How to Make Big Power on Low Octane
#1
How to Make Big Power on Low Octane
It's really quite simple making the power and designing an engine to run on low octane fuel is as simple as paying attention to the engines piston/head clearance AKA quench, the quench area is the area between the flat part of the piston and the flat part of the combustion chamber, some people also call this the 'squish band' but it all means the same thing, the domed or concave section of the cylinder head area is thew actual combustion chamber and is the place where you want (100%) of the combustion taking place, the air/fuel mixture doesn't explode inside the chamber but instead burns very rapidly, this rapid burn is also called the flame front, ideally the flame front propagates or rolls across the top of the piston forcing the piston down into the bore but if there is too much area between the flat sections of the piston and head this allows the burning a/f mixture to burn elsewhere besides the chamber and this is not good thing and allows for pre-ignition and/or detonation, the key is to minimize the squish band or quench area and force all of the A/F mixture into the Combustion chamber where it belongs, optimizing the squish band allows you to use moderate to high compression ratios and plenty of ignition advance with lower octane fuels, piston crown design comes into play and should be a mirror image of the combustion chamber area of the head. You may have heard the term "zero deck", what this means is any piston at TDC in the block is level with the top of the block, negative deck means the piston is below deck at TDC and positive deck means the piston is above the deck at TDC, having positive deck is not good and another block should be used but for most of you the problem will be excessive negative deck where the piston is not reaching the top of the block at TDC and when you combine a .035" compressed thickness head gasket you multiply the problem by moving the head even further away from the piston, when calculating piston/head clearance you must take rod stretch into account, aluminum rods stretch farther than steel rods so the quench must be adjusted accordingly, it you know how much rod stretch you will have at maximum rpm and you know what the compressed thickness of your head gasket is then it is easy to calculate the proper deck height for the piston which will give you minimum quench without fear of piston/head contact, it doesn't matter if you only have .001" clearance at 7,000rpm as long as it doesn't touch, piston to valve clearance is a whole other ball of string and must also be fed into the calculation but if you take your time and do it right you can build an 11:1 motor that'll run all day on 87 octane with full timing, you may have noticed an engine ion your travels that would ping if too much advance was dialed in, that would indicate excessive quench for the installed compression ratio, on the other hand you may have ran across an engine that would not make power without excessive advance, that indicates poor piston/head choice and very slow combustion, become one with the engine and you will begin to think like it does.. enjoy..
#2
#3
Modern engines built in the last few years are designed to use fuels with low octane ratings while putting out a lot of power (eg, the new 5.0L HO engine that comes in the Mustang GT). They achieve this through very careful head design. Some basic requirements:
A fast burn combustion chamber. Ideally, you want the combustion to occur instantaneously JUST after TDC so the expanding gas will act on the piston as early as possible. But in reality, it takes some time to combust the AF mixture in a typical chamber, and the timing is set to allow the maximum pressure to build up right at TDC. This produces a lot of waste heat, which promotes pre-ignition.
One trick is to put the spark plug in the center of the chamber, so the flame front has less distance to travel to consume all the fuel. In the best case, the centrally located plug will require half the time to combust all the AF than a plug located to the side of a chamber. The burn rate is doubled, and you can reduce timing advance appropriately. Chevrolet did this in the 1990 ZR1 engine. In the extreme case (F1 engines), the combustion occurs so rapidly and completely that there is no time for detonation to occur.
Making the chamber smaller (like making a large quench area) will also help increase burn rate. Obviously, you can't squish the AF so hard that it starts to diesel.
This is an oversimplification, as there are lots of details involved in a fast-burn chamber. But it's not an easy thing to implement on the typical wedge head engine.
A fast burn combustion chamber. Ideally, you want the combustion to occur instantaneously JUST after TDC so the expanding gas will act on the piston as early as possible. But in reality, it takes some time to combust the AF mixture in a typical chamber, and the timing is set to allow the maximum pressure to build up right at TDC. This produces a lot of waste heat, which promotes pre-ignition.
One trick is to put the spark plug in the center of the chamber, so the flame front has less distance to travel to consume all the fuel. In the best case, the centrally located plug will require half the time to combust all the AF than a plug located to the side of a chamber. The burn rate is doubled, and you can reduce timing advance appropriately. Chevrolet did this in the 1990 ZR1 engine. In the extreme case (F1 engines), the combustion occurs so rapidly and completely that there is no time for detonation to occur.
Making the chamber smaller (like making a large quench area) will also help increase burn rate. Obviously, you can't squish the AF so hard that it starts to diesel.
This is an oversimplification, as there are lots of details involved in a fast-burn chamber. But it's not an easy thing to implement on the typical wedge head engine.
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