arguments–work, at least in my experience. To paraphrase Proverbs 15:1, “A factual answer turneth away BS.”
“Life isn’t fair” is not a license to make it less fair. Is that what “grown up” means to you? To me, it’s an acknowledgement that things are not necessarily going to go your way, that not everyone is going to play by the rules. But that doesn’t mean I get a pass.
Regarding delaying the ignition timing, it actually creates more heat since the combusted fuel doesn’t expand as much. But at lower pressure and less time in the cylinder less heat is absorbed in to the cylinder head and piston and more heat is forced out the exhaust. Back in the 20s when engines had 4.5:1 compression power output was very low and I believe it was common for the exhaust to glow sligtly red!
An aluminum cylinder head can have higher compression or more agressive timing because it stays cooler.
Completely wrong. The pressure rise is the same, the amount of fuel burned is the same so the heat is the same. The timing of the peak pressure is later so the work the expansion can do is less.
As for 4.5 CR engines of the early era; I volunteer at an auto museum with cars from 1896 to 1995 Nearly all of these cars run and are exercised regularly. I’ve seen a number of the early cars run and none of them had exhaust manifolds glowing red. Valve timing is so conservative on a 4.5:1 early engine the mixture is completely combusted before the exhaust valves open.
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If all of the energy went to heat then the engine would put out zero power. The gasses cool as they expand, as energy is removed. It’s conservation of energy. According to this guy, delayed ignition timing is one reason why exhaust can glow red. https://www.youtube.com/watch?v=spjjNjRe5K0
Are the antique cars with < 4.5:1 compression ever run at full throttle for several minutes at a time?
Again, wrong. The combustion of fuel and air is ONLY about producing heat. That is its job.
The Otto cycle works as follows… The burning fuel creates heat, the heat expands the combustion gasses to increase cylinder pressure and that pressure pushes the pistons down, expands and cools. During that process, because the combustion chamber and cylinder walls are cooler, some of the combustion heat is lost to the coolant. Once the power stroke is complete, the exhaust valve opens, the residual pressure leaves past the exhaust valve and the rest is pushed out by the rising piston. The relative efficiency of the engine is a function of how effectively that expansion creates work and how little is lost to the coolant and the exhaust. The timing of the spark determines when in the power stroke the peak pressure occurs.
The tolerance between maximum power and detonation of preignition is 1-3 degrees of total engine rotation when the spark plug is firing 25 to 38 degrees or so BEFORE the piston is at top dead center (its highest point). Spark retard to reduce knocking is in the order of 3 to 5 degrees or so depending on the engine and the severity of knock.
Set the timing to fire at 0 degrees (or TDC) and you will get flame out the exhaust as well as red manifolds.
Yes, indeed they are. But let me share a video from Goodwood 2020. Watch this linked video. Tell me how many of these early 20th century cars have glowing red manifolds… (hint, the answer is “None”)
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Difference between carbon build up and a properly designed higher compression engine:
The carbon can build up in the wrong places, interfering with proper gas flow, cooling, etc., and
it can get hot enough to glow and cause ignition too soon and in the wrong places for proper flame propagation - a clean, higher compression engine won’t do this.